The current compost agronomy carries uncertainties
Variable rather than comprehensive benefits were identified by our global synthesis of current compost use. While compost application in croplands can be effective for SOC sequestration, it carries large uncertainty for crop yield and N2O emissions. Overall, 90% of all experiments detected improved SOC (Extended data Fig. 2b). Compost only (CO) or compost+mineral fertilizer (CM) increased SOC by 36.8% and 32.0% when compared to mineral fertilizer (MF) (Fig. 1a). This sharpens findings of previous meta-analyses with smaller sample sizes and geographic range that calculated SOC increase of over 40% (Extended data Table 2). The average crop yield in 61% of CO experiments was negatively affected with 10% lower yields than with MF (Fig. 1a, Extended data Fig. 2a). Although CM generated on average 15.7% higher yield than MF, 24% of experiments had lower yield (Fig. 1a, Extended data Fig. 2a). Of the 82 and 109 experiments with CO and CM that quantified yield and SOC, only 30.5% and 70.6% (25 and 77 experiments), respectively, found that both yield and SOC improved (Extended data Fig. 3).
Many studies contest the notion that organic soil amendments can mitigate GHG emissions because N2O emissions can increase in the presence of labile organic C26-27. Our results indicated that compost (CO, CM) did not stimulate N2O emissions in line with a previous meta-analysis based on a relatively small number of observations (Fig. 1a, Extended data Table 2). Compost strongly influences the variability of N2O emissions when compared to MF, ranging from 13.4% lower to 20.5% higher (CO, 95% CI) and from 8.0% lower to 26.6% higher (CM, 95% Cl) (Fig. 1a). Together, these findings are evidence that the potentially comprehensive benefits derived from compost use are not being realized.
Uncertainties at regional and temporal scales were detected. CO had lower yield than MF in most regions, except in Africa and South America where CO and MF had similar yields (Fig. 1b). CM improved crop yield in most regions, but the effect size varied greatly from a 48.6% increase in Africa to a 4.7% increase in Europe. CO increased SOC similarly across regions while the effect size of CM spanned from 86.5% SOC increase in Africa to 6.4% in Australia (only 6 observations, Fig. 1c). Strong regional variation was also observed for N2O emissions with lower emissions in Africa (CO), similar (CO, CM) in most regions (Asia, Europe, North America) and higher emissions in Australia (CM) compared to MF (Fig. 1d). It is worth noting that except for Asia (CM, 41 observations), most regions had few (6-15) observations for N2O emissions.
Our analysis did not confirm the often-reported positive relationship between the duration of compost application and crop yield (Extended data Fig. 4a). CO generated 10.9% lower yield than MF in short term use (<3 years) but produced similar yield as MF in medium-term (3-10 years) and long-term use (>10 years, only 12 observations) (Fig. 1e). In contrast, CM significantly increased yield in short-term (16.3%) and medium-term use (12.3%), while long-term use had similar yield as MF. Although CO and CM increased SOC, there was no obvious net accumulation of SOC in the longer term (Fig. 1f, Extended data Fig. 4b). Short-term compost use of CO had similar N2O emissions than MF, while long-term compost use appeared to stimulate N2O emissions by 22.8% (Fig. 1g). With CM, N2O emissions matched those of MF in short-term use and increased (26.9%) in medium-term use but were lower in long-term use (-19.2%).
Taken together, our global analysis robustly demonstrates that underperformance and variability, rather than comprehensive benefits, characterize compost use across regional and temporal scales. To innovate the use of compost, we propose a Precision Compost Strategy (PCS) that requires understanding of compost characteristics and their interactions with crop and biophysical settings.
Determinants of a Precision Compost Strategy
A boosted regression tree (BRT) analysis quantified 11 factors that impact the effect size of compost on yield and SOC (Fig. 2). To predict the effects, we considered site biophysical traits (crop type, soil texture, SOC, pH, temperature, precipitation), nitrogen (N) supply (relative supply with CO or CM to MF) and compost characteristics (C/N, C/P, pH, electrical conductivity EC). The correlation coefficient (R2) of the relationships between the model-predicted effects, and the measured effects on yield and SOC with CO or CM versus MF exceeded 0.80; i.e., together these factors explain over 80% of effects (Fig. 2c, f). A less comprehensive analysis was performed on N2O with fewer data available.
