Abstract
Grazing cover crops recouples crop and livestock production with the potential to enhance soil nutrient cycling processes. However, critical knowledge gaps on short-term N release dynamics and soil health outcomes upon adoption currently limit the application of integrated crop–livestock systems in organic vegetable production. The goal of this study was to characterize the benefits and potential tradeoffs of integrating sheep grazing of cover crops in organic vegetable production for soil ecosystem processes, N cycling, and underpinning shifts in microbial communities. We conducted a replicated experiment over 4 years of a vegetable rotation with three treatments: winter fallow, ungrazed cover crop, and grazed cover crop. We found that grazing cover crops did not significantly impact soil physical characteristics such as compaction. While organic carbon pools remained unchanged, grazing increased soil nitrogen in dissolved organic, inorganic, and microbial pools at key moments in crop production, exceeding levels observed under ungrazed cover crops. These increases were 22%, 36%, and 21%, respectively, at 0–15 cm; and 44%, 100%, and 22%, respectively, at 15–30 cm. Grazing did not lead to increased potentially leachable nitrate despite greater inorganic nitrogen pools during the cropping season. Cover crops, both grazed and ungrazed, lowered soil pH compared to fallow. Soil microbes responded rapidly to shifts in resources associated with grazing, with increased relative bacterial abundance, especially Gram (+) (+34%), and a decrease in the fungi:bacteria ratio (−64%) compared to no livestock integration. There was a trend toward less carbon in the particulate organic matter fraction (POM-C) after 4 years of winter grazing, highlighting the need for longer-term assessments. This research suggests that organic farmers can utilize grazing to strategically improve the timing of nitrogen release for their vegetable crops, with minimal tradeoffs in terms of physical properties, when grazing implementation follows best management practices.
Highlights
Sheep grazing of cover crops reconnects crop and livestock production, with benefits for nitrogen availability in organic vegetable systems.
Grazing enhanced nitrogen in dissolved organic, inorganic, and microbial pools during periods of peak crop demand.
Increased nitrogen availability was achieved without detectable increases in soil compaction or nitrate leaching risk.
Soil microbial biomass remained unchanged, but communities shifted to greater bacterial dominance and reduced fungi:bacteria ratios.
Cover crop grazing provides a practical management strategy to optimize nitrogen release timing in tilled systems, with an absence of short-term soil health tradeoffs.
1 Introduction
Large-scale specialization within our food system has led to the separation of crop and livestock production, where crops for human consumption and animal feed are mostly produced on prime farmland, while animals are raised in confinement operations or grazed in extensive rangeland systems (1, 2). As a result, nutrient cycling between animal and crop systems is largely disconnected across temporal and spatial scales. Crop systems underutilize animal waste to support crop growth, leading to higher fertilizer costs, losses of nutrients, and environmental degradation (3). Meanwhile, intensive livestock enterprises create valuable nutrients, yet excess concentrations lead to storage and pollution challenges (4), while extensive systems may not be able to support livestock production throughout the seasons without supplemental feed (5). Integrated crop–livestock systems (ICLSs), in which animals graze on cropland, offer an opportunity to reconnect landscape-scale nutrient cycles, conserving carbon (C) and nitrogen (N) within a system while providing forage. Animals in ICLSs consume unharvested plant biomass, such as cover crops, weeds, crop residues, or even crops themselves, in their early stage (dual-purpose crops) and return nutrients in urine and manure (6), cycling plant biomass nutrients in situ while lowering the risk of nutrient excesses from intensive operations and further expansion of agriculture. In large-scale ICLSs, there are few negative impacts on crop yields from grazing, compared to non-integrated systems (7). This type of diversification may be especially relevant to the growing organic vegetable industry, which could implement grazing to address key management challenges associated with fertility management and weed control (8).
Grazing influences key soil processes regulating soil organic carbon (SOC) and soil organic nitrogen (SON) cycling and stocks (9). Animal grazing alters soil inorganic N (SIN) cycling and loss potential by lowering the C/N ratios of plant residues through the conversion into manure and urine via ruminant digestion and CO2 respiration losses (10). Animal-derived nutrients in organic forms quickly mineralize, making N more plant bioavailable (11–13). Plant responses to grazing, both aboveground and belowground, also alter the quantity of C and N entering the soil (14–17). Although grazing leads to lower aboveground biomass inputs (18), root products (including biomass, exudates, and sloughed-off dead roots) often increase with grazing (18–21). Grazing can also keep plants at earlier developmental stages, when root exudation is greater, potentially contributing to greater dissolved organic C (DOC) inputs over the season (22). As such, grazing has the potential to increase soil organic carbon and significantly shift the distribution of carbon from particulate to more stable mineral-bound SOC (23).
Microbial communities are key drivers of C and N pools and shifts in the soil ecosystem, and grazing can have varying impacts on soil microbial community diversity, composition, and functioning according to grazing duration and intensity (24–26). In vegetable crop systems, these responses may be particularly dynamic because grazing often occurs within cover crop phases that are followed by tillage, residue incorporation, and rapid nutrient demand by subsequent cash crops. Grazing can alter microbial habitat and substrate availability by reducing standing residue inputs while returning more labile C and N through manure and urine deposition, potentially stimulating microbial biomass, bacterial activity, and short-term N mineralization (24–26). At the same time, repeated disturbance and residue removal may shift community structure away from fungal-dominated pathways and toward bacterial or Gram (+) bacterial groups that are better adapted to fluctuating resource availability and disturbance (27, 28). However, most studies to date have been performed in grasslands, which differ from cropland in terms of disturbances, management practices, and goals, leaving key knowledge gaps on understanding short-term nutrient pool dynamics, soil organic matter (SOM) accrual potential, and microbial responses to adoption of ICLSs.
Successful animal integration therefore requires context-specific and precise management of the timing and duration of grazing to maximize benefits and lower potential tradeoffs for soil health and nutrient status. For instance, while high-intensity grazing can increase compaction (29), research in no-till grain systems measured no biologically detrimental increase in bulk density with grazing at lower intensities (30), and integration can even decrease compaction in some cases (31). Aggregate stability has been shown to increase or remain unchanged with grazing (32–34). The absence of tradeoffs and potential benefits to soil physical properties, especially in tilled vegetable systems, may therefore outweigh the risks if grazing is managed following local best management practices, particularly regarding stocking rates and soil moisture when grazing. Studies examining the effect of ICLSs on net primary productivity (7, 31, 35), decomposition, and soil C and N pools have shown that a moderate grazing intensity can lead to C and N storage (36–38) and greater availability of labile C and N for grain crop production (25, 39). Therefore, moderate grazing on cropland could allow for a tightly coupled N cycle, which has a high proportion of N in SOM and less in inorganic forms, leading to low potential for N loss, yet adequate and well-timed N availability during the growing season (40).
