Improving agricultural soil health with feces-derived compost: a case study in northern Haiti

Ecological Sanitation (EcoSan) may help achieve multiple United Nations Sustainable Development Goals, such as SDG 2, SDG 6, and SDG 13, through circular management of local biogeochemical cycles. However, more information on the impact of the land application of human excreta-derived fertilizers such as thermophilically co-composted human feces on soil health, soil carbon stocks, and crop yield is necessary in the areas where EcoSan products are utilized. We conducted a sorghum growth trial comparing feces-derived compost application to synthetic fertilization and an unfertilized control over two consecutive cropping cycles on a farm in northern Haiti. We found that feces-derived compost, particularly when overapplied on the basis of total nitrogen to account for its slow mineralization over time, led to increases in bioavailable soil micronutrients. Feces-derived compost application led to modest increases in carbon (C) and nitrogen (N) concentration in the same 0–10 cm soil layer by the end of the second sorghum crop cycle. No experimental treatments crop yields differed from unfertilized soil in either cropping cycle. This research contributes to our understanding of the full life cycle benefits of source-separated sanitation strategies by showing short-term soil health and fertility benefits from the use of EcoSan compost.

Introduction

Achieving the United Nations Sustainable Development Goals (SDGs) by 2030 requires innovative approaches to resource management (Bleischwitz et al., 2018). Ecological Sanitation (EcoSan) is a circular approach to human excreta nutrient management, by which resources embedded in human excreta are safely recovered for reuse in agriculture (Langergraber and Muellegger, 2005). EcoSan offers a multi-benefit opportunity towards achieving SDG 6 (water and sanitation for all), SDG 2, (end hunger and achieve food security), and SDG 13 (take climate action). However, more research on EcoSan reuse products is necessary to gain a comprehensive understanding of the whole-system benefits of EcoSan, particularly as they pertain to achieving SDG 2 and 13.

As of 2020, 4.2 billion people lack access to safely managed sanitation, two billion of which have no access to basic sanitation (United Nations, 2020). SDG six will likely not be met by 2030 if rates of implementation do not increase (United Nations, 2020). However, sewered sanitation systems are unlikely to meet the sanitation needs of the Global South. Sewerage uses large amounts of fresh water, requires extensive infrastructure and upfront investment, and does not easily serve the often-informal pattern of urban development (Öberg et al., 2020). Container-based sanitation (CBS) is an approach to EcoSan that is helping to bridge the SDG six implementation gap. In CBS systems, feces and urine are source-separated in containers and transported to a local facility for waste treatment, processing, and reuse (Russel et al., 2019). CBS provides a climate-resilient, low-to-no water, decentralized approach to sanitation. An additional sustainability benefit of CBS is that these systems can also be integrated with resource recovery and reuse strategies that can recycle nutrients back to agricultural soils.

Current “linear” approaches to human excreta management ultimately release most of the nutrients embedded in human waste back to the environment. This leads to watershed pollution, including eutrophication and groundwater contamination, and atmospheric pollution, including greenhouse gas emissions that contribute to global climate change and stratospheric ozone depletion (Fowler et al., 2013; Trimmer and Guest, 2018). Improper waste treatment also leads to enteric disease (Orner and Mihelcic, 2018). However, the nutrients we excrete are the same plant-essential nitrogen (N), phosphorus (P), and potassium (K) that we apply as fertilizers (Jönsson et al., 2004; White and Brown, 2010; Mehta et al., 2015). If safely recovered and treated, human feces generated in Haiti could meet 13, 22, and 11% of the N, P, and K demand for major Haitian crop production (Ryals et al., 2021).

There is high potential for EcoSan reuse products to address SDG 2. However, there is a separate global economy dedicated to the extraction and production of inorganic fertilizers to feed the global population. Phosphorus and K fertilizers are mined from spatially heterogeneous and nonrenewable mineral deposits (Cordell and Neset, 2014; Cañedo-Argüelles et al., 2017). Inert atmospheric N is converted into reactive N through the Haber-Bosch process, the main source of N fertilizers globally (Fowler et al., 2013). This artificial nitrogen fixation process uses fossil fuels as a feedstock, making the production of N fertilizers a major contributor to climate change, accounting for 1.2% of global anthropogenic carbon dioxide emissions (Smith et al., 2020). While the overapplication of these inorganic fertilizers has various negative environmental impacts, their provision is also unequal (FAO, 2017). Smallholder farmers, a foundational part of the global food system, often lack access to synthetic fertilizer (Rapsomanikis, 2015). The circular approach to nutrient management in EcoSan not only has the potential to restore disrupted biogeochemical cycles but can also provide organic, nutrient-rich fertilizers to support underserved agroecological systems at a local level. The application of organic amendments, such as feces-derived compost, are also known to improve soil health (Urra et al., 2019). Improvement in soil health can lead to improved crop nutrition, and thus, better human health outcomes (Lehmann et al., 2020).

