Global environmental changes, such as increased atmospheric CO2 and nitrogen enrichment, threaten ecosystem services like carbon sequestration and biodiversity conservation. Nature-based solutions, such as organic fertilization, offer potential mitigation strategies. Inorganic fertilizers boost plant productivity but often lead to plant diversity loss. This study investigates whether organic fertilization offers an alternative, increasing biomass without sacrificing diversity. The research question centers on the comparative effects of organic and inorganic fertilization on aboveground biomass, plant diversity, and soil organic carbon (SOC) across diverse grassland and cropland ecosystems. Understanding these differences is crucial for developing sustainable land management practices that balance productivity and environmental protection. This is especially important in grasslands, which cover a significant portion of the Earth's land surface and play a vital role in carbon storage and biodiversity. The study's significance lies in its potential to inform the design and implementation of nature-based solutions aimed at improving multiple sustainability goals.

Previous research has extensively documented the impacts of inorganic fertilization on grasslands and croplands. While inorganic fertilizers increase aboveground biomass, they often reduce plant diversity. Several hypotheses attempt to explain this trade-off: the biomass-driven competition hypothesis, the niche dimension hypothesis, and the nitrogen detriment hypothesis. The biomass-driven competition hypothesis suggests that increased biomass from fertilization leads to greater competition for light, disadvantaging smaller plant species. The niche dimension hypothesis posits that fertilization reduces the number of belowground niches, limiting species coexistence. The nitrogen detriment hypothesis proposes that long-term nitrogen fertilization increases plant susceptibility to stress factors like soil acidification and ammonium toxicity. The relative importance of these mechanisms varies with environmental conditions. Few studies have directly compared the effects of organic versus inorganic fertilization on plant diversity, with inconsistent results. Similarly, the impact of both fertilizer types on soil organic carbon (SOC) remains uncertain, potentially varying based on climate, land use, and nutrient availability. This lack of consistent understanding motivates a comprehensive global meta-analysis to evaluate these differences.

This meta-analysis compiled data from 537 peer-reviewed publications on fertilization experiments in grasslands and croplands worldwide. The data collection process involved searching Web of Science and China National Knowledge Network databases using keywords related to fertilization, plant diversity, biomass, and SOC. Three selection criteria were applied: 1) studies must involve field experiments with ambient and nutrient addition treatments in semi-natural or natural grasslands or croplands; 2) studies must report means, standard errors, or standard deviations and sample sizes; 3) grassland studies reporting exotic plant species introduced by organic fertilization were excluded. Data were extracted from tables and figures using GetData Graph Digitizer software. The final dataset included over 5000 pairs of measurements comparing ambient conditions to fertilization treatments. The dataset included various types of inorganic fertilizers (urea, ammonium nitrate, etc.) and organic fertilizers (industrial organic fertilizers, livestock manure, compost). The meta-analysis employed the natural log-transformed response ratio (ln RR) to quantify the effect sizes of fertilization on aboveground biomass, plant diversity (species richness and Pielou's evenness), and SOC. A hierarchical model with inverse variance weighting was used to summarize response ratios, accounting for non-independence of observations from the same site. Linear mixed effects models investigated the relationships between fertilization responses and environmental gradients (mean annual temperature, soil properties, etc.), with study site as a random effect. A multi-model inference procedure was used to select the best-fitting models. Structural equation modeling (SEM) was used to disentangle direct and indirect effects of environmental factors and fertilization rates on biomass and species richness responses. Standardized Major Axis tests compared SOC responses between grasslands and croplands across environmental gradients.

The meta-analysis revealed significant effects of both organic and inorganic fertilization on aboveground biomass. Inorganic fertilization increased biomass by 42% and decreased species richness by 18% and evenness by 6%. Organic fertilization increased biomass by 56% without causing a significant decrease in plant diversity. Compared to inorganic fertilization, organic fertilization significantly increased species richness and evenness while maintaining higher biomass. Organic fertilization increased SOC by 19% relative to ambient conditions and 15% relative to inorganic fertilization. The response of aboveground biomass to organic fertilization in grasslands was positively influenced by mean annual temperature, soil total nitrogen, and fertilizer rate. The positive effect of organic fertilization on species richness in grasslands was associated with higher soil bulk density, water content, and cation exchange capacity. Conversely, the response of aboveground biomass to inorganic fertilization was positively related to nitrogen fertilizer rate, number of nutrients added, and soil water content, while higher inorganic fertilizer rates led to greater decreases in species richness. A partial regression analysis showed a weak negative link between biomass and species richness responses under organic fertilization in grasslands, while the link was significantly negative under inorganic fertilization. The effect of organic fertilization on SOC increased with mean annual temperature in grasslands but not in croplands. In grasslands, this positive effect was stronger in warmer regions, while in croplands, it was stronger in cooler regions. The effect of organic fertilization on SOC increased with fertilizer rate in both grasslands and croplands but decreased with increasing initial SOC, more so in croplands than in grasslands. The effect of organic fertilization on SOC was found to be equal in grasslands and croplands under comparable fertilization rates, after accounting for soil properties.

The findings challenge the assumption that increased biomass inevitably leads to biodiversity loss. Organic fertilization, unlike inorganic fertilization, demonstrates a potential pathway for increasing both biomass and biodiversity in grasslands, particularly in areas with sufficient soil moisture. The results provide limited support for the niche dimension and nitrogen detriment hypotheses. The lack of a negative relationship between biomass and species richness under organic fertilization suggests that mechanisms other than light competition are at play. The positive effect of organic fertilization on SOC, especially in warmer grasslands, highlights its potential as a climate change mitigation strategy. This difference in response between grasslands and croplands might be attributed to tillage practices, which are more common in croplands and can accelerate SOC decomposition. The study suggests that improved soil conditions, such as higher water holding capacity and nutrient availability, due to organic fertilization may contribute to biodiversity gains. The use of locally-sourced, non-contaminated manure is crucial to avoid introducing exotic plant species.

This global meta-analysis shows that organic fertilization represents a viable nature-based solution for enhancing grassland ecosystem services. It increases both aboveground biomass and SOC without negatively impacting plant diversity, unlike inorganic fertilization. The positive effect of organic fertilization on biodiversity is particularly pronounced in wetter grasslands. The potential for increased soil carbon sequestration with organic fertilization may be amplified under future warmer climates, particularly in grasslands. Future research should investigate the long-term effects of organic fertilization, its efficacy in different grassland types, and optimal application strategies to maximize both productivity and biodiversity benefits.

The meta-analysis relies on existing published data, which may be subject to publication bias and variability in experimental design. The selection criteria for including studies might have inadvertently excluded some relevant data. The effects of organic fertilization on exotic plant species need further investigation. The study focuses on the peak growing season and does not assess year-round effects. The long-term consequences of organic fertilization on SOC require further investigation.