The goal of this study was to elucidate how adoption of sheep grazing of winter cover crops in tilled, organic, vegetable cropping systems alters nutrient pool indicators of N cycling, N availability, microbial communities, and critical soil health indicators of relevance to growers during the livestock integration transition phase. We implemented a field experiment to compare three winter soil cover treatments over 4 years in an organic vegetable crop system: a) ungrazed cover crop (ungrazed CC), b) cover crop grazed by sheep (grazed CC), and c) winter fallow. We hypothesized that grazing would minimally impact soil physical properties while shifting nutrient bioavailability through 1) greater concentrations of bioavailable C and N [DOC and dissolved organic N (DON)] and 2) higher microbial biomass [microbial biomass C (MBC) and microbial biomass N (MBN)], which would 3) increase the proportion of stable C and 4) mitigate the risk of N loss via nitrate leaching. We also hypothesized that microbial community composition and activity would reflect the more labile nutrient status in the grazed system and rapidly shift upon implementation.
2 Materials and methods
2.1 Site description
The field trial was implemented at the Russell Ranch Sustainable Agriculture Facility at the University of California, Davis, in northern California (USA). The site is located in a Mediterranean climate characterized by wet, cool winters, with an average December temperature of 2.9 °C, and hot, dry summers, with an average July temperature of 33.7 °C (41). Average yearly precipitation is ~440 mm year−1, and the site received variable precipitation in the 2018–2019 (731 mm), 2019–2020 (295 mm), 2020–2021 (133 mm), and 2021–2022 (401 mm) field years (https://www.cimis.water.ca.gov, Station #006, Davis). The trial was located along the Putah Creek alluvial fan, which has two soil types: Yolo silt loam (fine-silty, mixed nonacid, thermic, Typic Xerorthents) and Rincon silty clay loam (fine, montmorillonitic, thermic, Mollic Haploxeralfs) (41). Textural analysis showed the site has a clay texture (27% sand, 32% silt, and 41% clay) in the top 30 cm.
2.2 Experimental design
The experiment was implemented in an organic certified field and arranged as a randomized complete block design with four replicates of three winter management treatments: ungrazed CC, grazed CC, and fallow in a 4-year vegetable rotation (Supplementary Figure 1A). Cover crops were seeded using a Tye drill in November of each year and relied on winter rainfall for germination and growth. Composted poultry litter was the only fertility added to the system and was applied in all treatments at cover crop seeding in the last 3 years of the experiment. In the spring, all beds were disced, leveled, and planted in the summer crop.
In Year 1, the cover crop mix (30% fava bean, 30% field pea, 20% vetch, and 20% oats) was grazed once, no compost was applied, and maize was grown. In Years 2 and 3, the cover crop mix (16% annual ryegrass, 16% common vetch, and 67% field pea) was grazed by sheep twice, and processing tomatoes were grown with poultry litter compost (1.8N-3P-2K % by weight) at rates of 9 and 20 t ha−1 in fall 2019 and 2020, respectively. In Year 4, the cover crop mix (75% cereal rye, 12.5% crimson clover, and 12.5% daikon radish) was grazed once, poultry manure was applied at a rate of 20 t ha−1, and cucumbers were grown. Shifts in cover crop composition and the increase in compost application rates were necessary to better meet organic fertility and forage needs as the system developed and bring management closer to common industry standards for organic vegetable production in the region. Tomatoes were transplanted, while maize and cucumber were direct-seeded. All crop systems were managed with tillage and irrigated using buried subsurface drip tape according to standard practices.
Sheep from the UC Davis Sheep Teaching and Research Facility were used for this experiment. Daily husbandry included rotational grazing across 15 acres of irrigated pasture. To meet precise nutritional demands, the primary forage feedstocks consisted of alfalfa and oat hay, supplemented with grain concentrates during periods of increased metabolic need such as late gestation, lactation, and breeding seasons. Stocking rates varied based on soil moisture and forage availability to achieve best practices and moderate grazing intensity, which is defined as a post-graze sward height of approximately 20 cm (42). The sheep were a mixture of Dorset, Suffolk/Hampshire crosses, and Dorset/Southdown crosses that were 1–5 years old. Sheep grazed for 8 days in Year 1 (37 sheep ha−1 day−1) and 1–3 days in Years 2–4. In Year 2, the first grazing event was stocked at 67 sheep ha−1 day−1, and the second event was stocked at 100 sheep ha−1 day−1. In Year 3, sheep were stocked at 65 and 75 sheep ha−1 day−1 during the first and second grazing events, respectively. In Year 4, sheep were stocked at 495 sheep ha−1 day−1 for one grazing event.
2.3 Soil sampling
We presented results for various soil physical, chemical, and biological properties and evaluated labile soil carbon and nitrogen pools closely in Years 2 and 3. In Year 4, a final assessment of C pools was conducted. Two subsamples per plot (taken ±1 m apart from each other) were collected using an auger and composited to form each soil sample. Soil was collected 30–40 cm from the center of the 3-ft. beds. In Years 2 and 3, half the plots had two samples per plot, while the other half had only one sample per plot. Unbalanced sampling within-plot samples were accounted for at the plot level in the statistical analysis. In Year 4, one sample was taken per plot. Bulk density samples (0–15 cm) were taken using a slide hammer at peak crop nutrient uptake. Soil was kept on ice and at 4 °C until processed and at −20 °C for phospholipid fatty acid (PLFA) analysis.
Soil samples in Year 2 (0–15, 15–30, and 30–60 cm) were collected and analyzed at four timepoints: a) prior to first grazing event (PreG1), b) prior to second grazing event (PreG2), c) at crop transplanting (CropT), and d) at peak crop nutrient uptake (NutrientU) (Supplementary Figure 1B). Soil samples in Year 3 (0–15 and 15–30 cm) were collected at a) CropT and b) NutrientU. Soil samples in Year 4 (0–15, 15–30, and 30–60 cm) were collected at NutrientU. PreG1 and PreG2 soil was collected within 9 days before grazing events; CropT soil was collected within 2 days after the crop was transplanted or seeded; NutrientU soil was collected within ±1 week of the crop’s peak nutrient demand.
2.4 Soil physical properties
Bulk density was calculated from the mass of the dry soil core and the core volume. Gravimetric water content (GWC) was determined by drying soil at 60 °C until stable weights were obtained.
Soil wet aggregate stability was determined using an automatic soil sieve with a rainfall simulator (Fritsch Analysette 3 Pro, Idar-Oberstein, Germany) (43). Briefly, 30 g of 8-mm sieved and then air-dried soil was sieved through the machine into four fractions of >2 mm (large macroaggregates), 2–250 µm (small macroaggregates), 250–53 µm (microaggregates), and <53 µm (silt and clay) with the amplitude set to 0.5 mm. First, the rainfall simulator was turned on for 15 s, and then the shaker was turned on for 15 s in conjunction with the rainfall simulator. The >2-mm fraction was manually sprayed with deionized water (DI) until it ran clear (~30–60 s), and then it was gently sprayed from the 2-mm sieve using DI into a metal loaf tin. Then, the rainfall simulator and shaker were turned on for 15 s simultaneously for the next two fractions. The four fractions were dried at 60 °C for 2 weeks until the weights stabilized.
The mean weight diameter, a weighted average of aggregate stability, was calculated using Equation 1 (44):
where Di is the average diameter of particles in each i-level aggregate fraction and Pi is the percentage of that fraction in each sample.