Sustainable Organic Integrated Livelihoods (SOIL) is a non-governmental organization operating in Haiti that provides a CBS EcoSan service, EkoLakay. Founded in 2006, SOIL’s EkoLakay service now serves over 2,600 households with safe and dignified access to sanitation (SOIL Haiti, 2024). SOIL’s work as a sanitation service is particularly salient, as rates of safely managed sanitation access in Haiti are extremely low. As of 2010, access to improved sanitation in Haitian urban areas is estimated at 24%, and as low as 10% in rural areas (Gelting et al., 2013). In Cap-Haïtien, the city in which SOIL is headquartered, and this study took place, it was estimated in 2019 that 1% of fecal sludge is treated (Biscan et al., 2021). SOIL utilizes thermophilic co-composting with sugarcane bagasse to transform source-separated fecal matter into a nutrient-rich, organic soil amendment, known to SOIL’s customers as Konpòs Lakay (Preneta et al., 2013). This process consists of the following steps. Fecal containers are collected, and the waste is transported to a centralized composting facility where it is dumped into pallet bins lined with sugarcane bagasse. The waste is covered with a layer of sugarcane bagasse to reduce smell and prevent the spread of disease vectors. Temperatures throughout the pile are monitored to ensure that piles meet the WHO standard of 120°F for seven consecutive days (WHO, 2013). Once this threshold has been met, piles are removed from the bins and turned beneath a hangar on flat open surfaces for 4–6 months, when decomposition is complete. Piles are then tested for the indicator E. coli before sieving and bagging the compost. This reduces public health risks, while also creating a value-added fertilizer that can be used in local agroecosystems.

The climate change mitigation potential of SOIL’s thermophilic fecal co-composting process has been rigorously studied. Research has shown SOIL’s co-composting process has the potential to mitigate 8.6 kg CO₂e kg⁻¹ of biological oxygen demand, accounting for the full sanitation cycle including avoided emissions (McNicol et al., 2020). This mitigation potential is mainly due to a large reduction in methane emissions due to compost pile aeration. This contrasts with high methane-emitting alternative waste fates in Haiti, such as waste stabilization ponds and unmanaged disposal sites (Ryals et al., 2019). This is further relevant as methane emissions from pit latrines, a wastewater management strategy that serves approximately one-quarter of the global population, are estimated to account for as much as 1% of global methane emissions (Reid et al., 2014; Van et al., 2019).

While the climate change mitigation potential of thermophilic fecal co-composting has been quantified, the land application of feces-derived compost is understudied in local agroecosystems. Ryals et al., 2021) demonstrated the potential of one application of Konpòs Lakay to elevate crop yield over six consecutive cropping cycles and increase soil carbon (C) content in a greenhouse growth experiment. However, a land application study on the impact of Konpòs Lakay application in Haitian agroecosystems is lacking in the literature. To drive local adoption of Konpòs Lakay, it is important to demonstrate its impact in local agroecological systems. Specifically, it is important to quantify the impact of Konpòs Lakay application on soil C, which has implications for soil C sequestration, a scalable and powerful climate change mitigation strategy (Trimmer et al., 2019). Additionally, the impact of Konpòs Lakay application on other soil health indicators, and crop production compared to other locally available fertilization regimes, is lacking (Ryals et al., 2021). To address this gap, we conducted a sorghum growth study using Konpòs Lakay. We chose sorghum as it is an important staple crop in northern Haiti and was recommended as a study crop by our Haitian co-authors. The objective of this study was to assess the effect of feces-derived compost application on sorghum production, soil health indicators, and soil C stocks on a farm in Cap-Haïtien, Haiti over two consecutive growing cycles. We assessed two cropping cycles as farms in northern Haiti typically crop two consecutive cycles annually, to accommodate the two rainy seasons. We were interested in whether successive Konpòs Lakay applications had an accrued effect on soil C stock, soil health indicators, and crop growth. We hypothesized that Konpòs Lakay application would lead to improved soil health outcomes due to the positive impact of C additions on soil biological, physical, and chemical processes (Larney and Angers, 2012). We expected Konpòs Lakay application to perform similarly to synthetic fertilization in Haitian agroecosystems, due to the restoration of C and macro- and micronutrients to degraded soil (Bargout and Raizada, 2013). We expected all fertilized treatments to outperform the unfertilized control.

