Stimulation of N₂O emission by manure application to agricultural soils may largely offset carbon benefits: a global meta‐analysis

Animal manure application as organic fertilizer does not only sustain agricultural productivity and increase soil organic carbon (SOC) stocks, but also affects soil nitrogen cycling and nitrous oxide (N₂ O) emissions. However, given that the sign and magnitude of manure effects on soil N₂ O emissions is uncertain, the net climatic impact of manure application in arable land is unknown. Here, we performed a global meta-analysis using field experimental data published in peer-reviewed journals prior to December 2015. In this meta-analysis, we quantified the responses of N₂ O emissions to manure application relative to synthetic N fertilizer application from individual studies and analyzed manure characteristics, experimental duration, climate, and soil properties as explanatory factors. Manure application significantly increased N₂ O emissions by an average 32.7% (95% confidence interval: 5.1-58.2%) compared to application of synthetic N fertilizer alone. The significant stimulation of N₂ O emissions occurred following cattle and poultry manure applications, subsurface manure application, and raw manure application. Furthermore, the significant stimulatory effects on N₂ O emissions were also observed for warm temperate climate, acid soils (pH < 6.5), and soil texture classes of sandy loam and clay loam. Average direct N₂ O emission factors (EFs) of 1.87% and 0.24% were estimated for upland soils and rice paddy soils receiving manure application, respectively. Although manure application increased SOC stocks, our study suggested that the benefit of increasing SOC stocks as GHG sinks could be largely offset by stimulation of soil N₂ O emissions and aggravated by CH₄ emissions if, particularly for rice paddy soils, the stimulation of CH₄ emissions by manure application was taken into account.