2.5 Soil chemical properties
Fresh soil was sent to a commercial laboratory (Ward Laboratories, Kearney, NE, USA) to evaluate pH (1:1 v/v method) and electrical conductivity (EC; mmho cm−1) using a 1:1 soil suspension with a 30-min reaction time. Soil macronutrients used as indicators of soil fertility were determined in air-dried soil using extraction methods followed by inductively coupled plasma optical emission spectroscopy (ICP–OES) analysis (45), including available P (the Olsen bicarbonate extraction, mg kg−1) and potassium, calcium, magnesium, and sodium (ammonium acetate extraction, mg kg−1). Cation exchange capacity (meq 100 g−1) was measured via ammonium acetate extraction and exchangeable cation displacement.
2.6 Soil inorganic nitrogen pools and cycling
Several inorganic N pool indicators of N cycling were measured. SIN was extracted from field-moist soil with 2 M KCl and analyzed for ammonium (NH4+) and nitrate (NO3−) contents (46, 47). Potentially mineralizable nitrogen (PMN) was measured at CropT in Year 2 using anaerobic incubation for 7 days (48).
Nitrate leaching was measured in the second year (2019–2020, Supplementary Figure 2) using anion exchange resin beads (AmberLite™ PWA5 Resin) “pucks” placed at a depth of 65 cm for 11 months, from October 2019 to September 2020, following the methods in Woodward et al. (49) (Supplementary Figure 2). Each plot had six pucks installed under an undisturbed soil column within 5 ft. of each other. Nitrate was extracted from the resin beads by taking a subsample of 6 g, adding 30 mL of 2 M KCl, and shaking on a reciprocal shaker for 1 h. Beads were allowed to settle, and the supernatant was decanted. Two more rounds of extraction using 2 M KCl were conducted. Extracts were analyzed for nitrate using colorimetry following Doane and Horwáth (46). Potentially leachable nitrate (PLN) was calculated by adding concentrations of all three extracts, which was extrapolated to the field scale using Equation 2 (49):
where PLN is potentially leachable NO3− (kg ha−1), MNO3–N is the mass of NO3− in each resin puck (g), and Ap is the area of the resin puck (0.00466 m2). At resin installation and removal, soil samples were taken at 0–15, 15–30, and 30–60 cm to obtain residual SIN.
2.7 Soil organic nutrient pools
Soil organic C and N pools (DOC, DON, MBC, and MBN) were measured using the chloroform fumigation extraction method on 50 g of fresh soil (50). Extracts were stored at −20 °C until analysis on a TOC/TNb analyzer (Vario TOC cube, Elementar, Langenselbold, Germany). Microbial biomass carbon and nitrogen were calculated as the difference in dissolved C and N fractions between the fumigated and unfumigated samples, divided by a conversion factor, as follows (Equation 3) (50):
2.8 Soil organic matter fractions
Soil organic matter fractions [mineral-associated organic matter (MAOM) and particulate organic matter (POM)] were measured on 10 g of 4-mm, air-dried soil via aggregate dispersion and particle size fractionation on a wet sieve (43). First, soil was shaken with 30 mL of 5% sodium hexametaphosphate and five glass beads for approximately 20 h on a reciprocal shaker to disperse aggregates. The soil slurry was poured on top of a 53-μm sieve on an automatic soil sieve with a rainfall simulator (Fritsch Analysette 3 Pro, Idar-Oberstein, Germany) and shaken with the rainfall simulator until the water ran clear (~30 s). The <53 μm (MAOM) and >53 μm (POM) fractions were oven-dried at 60 °C, weighed, ground, and analyzed for total organic carbon (TOC) and total organic nitrogen (TON) via combustion (ECS 4010, Costech Analytical Technologies, Inc., Valencia, CA). The concentration of carbon in each fraction [Mineral Associated Organic Carbon (MAOC) or particualte Organic Carbon (POC)] (Xi) was calculated using Equation 3:
where Fi is the oven-dried (OD) weight of each i-level fraction (either POM or MAOM) in grams, % Ci is the percent carbon of each i-level fraction, and SoilOD is the total OD weight of soil recovered after running the protocol. The OD weight recovered was between 98% and 102% of the initial air-dried soil weight. The concentration of nitrogen in each fraction (MAOM-N and POM-N) was calculated in the same manner.
2.9 Soil microbial community
Soil microbial community was characterized using a chloroform–methanol lipid extraction and gas chromatography (51) to complete PLFA profiling of Gram (+) bacteria (14:0 iso, 15:0 iso, 15:0 anteiso, 16:0 iso, 17:0 iso, and 17:0 anteiso), Gram-negative bacteria (16:1 ω7c, 17:1 ω8c, 18:1 ω7c, and 18:1 ω5c), arbuscular mycorrhizal fungi (16:1 ω5c), saprophytic fungi (18:2 ω6c), and actinomycetes (16:0 10-methyl, 17:0 10-methyl, and 18:0 10-methyl) by Ward Laboratories (Kearney, NE, USA). The total microbial biomass was the sum of the abundances of bacterial and fungal biomarkers up to 20 C chains and with no more than two branches (52).
2.10 Statistical analysis
Statistical analysis was completed in R version 4.3.0 (53). The variance of response variables was apportioned to a fixed effect (winter treatment) and random effects (plot and block), and the function “lme” in the R package “nlme” was used to construct linear mixed-effect models. For soil chemical and biological composition data, separate models were run for each depth per timepoint due to highly significant depth fixed effects. For response variables with multiple sampling timepoints, date was included as a fixed effect along with the respective interaction terms. For response variables measured at a single timepoint (PMN, PLN, TC, TN, MAOC, and POC), fixed effects included treatment, depth, and their interaction.
Potential outliers were detected using a graphical boxplot display. The rosnerTest was used with the suspected number of outliers, k, and outliers were removed if they were statistically confirmed and made biological sense. Response variables were transformed if residual plots indicated violated assumptions of heterogeneity and normality of residuals. Actinomycete biomass (Year 2 NutrientU) and PMN (Year 2 CropT) were transformed using a sqrt(x). Response variables were analyzed using analysis of variance (ANOVA) at a 5% type 1 error probability level. Adjusted post-hoc Tukey’s tests were used for mean comparisons in the R package “emmeans”. A heatmap was generated by reshaping and standardizing the PLFA data matrix using the melt() function in the reshape2 package and the scale() function for visualization in ggplot.
3 Results
3.1 Soil chemical and physical properties
Soil physical characteristics did not differ between treatments after 3 years of implementation. Bulk density was not impacted by cover crops or grazing (Figure 1A), and the mean weight diameter (MWD) of aggregates was not different between treatments (p = 0.6510) or depth (p = 0.143) (Figure 1B). Most soil chemical properties were not strongly impacted by cover crop or grazing during the transition phase (Supplementary Figure 3). Salinity (EC) was low across all treatments but was slightly higher in soils after cover crops at 0–15 cm in Year 2 (Supplementary Figure 3) but not in Year 3 (Figure 1C). Soil pH was lowered by cover crop, whether grazed or not, in Year 3 at peak nutrient uptake (Figure 1D, p = 0.0475). Soil CEC, P, K, Na, Ca, and Mg were not significantly altered by treatment at any timepoint (Supplementary Figure 3).