Effect of nitrogen. Nitrogen supply was the primary yield-determining factor, contributing 17.8% (CO) and 37.2% (CM) (Fig. 2a-b), in line with the notion that optimal N supply guarantees yield28. To match the yields achieved with MF, CO demands up to 50% more N than MF, and CM up to 50% less N. At similar N supply, CO generated 9.7% lower yield than MF, and CM 4.9% higher yield (Extended data Fig. 5a). The strong yield-enhancing effect of CM at similar N supply can be attributed to a higher N use efficiency when combining slower and faster release organic and inorganic N, respectively (Extended data Fig. 6). To generate higher yield than MF, overall, the proportion of compost-N to total-N supply may range from 10 to 80% (Extended data Fig. 7a). Soils with different SOC demand different N regimes; for example for greatest benefits, the proportion of compost-N should be no more than 30% in very low SOC soil (<5.0g SOC/kg soil), but can approach 50% in high SOC soil (>15.0g/kg) (Extended data Fig. 7b,e).
Nitrogen supply was the third strongest factor for increasing SOC, contributing 15.4% (CO) and 13.1% (CM) (Fig. 2d-e). Nitrogen supply determines the rates of SOC accumulation and decomposition29, and both insufficient30 and excess N31 can prevent SOC accumulation. SOC benefits with CM peaked when 6.5-times more N than MF was supplied (Extended data Fig. 8b), highlighting that optimal N supply will maximize net SOC accumulation. The balance between C and N input is an important factor for SOC accumulation32, and the highest relative SOC benefit was observed with a ratio of C:N input of around 5.0 in very low SOC soil (<5.0g C/kg soil), 10-15 in low SOC soil (5.0-10.0g/kg), and 15-20 in moderate and high SOC soil (>10.0g/kg) (Extended data Fig. 9).
Effective reduction of N2O emissions also relies on optimal N supply33. IPCC Tier 134 accounting assumes that 1.0% of N fertilizer is emitted as N2O, similar to our calculated average of MF (0.9%) but twice the calculated averages of CO (0.44%) and CM (0.49%) (Extended data Table 3). Rather than assuming a linear increase of N2O emissions with N supply, exponential increases can occur35. To lower N2O emissions below those of MF, CO should supply 2.9-times less, and CM 1.2-times less N, than MF (Extended data Fig. 8c). Promisingly, at similar N supply CM resulted in 17.0% lower N2O emissions than MF (Extended data Fig. 5c).
In summary, N is a strong determinant of the effects of compost on yield, SOC and N2O emissions and trade-offs with higher N supply have to be considered. Sufficient N supply guarantees yield benefits but carries uncertainties for SOC and N2O emissions mitigation. Optimal N supply with compost use must be accurately identified for different cropping systems (e.g., paddy vs upland production), application methods (e.g., subsurface vs surface) and compost characteristics (e.g., cattle vs poultry manure feedstocks). Similarly, initial soil SOC must inform the ratio of C and N input and the proportion of compost N to total N input.
Effects of crop and site characteristics. Compost effects depend on crop type, soil properties (texture, initial SOC, pH) and climate (mean annual temperature MAT, mean annual precipitation MAP), which together contribute almost 50% to yield (CO 45.8, CM 46.0%, Fig. 2a-b) and 60% or more to SOC (CO 60.0, CM 65.0%, Fig. 2d-e).
