Diversified legume-based crop rotations enhance nutrient cycling and long-term productivity in tropical no-tillage systems

Abstract

Diversified no-tillage crop successions are key strategies for improving soil health, productivity, and sustainability in tropical environments. However, adoption of diversified cropping systems remains limited due to economic, cultural, and technical constraints. This study aimed to evaluate how long-term diversified crop rotations, differing in legume inclusion and mineral phosphorus (P) and potassium (K) fertilization, influence system nitrogen (N), P, and K budgets, biomass production, and crop productivity in mature tropical no-tillage systems (NTS). Using a 10-year field experiment established on a tropical Ferralsol in southern Brazil, we evaluated six long-term crop rotations that differed in crop diversification, inclusion of Fabaceae cover crops, and phosphorus and potassium mineral fertilization. Total system N, P, and K budgets were quantified by integrating nutrient inputs, crop uptake, grain nutrient export, biomass nutrient cycling, and net soil nutrient balances. Intensive legume-based rotations increased crop productivity compared with low-diversity, non-legume rotations (specifically, the two-crop black oat/common bean succession). Vetch-based rotations under optimal or suboptimal mineral fertilization, as well as unfertilized vetch rotations, produced the highest cumulative grain yields (∼52,000 kg ha^-1), representing a 38% increase (a 1.38-fold increase) over the black oat/common bean rotation (∼38,000 kg ha^-1). This response was associated with a 40% increase in aboveground biomass and grain yield when maize followed hairy vetch rather than black oat. Legume-intensive rotations under optimal mineral fertilization or without fertilization maintained a strongly positive N budget (+559 kg ha^-1), whereas the low-diversity black oat/common bean rotation showed a net N deficit. Conversely, all unfertilized systems, including the highly productive unfertilized hairy vetch-based rotations, exhibited strongly negative P and K budgets, indicating a progressive risk of soil fertility depletion. Only suboptimal mineral-fertilized vetch rotations partially offset these nutrient deficits in highly crop-productive systems. The study highlights the role of legume-based crop rotations as a key strategy to sustain a positive N budget, thereby supporting nutrient cycling and crop productivity under tropical NTS. Maintaining nutrient balance remained dependent on coupling biological N fixation with balanced mineral P and K fertilization to offset nutrient export deficits in high-yield systems and to reduce the risk of long-term soil fertility depletion.

Highlights

1 Introduction

No-tillage systems (NTS), internationally recognized as conservation agriculture, have become the predominant cropping system in Brazil, covering approximately 33 million hectares, or 61% of the country’s cultivated land (Fuentes-Llanillo et al., 2021). Under tropical conditions, their widespread adoption has yielded substantial soil conservation benefits, including reduced soil erosion, increased soil carbon stocks, improved soil aggregation, greater water infiltration, and higher crop productivity when the three fundamental principles of conservation agriculture (minimal soil disturbance, permanent soil cover, and diversified crop rotations) are implemented simultaneously (Kassam et al., 2014; Brevilieri et al., 2024).

Despite these advances, sustaining high crop productivity in tropical NTS remains challenging because most agricultural soils are highly weathered Ferralsols with low natural fertility, low base saturation, high phosphorus-fixation capacity, and limited nutrient reserves. Although liming and fertilizer application alleviate some of these constraints during the establishment phase of NTS, sustaining productivity over decades increasingly depends on efficient biological nutrient cycling and the continuous replenishment of nutrients removed by grain harvest (Dias et al., 2025). Consequently, understanding how different crop rotations regulate nutrient dynamics has become essential for evaluating the long-term sustainability of conservation agriculture in tropical conditions.

Crop diversification is widely recognized as a key driver of nutrient cycling in no-tillage systems. Cover crops vary markedly in biomass production, residue quality, rooting depth, nutrient acquisition strategies, and residue decomposition rates, thereby influencing nutrient availability for subsequent cash crops (Amado et al., 1998; Anghinoni and Vezzani, 2021). In particular, Fabaceae species can substantially increase nitrogen (N) availability through biological nitrogen fixation (BNF), whereas deep-rooted species such as forage radish enhance nutrient redistribution throughout the soil profile. Together, these complementary functional traits can improve nutrient use efficiency, increase biomass production, and sustain crop performance over time.

Nitrogen dynamics have attracted particular attention because N is often the most limiting nutrient for crop production in tropical no-tillage systems. Grass residues with high C ratios, such as black oat, decompose slowly and may temporarily immobilize mineral N, reducing its availability during the critical early stages of maize growth (Kuo and Jellum, 2002; Ramalho et al., 2020). In contrast, legume residues generally have lower C ratios and contribute substantial amounts of biologically fixed N, thereby accelerating residue decomposition and increasing N availability for subsequent crops (Amado et al., 1998; Boddey et al., 2008). Beyond increasing N supply, these processes can indirectly stimulate the uptake and cycling of other macronutrients, ultimately supporting greater biomass production.