Methods

Study site

A sorghum growth experiment was established at a farm under the management of Université Anténor Firmin (UNAF) in Cap-Haïtien, Haiti (19°43′21.9″N 72°09′51.9″W) in January 2022. The regional climate is characterized as tropical, with an average annual temperature of approximately 25.7 °C and an average annual precipitation of 1,256 mm (Cap-Haitien Climate, 2025). The site had not received any recent fertilization. Management in the previous year included rotational crops, a fallow period, and cassava cultivation. The soil particle size analysis was done on a composite soil sample collected from 0–30 cm using the hydrometer method according to Gee and Bauder (1986). This soil consisted of 24% clay, 36.4% sand and 39.6% silt and was classified as loam according to USDA Soil Survey Manual (2017). A lack of local soil survey data made further classification difficult.

Experimental design and management

The study was laid out in a randomized block design with five treatments; each replicated five times. Treatments included Konpòs Lakay applied at 100% of the crop N demand (hereafter Compost), Konpòs Lakay applied at 150% of the crop N demand (hereafter Compost 150), 50% of the crop N demand from synthetic fertilizer and 50% from Konpòs Lakay (hereafter NPK-Compost), 100% of the crop N demand from synthetic fertilizer (hereafter NPK), and an unfertilized control (Control). All amendments were applied at 76 kg-N ha^-1, except for Compost 150, on a dry mass basis. 76 kg-N ha^-1 is the sorghum N application rate typical in Haiti. Compost 150 was tested to account for the documented slow mineralization rate of nitrogen in compost (Amlinger et al., 2003). Amendments were applied based on total N content. Compost application supplied 510 kg-C ha^-1 while Compost 150 application supplied 765 kg-C ha^-1, on a dry mass basis. 2.21 Mg ha^-1 of Konpòs Lakay was applied for the Compost treatment, while 3.31 Mg ha^-1 of Konpòs Lakay was applied for the Compost 150 treatment, on a dry mass basis. This information is summarized in Table 1. Synthetic fertilization consisted of a 20–20–10 NPK fertilizer applied at 30 kg-N ha^-1 at planting, with a urea sidedress applied at 46 kg-N ha^-1 5 weeks later, based on local agronomic recommendations for synthetic fertilization. Relevant physiochemical properties of the compost are shown in Table 2. Plots were weeded and hand-tilled prior to planting the first cycle. Amendments were surface applied and then spread evenly with a rake prior to planting.

Table 1

Table 1. Summary of experimental treatments. All Konpòs Lakay treatments were applied based on total N content and applied on a dry mass basis. Synthetic fertilization consisted of a 20–20–10 NPK fertilizer applied at 30 kg-N ha^-1 at planting, with a urea sidedress applied at 46 kg-N ha^-1 5 weeks later.

Table 2

Table 2. Compost physicochemical analysis. All results are reported on a dry mass basis except moisture and solids content. Complete TMECC methods can be found in the Test Methods for the Examination of Composting and Compost (U.S. Composting Council and U.S. Department of Agriculture, 2015).

The study was done over two consecutive sorghum cropping cycles, each of which lasted approximately 4 months. The first cycle lasted from February to June 2022, and the second cycle lasted from June to October 2022. A semi-dwarf sorghum cultivar, Tinen-2, was grown in this study. Seeds were provided by La Brasserie Nationale d’Haiti. The cultivar was bred for yield intensification with increased planting density and fertilization, and for aphid resistance. The line was developed specifically for Haiti (Pressoir, 2024). Each plot was 2.5 m by 4 m. A 1.4 m buffer was allocated between plots, which was planted with one row of sorghum in the first cycle, but not planted for the second cycle to allow easier movement between the plots. We chose to use a buffer to keep treatment effects spatially separate. An approximately 0.5 m unplanted aisle was left between each block. Each plot consisted of four rows with approximately 16 plants per row. Each row was 70 cm apart, and plants within rows were approximately 25 cm apart. This planting density was chosen based on past local research (Aristil, 2019). Two seeds were planted manually per hole and thinned a week after emergence, for a total of 64 plants per plot. Chlorpyrifos pesticide was applied at the recommended rate based on local practice in locations with aphid infestations during both cycles. The crop was watered with a bucket three times per week unless it rained heavily, based on typical irrigation practices at the university farm. Heavy bird grazing was problematic in both cycles. A scarecrow was used in both cycles, in addition to paper covers on the panicles, and reflective tape in the second cycle. Amendments were applied prior to planting in the same plots for the first and second cycle.

