A global meta-analysis on the effects of organic and inorganic fertilization on grasslands and croplands

Global environmental change (e.g., rising atmospheric CO2 and nutrient enrichment) threatens ecosystem functions, spurring interest in nature-based solutions to sustain biodiversity and enhance carbon sequestration. Inorganic fertilizers are widely used in grasslands and croplands to boost productivity but often reduce plant diversity, especially as nutrient loads increase. It is unclear whether organic fertilization can avoid this tradeoff by increasing biomass without eroding diversity, given mechanistic hypotheses for diversity loss under fertilization: biomass-driven light competition, reduced niche dimensionality, and nitrogen-related detriments (e.g., acidification, ammonium toxicity, microbiome shifts). Organic fertilizers, derived from plant/animal residues, can improve soil properties (SOC, cation exchange, water holding capacity) differently from inorganic fertilizers. Understanding whether these differences alter diversity outcomes and carbon sequestration is crucial, and may depend on climate, soil moisture, nutrient availability, land use, and management. The authors compiled a global dataset of fertilization experiments in grasslands and croplands to test hypotheses that: (1) organic fertilization increases aboveground biomass more than inorganic fertilization in grasslands; (2) if light competition or reduced belowground niches dominate, organic fertilization would also reduce diversity; (3) if nitrogen detriment dominates, organic fertilization would not reduce diversity; and (4) organic fertilization in croplands would produce SOC gains comparable to those in grasslands. They further evaluated environmental conditions under which tradeoffs among biomass, SOC, and diversity are minimized.

Prior work shows inorganic nitrogen often increases biomass but reduces plant diversity in grasslands, with stronger losses at higher nutrient inputs. Three mechanisms have been proposed for diversity loss under fertilization: (i) biomass-driven light competition excluding short-statured species; (ii) niche dimension reduction due to fewer limiting resources; and (iii) nitrogen detriments including soil acidification, ammonium toxicity, and microbiome shifts. Evidence for the niche dimension hypothesis has been mixed, and recent tests found limited support. Organic fertilizers differ from inorganic fertilizers by supplying organic matter and multiple macro/micronutrients, enhancing SOC, cation exchange capacity, water-holding capacity, and microbial activity, and buffering pH—factors that may mitigate diversity loss. However, reported effects of organic fertilization on diversity vary (decreases, increases, or no effect), likely modulated by soil pH, moisture, and fertility. SOC is the largest terrestrial carbon pool; its response to fertilization is uncertain and context-dependent (climate, land use, nutrient status). Meta-analyses suggest manure can increase SOC substantially in both croplands and grasslands, but land-use differences and management (e.g., tillage) may modulate outcomes.