Unlike nitrogen, phosphorus (P) and potassium (K) cannot be replenished by biological fixation and therefore require different management strategies. In tropical Ferralsols, P availability is constrained by strong adsorption to Fe and Al oxides, whereas K is highly susceptible to redistribution and leaching because it occurs predominantly in exchangeable forms (Nascimento et al., 2024; Olibone and Rosolem, 2010). Consequently, highly productive cropping systems may progressively deplete soil P and K reserves when nutrient exports consistently exceed fertilizer inputs, even in well-managed no-tillage systems. These contrasting nutrient dynamics underscore the importance of evaluating whole-system nutrient budgets rather than considering individual nutrient fluxes in isolation.

We hypothesized that integrating Fabaceae cover crops into diversified crop rotations would increase biological nitrogen fixation, enhance biomass production and nutrient cycling, and thereby improve long-term crop productivity in long-established tropical no-tillage systems. We further postulated that maintaining nutrient balance in these highly productive systems would require coupling biological N fixation with replenishment of mineral P and K to compensate for nutrient exports via grain harvest. Therefore, the objective of this study was to evaluate, using a decade-long field experiment established on a tropical Ferralsol, how diversified crop rotations that differ in legume inclusion and mineral P and K fertilization affect whole-system nitrogen, phosphorus, and potassium budgets, nutrient cycling, biomass production, and crop productivity in long-established tropical no-tillage systems.

2 Methods

2.1 Study area and soil characteristics

The experiment was conducted on a commercial farm in Cruz Alta, Rio Grande do Sul, Brazil (28°36′113″ S; 53°42′035″W), a pioneer in NTS adoption, from April 1997 to May 2007. The region has a mean annual temperature of 19 °C and average annual precipitation of 1,871 mm, with rainfall distributed throughout the year and recurrent dry periods during the summer months (December-February). The soil is a clay Typic Haplorthox (US Soil Taxonomy). The experimental area was converted from native vegetation to cropland in 1961 and was initially cultivated with wheat. In 1971, a wheat–soybean succession was established, and wheat residues were routinely burned to eliminate them and facilitate soybean sowing. No-tillage management was adopted in 1986, resulting in a well-established no-tillage system before the start of the present long-term experiment.

Before the experiment was established, soil samples were collected from the 0–10 cm layer for chemical characterization. Available P and exchangeable K were determined using the Mehlich^-1 extraction method, following standard procedures adopted for Southern Brazilian soils. The soil presented the following properties: 550 g kg^-1 clay; pH (H₂O) 5.1; 38 g kg^-1 organic matter; 17.2 mg dm^-3 available P; 162 mg dm^-3 exchangeable K⁺; 5.0 cmolc dm^-3 Ca²⁺; 2.0 cmolc dm^-3 Mg²⁺; and 0.7 cmolc dm^-3 Al³⁺.

2.2 Experimental design and sampling procedures

The experiment comprised crop successions that included winter and summer cover crops and/or short-season cash crops, interspersed with grain crops. The species used were: black oat (Avena strigosa), forage radish (Raphanus sativus var. oleiformis), and common vetch (Vicia sativa L.) as winter cover crops; forage radish, sunn hemp (Crotalaria juncea L.), and common bean (Phaseolus vulgaris L.) as summer cover crops and/or short-season crops; and maize (Zea mays), wheat (Triticum aestivum), and soybean (Glycine max L.) as grain cash crops with or without P and K fertilization. The rotational sequence consisted of a winter cover crop (April), maize (September), a summer cover crop or short-season crop (February), wheat (June), and soybean (November).

The experiment used a randomized complete block design with four replicates. Treatments consisted of the following crop successions: T1: forage radish-maize-forage radish-wheat-soybean; T2: oat-maize-bean-wheat-soybean; T3: fertilized vetch-maize-forage radish-wheat-soybean; T4: fertilized vetch-maize-sunn hemp-wheat-soybean; T5: vetch-maize-forage radish-wheat-soybean; and T6: vetch-maize-sunn hemp-wheat-soybean. The crop rotations were designed to diversify root systems and incorporate Fabaceae species capable of biological nitrogen fixation, thereby increasing N inputs to the system. Each rotation cycle lasted 2 years, yielding five complete cycles over the 10-year experimental period, with the same crop species and rotation sequence maintained throughout the study. A detailed description of each crop rotation and the corresponding cropping periods is provided in Supplementary Table S1. Results are presented by rotation cycle to account for the complete crop succession within each 2-year rotation (Supplementary Figure S1).