Figure 1
3.2 Soil N dynamics and availability
Cover crops and grazing significantly altered SIN pools compared to fallow. Prior to grazing in Year 2, when the cover crop was actively growing, cover crops reduced residual SIN at 0–15 and 15–30 cm, with no treatment differences observed at 30–60 cm (Figure 2A). After the first grazing event, the cover crop effect persisted with grazed and ungrazed CC having significantly less SIN at 15–30 and 30–60 cm. At crop transplanting, after cover crop incorporation, grazed CC and ungrazed CC plots had greater SIN than fallow at 0–15 cm (+19 mg N kg−1 dry soil, p = 0.0345). Ungrazed CC soil had greater PMN at 0–15 cm than the other two treatments at that timepoint (Figure 2B). At peak nutrient uptake, the cover crop effect persisted with grazed CC and ungrazed CC plots having greater SIN than fallow at 0–15 and 15–30 cm. Shifts in SIN did not lead to significant differences in PLN between the treatments (Figure 2C).
Figure 2
Three years after treatment implementation, non-grazed CC increased the SIN pool at crop transplanting (15–30 cm) by threefold (+6 mg N kg−1 dry soil compared to fallow; p = 0.0394) (Figure 2D). At peak nutrient uptake, grazed CC had four times the SIN compared to fallow (0–15 cm) (+34 mg N kg−1 dry soil; p = 0.0442), whereas the ungrazed CC plots had intermediate levels of SIN (35.26 mg N kg−1 dry soil). At the 15–30-cm depth, SIN in grazed CC plots was three times greater than in fallow (26.57 compared to 7.92 mg N kg−1 dry soil; p = 0.0394).
3.3 Dissolved organic pools and microbial biomass
DOC in the topsoil (0–15 and 15–30 cm) was not altered by treatment at any timepoint 2 and 3 years after implementation (Figures 3A, B). In Year 2, grazed CC had significantly lower DON at 0–15 cm than fallow pre-grazing (p = 0.0299), showing a residual grazing effect from Year 1 (Figure 3C). At crop transplanting, the trend reversed, and grazed CC had significantly higher DON than fallow at 15–30 cm (p = 0.0168), with ungrazed CC being intermediate. At peak crop nutrient uptake, the cover crop effect persisted, and the ungrazed CC treatment had greater DON at 0–15 cm compared to fallow (p = 0.0436), with grazed CC showing intermediate levels. In Year 3, at peak nutrient uptake, grazed CC plots had approximately double the concentration of DON than the fallow plots at 0–15 (p = 0.0412) and 15–30 cm (p = 0.0071), with ungrazed CC being indistinguishable from both (Figure 3D).
Figure 3
Two years after implementation, grazed CC had significantly greater MBC than fallow at 0–15 cm at crop transplanting (p = 0.0426; Figure 4A). By peak nutrient uptake, this trend was no longer apparent (Figure 4A). In Year 3, at crop transplanting, there was a non-significant trend toward lower MBC at 15–30 cm with grazing (p = 0.46276), with this trend reversing at peak crop nutrient uptake (Figure 4B). There were no differences in MBN between winter treatments at any timepoint in Year 2 (Figure 4C). In Year 3, at peak nutrient uptake, cover crops doubled MBN at 0–15 cm compared to fallow, and at 15–30 cm, grazing doubled MBN compared to fallow (p = 0.0175) (Figure 4D). The MBC/MBN ratio was not impacted by treatment, while the DOC/DON ratio tended to be lower at peak nutrient uptake in the CC treatments compared to fallow (Figures 5A, B).
Figure 4
Figure 5
3.4 Total C and N fractions
Total soil C, N, MAOM, and POM all decreased with depth (Figures 5C–F). In Year 4, we found no significant differences in total soil C, total soil N, or C in MAOM across depths. Notably, carbon in the POM fraction was lower at 0–15 cm in the CC treatments (Figure 5F).
3.5 Soil microbial communities
Shifts in the biomass of microbial groups due to grazing and cover crops as determined with PLFA markers were generally not significant (Supplementary Figure 4) in Years 2 and 3. The diversity of PLFA markers was also unaffected by treatments across years (data not shown). Although microbial PLFA diversity and biomass remained unchanged in both years (Supplementary Figure 5), significant differences were observed in the fungal-to-bacterial (F:B) ratio and the Gram (+)-to-Gram negative (GP: GN) ratio during peak nutrient uptake in Year 3 (Figure 6). Grazing decreased the fungal:bacterial ratio, suggesting that bacteria became dominant under grazing treatment (Figure 6A). Grazing was also associated with an increase in the GP: GN ratio (Figure 6B).
Figure 6
4 Discussion
4.1 Grazing had negligible impacts on soil physical properties in the short term, if well managed
While compaction when grazing remains a main concern when adopting this practice, we observed no differences in soil compaction or structure with livestock integration. The absence of shifts in soil aggregation aligns with one ICLS study conducted under conventional tillage, which reported that despite soil penetration resistance being greater, aggregate stability was unaffected by grazing (34). Optimal grazing management in our experiment limited short-term potential tradeoffs for soil compaction, but frequent tillage coupled with the short experimental period may have hampered aggregate formation and our ability to detect positive shifts in MWD, which have been observed after 30 years of ICLS implementation (27). In less disturbed systems such as grasslands, bulk density has been observed to increase or be unaltered with grazing (9). Our systems were tilled, and strong interactions of treatments with tillage practice likely regulate these physical properties. Different outcomes may be found in no-till systems, which generally maintain more porous soil structures and ecologies at the soil surface. While the short-duration, high-density grazing operations practiced in this study minimized negative externalities, they may not be practical on a working farm where a farm’s land base may be insufficient to limit the grazing period for a herd to 3 days or less per season. This may be more feasible in systems contracting grazing services, bringing animals onto a farm specifically for cover crop grazing.
4.2 Grazing enhances the timing of N release without increasing leaching potential
Cover crop use and grazing provided an opportunity for balancing common tradeoffs in N cycling and availability in organic vegetable systems (54). We found that cover crops (which can take up between 29 and 77 kg N ha−1; 55) decreased SIN during the cover crop phase and increased both organic and inorganic N pools during the cropping season. Although there was reduced SIN during cover crop growth, we did not observe a reduction in nitrate leaching with CC or with grazing. In contrast, one study found significantly higher NO3−–N below the rooting zone after 3 and 4 years of livestock integration (56). Inorganic N from urine could be a source of N loss and could contribute to rapid mineralization of organic C and N, as urea has high loss potential if not synchronized with crop demand (11, 57). Studies in extensive range systems have shown that grazing can lead to N immobilization in microbial biomass, allowing for later synchronous release of N with plant demand and no additional NO3− leaching concerns (58, 59). In our trial, the average mean leachable nitrate was 32.6 kg N ha−1, which is comparable to mean leaching values between 21.8 and 58.2 kg ha−1 previously observed at our site (49). The lower leaching potential of the silt loam texture at our site (60) combined with well-managed drip irrigation likely minimized deep percolation, and thus, cover crops provided less mitigation of nitrate leaching than they may have under more vulnerable environments. High amounts of soil NO3−, in conjunction with soil C, can also lead to denitrification and N2O emissions, which could be increased with grazing (61), although this loss pathway was not evaluated in our study.