Plant growth indicators and productivity

Plant growth indicators were measured throughout the study. Germination, plant height, stem circumference, number of leaves, length of the longest leaf, and width of the widest leaf were monitored for 10 plants per plot in three out of five blocks. During the first cycle, height and number of leaves were monitored 4 times per week from emergence to the 3-leaf stage (the first 2 weeks of the growing season). Growth indicator monitoring was reduced to four times over the course of the first 2 weeks of the second cycle due to logistical constraints. After the 3-leaf stage, all indicators were monitored weekly until harvest. Final plant survival was surveyed approximately 2 months before harvest.

At harvest, five plants were chosen randomly from each plot in blocks two, three, and four. Selected plants were at least three plants into the plot and within the middle two rows. Plants were harvested above the adventitious root with a machete. The panicle was cut off and bagged. Biomass samples were dried in the sun for 3 days and later weighed. Between cycles, the residue was removed, and the field was hand tilled carefully to remove the roots and any weeds. Plants were harvested based on seed maturity, approximately 110 days after germination.

Soil health indicators

Soils were sampled for analysis prior to planting, immediately prior to the first harvest, and immediately prior to the second harvest. Three random sampling locations were flagged within each plot at the beginning of the study based on the intended planting scheme. Sampling locations were adjacent to plants, at least three plants into the plot lengthwise and within the two innermost rows. Samples were at least three plants apart from one another. For the baseline and first cycle sampling, soils were sampled with a 5.5 cm diameter PVC corer from 0–10 cm and with a soil knife from 10–30 cm. For the second cycle, soils were sampled with a soil knife from 0–10 cm. Soils were air dried and stored for later analysis. Most soil health assessment protocols were adapted from the Soil Health Evaluation Manual from the Soils Cross Cutting Project, and are described as follows (Vanek, Fonte, and Magonziwa, 2018). Wet aggregate stability was measured with the method outlined in the Soil Health Evaluation Manual. Briefly, a 70 g composite soil sample from each plot from the 0–10 cm fraction was allowed to completely soak for 5 min in a 2 mm sieve with aggregates larger than 8 mm removed. The sample was sieved at 50 beats per minute for 2 min using a smartphone metronome app. The aggregates remaining on the sieve were dried and weighed. The soil that washed through the sieve was transferred to a 250 μm sieve and processed following the same procedure. Soil bulk density was measured by weighing the 0–10 cm sample and taking the soil moisture content in triplicate with a moisture analyzer. Bioavailable nutrients were measured using Plant Root Simulator (PRS®) ion exchange membrane probes (Western Ag Innovations, Saskatoon, Saskatchewan, Canada). Bioavailable nutrients measured included Al, B, Ca, Cd, Cu, Fe, K, Mg, Mn, NO₃⁻-N, NH₄⁺, P, Pb, S, and Zn. Three pairs of anion and cation probes were inserted in a randomized zigzag design in each plot immediately after planting. Probes were inserted approximately 10 cm into the soil. The probes were collected 2 weeks after installation, cleaned, and shipped to the manufacturer for total ion analysis. The three pairs per plot were analyzed as one average sample by the manufacturer. A control probe sample was also analyzed. Soil C and N were measured via elemental analysis using a Costech 4,010 Elemental Analyzer (Costech Analytical Technologies Inc., Valencia, CA) on a composite sample for each depth increment.

Soils were also sampled monthly at a depth of 5 cm for additional soil health indicators, including pH, basal respiration, soil moisture, and EC. Soils were sampled following the same criteria as outlined above. Monthly soil samples were taken systemically on each occasion, approximately 30 cm from the center of the sampling area. At the next sampling event, the flag was moved approximately 10 cm down clockwise, and soil was sampled from that area. The three random samples were composited in-field and bagged. Soil pH was measured with a subsample of each monthly soil sample using the 1:2 soil:DI water method (Thermo Scientific™ Expert pH Pocket Tester, Thermo Fisher Scientific Inc., USA). Soil basal respiration was measured with a separate subsample of each monthly soil sample using the Solvita CO₂ Burst Test and the Solvita Digital Color Reader (Woods End Laboratories, Inc., Mount Vernon, ME, USA). Soil moisture was measured in-field during monthly sampling events, at one random location per plot (Extech Instruments Soil Moisture Meter Model MO750, Teledyne FLIR LLC, USA). Soil electrical conductivity (EC) and temperature was also measured in-field during monthly sampling events, at one random location per plot (Hanna Instruments GroLine HI98331, Hanna Instruments USA, Smithfield, RI, USA).