Each experimental plot measured 225 m2 (7.5 × 30.0 m). Phosphorus and potassium fertilization was applied only to T3 and T4 during vetch establishment, totaling 324 kg ha^-1 of P₂O₅ and 324 kg ha^-1 of K2O, applied gradually over the experimental period. The sources of P₂O₅ and K2O were Triple superphosphate (TSP) at 46% and potassium chloride (KCl) at 63%, respectively. Fertilizer rates were adjusted based on the initial soil chemical analysis and in accordance with regional fertilizer recommendations for grain production. No mineral N fertilizer was applied to any treatment to evaluate biologically mediated N inputs under contrasting crop successions. Crop establishment, sowing density, and crop management practices followed standard regional technical recommendations for each species. All crop management was performed by the farmer (Ulfried Arns) using standard farm machinery and regional management practices to capture treatment effects under commercial farm conditions.

2.3 Crop productivity

Aboveground dry biomass was determined from plant samples collected within a 1 m2 area, with three subsamples per plot. For row crops such as maize, sampling areas were positioned to include representative plant rows and inter-row spacing within each plot. Samples were oven-dried at 55 °C to constant weight, and results were expressed as Mg ha^-1. Winter and summer cover crops were sampled near full flowering, whereas wheat, soybean, maize, and common bean were sampled at physiological maturity.

The aboveground dry biomass of wheat, soybean, and maize was measured directly at the soil surface during the 2004, 2004/05, and 2005/06 growing seasons, respectively. In the remaining years, biomass was estimated using the harvest index (HI), defined as the ratio of grain yield to total aboveground biomass. A HI of 0.50 was assumed for common bean. These estimates were used to provide system-level assessments of biomass accumulation and nutrient budgets over the entire experimental period.

Because direct measurements of aboveground biomass were available for only part of the experimental period, harvest index values reported in the literature were used to estimate biomass production for the remaining growing seasons. This approach enabled reconstruction of cumulative biomass production and whole-system nutrient budgets over the 10-year experimental period. Although fixed harvest index values may introduce uncertainty due to interannual climatic variability and genotype-specific responses, the same estimation procedure was applied consistently across all treatments. Therefore, these estimates provide a robust basis for comparing long-term treatment effects rather than for obtaining precise annual biomass values.

Root biomass contribution was estimated using species-specific proportions relative to aboveground biomass: 27.5% for maize, 25% for oat, radish, vetch, and sunn hemp, and 20% for soybean, wheat, and common bean (Bayer and Mielniczuk, 1996; Da Ros, 2004). Grain yield for maize, wheat, and soybean was determined manually from 1 m2 harvest areas, with three subsamples per plot, and results were expressed as kg ha^-1 at 13% grain moisture.

2.4 Internal nutrient partitioning, translocation, and crop residue retention indices

Concentrations of N, P, and K in the aboveground biomass of winter and summer cover crops, as well as in the grains of wheat, soybean, and maize, were determined following the analytical protocols of Tedesco et al. (1995). Plant tissue samples were subjected to sulfuric acid digestion for N determination and to nitric-perchloric acid digestion for P and K determination. Total vegetative nutrient accumulation (whole-plant extraction) was calculated from the respective nutrient concentrations and dry biomass production. For grain crops, nutrient extraction values were calculated using the integration coefficients proposed by Pauletti (1998), enabling an integrated assessment over the full five-cycle experimental period.

To evaluate the physiological efficiency of nutrient translocation from vegetative source organs to the economic sink (grain yield), internal partitioning was quantified using the Nutrient Harvest Index (NHI, PHI, and KHI for N, P, and K, respectively) as follows:

Conversely, the portion of nutrients retained within the cropping system to support no-tillage surface mulch and long-term biological cycling was quantified as the residual biomass percentage, defined as:

For non-harvested cover crops, nutrient retention was assumed to be 100% of total nutrient uptake. Biological nitrogen fixation by hairy vetch, sunn hemp, soybean, and common bean was estimated from total plant N uptake (shoot + estimated root contribution) using standard literature coefficients (Vargas and Hungria, 1997; Jantalia, 2005) and was interpreted as an indicator of biologically mediated N inputs across crop successions.

2.5 Statistical analysis

Mean values and standard deviations were calculated for all variables. Data were analyzed using analysis of variance (ANOVA), and treatment means were compared with Duncan’s test at p < 0.05. Normality and homogeneity of variances were assessed prior to ANOVA using residual analysis. Statistical analyses were performed using SAS software (SAS Institute, 1989). Because the study focused on system-level responses across complete crop succession cycles, results are presented by rotation cycle and cumulatively over the experimental period.

