The effect of substituting inorganic fertilizer with manure on soil N₂O and CH₄ emissions and crop yields: A global meta-analysis

The replacement of inorganic fertilizers with manures is gaining increased attention as a strategy for developing sustainable agroecological farming systems. Substituting synthetic N fertilizers with manure has been proposed to affect field N₂O emissions and crop yields. However, the global effects of such substitution practices on the trade-offs between crop yield, net greenhouse gas (GHG) emissions, and GHG intensity (GHGI) remain controversial. This research evaluates the impact of substituting inorganic fertilizers with manure on crop yields, GHG emissions, and GHGI under varying conditions. Additionally, it examines the influence of different factors on these outcomes. This study conducted a global meta-analysis (119 articles, 856 observations) to quantify the effects of manure substitution on N₂O and CH₄ emissions and crop yields, considering variations in manure characteristics, climatic conditions, and soil properties. The findings revealed that substituting inorganic fertilizers with manure significantly increased CH₄ emissions, while N₂O emissions decreased, though the reduction was not statistically significant. The effects on GWP and crop yields were minimal. A significant decrease in soil N₂O emissions was observed under the following conditions: mean annual temperature (MAT) > 15°C, rice cropping systems, composted manure application, or manure with a C:N ratio of 8–16. A significant increase in soil CH₄ emissions was observed in clay soils, soils with SOC > 15 g C kg⁻¹ , MAT > 15°C or < 10°C, a mean annual precipitation (MAP) > 800 mm or < 400 mm, vegetable cropping systems, total N application rates of 150–300 kg N ha⁻¹ , a manure N concentration of > 66 %, or with raw manure applications. A significant increase in crop yield was observed in vegetable cropping systems or when manure N percentage was between 33 % and 66 %. The GHGI advantage of substituting manure fertilizer increases as SOC levels rise. These findings contribute to optimizing management practices for reducing GHG emissions and enhancing crop yields, supporting sustainable agriculture. • Manure substitution significantly increased CH₄ emissions. • Composted manure, a C:N ratio of 8–16, MAT > 15°C, or rice cropping systems reduced N₂O emissions with manure substitution. • Vegetable cropping systems or a manure N content of 33 % - 66 % increased yield with manure substitution. • The GHGI advantage of manure substitution increases as SOC levels increase.