CO effects on yield differed significantly between crop types (Extended data Table 5) with lower yields compared to MF in vegetable (-18.8%), grain (-12.6%) and feed crops (-10.1%), variable yields in root and tuber crops (-6.6 to 4.9%), and higher yield in fruit crops (11.6%, only 7 observations) (Fig. 3a). Thus, CO benefits crops with longer growing periods and/or lower nutrient demand but not fast growing and nutrient demanding crops. The consistent yield increases achieved with CM, ranging from 13.1% in grain crops to 24.6% in fruit crops (Fig. 3a), confirm that the nutrient limitations observed with CO are preventable in all major crops. SOC benefits with CO depended on crop types with highest SOC gains observed in root and tuber (101.2%), feed (98.8%) and fruit crops (56.4%), and lower gains in grain (28.4%) and vegetable crops (27.2%). SOC benefits with CM were similar in root and tuber (36.9%), fruit (36.2%) and grain crops (32.1%), and greater than in vegetable crops (15.8%) (Extended data Table 5, Fig. 3g). The relatively lower SOC benefits with vegetable crops can be attributed to higher initial SOC content in soils under vegetable production36, and potentially accelerated SOC decomposition in the presence of high N supply37.
Yield and SOC were strongly influenced by soil properties (P<0.001; Extended data Tables 5-6). Compost benefited crops more on poorer textured soils (sandy, clay) and less on favorably textured soils (clay-loam, loam). While CO generated similar yield as MF on poorer textured soils, it strongly reduced yield on more favorably textured soils (clay-loam -16.1%; loam -32.5%) (Fig. 3b). CM strongly boosted yield (41.3 and 39.2%) on sandy and clay soils, but not on clay-loam and loam. The beneficial effects of compost on poorer textured soils can be attributed to improved soil physico-chemical properties (Extended data Fig. 6). SOC increased in the order sandy soil (CO 83.0%, CM 155.1%) > clay soil (CO 37.0%, CM 40.1%) > clay-loam soil (CO 24.1%, CM 23.5%) > loam soils (CO 20.8%, CM 16.3%) (Fig. 3h). The pronounced effect on sandy soils is expected as these soils generally have low initial SOC and therefore much potential for SOC gain. The strong SOC benefit on clay soil may be explained by the mineral matrix protecting organic C from microbial degradation38.
We detected notable trade-offs of compost benefits on yield and SOC. In very low SOC soil (<5.0 g C/kg soil), compost resulted in the lowest yield (CO -19.9%, CM 15.6%), but the largest SOC benefits (CO 58.7%, CM 55.8%) (Fig. 3c,i). In high SOC soil (>15.0 g C/kg), compost generated comparatively the highest yields (CO -5.5%, CM 25.7%), but lowest SOC benefits (CO 23.7%, CM 10.9%). The low yield benefit in low SOC soil can be explained by low microbial activity resulting in slow decomposition and nutrient release39. The lowest SOC benefits occurred on high SOC soil possibly due to the negative relationship between the initial SOC of soil, a primary driver (CO 19.2%, CM 24.2%, Fig. 2d-e), and SOC benefits40. Thus, depending on initial SOC levels, specific compost must be chosen to achieve synergistic benefits as outlined below.
Compost benefitted yield more on acidic soil (pH<6.0) than alkaline soil (pH>8.0) or more neutral soil (pH 6.0-8.0). CO generated similar yield as MF on acidic and alkaline soil, and lower yield on neutral soil (-18.7%, Fig. 3d), while CM had the strongest benefit on acidic (27.8%) and neutral soil (11.0%) and similar yield as MF on alkaline pH soils. SOC benefits ranged from acidic soil (CO 77.6%, CM 60.5%), alkaline soil (CO 37.8%, CM 41.7%) to neutral pH soil (CO 27.2%, CM 22.2%) (Fig. 3j). Together, this confirms a pH amelioration effect of compost on soils with non-optimal pH for crops (Extended data Fig. 6), which is most pronounced for acidic soil as compost is generally alkaline (Extended Data Table 1).