Multivariate relationships among the evaluated parameters and crop successions were examined using Principal Component Analysis (PCA). Before analysis, data were standardized to zero mean and unit variance (z-score scaling) to account for differences in variable magnitudes and measurement units. PCA was performed using the FactoMineR package (Husson et al., 2016). The biplot was constructed from the first two principal components (PC1 and PC2), which explained the largest proportion of total variance. 95% confidence ellipses were calculated for each treatment to identify clustering patterns among crop successions.

Furthermore, simple linear regression analyses were conducted to identify the primary quantitative drivers of overall system performance. The coefficient of determination (R²) was calculated for each nutrient-related variable against cumulative dry matter production and cumulative grain yield. Predictors were then ranked by their R² values, and statistical significance was assessed at p < 0.05. To aid visual interpretation, the five variables with the highest R² values for each target variable were highlighted in the ranking chart.

3 Results

3.1 Dry matter production of the crops

Crop succession significantly affected biomass production throughout the 10-year experimental period (Table 1; Figure 1). Winter cover crop biomass varied markedly among crop rotations. Forage radish (T1) produced the highest biomass in the first rotation cycle but declined progressively over time due to phytosanitary constraints, resulting in the lowest cumulative winter biomass (17.3 Mg ha^-1). In contrast, black oat (T2) maintained higher biomass throughout most of the experimental period and accumulated the highest winter biomass (26.5 Mg ha^-1). A pronounced decline in winter biomass was observed across all treatments during the fifth rotation cycle.

FIGURE 1

Maize biomass was consistently higher in diversified legume-based rotations (T3-T6) than in the simple rotations (T1 and T2). Cumulative maize biomass reached 40.3–42.4 Mg ha^-1 in T3-T5, representing an approximately 40% increase compared with T2, which had the lowest biomass production. Summer cover crop biomass also varied substantially among rotations. The common bean rotation (T2) produced the lowest biomass throughout the study, whereas the fertilized rotations (T3 and T4) accumulated the greatest summer biomass (28.2 and 27.9 Mg ha^-1, respectively), approximately sixfold higher than T2. Biomass production declined during the last two rotation cycles in most treatments.

Wheat biomass was highest in T2, whereas soybean biomass varied only slightly among treatments, with the highest cumulative production in T3. Consequently, total biomass production clearly distinguished the crop rotation systems. Diversified rotations that included hairy vetch in winter and either oilseed radish or sunn hemp in summer (T3-T6) consistently outperformed the simple rotations, with the fertilized systems (T3 and T4) achieving the highest cumulative biomass production (127.4 and 125.5 Mg ha^-1, respectively), approximately 30% greater than T2 (Figure 1).

3.2 Grain yield

Crop succession significantly affected grain yield over the 10-year experimental period (Table 2; Figure 2). Maize responded most strongly to the crop rotation systems. Treatments including hairy vetch (T3-T6) consistently outperformed the black oat rotation (T2), particularly from the third rotation cycle onward. Cumulative maize grain yield was highest in T4 and T5 (29.4 and 29.5 Mg ha^-1, respectively), representing approximately a 40% increase compared with T2.

FIGURE 2

Wheat grain yield showed smaller differences among treatments than maize. Nevertheless, T3 consistently ranked among the most productive rotations and achieved the highest cumulative wheat yield (10.6 Mg ha^-1), whereas T2 recorded the lowest cumulative production (6.8 Mg ha^-1). Soybean grain yield was less responsive to crop succession during the first two rotation cycles but increased under diversified legume-based rotations in subsequent cycles. Consequently, T5 achieved the highest cumulative soybean yield (13.8 Mg ha^-1), representing increases of 9% and 12% relative to T1 and T2, respectively.

Cumulative grain yield clearly differentiated the crop rotation systems (Figure 2). Rotations that included hairy vetch as the winter cover crop and either oilseed radish or sunn hemp in summer (T3-T5), with or without P and K fertilization, achieved the highest cumulative grain yields (51.7–52.4 Mg ha^-1). By contrast, the black oat/common bean rotation (T2) consistently produced the lowest grain yield (37.9 Mg ha^-1).

3.3 Addition and plant uptake of N, P, and K

Nitrogen inputs varied markedly among crop rotations (Table 3). Rotations including hairy vetch and sunn hemp (T4 and T6) received the greatest cumulative N inputs, whereas the simple rotations (T1 and T2) had the lowest values. Mineral P and K fertilization increased cumulative N inputs by approximately 9% in T3 and T4 compared with their respective unfertilized counterparts (T5 and T6). Estimated soil-derived N inputs were also higher in the rotations combining hairy vetch and oilseed radish (T3 and T5).