We expected grazing to accelerate the synchronous release of bioavailable N with summer row crop demand, as grazing alters the quantity and quality of organic inputs (12, 39). Although not measured, grazing at light-to-moderate intensities can increase root turnover and root mass (62, 63) and plant exudation, particularly in grass species (14–16), and keep plants at stages with less lignin, which decompose faster than ligneous plants (39). In addition, nutrient recycling via animal waste likely provided nitrogen inputs that heavily influence soil nitrogen cycles. Both urine and manure are potential sources of nitrogen with varying rates of mineralization, which may help enhance bioavailable N (13, 64). Because manure supports soil microbial heterotrophy, it can also contribute to the storage of N in SOM and microbial pools (65). We found that cover crop effects on nutrient pools and retention were generally stronger than grazing effects in the short term. Residue incorporation via tillage in both systems may have partially masked or interacted with treatment-associated shifts in biomass inputs, residue quality, and organic depositions. As such, we observed starker differences when comparing grazed to fallow plots, and grazing still has the potential to enhance the benefits of cover crops in highly disturbed systems. Cover crop grazing did not alter soil DOC levels, while DON tended to be significantly higher during the cropping stage in soils under grazed CC compared to fallow. Grazing cover crops also lowered MBC and MBN during the cover crop stage but increased both pools during the crop stage compared to fallow. This indicates that cover crops alone may not optimize the tradeoff between N supply and retention, whereas grazing magnified the benefits of cover crops for N availability in season (Figures 4, 5). After 3 years of grazing, early in the season, there was more SIN in ungrazed CC than in grazed CC, but higher bioavailable N at peak nutrient uptake with grazing was achieved, suggesting that grazing allows for more synchronous release of N with crop demand. However, interaction between tillage, cover crop composition, increase in compost application rate in the last year of the trial, and direct and indirect impacts of animals on soil processes likely regulated yearly variations in the effect sizes attributed to grazing.
Understanding outcomes is further challenged by the net effect of shifts in labile N on organic nutrient pools, as it contributes to both DOC and DON formation and consumption processes (66). In one study, increasing labile N was correlated with increased DOC concentrations (66), whereas in another study, a lower soil DOC/DON ratio reflected a nutrient-rich system (67). We found a lower DOC/DON ratio with grazing to a ratio closer to that of MBC/MBN (Figure 5). Although not significant, trends toward a lower MBC/MBN ratio in grazed CC compared to ungrazed CC could be related to the decreased fungal/bacterial ratio with grazing (Figure 6A), as fungi have a higher C/N ratio (14–16) than bacteria (9–11) (68).
4.3 Soil microbial community changes with grazing, reflecting alterations in nutrient status
Soil biological properties can have detectable responses to grazing sooner than physicochemical changes (69), and we expected grazing to shift toward a more diverse microbial community (24). Diversity metrics were not affected (data not shown); however, the fungal/bacterial ratio was lower with grazing, possibly driven by less saprophytic and arbuscular mycorrhizal (AM) fungi, the two main groups represented in fungal PLFA measurements. In general, AM fungal abundance is low under high-nutrient, disturbed conditions, such as tilled crop production. Shifts in residue lability due to grazing could have also contributed to a lower fungal/bacterial ratio (27, 28), although other research has found that grazing can increase saprophytic fungal biomass in the topsoil (70). In our study, grazing increased the Gram (+)/gram (−) ratio. This may be associated with the ability of Gram (+) bacteria to be more adapted to decomposing complex POM and recalcitrant substrates, when N availability is high (71), and their ability to proliferate with disturbance, while Gram (−) bacteria tend to be associated with lower-disturbance regimes (28, 72).
4.4 Grazing had limited effects on MAOM but showed early signs of POM-C depletion
Alterations to chemical, biological, and physical soil properties through grazing may ultimately impact SOM and C storage pools in both the short and long term. Bioavailable C and N, once assimilated by microbes into MBC and MBN under low N limitations due to grazing, could contribute to the MAOM fraction through microbial growth and death (soil microbial carbon pump) (73, 74). Soils in this study are high in silt and clay and therefore have potentially greater capacity to transform and store OM inputs in MAOM over the long term (13, 75, 76). Although we found that MAOM was not altered by grazing, there was a trend toward less % C in the POM fraction of the topsoil after 4 years of grazing. Another study similarly found lower POM-C at 0–3 cm with grazing than without grazing (77). Plant residues contribute to the POM fraction, which is made up of larger insoluble molecules that require depolymerization prior to plant or microbial assimilation and are associated with the fraction of soil larger than 20–63 µm (73). Aboveground biomass removal with grazing, along with uncertain alterations in belowground biomass and turnover, could have contributed to this early decrease in POM. Although the mechanisms by which roots contribute to C stabilization (recalcitrance, occlusion, and interaction with metals and minerals) and destabilization (priming effects) are increasingly known (78), the balance of how root quality impacts these two processes (79) and how grazing impacts root quality are still unclear. In order to increase total soil C, depending on the C saturation potential of a soil, management practices may need to focus on increasing POM, MAOM, or both (75). Since higher-intensity grazing (sward height of <10 cm) has been associated with SOM degradation (39) and compaction, it is critical to manage grazing optimally to harness potential benefits for soil health and C storage.
5 Conclusion
We aimed to understand how grazing of cover crops impacts bioavailable soil C and N pools and whether it is an effective practice for timely N release from cover crops. Our research shows that the common tradeoffs (i.e., compaction and nitrate leaching) associated with grazing cover crops were minimal to none, and animal integration amplified some benefits of cover crops, particularly timely N provisioning. Grazing increased organic forms of N (i.e., DON and MBN), which are especially important for organic farmers who rely on the soil’s capacity to cycle organic nutrients into inorganic forms. It remains unclear how shifts in microbial composition toward more bacteria will impact long-term outcomes in this system, or if the decrease in POM we observed will eventually lead to greater MAOM storage or simply a decrease in TC. Nevertheless, sheep grazing remains a viable option for organic farmers who are looking to diversify their agroecosystem, increase the biological activity and N availability in their soil, produce food with fewer external inputs, and close the nutrient gaps in our currently disconnected crop and livestock production systems. We anecdotally observed less weed pressure in our grazed plots during the crop stage and changes in weed community composition away from the dominant monocot weed (Echinochloa crus-galli) toward dicots (data not shown). Grazing may have stimulated earlier germination of weeds, increased seed predation, or otherwise impacted seed viability, providing potential additional benefits to organic growers.
Statements
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Ethics statement
The animal studies were approved by IACUC, University of California Davis. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent was obtained from the owners for the participation of their animals in this study.
Author contributions
SW: Data curation, Conceptualization, Visualization, Writing – original draft, Formal analysis, Investigation, Methodology. FN: Investigation, Writing – original draft. SC: Investigation, Writing – review & editing. CC-K: Writing – review & editing, Investigation. NT: Writing – review & editing, Conceptualization, Investigation. NJ: Writing – review & editing, Visualization. AP: Project administration, Funding acquisition, Writing – review & editing, Conceptualization, Methodology. AG: Funding acquisition, Writing – review & editing, Conceptualization, Supervision, Project administration.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This work was supported by competitive grants from the California Department of Food and Agriculture (#19-001-021) to AG and USDA_SCRI program (AM200100XXXXG032 and AM190100XXXXG08) to AP.
