Full Manure Recycling Risks an 18% Rise in China's Cropland N 2 O Emissions Without Improved Management

Cropland fertilization is the largest anthropogenic source of nitrous oxide (N2O-N) emissions in China and could be mitigated through manure substitution. However, accurate quantification of N2O-N losses under different combinations of manure and chemical fertilizer applications remains challenging due to limited national data, oversimplified methods of estimation, and neglect of the nitrogen (N) source (chemical-N and manure-N) interaction effects under mixed fertilization on N2O-N production processes. Using a dataset consisting of 2186 observations across China, we developed N2O-N models for fertilized (chemical fertilizer and manures) croplands incorporating climate, soil, fertilization and cropping variables, with particular emphasis on the manure component, through machine-learning approaches. Model simulations estimated a median N2O-N flux of 1.1 kg ha-1 and a total national N2O-N loss of 243.7 Gg in 2019, with a hotspot in Northcentral China. During 2000-2019, N2O-N losses were driven mainly by annual total N input. By 2050, merely pursuing 100% manure recycling without considering improved manure-N use efficiency could lead to an 18% increase in N2O-N losses compared with 2019. Hence, this study provides a high-resolution modeling framework to predict how developments in manure substitution will affect the magnitude and spatial distribution of N2O-N losses. We also demonstrate that future manure substitution policies will need to promote improved manure nutrient management at the same time as greater manure recycling rates.