Over the 10-year experimental period, the fertilized treatments (T3 and T4) received cumulative P and K inputs totaling 324 kg ha^-1. These rotations also exhibited the greatest N and P uptake, exceeding the corresponding unfertilized legume-based systems by 6%–14%. Under unfertilized conditions, rotations that included Fabaceae species in both winter and summer (T5 and T6) substantially increased N uptake compared with the simple black oat/common bean rotation (T2).

Potassium uptake followed a similar pattern. The greatest cumulative K uptake occurred in the rotations combining hairy vetch and oilseed radish (T3 and T5), whereas T2 consistently had the lowest K uptake. Relative to T2, the unfertilized legume-based rotations (T5 and T6) increased K uptake by 73% and 38%, respectively. P and K fertilization further increased K uptake by 5.7% in T3 compared with its unfertilized counterpart (T5).

3.4 Nutrient export, cycling, and budget of N, P, and K

Nutrient export patterns for N, P, and K were consistent across the crop rotations (Figure 3). Rotations including hairy vetch (T3-T6) had the highest cumulative nutrient exports, whereas T2 consistently had the lowest exports of all three nutrients. No significant differences in nutrient export were observed between fertilized and unfertilized hairy vetch-based rotations.

FIGURE 3

The fertilized rotations (T3 and T4) showed the greatest N and P cycling, whereas T3 exhibited the highest K cycling. Compared with T2, the unfertilized legume-based rotations (T5 and T6) substantially increased N, P, and K cycling, indicating greater nutrient return to the production system.

Apparent N balances over the 10-year experimental period ranged from negative values in the simple rotations to strongly positive values in the legume-based systems (Figure 3). The highest apparent N balances were recorded in T4 and T6, whereas the fertilized rotations (T3 and T4) maintained less negative P and K balances than the unfertilized systems, partially offsetting nutrient depletion associated with grain harvest.

The Nutrient Harvest Index (NHI) also differed among crop rotations (Figure 4). The simple rotations (T1 and T2) generally had the highest NHI values for N, P, and K, whereas the diversified hairy vetch-based rotations (T3-T6) allocated a smaller proportion of these nutrients to grain production. Potassium consistently had lower NHI values than N and P across all treatments, indicating greater retention of K in vegetative biomass. Although NHI varied among nutrients and crop rotations, these results demonstrate contrasting patterns of nutrient partitioning across the evaluated production systems.

FIGURE 4

3.5 Multivariate analysis of crop rotation and nutrient dynamics

Principal component analysis (PCA) was used to examine multivariate relationships among cumulative productivity and nutrient dynamics across the six crop successions over the 10-year experimental period. The first two principal components (PC1 and PC2) together explained 84.1% of the total variance, with PC1 and PC2 accounting for 65.5% and 18.6%, respectively (Figure 5). PC1 showed strong positive loadings on biomass (0.292), grain yield (0.268), N, P, and K exports (0.281, 0.277, and 0.282, respectively), N and P cycling (0.294 and 0.293), and biological N fixation (0.259), clearly representing a gradient of overall system productivity and internal nutrient turnover. In contrast, PC2 was largely defined by contrasting positive loadings on P and K balances (0.463 and 0.385), available P (0.387), and mineral P and K inputs (both 0.315), against negative loadings on grain yield (−0.263) and nutrient exports (ranging from −0.185 to −0.235), thereby distinguishing systems with nutrient surplus from those with higher nutrient export efficiency. The resulting treatment ordination revealed three distinct clusters: the fertilized legume-based systems (T3 and T4) occupied the positive extreme of PC1, reflecting enhanced biomass production and nutrient cycling; the non-legume systems (T1 and T2) were positioned at the opposite end, indicating lower overall productivity; and the unfertilized legume systems (T5 and T6) were situated intermediately, with separation along PC2 primarily driven by lower P and K balances, highlighting the role of external nutrient inputs in modulating long-term nutrient budgets.

FIGURE 5

3.6 Ranking of the main drivers of system productivity

To determine which nutrient dynamics governed the long-term performance of cropping systems under NTS, all nutrient-related variables were ranked by their coefficient of determination (R2) for cumulative dry matter production and cumulative grain yield (Figure 6). All top-ranked predictors showed a highly significant linear relationship (p < 0.05) with system productivity.

FIGURE 6

For both cumulative dry matter and cumulative grain yield, the top five indicators were dominated by variables associated with extraction, cycling, and nutrient input. Extraction and cycling of N and K emerged as among the strongest predictors, highlighting their strong association with long-term system productivity over the 10 years. In contrast, variables related to the net nutrient budget, while important for long-term soil fertility, showed relatively lower R2 values, indicating that crop performance in soils with satisfactory initial fertility was more dependent on overall nutrient uptake and cycling than on the net nutrient surplus remaining in the soil.