Compost benefits were greater in tropical climate (MAT>20°C) with comparatively higher yields (CO -7.1%, CM 20.1%), than in cool climate (MAT<10°C; CO -20.1%, CM 8.8%) (Fig. 3e). SOC showed a similar trend with highest benefits in tropical (CO 44.2%, CM 35.9%) and warm climates (10°C <MAT<20°C) (CO 41.1%, CM 33.4%) than in cool climate (CO 24.5%, CM 21.9%) (Fig. 3k). The lower benefits of yield and SOC with compost use in cool climates is likely due to the slower nutrient release39 and relative higher initial SOC in these soils41.
Compost generated higher benefits in arid (MAP <500mm y-1) and semi-arid climates (MAP 500-1000mm y-1) than humid climate. CO produced similar yield as MF in arid climate, but lower yields in semi-arid (-10.6%) and humid climate (-16.7%; MAP >1000mm y-1, (Fig. 3f). In contrast, CM benefitted yield more in semi-arid (21.3%) and humid climates (12.5%) than in arid climate (3.9%). CO benefitted SOC more in arid (55.3%) than in semi-arid (31.9%) and humid climates (34.3%) (Fig. 3l). SOC benefits with CM were higher in arid and semi-arid (34.2, 40.4%) than in humid climates (18.6%). The stronger benefits of compost in drier climate can be largely explained by improved soil water relations (Extended data Fig. 6), and SOC depleted agricultural soils41. In humid regions, compost only (CO) had lowest yield with possible reasons including fast growth of tropical crops and associated high nutrient demand, and heavy nutrient leaching form high rainfalls42.
N2O emissions were not significantly impacted by the factors outlined above. Emissions were mostly lower or matched those with MF (Extended data Fig. 10), likely because compost mostly generates less inorganic N which limits nitrification and denitrification43.
We show that the properties of the crop-soil-climate system significantly influence how composts benefit yield and SOC, and how current compost use generates most benefit in crop systems with long growing periods, poorly textured and/or acidic soils, or warm, dry climates (Extended data Fig. 11). Questions remain about how to achieve the best outcomes with compost use. SOC gains in low SOC soil seem easily achieved but can SOC levels also be improved in high SOC soil and realize synergistic effects for yield and SOC?To address these questions, we explored the effects of compost characteristics.
Effects of compost characteristics. Together, compost characteristics (C/N, C/P, pH, EC) contributed 36.3% (CO) and 17.1% (CM) to yield (Fig. 2a-b) and 24.5% (CO), 21.9% (CM) to SOC (Fig. 2d-e).
The C/N ratio of CO emerged as the second most important factor (14.8% contribution) for crop yield (Fig. 2a). Generally, CO with low C/N ratio (<10.0) generated the same yield as MF and a high C/N ratio (>10.0) reduced yield (-7.5 to -16.9%, Fig. 4a). In contrast, CM with high C/N ratio produced 10.8-18.1% higher yields than MF (Fig. 4a), confirming that nutrient limitation imposed by CO can be offset by mineral fertilizer addition. Notably, the effect of compost C/N ratio on yield was impacted by initial SOC. In soil with very low initial SOC (<5.0g/kg soil), low C/N compost had the strongest yield benefits (CO 11.5%, CM 35.8% above MF), while in soil with high initial SOC (>15.0g/kg) high C/N compost achieved best outcomes with similar (CO) and 32.8% higher (CM) yield than MF (Fig. 4i-j). Low SOC soils generally have a lower inherent nutrient status and low C/N compost supplies more N to crops44, while high SOC soils with higher nutrient status and higher C/N compost benefit N relations, including initial N immobilization by microbes45-46.
Similarly, SOC benefit of compost was impacted by the initial soil SOC. Low C/N compost nearly doubled SOC on very low initial SOC soil (CO 106.2%, CM 88.3%) compared to high C/N compost (CO 40.3%, CM 34.7%) (Fig. 4k). On high SOC soil, this reversed as high C/N compost benefitted SOC more (CO 41.8%, CM 21.7%) than low C/N compost (CO 20.6%, CM 4.2%) (Fig. 4l). This confirms that depending on SOC status, soils require compost with different C/N stoichiometry. Nitrogen limitation in low SOC soil demands compost with lower C/N ratio, whereas C limitation in higher SOC soil requires higher C/N compost47-48. A further consideration is that higher SOC soil with higher C/N ratio has a higher fungal/bacterial ratio, which is favoured by higher C/N compost which in turn can favour SOC build up49.
