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 = 15Nlabeled 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.
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