Acknowledgments
We wish to thank the UC Davis staff at Russell Ranch and the Animal Science Sheep Barn, along with staff and students of the Gaudin and Pires Labs, for creating a supportive research environment.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The author AG declared that they were an editorial board member of Frontiers at the time of submission. This had no impact on the peer review process and the final decision.
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The author(s) declared that generative AI was not used in the creation of this manuscript.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fsoil.2026.1825405/full#supplementary-material
References
1
Michaud-WintersCHaroACallahanSBigelowDP. Major uses of land in the United States 2017 (Report no. EIB-275). Washington, DC: U.S. Department of Agriculture, Economic Research Service (2024).
2
NaylorRSteinfeldHFalconWGallowayJSmilVBradfordEet al. Losing the links between livestock and land. Science. (2005) 310:1621–2. doi: 10.1126/science.1117856
3
WatsonCAToppCFERyschawyJ. Linking arable cropping and livestock production for efficient recycling of N and P. In: Agroecosystem Diversity. London, UK: Elsevier Inc (2019). p. 169–88. doi: 10.1016/b978-0-12-811050-8.00010-8
4
GoyetteJOBotrelMBillenGGarnierJMarangerR. Agriculture specialization influence on nutrient use efficiency and fluxes in the St. Lawrence Basin over the 20th century. Sci Total Environ. (2023) 856:1–13. doi: 10.1016/j.scitotenv.2022.159018
5
de Faccio CarvalhoPCSavianJVDella ChiesaTde Souza FilhoWTerraJAPintoPet al. Land-use intensification trends in the Rio de la Plata region of South America: Toward specialization or recoupling crop and livestock production. Front Agric Sci Eng. (2021) 8:97–110. doi: 10.15302/J-FASE-2020380
6
GarrettRDNilesMTGilJDBGaudinACMChaplin-KramerRAssmannAet al. Social and ecological analysis of commercial integrated crop livestock systems: Current knowledge and remaining uncertainty. Agric Syst. (2017) 155:136–46. doi: 10.1016/j.agsy.2017.05.003
7
PetersonCADeissLGaudinACM. Integrated crop-livestock systems achieve comparable crop yields to specialized systems: a meta-analysis. PLoS One. (2020) 15:1–25. doi: 10.1371/journal.pone.0231840
8
MacLarenCStorkeyJStraussJSwanepoelPDehnen-SchmutzK. Livestock in diverse cropping systems improve weed management and sustain yields whilst reducing inputs. J Appl Ecol. (2019) 56:144–56. doi: 10.1111/1365-2664.13239
9
PiñeiroGParueloJMOesterheldMJobbágyEG. Pathways of grazing effects on soil organic carbon and nitrogen. Rangeland Ecol Manage. (2010) 63:109–19. doi: 10.2111/08-255.1
10
SoussanaJFLemaireG. Coupling carbon and nitrogen cycles for environmentally sustainable intensification of grasslands and crop-livestock systems. Agric Ecosyst Environ. (2014) 190:9–17. doi: 10.1016/j.agee.2013.10.012
11
HaynesRJWilliamsPH. Nutrient cycling and soil fertility in grazed pature ecosystem. Adv Agron. (1993) 49:119–99. doi: 10.1016/S0065-2113(08)60794-4
12
LazickiPGeisselerDLloydM. Nitrogen mineralization from organic amendments is variable but predictable. J Environ Qual. (2020) 49:1–13. doi: 10.1002/jeq2.20030
13
SørensenPJensenES. Mineralization of carbon and nitrogen from fresh and anaerobically stored sheep manure in soils of different texture. Biol Fertil Soils. (1995) 19:29–35. doi: 10.1007/BF00336343
14
HamiltonEWFrankDAHincheyPMMurrayTR. Defoliation induces root exudation and triggers positive rhizospheric feedbacks in a temperate grassland. Soil Biol Biochem. (2008) 40:2865–73. doi: 10.1016/j.soilbio.2008.08.007
15
HollandJNChengWCrossleyDA. Herbivore-induced changes in plant carbon allocation: Assessment of below-ground C fluxes using carbon-14. Oecologia. (1996) 107:87–94. doi: 10.1007/BF00582238
16
PatersonESimA. Effect of nitrogen supply and defoliation on loss of organic compounds from roots of Festuca rubra. J Exp Bot. (2000) 51:1449–57. doi: 10.1093/jxb/51.349.1449
17
ZhouGZhouXHeYShaoJHuZLiuRet al. Grazing intensity significantly affects belowground carbon and nitrogen cycling in grassland ecosystems: a meta-analysis. Global Change Biol. (2017) 23:1167–79. doi: 10.1111/gcb.13431
18
DaiLGuoXKeXZhangFLiYPengCet al. Moderate grazing promotes the root biomass in Kobresia meadow on the northern Qinghai–Tibet Plateau. Ecol Evol. (2019) 9:9395–406. doi: 10.1002/ece3.5494
19
RobertsAJJohnsonNC. Effects of mob-grazing on soil and range quality vary with plant species and season in a semiarid grassland. Rangeland Ecol Manage. (2021) 79:139–49. doi: 10.1016/j.rama.2021.04.008
20
WilsonCHStricklandMSHutchingsJABianchiTSFlorySL. Grazing enhances belowground carbon allocation, microbial biomass, and soil carbon in a subtropical grassland. Global Change Biol. (2018) 24:2997–3009. doi: 10.1111/gcb.14070
21
ZhangYWangZLiuPWangC. Mixed cattle and sheep grazing reduces the root lifespan of the community in a desert steppe. Ecol Indic. (2022) 143:1–10. doi: 10.1016/j.ecolind.2022.109422
22
SixJBossuytHDegryzeSDenefK. A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics. Soil Tillage Res. (2004) 79:7–31. doi: 10.1016/j.still.2004.03.008
23
PrairieAMKingAECotrufoMF. Restoring particulate and mineral-associated organic carbon through regenerative agriculture. Proc Natl Acad Sci. (2023) 120:1–10. doi: 10.1073/pnas.2217481120
24
IlmarinenKMikolaJVestbergM. Do interactions with soil organisms mediate grass responses to defoliation? Soil Biol Biochem. (2008) 40:894–905. doi: 10.1016/j.soilbio.2007.11.004
25
TianLDellEShiW. Chemical composition of dissolved organic matter in agroecosystems: correlations with soil enzyme activity and carbon and nitrogen mineralization. Appl Soil Ecol. (2010) 46:426–35. doi: 10.1016/j.apsoil.2010.09.007
26
YaoZShiLHeYPengCLinZHuMet al. Grazing intensity, duration, and grassland type determine the relationship between soil microbial diversity and ecosystem multifunctionality in Chinese grasslands: A meta-analysis. Ecol Indic. (2023) 154:110801. doi: 10.1016/j.ecolind.2023.110801
