Contributions of Different N Sources to Crop N Nutrition in a Chinese Rice Field

N availability is one of the most important factors limiting crop yield enhancement.The recovery of applications of 15 N-labeled fertilizer and crop residues in a rice-wheat cropping system was determined for up to 6 consecutive growing seasons.The crop residues from the previous season were either incorporated or removed as two different treatments.Our results showed that 16.55%-17.79% (17.17% on average) of the fertilizer N was recovered in the crop during the first growing season,suggesting that more than 80% of crop N was not directly from the N fertilizer.When 15 N-labeled residues were applied,12.01% was recovered in the crop in the first growing season.The average recoveries of fertilizer N and crop residue N in the soil after the first growing season were 33.46% and 85.64%,respectively.N from soil organic matter contributed approximately 83% of the N in the crop when 15 N fertilizer was applied or 88% when crop residues were applied.There was a larger difference in the total 15 N recovery in plant and soil between N applications in the forms of fertilizer and crop residues.Incorporation of crop residues following the 15 N fertilizer application did not significantly promote 15 N recovery in the crop or soil.On average,only additional 1.94% of N for the fertilizer-applied field or 5.97% of N for the crop residue-applied field was recovered by the crops during the 2nd and 3rd growing seasons.The total recoveries of 15 N in crop and soil were approximately 64.38% for the fertilizer-applied field after 6 growing seasons and 79.11% for the crop residue-applied field after 5 growing seasons.Although fertilizer N appeared to be more readily available to crops than crop residue N,crop residue N replenished soil N pool,especially N from soil organic matter,much more than fertilizer N.Therefore,crop residue N was a better source for sustaining soil organic matter.Our results suggested that the long-term effect of fertilizer or crop residues on N recovery were different in the crop and soil.However,there was little difference between the practices of crop residue incorporation and residue removal following the N fertilizer application.

Optimization of nitrogen fertilizer rate under integrated rice management in a hilly area of Southwest China

China has the world’s highest nitrogen(N)application rate,and the lowest N use efficiency(NUE).With the crop yield increasing,serious N pollution is also caused.An in-situ field experiment(2011–2015)was conducted to examine the effects of three N levels,0(i.e.,no fertilizer N addition to soil),120,and 180 kg N ha-1,using integrated rice management(IRM).We investigated rice yield,aboveground N uptake,and soil surface N budget in a hilly region of Southwest China.Compared to traditional rice management(TRM),IRM integrated raised beds,plastic mulch,furrow irrigation,and triangular transplanting,which significantly improved rice grain yield,straw biomass,aboveground N uptake,and NUE.Integrated rice management significantly improved 15N recovery efficiency(by 10%)and significantly reduced the ratio of potential15N loss(by 8%–12%).Among all treatments,the 120 kg N ha-1 level under IRM achieved the highest 15N recovery efficiency(32%)and 15N residual efficiency(29%),with the lowest 15N loss ratio(39%).After rice harvest,the residual N fertilizer did not achieve a full replenishment of soil N consumption,as the replenishing effect was insufficient(ranging from-31 to-49 kg N ha-1).Furthermore,soil surface N budget showed a surplus(69–146 kg N ha-1)under all treatments,and the N surplus was lower under IRM than TRM.These results indicate IRM as a reliable and stable method for high rice yield and high NUE,while exerting a minor risk of N loss.In the hilly area of Southwest China,the optimized N fertilizer application rate under IRM was found to be 100–150 kg N ha-1.