Methods constraints

Due to political and civil unrest in Haiti in late 2022, our data collection methods were modified during the second cropping cycle. To reduce time spent in the field for the sake of UNAF intern worker safety, we used modified protocols. Only the 0–10 cm soil samples were collected, with a soil knife rather than bulk density corer. Soil aggregate stability was not measured, and the final basal respiration measurement was also not taken. Plant growth indicators were monitored less frequently in the second cycle. Due to the lack of complete bulk density data for the second cycle, we represent soil C data as C concentration rather than C stocks. Due to heavy bird grazing in both cycles, we were unable to accurately measure yield. Thus, total biomass, stalk biomass, and seed head biomass with seeds removed are reported.

Statistical analysis

All statistical analyses were performed in R Statistical Software (v 4.1.3; R Core Team 2022). All data were tested for normality prior to statistical analysis with Shapiro Wilk tests and QQ plots. Analysis of variance (ANOVA) tests with post hoc Tukey’s HSD tests were used to test for treatment differences by cycle on bioavailable nutrients, harvest data, plant indicator data, and all soil physiochemical data. To account for the urea sidedress in NPK treatments, and the differing P and K contents of the amendments, these bioavailable nutrient data were normalized by application rate. T-tests were used to test for differences within treatment between the baseline and first cycle. Linear mixed effect models were used to compare time series data between treatments within each cycle (plant growth, soil pH, EC, moisture, and temperature), with treatment and date as fixed effects and block as a random effect. Pairwise Tukey’s Honestly Significant Difference (Tukey’s HSD) post hoc comparisons between treatments by date were performed using the emmeans function in the emmeans package. Due to irregular data collection time points for plant growth indicators and soil physiochemical data between the two cycles, treatments are not compared statistically between cycles. Trends comparing the two cycles are discussed when relevant. The difference in the lagged value for plant growth and survival data was used as the predictor variable to address temporal autocorrelation. For example, the difference between plant height on a date and the previous date was calculated to perform a linear mixed effect model on plant height data. Relevant statistical tables are found in the supplemental material. All other data is represented graphically in the supplemental material, with a note of significance in the figure caption and/or main text. ANOVA or t-test p values are reported in the main text, and the reader is referred to post hoc test p values in the supplemental material when relevant. All reference to statistical significance considers p < 0.05.

Results and discussion

Compost application and bioavailable nutrients

Compost application significantly increased bioavailable soil nutrients, particularly after the second application. Few significant differences between treatments were observed for bioavailable nutrients during the first cycle. In the second cycle, both compost application rates led to significant increases in bioavailable soil sulfur (S) compared to other treatments (Figure 2).

NO₃⁻, NH₄⁺, P, and K bioavailability

NO₃⁻, NH₄⁺, and K did not differ significantly for any of the treatments in the first or second cycle. This demonstrates that the supplication of these nutrients from feces-derived compost is comparable to synthetic fertilizer. However, as bioavailable nutrients were measured in the first 2 weeks of the study, these data do not capture the 5-week urea sidedress application. This was a methodological error, and care should be taken when interpreting these bioavailable nitrogen data.

First and second cycle NPK bioavailable P was significantly higher than Compost, Compost 150, and Compost-NPK, with 0.11 ± 0.21 μg-P^-1 kg-P applied^-1 14 days^-1 (Figure 1). This indicates that the 20–20–10 fertilizer was more immediately bioavailable than the compost treatments. A study of Browntop millet nutrient uptake for various synthetic fertilization treatments found the 20–20–10 treatment had similar grain-P uptake to the other synthetic fertilizers, indicating the phosphorus availability of 20–20–10 in a similar crop (Sukanya et al., 2021). Future research should explore the impact of successive feces-derived compost applications on soil P bioavailability, as increases in surface soil P after multiple years of repeated cattle manure applications are well documented (Qian et al., 2004; Weyers et al., 2016).

Figure 1

Figure 1. Bioavailable soil N, P, and K in the first and second cropping cycles (n = 5). Letters represent significant differences between treatments in the second cycle, normalized by nutrient application rate. If treatments share a letter do not differ significantly at p < 0.05.