27
BansalSChakrabortyPKumarS. Crop–livestock integration enhanced soil aggregate-associated carbon and nitrogen, and phospholipid fatty acid. Sci Rep. (2022) 12:1–13. doi: 10.1038/s41598-022-06560-6
28
LangeMKoller-FranceEHildebrandtAOelmannYWilckeWGleixnerG. How plant diversity impacts the coupled water, nutrient and carbon cycles. In: Advances in Ecological Research, vol. 61. London, UK: Elsevier Ltd (2019). doi: 10.1016/bs.aecr.2019.06.005
29
LaiLKumarS. A global meta-analysis of livestock grazing impacts on soil properties. PLoS One. (2020) 15:1–17. doi: 10.1371/journal.pone.0236638
30
da SilvaHAde MoraesAde Faccio CarvalhoPCda FonsecaAFCairesEFdos Santos DiasCT. Chemical and physical soil attributes in integrated crop-livestock system under no-tillage. Rev Ciencia Agron. (2014) 45:946–55. doi: 10.1590/s1806-66902014000500010
31
McKenzieSCGooseyHBO’NeillKMMenalledFD. Impact of integrated sheep grazing for cover crop termination on weed and ground beetle (Coleoptera:Carabidae) communities. Agric Ecosyst Environ. (2016) 218:141–9. doi: 10.1016/j.agee.2015.11.018
32
Acosta-MartínezVZobeckTMAllenVG. Soil microbial, chemical and physical properties in continuous cotton and integrated crop–livestock systems. Soil Sci Soc Am J. (2004) 68:1875–84. doi: 10.2136/sssaj2004.1875
33
BrewerKMGaudinACM. Potential of crop-livestock integration to enhance carbon sequestration and agroecosystem functioning in semi-arid croplands. Soil Biol Biochem. (2020) 149:107936. doi: 10.1016/j.soilbio.2020.107936
34
FranzluebbersAJStuedemannJA. Soil physical responses to cattle grazing cover crops under conventional and no tillage in the Southern Piedmont USA. Soil Tillage Res. (2008) 100:141–53. doi: 10.1016/j.still.2008.05.011
35
AllanCJJonesBFalkinerSNicholsonCHydeSMauchlineSet al. Light grazing of crop residues by sheep in a Mediterranean-type environment has little impact on following no-tillage crops. Eur J Agron. (2016) 77:70–80. doi: 10.1016/j.eja.2016.04.002
36
AbdallaMHastingsAChadwickDRJonesDLEvansCDJonesMBet al. Critical review of the impacts of grazing intensity on soil organic carbon storage and other soil quality indicators in extensively managed grasslands. Agric Ecosyst Environ. (2018) 253:62–81. doi: 10.1016/j.agee.2017.10.023
37
da SilvaFDAmadoTJCBredemeierCBremmCAnghinoniIde Faccio CarvalhoPC. Pasture grazing intensity and presence or absence of cattle dung input and its relationships to soybean nutrition and yield in integrated crop-livestock systems under no-till. Eur J Agron. (2014) 57:84–91. doi: 10.1016/j.eja.2013.10.009
38
Medina-RoldánEArredondoJTHuber-SannwaldEChapa-VargasLOlalde-PortugalV. Grazing effects on fungal root symbionts and carbon and nitrogen storage in a shortgrass steppe in Central Mexico. J Arid Environ. (2008) 72:546–56. doi: 10.1016/j.jaridenv.2007.07.005
39
AssmannJMAnghinoniIMartinsAPde Andrade CostaSEVGKunrathTRBayerCet al. Carbon and nitrogen cycling in an integrated soybean - beef cattle production system under different grazing intensities. Pesquisa Agropecuária Bras. (2015) 50:967–78. doi: 10.1590/S0100-204X2015001000013
40
BowlesTMHollanderADSteenwerthKJacksonLE. Tightly-coupled plant-soil nitrogen cycling: Comparison of organic farms across an agricultural landscape. PLoS One. (2015) 10:1–24. doi: 10.1371/journal.pone.0131888
41
WolfKMTorbertEEBryantDBurgerMDenisonRFHerreraIet al. The century experiment: the first twenty years of UC Davis’ mediterranean agroecological experiment. Ecology. (2018) 99:503. doi: 10.1002/ecy.2105
42
de Albuquerque NunesPABredemeierCBremmCCaetanoLAMde AlmeidaGMde Souza FilhoWet al. Grazing intensity determines pasture spatial heterogeneity and productivity in an integrated crop-livestock system. Grassland Sci. (2019) 65:49–59. doi: 10.1111/grs.12209
43
BrewerKMMuñoz-ArayaMMartinezIMarshallKNGaudinACM. Long-term integrated crop-livestock grazing stimulates soil ecosystem carbon flux, increasing subsoil carbon storage in California perennial agroecosystems. SSRN Electron J. (2022) 438:1–21. doi: 10.2139/ssrn.4258595
44
van BavelCHM. Mean weight-diameter of soil aggregates as a statistical index of aggregation. Soil Sci Soc Am J. (1950) 14:20–3. doi: 10.2136/sssaj1950.036159950014000c0005x
45
HaneyRLHaneyEBHossnerLRArnoldJG. A new soil extractant for simultaneous phosphorus, ammonium, and nitrate analysis. Commun Soil Sci Plant Anal. (2006) 37:1511–23. doi: 10.1080/00103620600709977
46
DoaneTAHorwáthWR. Spectrophotometric determination of nitrate with a single reagent. Anal Lett. (2003) 36:2713–22. doi: 10.1081/AL-120024647
47
MirandaKMEspeyMGWinkDA. A rapid, simple spectrophotometric method for simultaneous detection of nitrate and nitrite. Nitric Oxide. (2001) 5:62–71. doi: 10.1006/niox.2000.0319
48
DrinkwaterLECambardellaCAReederJDRiceCW. Potentially mineralizable nitrogen as an indicator of biologically active soil nitrogen. In: Methods for Assessing Soil Quality. Madison, WI, USA: Soil Science Society of America (1996). p. 217–29. doi: 10.2136/sssaspecpub49.c13
49
WoodwardERaij-HoffmanIScowKTautgesN. Alfalfa reduces winter nitrate leaching relative to organic and conventional annual vegetable systems: resin bag field measurements and modeling with HYDRUS-1D. J Soil Water Conserv. (2022) 77:450–65. doi: 10.2489/JSWC.2022.00155
50
HorwathWRPaulEA. Microbial biomass. In: Methods of Soil Analysis. Part 2 - Microbiological and Biochemical Properties. Madison, WI, USA: SSSA (1994). p. 753–73.
51
BuyerJSSasserM. High throughput phospholipid fatty acid analysis of soils. Appl Soil Ecol. (2012) 61:127–30. doi: 10.1016/j.apsoil.2012.06.005
52
McClellandSCSchipanskiME. Soil organic carbon sequestration mediated by plant–microbe interactions after compost application. Ecosphere. (2025) 16:e70267. doi: 10.1002/ecs2.70267
53
R Core Team. R: A Language and Environment for Statistical Computing. Vienna, Austria: R Foundation for Statistical Computing (2023). Available online at: https://www.R-project.org/.
