Fate of Nitrogen from Organic and Inorganic Sources in Rice-Wheat Rotation Cropping System

The lower availability of N is one of the most important limiting factors impeding crop yield enhancement among the various factors that affect crop yield under the multiple-cropping agroecosystem in China. In this study, the recovery of a single application of 15N-labeled fertilizer or residues in rice-wheat cropping system was determined, in order to provide theoretical foundation for the nitrogen management in sustainable agricultural production. A continuous trace experiment was conducted for 15N microplots by using randomized block design with four treatments and four replications (T1=15N-labeled fertilizer with crop residue incorporation, T2 = 15N-labeled residues, T3=14N fertilizer to generate unlabeled crop residue, and T4 =15N-labeled fertilizer without crop residue incorporation). Our results showed that, on average, 17.17 and 12.01% of crop N was derived from N fertilizer and 15N-labeled residues, respectively during the first growing season, suggesting that approximately 82.83 or 87.99% of crop N was derived directly from soil N pool. There was a larger difference in the 15N recovery pattern in crop when N was applied as fertilizer or residues, i.e., most of crop N derived from N fertilizer was absorbed in the first growing season (92.04%), and the relevant value was 38.03% when 15N-labeled residues were applied. This implied that most of N fertilizer was recovered in the present cropping season, while a longer residue effect will be found for 15N-labeled residues. Thus, the average recovery of N fertilizer and N residue in the soil after the first growing season was 33.46 and 85.64%, respectively. The recovery of applied N in soil when N was applied as residues was significantly higher than that when N was applied as fertilizer. There was a larger difference in the total 15N recovery in plant and soil when N was applied as fertilizer or residues. By the end of the fifth or sixth cropping season, the total 15N recovery in plant and soil when N was applied as fertilizer or residues were estimated at 64.38 and 79.11%, respectively. On the contrary, there was little difference between the practices of residue incorporation and residue removal following the N fertilizer application. N fertilizer appeared to be more readily available to crops than residue-N, and residue-N replenished soil N pool, especially N in soil organic matter, much more than N fertilizer after six growing seasons. Therefore, residue-N is a better source for sustaining N content of soil organic matter. Thus, one possible management practice is to use both organic and inorganic N sources simultaneously to improve the use efficiency of N while protecting the sustainability of soil.