Acidic (pH<6.0) compost often resulted in less benefit, while neutral (pH 6.0-8.0) and alkaline (pH>8.0) compost tended to have better outcomes. Acidic compost had the lowest benefits with -24.3% yield (CO) and similar yield (CM) as MF (Fig. 4c), while alkaline (pH>8.0) CO and neutral (pH 6.0-8.0) CM generated best outcomes with similar (CO) and 25.7% higher (CM) yield than MF. Immature compost generally has higher acidity due to the presence of organic acids which can negatively affect crops, while mature compost is often weakly alkaline with a higher content of soluble N and other yield-benefitting substances (e.g., humic compounds)9. Overall, alkaline compost benefitted SOC more (CO 52.2%, CM 46.4%) than acidic compost (CO 23.0%, CM 25.2%) (Fig. 4g), in line with greater yield with alkaline compost (Fig. 4c). On acidic soil, a higher pH of CO had a significant (P<0.01) positive relationship with yield and SOC (Fig. 4m), confirming the pH amelioration effect. We examined if acidic compost has most benefits on alkaline soil, and while the limited data did not support this notion, recommend further exploration.
Compost EC significantly affected crop yield with CO (Extended data Table 5). CO with low EC (<2mS/cm) increased yield by 16.8%, while high EC (>4mS/cm) reduced yield (-26.5%) (Fig. 4d). The use of high EC compost, especially in larger amounts and over longer times as often practiced when compost is the only nutrient source (e.g., some organic production systems), causes salt accumulation in soil and diminishes yield as crops suffer water stress and salt toxicity50. High EC compost also negatively impacted the soil biological community and nutrient cycling51. Compost EC weakly affected SOC (CO P=0.058, CM P=0.078, Extended data Tables 5-6) with low EC compost more strongly benefitting SOC (CO 66.9%, CM 30.5%) than high EC compost (CO 28.7%, CM 12.5%, Fig. 4h). Reasons for declining SOC benefits with increasing EC include reduced crop growth (Fig. 4d) and associated lower input of crop residues and negative impact on soil aggregation and protection of SOC from microbial degradation52.
N2O emissions were lower or matched those with MF and were not significantly impacted by compost characteristics (Extended data Fig. 12). We could not perform a group heterogeneity (QB) analysis for EC, with only one subgroup with CO and no studies with CM. However, CO with high EC stimulated N2O emissions (25.4% above MF) (Extended data Fig. 12d), likely owing to a combination of factors that include inhibited crop growth, resulting N surplus, reduced soil aggregation and poorer soil aeration, all of which enhance the processes leading to N2O emission50-52.
In summary, the findings confirm our hypothesis that compost characteristics significantly affect the responses of the tested response variables in cropping systems. Matching the initial soil SOC, pH, EC with a specific compost is crucial to obtain the desired outcomes, and further research has to fill the knowledge gaps where current data cannot examine all scenarios.
A Precision Compost Strategy for sustainable agriculture
PCS is a conceptual innovation to advance the effective use of diversified composts in today’s cropping systems that demand inputs of organic matter and nutrients. We envisage PCS to guide a systematic approach to achieve superior outcomes by matching compost characteristics, cropping system properties and application methodology (Fig. 5). Three principal steps in a PCS comprise (i) diagnosing local biophysical conditions, (ii), designing and producing specific composts targeting crop-soil-climate, (iii) supplying composts with optimal carbon and nutrient supplies (N, P, other essential and beneficial nutrients). Based on these principles, we estimated the potential benefits across global regions (Table.1, Supplementary Information).
