Optimizing nitrogen fertilization rate to enhance soil carbon storage and decrease nitrogen pollution in paddy ecosystems with simultaneous straw incorporation

Nitrogen fertilization (NF) is an important agricultural management practice that greatly affects soil carbon (C) storage through regulating the balance of soil C input (straw biomass) and output (ecosystem respiration), especially for agroecosystems with simultaneous straw incorporation (SI). However, little is known about how different NF rates affect soil C balance (net ecosystem carbon budget, NECB) in flooded paddy ecosystems with SI. We conducted a two-year field experiment in a typical rice-wheat cropping system in southern China to investigate the response of soil C input (net primary production (NPP) and SI), output (ecosystem respiration and methane emissions), and NECB and N losses under five different NF rates (0/0, 120/90, 180/135, 240/180 and 300/225 kg N ha(-1) for the rice/wheat season). Straw harvested from previous cropping season was chopped and fully incorporated into the soil prior to the next season. Our results showed that all treatments had a positive NECB value ranged between 4057-14380 kg C ha(-1) yr(-1). Compared with the no NF, NF treatments clearly increased the annual NECB by 17.2 %-185 % depending on NF rate. A 20 % reduction relative to the local NF rate (525 kg N ha(-1)) increased annual crop yield by 4.1 %-6.7 % and NECB by 9.6 %-16.8 %, which was attributed to the increase in NPP (2.6 %-6.2 %) and straw C input (3.8 %-6.1 %), and the decrease in ecosystem respiration (1.8 %-2.1 %) and methane emissions (8.4 %-10.4 %). Moreover, total reactive N losses were significantly decreased by 21 %. However, further reducing the rate by 40 % from local NF rate significantly decreased crop yield by 12.7 %-14.9 % and NECB by 16.7 %-19 %, mainly due to the significant decrease in soil C input through NPP by 14 %-18 % and straw C input by 7.4 %-15.8 % as the inadequate N supply impaired crop growth. Our results suggest that optimizing NF rate is effective in further enhancing soil C storage, decreasing N pollution and sustaining food security in paddy ecosystems with simultaneous SI.