54
WhiteCMDuPontSTHautauMHartmanDFinneyDMBradleyBet al. Managing the trade off between nitrogen supply and retention with cover crop mixtures. Agric Ecosyst Environ. (2017) 237:121–33. doi: 10.1016/j.agee.2016.12.016
55
KayeJFinneyDWhiteCBradleyBSchipanskiMAlonso-AyusoMet al. Managing nitrogen through cover crop species selection in the U.S. mid-Atlantic. PLoS One. (2019) 14:1–23. doi: 10.1371/journal.pone.0215448
56
FranzluebbersAJStuedemannJA. Soil-profile distribution of organic C and N after 6 years of tillage and grazing management. Eur J Soil Sci. (2013) 64:558–66. doi: 10.1111/ejss.12057
57
WilliamsRHHaynesRJ. Comparison of initial wetting pattern, nutrient concentrations in soil solution and the fact of 15N labelled urine in sheep and cattle urine patch areas of pasture soil. Plant And. (1994) 162:49–59. doi: 10.1007/BF01416089
58
FrankDAGroffmanPMEvansRDTracyBF. Ungulate stimulation of nitrogen cycling and retention in Yellowstone Park grasslands. (2000) 123:116–21. doi: 10.1007/s004420050996
59
HoogendoornCJNewtonPCDDevantierBPRolleBATheobaldPWLloyd-WestCM. Grazing intensity and micro-topographical effects on some nitrogen and carbon pools and fluxes in sheep-grazed hill country in New Zealand. Agric Ecosyst Environ. (2016) 217:22–32. doi: 10.1016/j.agee.2015.10.021
60
NouriALukasSSinghSSinghSMaChadoS. When do cover crops reduce nitrate leaching? A global meta-analysis. Global Change Biol. (2022) 28:4736–49. doi: 10.1111/gcb.16269
61
LazcanoCGonzalez-MaldonadoNYaoEHWongCTFMerrileesJJFalconeMet al. Sheep grazing as a strategy to manage cover crops in Mediterranean vineyards: short-term effects on soil C, N and greenhouse gas (N2O, CH4, CO2) emissions. Agric Ecosyst Environ. (2022) 327:107825. doi: 10.1016/j.agee.2021.107825
62
MapfumoENaethMABaronVSDickACChanasykDS. Grazing impacts on litter and roots: Perennial versus annual grasses. J Range Manage. (2002) 55:16–22. doi: 10.2307/4003258
63
McinenlyLEMerrillEHCahillJFJumaNG. Festuca campestris alters root morphology and growth in response to simulated grazing and nitrogen form. Funct Ecol. (2010) 24:283–92. doi: 10.1111/j.1365-2435.2009.01642.x
64
HoogendoornCJBetteridgeKCostallDALedgardSF. Nitrogen concentration in the urine of cattle, sheep and deer grazing a common ryegrass/cocksfoot/white clover pasture. N Z J Agric Res. (2010) 53:235–43. doi: 10.1080/00288233.2010.499899
65
LovellRDJarvisSC. Effect of cattle dung on soil microbial biomass C and N in a permanent pasture soil. Soil Biol Biochem. (1996) 28:291–9. doi: 10.1016/0038-0717(95)00140-9
66
FilepTRékásiMFactors controlling dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and DOC/DON ratio in arable soils based on a dataset from Hungary. Geoderma. (2011) 162:312–8. doi: 10.1016/j.geoderma.2011.03.002
67
CooksonWROsmanMMarschnerPAbayeDAClarkIMurphyDVet al. Controls on soil nitrogen cycling and microbial community composition across land use and incubation temperature. Soil Biol Biochem. (2007) 39:744–56. doi: 10.1016/j.soilbio.2006.09.022
68
McGillWBHuntRGWoodmanseeRGReussJO. Phoenix, a model of the dynamics of carbon and nitrogen in grassland soils. Ecol Bulletins. (1981) 33:49–115. Available online at: https://www.jstor.org/stable/45128653.
69
SekaranUKumarSGonzalez-HernandezJL. Integration of crop and livestock enhanced soil biochemical properties and microbial community structure. Geoderma. (2021) 381:1–10. doi: 10.1016/j.geoderma.2020.114686
70
AlvesLAVelosoMGDenardinLGOJoãoFPMFilippiDSaccol de SáEet al. Grazing, liming, and fertilization: Shifts on soil fertility and microbial community in a no-till sheep-soybean integrated system. Appl Soil Ecol. (2023) 188:1–10. doi: 10.1016/j.apsoil.2023.104893
71
OrwinKHDickieIAHoldawayRWoodJR. A comparison of the ability of PLFA and 16S rRNA gene metabarcoding to resolve soil community change and predict ecosystem functions. Soil Biol Biochem. (2018) 117:27–35. doi: 10.1016/j.soilbio.2017.10.036
72
KlumppKFontaineSAttardELe RouxXGleixnerGSoussanaJ-F. Grazing triggers soil carbon loss by altering plant roots and their control on soil microbial community. J Ecol. (2009) 97:876–85. doi: 10.1111/j.1365-2745.2009.01549.x
73
LavalleeJMSoongJLCotrufoMF. Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century. Global Change Biol. (2020) 26:261–73. doi: 10.1111/gcb.14859
74
ZhuXJacksonRDDeLuciaEHTiedjeJMLiangC. The soil microbial carbon pump: from conceptual insights to empirical assessments. Global Change Biol. (2020) 26:6032–9. doi: 10.1111/gcb.15319
75
AngstGMuellerKECastellanoMJVogelCWiesmeierMMuellerCW. Unlocking complex soil systems as carbon sinks: Multi-pool management as the key. Nat Commun. (2023) 14:1–8. doi: 10.1038/s41467-023-38700-5
76
MoukanniNBrewerKMGaudinACMO’GeenAT. Optimizing carbon sequestration through cover cropping in mediterranean agroecosystems: synthesis of mechanisms and implications for management. Front Agron. (2022) 4. doi: 10.3389/fagro.2022.844166
77
FranzluebbersAJStuedemannJA. Early response of soil organic fractions to tillage and integrated crop–livestock production. Soil Sci Soc Am J. (2008) 72:613–25. doi: 10.2136/sssaj2007.0121
78
BaileyVLPriesCHLajthaK. What do we know about soil carbon destabilization? Environ Res Lett. (2019) 14:1–15. doi: 10.1088/1748-9326/ab2c11
79
PoirierVRoumetCMunsonAD. The root of the matter: linking root traits and soil organic matter stabilization processes. Soil Biol Biochem. (2018) 120:246–59. doi: 10.1016/j.soilbio.2018.02.016
Summary
Keywords
agroecology, cover crop grazing, ICLS, integrated crop-livestock systems, nitrogen cycling, organic vegetable, soil health
Citation
Williams SR, Neill F, Cheong S, Chandler-Khayd C, Tautges N, Ji N, Pires AFA and Gaudin ACM (2026) Grazing of cover crops improves soil nitrogen dynamics in organic vegetable systems with minimal soil health tradeoffs. Front. Soil Sci. 6:1825405. doi: 10.3389/fsoil.2026.1825405
Received
07 March 2026
Revised
24 June 2026
Accepted
13 July 2026
Published
11 August 2026
Volume
6 - 2026
Edited by
Gilles Joseph Lemaire, INRAE Nouvelle Aquitaine Poitiers, France
Reviewed by
Tangriani Simioni Assmann, Technological Federal University of Paraná - UTFPR, Brazil
Zhine Wang, Saint Louis University, United States
Updates
Copyright
© 2026 Williams, Neill, Cheong, Chandler-Khayd, Tautges, Ji, Pires and Gaudin.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Amélie C. M. Gaudin, agaudin@ucdavis.edu
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.