Data collection: The authors systematically searched peer-reviewed literature (Web of Science, CNKI) up to 30 October 2022 using combinations of nutrient addition and response terms (biomass, diversity, SOC) and ecosystem terms (grassland/herbaceous/cropland). Inclusion criteria: (1) field experiments in semi-natural/natural grasslands or croplands with ambient and fertilized treatments; (2) reported means with SE/SD and sample sizes; (3) grassland studies with exotic plant species introduced by organic fertilization were excluded. Data were extracted from tables or digitized from figures. The final dataset included 537 publications across all continents except Antarctica. Grasslands: 1540 pairs (ambient vs fertilization) for aboveground biomass, 1625 for species richness, 191 for Pielou evenness, 799 for SOC under inorganic fertilization; 350 for biomass, 155 for richness, 89 for evenness, 388 for SOC under organic fertilization. Croplands: 367 SOC pairs under organic fertilization. Plant diversity was not analyzed in croplands due to low species numbers in managed systems. Fertilizer definitions: Inorganic included urea, ammonium nitrate, calcium nitrate, super-phosphate, ammonium phosphate dibasic, sodium dihydrogen phosphate, potassium sulphate, potassium chloride (no liquid ammonia). Organic included industrial organic fertilizers, livestock manures, and composts; 12% industrial (thermally fermented), 88% livestock/compost (composted, air-dried). In 12% of organic experiments, inorganic NPK were also added. About 97% of grassland experiments lasted <10 years; 74% of cropland experiments lasted >10 years. Effect sizes and meta-analysis: Effects quantified as natural log response ratio (lnRR = ln(Yt/Yc)) for Org vs Amb, Inorg vs Amb, and Org vs Inorg (same experiments). A hierarchical random-effects meta-analysis with inverse-variance weighting (site as random factor) estimated weighted mean lnRR++ and 95% CIs using rma.mv (R metafor 4.4.0); CIs via bootstrapping; significance inferred when CIs excluded zero. Publication bias assessed via Egger’s test and trim-and-fill. Percent change computed as (e^{lnRR++} − 1) × 100%. Environmental covariates: Mean annual temperature (WorldClim 2), soil properties at 0–30 cm (SoilGrids: SCEC, total N, pH, bulk density, organic C density, sand), soil water content (ERA5-Land 0–28 cm, mean 1982–2022), and fertilizer rates. Species richness used as primary diversity metric for gradients due to larger sample size. Models across gradients: Linear mixed-effects models (lme4, lmerTest) related lnRR to environmental moderators with study site as random effect. Multi-model inference (MuMIn dredge, AIC) selected best predictors; random forest (randomForest with rfPermute) corroborated significant predictors. Structural equation modeling (lavaan) disentangled direct/indirect effects on biomass and richness responses; model fit evaluated per standard criteria. Standardized Major Axis Tests (and OLS) compared slopes of SOC responses vs drivers (MAT, organic rate, initial SOC, nutrients, texture, pH) between grasslands and croplands. Data and code availability: All data and R code available at Figshare (10.6084/m9.figshare.25493419).

- Global mean effects in grasslands: - Inorganic fertilization increased aboveground biomass by 42% (p<0.001) vs ambient, but decreased species richness by 18% (p<0.001) and evenness by 6% (p<0.001). It increased SOC by 2% (p<0.001). - Organic fertilization increased aboveground biomass by 56% (p<0.001) vs ambient, without reducing plant diversity (richness +4%, p=0.181; evenness +0.02%, p=0.986). It increased SOC by 19% vs ambient (p<0.001) and by 15% vs inorganic (p<0.001). Compared with inorganic, organic increased richness and evenness by 10% (both p<0.001) while maintaining higher biomass. - Environmental gradients in grasslands under organic fertilization: - Biomass gains increased with mean annual temperature (MAT), soil total N, and organic fertilizer rate (all p≤0.001). - Species richness increased with higher soil bulk density, soil water content (SWC), and cation exchange capacity (p≤0.038), indicating mesic and higher-capacity soils promote diversity gains under organic inputs. - Environmental gradients under inorganic fertilization: - Biomass increases were larger with higher N fertilizer rates, more nutrients added, and higher SWC (p≤0.05). - Species richness declined more steeply with increasing N and P fertilizer rates (p<0.001). - Biomass–diversity relationship: - Under organic fertilization, the link between biomass and richness responses was weak and nonsignificant (slope −0.11, p=0.61, n=25). - Under inorganic fertilization, richness declined with increasing biomass response (slope −0.42, p<0.001, n=229); slope was significantly steeper than under organic (Pslope difference=0.023). - SOC responses to organic fertilization across land uses and climate: - At MAT ≤15°C, SOC increased more in croplands than grasslands (32% vs 14%). At MAT >15°C, SOC increased more in grasslands than croplands (62% vs 34%). - In grasslands, SOC response increased with MAT (standardized slope ~0.32, p=0.002) after accounting for soil and rate effects; in croplands it decreased with MAT (slope ~−0.23, p=0.013). - SOC response increased with organic fertilizer rate in both land uses (p≤0.031); slopes did not differ significantly between grasslands and croplands. - SOC response declined with higher initial SOC in both land uses, more strongly in croplands (p<0.001; Pslope difference=0.001). Under similar initial SOC, grasslands showed larger SOC gains. - Additional soil moderators: positive effect with higher sand content in grasslands (p=0.023); negative with soil total N (croplands, p=0.041) and marginally with higher pH (croplands, p=0.053). - Mechanistic implications: - Results do not support the niche dimension hypothesis for species richness; number of nutrients added was not selected in best models for richness under organic or inorganic treatments. - Patterns are consistent with nitrogen detriment mechanisms (acidification, ammonium toxicity) driving diversity loss under inorganic N and P addition, while organic inputs buffer soils, release nutrients gradually, and maintain or increase diversity, especially in wetter grasslands.