4 Discussion

4.1 Legume-driven nitrogen economy and system productivity

The central contribution of this 10-year investigation extends beyond conventional crop yield assessments by providing a long-term, whole-system nutrient budget for diversified NTS rotations on tropical Ferralsol. By integrating nutrient addition, cycling, and export across five complete crop rotation cycles, this study offers a systems-scale evaluation of nutrient dynamics that are rarely captured in long-term NTS research. Most previous studies have focused on short-term indicators such as biomass quality, seasonal nutrient release, or fertilizer-use efficiency, without assessing how these processes accumulate over time (Carvalho et al., 2024).

The decade-long perspective adopted here enables detection of slow but structurally consequential processes, including progressive nutrient depletion linked to continuous grain export and contrasting nutrient balance trajectories across crop successions. These long-term patterns are particularly relevant for assessing agroecosystem sustainability under tropical no-tillage systems, especially in the context of global soil conservation targets and the Sustainable Development Goals 2, 12, and 15 (Hilmi et al., 2023).

The T2 succession (oat-maize-bean-wheat-soybean) illustrates how the structural recalcitrance of cover-crop residues can modulate nutrient cycling in long-term NTS. Oat residues are rich in fibrous, lignin-rich components that decompose slowly when left as a surface mulch (Chopda et al., 2020). Under these conditions, soil microorganisms increase their demand for inorganic N (NH⁴⁺, NO^3-) to sustain the breakdown of these carbon-rich but poorly labile substrates, temporarily intensifying microbial N immobilization (Yin et al., 2024). This temporary microbial demand for N may reduce plant-available N during the initial growth stages of maize, a period when adequate N uptake is essential for leaf area expansion and early plant development.

Experimental evidence from Brazilian NTS also shows that maize productivity after black oat residues is strongly dependent on N availability and residue management, with insufficient N supply causing significant yield limitations (Weber and Mielniczuk, 2009; De Oliveira Ferreira et al., 2009). This interpretation is supported by the ranking of productivity indicators (Figure 6). The high coefficients of determination (R2) for the top five predictors of cumulative dry matter and grain yield confirm that system output is tightly coupled with the capacity of the crop succession to extract and cycle nutrients, particularly N.

The prominence of variables related to N and K cycling and extraction among these top predictors supports our hypothesis that intensive legume-based rotations are the primary drivers of system productivity. Unlike simple rotations like T2, which rely heavily on native soil nutrient reserves and are periodically constrained by microbial immobilization, legume-integrated systems (such as T3, T4, T5, and T6) maintain a continuous supply of biologically fixed N. This dynamic flux fulfills immediate crop demands while also boosting the biological cycling of other macronutrients, improving overall nutrient circulation within the system. The regression analysis indicates that the sheer volume of nutrients moving through the soil-plant system (cycling and uptake), driven largely by the high-quality biomass of Fabaceae cover crops, is strongly associated with long-term crop productivity over a decade in tropical Ferralsols, outweighing even the net nutrient budget in terms of immediate yield response.

The PCA reinforced these patterns by separating the simple oat-based succession (T2) from the legume-based systems along the productivity-related axis, supporting the strong association among biological N supply, nutrient cycling, and long-term system productivity (Figure 5). The consequences of this early N immobilization may have extended beyond the maize phase, contributing to lower biomass production and nutrient cycling throughout the succession. Low early-season N availability may also restrict crop development and nutrient uptake, particularly in systems without mineral N fertilization (Walne et al., 2024). We acknowledge that the physiological and microbial mechanisms underlying these responses were not directly evaluated in the present study. However, the consistently lower biomass production, nutrient accumulation, and negative N budget observed in T2 suggest that residue quality and low N availability are likely to constrain system performance over time. Long-term observations under tropical NTS reveal that when nutrients, especially nitrogen, are not replenished enough, soil organic carbon levels drop and the system’s resilience weakens. This can limit crop yields, even if conservation practices are in place (Weber and Mielniczuk, 2009; De Oliveira Ferreira et al., 2024).

All rotations incorporating Fabaceae (T3, T4, T5, and T6) modified the soil N budget by introducing biological N inputs through symbiotic N2 fixation. Legume residues tend to contain more labile carbon compounds and a higher proportion of readily decomposable organic matter, which may decompose more rapidly under NTS (Li et al., 2023). This faster turnover can enhance the release of plant-available N, while N derived from BNF helps meet the demands of microbes and plants (Nair et al., 2022). In vetch-based rotations, the combined effect of biologically fixed N and accelerated breakdown of legume residues was associated with higher maize productivity and cumulative grain yield relative to T2 (Weber and Mielniczuk, 2009; Yang et al., 2021). These findings emphasize the crucial role of legume integration in maintaining nitrogen supply over the long term in tropical NTS.