S and micronutrient bioavailability

Second cycle Compost 150 bioavailable S was significantly higher than the Control and NPK, with 19.62 ± 10.01 μg-S 10 cm^-2 14 days^-1 observed (Figure 2). Compost and NPK-Compost did not differ significantly from any other treatments, with 14.56 ± 4.66 μg-S 10 cm^-2 14 days^-1 and 18.69 ± 9.71 μg-S 10 cm^-2 14 days^-1 observed, respectively. Soil available S increased significantly for Compost between cycles (Supplementary Table S1). These results are important as soil S deficiency is widespread, particularly in low-income countries (Behera et al., 2021), though S fertilization is important for increasing sorghum yield and crop quality (Sahrawat et al., 2008). Further, the need to recycle S-rich organic materials such as feces as fertilizers to reduce reliance on mineral fertilization has been identified, particularly as the supply of sulfuric acid decreases while the global economy moves towards decarbonization (Lisowska et al., 2022; Maslin, Van Heerde, and Day, 2022). Increase in soil bioavailability of other important crop nutrients such as Fe, Mg, Ca, Mn, and Cu were observed in the second cycle in compost treatments (Supplementary Tables S1, 2). Some treatments saw significant changes in bioavailability of micronutrients between the first and second cycle, including Fe with NPK-Compost application (p = 0.025), Mg with NPK application (p = 0.024), Mn with NPK-Compost application (p = 0.001), and Zn with Compost application (p = 0.005) and for the Control (p = 0.026). These results are expressed in part in Figure 3 and in full in Supplementary Table S1. These results further demonstrate the potential of feces-derived compost to supply critical crop nutrients in Haitian agroecosystems.

Figure 2

Figure 2. Bioavailable soil S and micronutrients in the first and second cropping cycles (n = 5). Letters represent significant differences between treatments in the second cycle. If treatments share a letter, they do not differ significantly at p < 0.05.

Figure 3

Figure 3. Bioavailable soil heavy metals in the first and second cropping cycles (n = 5). No significant differences were detected between treatments for either cycle at p < 0.05.

Heavy metal bioavailability

No treatments differed significantly for either cycle in the bioavailability of any heavy metals that were tested (Pb, Cd, Cu, Fe, Mn, and Zn). As noted, Cu, Fe, and Mn bioavailability increased for compost treatments in the second cycle, and Zn bioavailability increased between cycles across treatments. While Cu, Fe, Mn, and Zn are important plant micronutrients, they can cause plant toxicity when present in excess quantities in the soil (Arif et al., 2016). The bioavailability of Pb, a heavy metal of common concern, increased significantly for NPK between the first and second cycles (p = 0.041) (Supplementary Table S1). Heavy metal accumulation is a topic of concern with fecal recovery and reuse and is well-explored in the literature (Nunes et al., 2021). However, the source of heavy metals in sewage sludge is mainly derived from mixed-stream treatment processes in wastewater treatment systems that mix various waste effluents with human excreta (Sörme et al., 2003; Houhou et al., 2009; Duan et al., 2015). Our results demonstrate that application of composted, source-separated fecal waste does not lead to significant bioavailability of heavy metals.

Harvest biomass

No statistically significant differences were observed between treatments for either cycle for total biomass, stalk biomass, or seed head biomass (Figure 4; Supplementary Figures S1, 2; Supplementary Tables S3–8). There was also no statistical significance when comparing treatments across cycles (Supplementary Tables S9–11). This is likely due to the high native C and N status of the soil (Figure 5), indicating good baseline soil fertility. If the field was cropped for subsequent cycles, we might expect treatment differences in the longer term. However, the results of this short-term study indicate that feces-derived compost treatments perform as well as synthetically fertilized treatments, but no better than the unfertilized control, in a Haitian agricultural loam.

Figure 4

Figure 4. Total dry biomass for the first and second cycle. Data is expressed as the average ±standard deviation (n = 5). No significant differences were detected between treatments for either cycle at p < 0.05.

Figure 5

Figure 5. Soil C and N content in the 0–10 cm depth increment for the baseline, first cycle, and second cycle soil sampling. The 10–30 cm depth was not sampled in the second cycle due to threats to worker safety. Data is expressed as the average ±standard deviation (n = 5).

Plant monitoring indicators

In the first cycle, no treatments differed significantly in plant survival rate (p = 0.87, Supplementary Table S12). This trend was the same for the second cycle (p = 0.96, Supplementary Table S13). Plant survival increased across treatments between cycles (Supplementary Figure S3). This increase was significant for NPK (p = 0.028) and NPK-Compost (p = 0.035) (Supplementary Table S14). However, both Compost and Compost 150 treatments had significantly lower initial germination than other treatments during some dates in the first month of the cropping cycle (Supplementary Figure S3). This was true for both cycles, though more pronounced in the first cycle. However, as plant survival did not differ significantly between treatments at the end of either cycle, this may not be of concern. The impact of feces-derived compost application on germination and plant survival should be further studied with other crops and soil types.