The meta-analysis shows that organic fertilization can break the typical tradeoff observed with inorganic fertilization—namely, increased biomass at the expense of plant diversity—by enhancing aboveground production and SOC without reducing diversity in grasslands. Gains in species richness under organic fertilization were favored in soils with higher water content, bulk density, and cation exchange capacity, suggesting that improved soil physical and chemical properties and greater soil moisture mitigate light-competition or water-stress pathways that would otherwise reduce diversity as biomass increases. The lack of support for the niche dimension hypothesis (no stronger diversity losses with more nutrients under organic inputs) and the stronger diversity declines with higher inorganic N and P rates align with nitrogen detriment mechanisms. Organic amendments likely buffer pH, enhance cation exchange and water retention, and supply multiple nutrients more gradually, supporting both plant and microbial communities, thereby maintaining diversity even as biomass rises. SOC responses reveal land-use and climate contingencies: organic fertilization boosts SOC in both land uses, but the benefit shifts with temperature—croplands show larger gains in cooler regions, while grasslands outperform in warmer regions. Management (tillage, irrigation) likely modulates these patterns: tillage in croplands can accelerate decomposition, especially in warm climates, dampening SOC accrual; grasslands, typically untilled and unirrigated, may exhibit stronger SOC gains with warming due to increased inputs and moisture dynamics. The observed increase in SOC with organic fertilizer rate and the diminishing returns at high initial SOC inform where and how much to apply for maximal sequestration. Overall, results support organic fertilization as a nature-based solution that simultaneously enhances forage production and soil carbon storage while conserving native plant diversity, with particularly favorable outcomes in mesic grasslands and under warming in many regions.

This global synthesis of 537 experiments demonstrates that organic fertilization increases aboveground biomass and soil organic carbon without concomitant losses of plant diversity in grasslands, contrasting with inorganic fertilization, which elevates biomass but reduces species richness and evenness. Organic fertilization also enhances SOC more than inorganic fertilization and exhibits context-dependent advantages across climate and land uses—especially strong SOC gains in warmer grasslands. These findings position organic fertilization as a practical nature-based solution for improving grassland ecosystem services, including forage production, biodiversity conservation, and carbon sequestration. Future research directions include: (1) long-term (>10–20 years) experiments in both grasslands and croplands to quantify durability of diversity and SOC responses; (2) explicit tests of management interactions (e.g., tillage, irrigation, grazing intensity, harvest frequency) with organic inputs; (3) systematic assessment of organic fertilizer quality (e.g., C:N ratios, macro/micronutrient profiles) and application timing on biodiversity and SOC; (4) evaluation of strategies to prevent exotic species introduction and contamination in organic amendments; and (5) region-specific optimization frameworks that integrate climate, soil properties, and initial SOC to maximize co-benefits and minimize tradeoffs.

- Plant diversity effects were analyzed only in grasslands; croplands were excluded due to low crop species numbers, limiting generalization of diversity results to agricultural systems. - A subset of organic fertilization studies (12%) included additional inorganic NPK, potentially confounding pure organic effects, though analyses accounted for treatment contrasts and covariates. - Most grassland experiments were short-term (<10 years), whereas many cropland experiments were longer (>10 years), introducing potential duration-related biases in response magnitudes. - Measures of organic fertilizer quality (e.g., C:N) were available only in subsets (about 40% for SOC analyses; ~6% for detailed nutrient profiles), constraining inference on amendment quality effects. - Meta-analysis relies on published data; despite Egger’s tests and trim-and-fill procedures, residual publication bias and non-independence (handled via random effects) may persist. - Environmental covariates were derived from global datasets (WorldClim, SoilGrids, ERA5-Land) and may not fully capture site-level heterogeneity or management nuances (e.g., tillage intensity, irrigation regimes).