Achieving N sufficiency with a legume cover crop does not ensure long-term nutrient sustainability. In the high-yield rotations T5 and T6, the absence of P and K inputs intensified negative nutrient balances due to continuous nutrient export, particularly in systems lacking biological or atmospheric replenishment pathways. In these unfertilized but productive systems, grain removal consistently exceeded nutrient returns via residues, resulting in a pronounced negative budget, including a P deficit of −380 kg ha^-1. This pattern shows how soil nutrients are depleted over time, with short-term productivity maintained by extracting limited P and K reserves (McDowell et al., 2025; Das et al., 2022). As these negative balances persist, they can threaten the long-term sustainability of highly productive NTS systems by gradually reducing the easily accessible soil nutrients. Such ongoing nutrient losses directly impair the goals of resource-efficient farming outlined in SDGs 12 and 13.

Also, it is well established that high Fe/Al oxide content creates an environment dominated by ligand exchange and strong inner-sphere adsorption, thereby sequestering P in forms with slow turnover (Anil et al., 2022). Although legume-derived organic matter contributes organic acids and low-molecular-weight anions that compete with phosphate for sorption sites, these processes generally increase P availability in the short term but cannot offset large export-driven deficits (Schwerdtner and Spohn, 2022). Therefore, under long-term high-yield conditions, external P replenishment remains essential to sustain more balanced nutrient budgets in tropical Ferralsols.

The PCA captures this productivity gap, with PC1 serving as a proxy for overall system productivity linked to nutrient availability. The extreme isolation of T2 on the negative side of this axis suggests that N limitation induced by oat residues was consistently associated with lower system performance throughout the experiment. By contrast, the clustering of all legume-based systems on the positive side of PC1 indicates that biological N inputs from vetch and sunn hemp were strongly associated with greater biomass production, more efficient nutrient cycling, and higher grain yield.

This decoupling between biomass production and nutrient allocation to grain is further supported by the long-term Nutrient Harvest Index (NHI). Legume-based systems generally exhibited lower or intermediate NHI values than the simple rotations, indicating that a greater proportion of absorbed nutrients remained in vegetative biomass rather than being exported through grain harvest (Figure 4). The lower NHI values observed in the legume-based rotations indicate that a substantial proportion of absorbed nutrients remained in crop residues rather than being exported through grain harvest. Under nutrient-poor Ferralsols, this mechanism partially offsets nutrient depletion by promoting residue-mediated nutrient recycling (Dutta et al., 2024), even though productive systems increasingly rely on native soil P and K reserves in the absence of mineral fertilization.

The lower or intermediate NHI values observed in T5 and T6 indicate that a substantial proportion of absorbed nutrients remained in crop residues rather than being exported through grain harvest. In unfertilized systems on nutrient-poor Ferralsols, this dynamic offers a nuanced view of the “mining strategy.” Legumes (vetch and sunn hemp), by driving system productivity through abundant Biological Nitrogen Fixation (BNF) inputs, increased cash crops’ dependence on native soil P and K reserves (Figure 7). However, because a significant fraction of these mobilized nutrients remains in vegetative biomass (as evidenced by the lower NHI in T5/T6 for P and N than in T1/T2), the nutrients are recycled via straw rather than being completely exported. Conversely, the fertilized intensive rotations (T3 and T4) exhibited the lowest NHI values overall for N (∼40%), P (∼47–49%), and K (∼13–14%). This indicates that when mineral P and K are replenished, plants retain an even greater proportion of nutrients in their vegetative structures, contributing to a robust nutrient cycling loop through the straw. This replenishment is essential on Ferralsols to sustain mature No-Till Systems (NTS) and avoid intensifying soil fertility depletion over time, promoting a more sustainable nutrient-coupling strategy rather than relying on the progressive mining of soil reserves.

FIGURE 7

4.2 Phosphorus and potassium dynamics: the cost of high productivity

Potassium dynamics in these rotations were driven by mechanisms distinct from those governing P behavior. In highly weathered Ferralsols, K remains largely in the exchangeable and solution phases, cycling rapidly through ionic desorption, leaching, and redistribution of crop residues (Hinsinger et al., 2021). Deep-rooted cover crops such as radish (T3) may have contributed to K redistribution within the soil profile (Cottney et al., 2025), as reported for Brachiaria in the Brazilian Cerrado (Rosolem et al., 2019). While this mechanism may enhance short-term K availability, it also increases K export through grain, accelerating long-term depletion. In soils with intrinsically low K-buffering capacity, such as sandy tropical soils, this sustained export may exceed natural replenishment rates, leading to progressively negative K balances over time and potentially weakening system resilience to yield stability, drought, and cation imbalances (Souza Junior et al., 2022). From a sustainability perspective, this represents a potential limitation for high-yield NTS aligned with SDG 2 (food security).