Stem circumference, leaf number, length of the longest leaf, and width of the widest leaf, did not differ significantly between treatments for either cycle. P values are shown in Supplementary Tables S15-22, and data is visualized in Supplementary Figures S4-7. Plant height did not differ significantly between treatments for the first cycle (p = 0.18, Supplementary Table S23). In the second cycle, treatments did differ significantly for plant height (p = 2.42E-03, Supplementary Table S24). Compost 150 and Compost were significantly lower than the Control across the cycle (Supplementary Table S25).

Soil bulk density and aggregate stability

No significant differences in soil bulk density were observed between treatments for the baseline or first cycle for the 0–10 cm depth (Supplementary Figure S9). However, when comparing bulk density for each treatment between the two cycles, a significant reduction in bulk density in the second cycle was observed for Compost, Compost 150, and NPK (Supplementary Figure S9; Supplementary Table S26). This amounted to a 12.9% ± 8.64% reduction for Compost, 15.8% ± 12.4% reduction for Compost 150, and 16.0% ± 6.96% reduction for NPK.

While reduced bulk density in compost-amended soils is well established (Brown and Cotton, 2011), our results do not demonstrate short term bulk density improvements with compost treatments, since NPK-Compost did not lead to a significant reduction and NPK had the lowest second cycle bulk density. We did not see significant treatment differences for aggregate stability at 0–10 cm for both aggregates less than 250 μm and those larger than 2 mm (Supplementary Figures S10, 11). However, aggregates less than 250 μm generally decreased across treatments between cycles, while those larger than 2 mm generally increased, implying enhanced aggregation between cycles. Future research should explore the impact of successive feces-derived compost applications on agricultural soil bulk density, particularly as it compares to synthetic fertilization, to understand if longer term management with feces-derived compost can improve soil bulk density.

Soil pH, EC, moisture, temperature, and basal respiration

Soil pH did not differ significantly between treatments during the first cycle (p = 0.06, Supplementary Table S27), but differences were significant in the second cycle (p = 0.02, Supplementary Table S28). Second cycle Compost 150 soil pH averaged 6.78 ± 0.31, which was significantly lower than NPK-Compost at 6.90 ± 0.27 (p = 0.011, Supplementary Table S29). Though both pH ranges are within the optimal range for sorghum production (Butchee et al., 2012), farmers might consider NPK-Compost for crops that require more alkaline soil conditions. Further research on changes in soil pH with repeated application of feces-derived compost should be pursued, as the impact of repeated organic amendment application on soil pH varies across studies (Diacono and Montemurro, 2011). This is important as the availability of heavy metals is inversely correlated with soil pH (Diacono and Montemurro, 2011), which may be an important factor that determines best practices for feces-derived compost application, as seen with the nonsignificant increase of Cu and Pb bioavailability with repeated Compost 150 application in this study.

There were no statistical differences in soil basal respiration flux. Fluxes were significantly lower across treatments in the second cycle compared to the first (Supplementary Table S33). Soil EC, moisture, and temperature did not differ significantly between treatments for either cycle. P values are shown in Appendix C SI; Supplementary Table S34–39, and data is visualized in Appendix C SI; Supplementary Figures S14–16.

Soil C and the climate change mitigation potential of feces-derived compost application

