Impacts of dairy manure and synthetic fertilizers on greenhouse gas emissions and crop yields: A global meta-analytical comparison.

Dairy manure (DM) is commonly used as an organic nutrient source to improve soil fertility. However, concerns remain about its potential to increase greenhouse gas emissions and reduce crop yields compared to synthetic fertilizers (SF). This global meta-analysis evaluated and compared the effects of DM and SF applications on greenhouse gas emissions and crop yields, considering different soil properties (texture, pH), nitrogen application rates, crop types, water managements, and climate zones. Additionally, the influence of different DM types and placement methods were examined. Results showed no significant differences in methane emissions and grain yield between DM and SF across most conditions. However, fine-textured soils exhibited higher soil carbon dioxide emissions with DM application. Elevated soil carbon dioxide emissions were also observed with DM in alkaline soils (pH 7.4-7.6), while slightly acidic soils (pH 5-6.5) favored higher crop yields. Lower DM application rates (≤200 kg N ha-1) reduced crop biomass and increased soil carbon dioxide emissions, though grain yields remained comparable to SF. Crop-specific effects included higher nitrous oxide emissions from barley (Hordeum vulgare L.) and lower corn (Zea mays L.) yields under DM than SF. Higher soil carbon dioxide emissions were associated with DM in cooler climates and under irrigation. Among DM types, liquid DM increased nitrous oxide emissions, whereas composted DM enhanced crop yields. Sub-surface DM application mitigated nitrous oxide emissions while maintaining yields. Findings suggest that DM can be effectively utilized without increasing greenhouse gas emissions or compromising crop yields compared to SF, provided that site-specific factors and trade-offs are carefully managed.