Treatments T3 and T4, both integrating vetch and receiving P + K fertilization, were the only rotations that sustained high productivity while maintaining more balanced nutrient budgets. External nutrient inputs partially offset export-driven nutrient removal, thereby supporting higher cumulative biomass production and nutrient uptake. These inputs also resulted in an 8%–9% higher N input in T3 and T4 than in T5 and T6. We suggest that improved nutrient availability contributed to the performance of legume-based rotations, although the physiological mechanisms underlying these responses were not directly evaluated in this study. Together, biological N inputs and targeted mineral fertilization represent an important strategy for sustaining productivity while preserving nutrient balance.

4.3 Cover crop functional diversity and nutrient cycling

The superior performance of rotations T3 and T4 can be attributed to the synergistic interaction among legume-driven N inputs, the functional diversity of cover crops, and the continuous replenishment of P and K, which together sustained nutrient sufficiency throughout the full 10-year cycle. Both successions begin with vetch, whose high biological N fixation rate and rapid residue turnover may enhance early-season N availability and minimize microbial N immobilization during maize establishment, an effect widely documented for Fabaceae species in NTS (Pes et al., 2022; Carvalho et al., 2024). In T3, the inclusion of forage radish was also associated with greater K cycling and biomass accumulation throughout the experimental period (Inagaki et al., 2021).

In contrast, T4 benefited from the functional attributes of Sunn Hemp, which provide high-quality biomass, sustained N inputs, and greater biomass returned to the soil system (Carvalho et al., 2024). The multivariate space occupied by T3 and T4 also underscores the importance of functional diversity in cover crop selection. For instance, the inclusion of forage radish in T3 coincided with the highest K cycling values observed across the evaluated successions, whereas the use of Sunn Hemp in T4 was associated with a sustained positive N budget and high cumulative biomass production. We acknowledge that the specific physiological and rhizosphere mechanisms underlying these responses were not directly evaluated in the present study. However, the consistent separation of T3 and T4 in the PCA indicates that combining distinct cover crop functional traits with nutrient replenishment contributed to greater long-term system productivity and nutrient cycling.

In both rotations, P and K fertilization inputs were critical, partially offsetting export-driven nutrient removal and sustaining greater cumulative nutrient uptake over time (Dutta et al., 2024). These external inputs were also linked to greater N accumulation in the legume-based systems, suggesting that nutrient availability may have favored the overall performance of these rotations (Johnson et al., 2022). Consequently, T3 and T4 maintained less negative P and K budgets, higher cumulative N inputs, and consistently higher grain productivity than unfertilized systems. We therefore interpret these results as evidence that long-term sustainability in tropical NTS arises not merely from legume integration but from the functional coupling of biological N supply with targeted replenishment of P and K fertilization, producing a nutrient budget that aligns with global soil stewardship frameworks and SDGs 2, 12, and 15.

4.4 Integrated sustainability: synergies between biological and mineral inputs

Finally, these findings do not support the assumption that legume integration alone can sustain long-term nutrient balance in high-yield NTS systems. While legumes effectively address N constraints, they also intensify P and K export by enabling higher grain yields. Thus, the sustainability of productive tropical NTS depends on integrating biological inputs with targeted replenishment of mineral fertilizers. Intensive legume-based rotations supply renewable N and contribute to nutrient turnover within the system, whereas P and K fertilization remain essential to offset export-driven removal and maintain long-term nutrient stocks (Okumu et al., 2023). In this sense, the combined strategy observed in T3 and T4 represents a technically robust and environmentally aligned model for sustainable NTS intensification, consistent with international soil stewardship commitments and the Sustainable Development Goals.

5 Conclusion

This 10-year study demonstrated that integrating Fabaceae into NTS rotations substantially increased N inputs and sustained long-term productivity on tropical Ferralsols. Legume-based systems, particularly those with vetch and sunn hemp, achieved positive N budgets and increased cumulative grain yield by about 38% compared with the non-legume succession.

However, higher productivity also intensified nutrient export via grain removal. Unfertilized high-yield systems developed strongly negative P and K budgets, indicating that biological N inputs alone were insufficient to sustain nutrients over the long term. Thus, the original hypothesis was only partially supported.

Integrating legume-derived N inputs with targeted replenishment of P and K fertilization was the most effective strategy for sustaining high productivity while maintaining more balanced nutrient budgets over the 10 years. Overall, long-term sustainability in tropical NTS depends on coupling biological N inputs with mineral fertilization to offset export-driven nutrient losses in high-yield systems.

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