In Africa, especially Sub-Saharan regions, yield and SOC are generally low and heavily constrained by nutrient input. Developing compost as a source of nutrients could be a critical strategy to improve soil fertility and crop productivity with best yield achieved with the combined input of organic matter and nutrients53. To deliver nutrients and build SOC, low C/N compost should be a priority. Suitable feedstocks for low C/N compost include crop residues (N-rich legume biomass), animal manure (poultry, pig), and municipal wastes (food waste, humanure). Low-cost composting techniques, such as open compost rows, can be implemented with limited infrastructure. Compost is likely to have most impact in drier regions where increased SOC can partially mitigate the impacts of climate change induced altered rainfall and temperatures21. We estimate that precision composting in Africa’s croplands can increase cereal productivity by 82.5 Tg (40% of current production) and that annually 46.7 Tg C can be sequestered into SOC, which amounts to a 3.9‰ rate that approximates the “4 per 1000 initiative” for climate change mitigation (Table 1).
In Asia, we focus on China where most research on compost has been performed, crop yield is moderate, and low SOC and high N2O emissions demand action. Contrary to Africa, China suffers nutrient excess28. To maximize compost benefits, reducing N input in cropping systems should be the first step, as this negatively impacts SOC buildup and stimulates N2O emissions (Extended data Fig. 8b-c). Toxins in compost are derived from intensive livestock and industrial wastes, and high-quality compost requires clean feedstocks (separating clean and contaminated feedstocks) and controlling composting processes (e.g., adding heavy metal fixatives, ensuring adequate maturation periods22). For example, northeast China’s high SOC soils demand high C/N compost and Southern China’s acidic soils need alkaline compost. Since China has a considerable soil phosphorus (P) surplus54, a P-based strategy should be adopted for compost use to avoid P oversupply55. We estimated that compost use in China can increase cereal production by 24.8 Tg (4% of current production) and annually sequester 27.9 Tg C into SOC, which amounts to a sequestration rate of 3.3‰ (Table 1).
Europe and USA have high crop yield and SOC, with moderate, stable or declining N2O emissions. These regions have globally the most advanced nutrient and crop management28 but can further benefit from an advanced compost agronomy (e.g., crop rotation, conservation tillage, manure recycling), to lead the way towards a Precision Compost Strategy to accurately match the needs of crops and avoid current N excess and pollution. Developing high C/N compost could be an important strategy to increase SOC. Suitable techniques and feedstocks for high C/N compost can be solid-liquid separation of livestock slurries that favor solid fractions with high C/N ratio, adding C-rich wastes (municipal green waste, forest industry wastes) as well as considering nutrient-dense food waste for nutrient delivery56. We estimate that compost use can increase crop productivity by 9% (Europe 47.0 Tg) and 14% (USA 60.6 Tg) above current production, and that annually 20.7 Tg C (Europe) and 19.6 Tg C (USA) can be sequestered into SOC, amounting to annual C sequestration rates of 1.0‰ (Europe, USA) (Table 1).
We estimate that globally, a Precision Composting Strategy can increase cereal production by 354.5 Tg, which is 1.7-times Africa’s current production (Table 1). Composts in cropping lands could annually sequester 170.3 Tg C into soils, which achieves 17.0-22.7% of the total C sequestration potential of global croplands (0.75–1.0 Pg yr-1)57. Our estimated compost benefits on climate mitigation did not consider the wider environmental impact that include GHG emissions in upstream mineral fertilizer producing and composting processes. However, recent life cycle assessments (LCA) have demonstrated GHG savings with compost use based on manufacturing less mineral fertilizer, sequestering more SOC and curbing N2O emissions58-60.
Recycling biowastes into cropland soil is attracting attention as a win-win strategy for mitigating environmental impacts of cropping and enabling sustainable high production agriculture. The challenge is how to recycle and use biowaste efficiently. We present evidence that a global Precision Composting Strategy is a vital component of sustainable agriculture. This strategy could be accepted worldwide but has to after overcome various barriers (Supplementary Information) to advance circular agriculture with greater crop productivity, higher soil fertility, greater resilience against climate change, and circularity of material flows in support of Sustainable Development Goals.