No significant differences between treatments were observed for soil C at the 0–10 cm depth or 10–30 cm depth for any cycle (Supplementary Table S40). A reduction in average soil C for all treatments at the 10–30 cm depth was observed between the baseline and first cycle sampling (Supplementary Table S41). This may be due to soil C loss from initial tillage prior to planting the first cycle (Haddaway et al., 2017). This loss was significant for NPK, amounting to an −12.8% ± 4.75% (p = 0.016). This is likely due to the documented depletion of soil organic C in the subsoil with synthetic fertilization across agroecosystems (Khan et al., 2007). This may have longer term implications for continual synthetic fertilization of Haitian soils for soil C stocks. A positive, nonsignificant trend in soil C concentration in 0–10 cm soil was observed for Compost 150 and NPK-Compost over the course of the study, while NPK saw a decline in 0–10 cm soil C over the course of the study (Figure 5). Compost 150 0–10 cm C concentration increased by an average of 4.01% ± 4.4% by the first cycle, and 2.92% ± 7.95% by the second cycle. Similarly, NPK-Compost increased by 1.27% ± 10.37% by the first cycle and 3.68% ± 10.11% by the second cycle. This contrasts with NPK increasing by 0.18% ± 7.96% by the first cycle and decreasing by −10.1% ± 2.99% by the second cycle. Compost increased by 12.23% ± 7.48% by the first cycle and decreased by −9.9% ± 3.75% by the second cycle, and the Control increased by 9.66% ± 23.39% by the first cycle and decreased by −7.34% ± 20.18% by the second cycle. While the variability in these data is high, these results may indicate the potential of Compost 150 or NPK-Compost to increase soil C stocks in the topsoil in the long term. While soil C concentration has implications for soil C sequestration, more data is necessary to scale the climate change mitigation potential of feces-derived compost application. Studies estimating the soil C sequestration potential of organic amendment application on agricultural land often utilize long term field study data or modeling to estimate the net climate change mitigation potential at the decadal or centennial time scale (Jarecki and Rattan, 2003; Ryals et al., 2015). Additional spatially and temporally robust GHG flux measurements after feces-derived compost application, deep soil C measurements over multiple years of a given compost application practice, and complete bulk density and/or equivalent soil mass data sets are necessary to determine the net climate change impact of feces-derived compost application on cropland. Feces-derived compost application on different crop types, soil degradation statuses, land management scenarios, and regions should also be explored.

Soil N

No significant differences between treatments for soil N at the 0–10 cm depth or 10–30 cm depth were observed for any cycle (Supplementary Table S42). Soil N at the 10–30 cm depth declined across treatments between the baseline and first cycle (Supplementary Table S43). This is also likely due to microbial activity being stimulated by hand tillage used to prepare the field for the first cycle (Song et al., 2022). This loss was significant for NPK at −14.2% ± 6.90% (p = 0.011). This is consistent with findings on the long term loss of soil N in the subsoil after synthetic fertilizations across agroecosystems, attributed to fertilizer-N stimulation of microbial C usage and native soil N mineralization (Mulvaney et al., 2009; Gardner and Drinkwater, 2009). This may have longer term impacts on native soil N depletion and ultimately crop quality with continual synthetic fertilization in Haitian agroecosystems.

Conclusion

CBS EcoSan systems have the potential to help achieve multiple SDGs, including SDGs 6, 2, and 13. This research demonstrates the potential of a CBS EcoSan reuse product, feces-derived compost, to enhance key soil health indicators in a Haitian agroecosystem over two consecutive sorghum cropping cycles. Feces-derived compost applied at 150% of sorghum N demand significantly increased bioavailable soil S after the second application when compared to synthetic fertilization and unfertilized soil. Feces-derived compost application led to moderate increases in 0–10 cm C and N content at the end of the second sorghum crop cycle. This has implications for long term positive soil health impacts with repeated feces-derived compost application in Haitian agroecosystems. However, further research is necessary on the tradeoffs between improved soil health indicators, soil C stocks, and ultimately climate change mitigation potential. Feces-derived compost application should be studied longer term in multiple Haitian agroecosystems.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

EB: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing. PW: Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing – review and editing. JJ: Conceptualization, Data curation, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing – original draft, Writing – review and editing. CR: Conceptualization, Methodology, Project administration, Resources, Supervision, Validation, Writing – review and editing. YB: Investigation, Methodology, Resources, Validation, Writing – review and editing. SK: Conceptualization, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing – original draft, Writing – review and editing. WF-J: Formal Analysis, Resources, Validation, Writing – review and editing. RR: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing – review and editing.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Acknowledgements

The authors are overwhelming grateful to Eldine Dacéus, Jules Camille Janvier, Limane Joachim, Fledine Joseph, and Samuel Predinor of the Université Anténor Firmin du Cap-Haïtien, Haiti for their essential support in data curation, methodology, investigation, supervision, and resource provision during this study. This work would have been impossible without their dedication, insight, and creativity. The authors also thank Bridj Ozeris for facilitating interviews in Kreyól to reflect on the research process with Haitian co-authors, Michèle Heeb for helping coordinate the use of SOIL lab equipment, and to Melisa Quintana for helping process soil samples. EB thanks the Rotary District 5220, UC Davis Global Fellowship for Agricultural Development, and the UC Merced Department of Life and Environmental Sciences for supporting this work.

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.

Generative AI statement

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/fenvs.2025.1705344/full#supplementary-material

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