Excessive fertilizer application accelerates the greenhouse gas emissions intensity of major croplands in China

Stabilizing greenhouse gas (GHG) emissions from croplands as agricultural demand grows is critical for global climate mitigation and sustainable development. As potential foci for balancing crop production and GHG emissions, large uncertainty yet exists in the spatial and crop-wise patterns of GHG emissions intensity and its attribution. Here we investigated the GHG emissions intensities for wheat, maize, and rice in China during 1949-2012 and the underlying mechanisms, through a combined use of meta-analysis, which covers a vast lot of site experiments, and the process-based Dynamic Land Ecosystem Model (DLEM-AG2.0). The results showed that China achieved significant increase in crop yield, but at the expense of high GHG emissions. GHG emissions intensities of wheat, maize, and rice during 1949-2012, on average, were 0.095 , 0.047, and 1.308 kg CO₂-eq kg^-1, with a significant increase rate of 1.66, 0.46, and 3.58 10^-3 kg CO₂-eq kg^-1, respectively. Nitrogen fertilizer was the dominant contribution factor to GHG emissions intensity in the northern China, and had raised its weight in the southern China in the 2000s, where nitrogen deposition was the most important controlling factor. N fertilizer-induced GHG emissions intensity increased remarkably during 2000-2012 across all crops. The increasing rates were 37.9, 19.9, and 28.5 10^-3 kg CO₂eq kg^-1 yr^-1, respectively, being 5.0, 1.7, and 2.7 times of those during 1949-1999. Further analysis indicated that the current fertilizer inputs for wheat, maize and rice were 36.5%, 78.6%, and 48.1% higher than the optimal nitrogen fertilizer use (when the yield reaches the maximum), leading to increases in GHG emissions by 70.2%, 72.3%, and 88.0%. Over-fertilization were found mainly in the winter wheat-summer maize rotation area in the North China Plain and the semiarid regions, and the winter wheat-rice rotation area in the middle and lower reaches of the Yangtze River and southwest China. The excessive nitrogen fertilizer input constitutes a growing threat to China's agroecosystem sustainability. Optimized fertilization, especially in the high-nitrogen fertilizer and water-deficient areas in northern China, and improved managements are preferable in the efforts of increasing crop yield and GHG mitigation.