Fate Of N Fertilizer And N Balance In Different Cropping Systems In Purple Soil Areas Of The Upper Reaches Of Yangtze River

Under the current production levels, China needs to produce at least 20% more grains in order to meet the future peak population requirement. Meanwhile, China has been the largest N fertilizer producer and consumer in the world. In China, less than half of N fertilizer was utilized by crops, the others were lost through different N loss pathway into the environment, causing many negative impacts, and a tremendous contradiction on food security, and environment security under improper use of N fertilizers. The Sichuan Basin (major part of Sichuan and Chongqing Provinces) is not only the major food crops producing area in the upper reaches of Yangtze River, but also the most important ecological barrier in western China with sensitive ecosystems, agricultural inputs and eco-environment degradation.This study was conducted in Jiangjin and Yanting Experimental Stations in the upper reaches of the Yangtze River in the Sichuan Basin, focused on the N fertilizer use, soil N residue and N loss in different rotation systems (e.g., Maize-Wheat (MW), Rice-Wheat (RW), Rice-Flooded fallow (RF)) under similar climatic and soil conditions.15N enriching isotope tracer technology was used for quantifying the total N use efficiency, soil residue and loss rate of N fertilizer in different rotation systems. Vented PVC chamber and static close-chamber methods were used to measure ammonia volatilization and N2O emission in different rotation systems, respectively. Meanwhile, in situ micro-lysimeter method was used to collect N leaching samples in different rotation systems. Also, crop yield, N utilization and losses were evaluated and compared among RF, RW and MW systems.The results are shown as follows:1) Fertilizer N was the key factor to ensure stable crop yields in different rotation systems in the upper reaches of Yangtze River. No significant yield differences were found between Neon (conventional N treatment) and Nopt (optimized N treatment) under MW, RW and RF systems, indicating that fertilizer N rate can be optimized or reduced without yield decrease under the same management practices.2) Total grain yield and aboveground biomass in RW were the highest among the three rotation systems. Crop yields were 30.3% and 23.4% higher in Neon and Nopt in RW than those in MW, and aboveground biomass yields were 18.9% and 9.4% higher in corresponding treatments than those in MW.Total N uptake by crop aboveground was in the order of MW> RW> RF, and the absorbed 15N labelled fertilizer was ranked by RW> MW> RF in either Neon or Nopt, indicating that under optimized fertilization conditions, crops would use more N from soil and surrounding environment, and in the traditional fertilization conditions, crops was even more dependent on fertilizer N.3) In MW and RW, residue and loss of 15N in Neon were significantly higher than those in Nopt. Large amount of fertilizer N applied significantly increased the amount of fertilizer N loss and its environmental risk. Under optimized N treatment, aboveground N use efficiency (NUE) showed the order of RW (42.3%)> MW (37.2%)> RF (36.2%), the 15N residue in 0-50 cm soil and underground portion showed the sequence of RF (38.1%)> MW (32.0%)> RW (28.0%), and 15N loss showed MW (30.8%)> RW (29.8%)>RF (25.7%). Under conventional N treatment, aboveground NUE showed MW (37.2%)> RW (32.8%)> RF (25.7%), the 15N residue in 0-50 cm soil and underground portion showed MW (30.4%)> RF (29.1%)> RW (22.8%), and 15N loss was RF (45.2%)> RW (39.2%)> MW (35.2%). Fertilizer NUE was significantly higher in RW than MW, and the absolute amount of N loss was lower in RF than RW and MW rotation systems because of the single cropping mode for RF. Soil 15N residue in winter wheat season could be used only by 2.1% and 2.7% of N previously applied in the summer maize and rice season, and there were still 17% and 20% of these residual N remaining in soil, about 11.0 and 15.6 percents of these residual N would be lost during the summer maize and rice season (the next season after N application).4) Ammonia volatilizations in Jiangjin under all three rotation systems were very low, only accounted for 1-2% of total fertilizer N applied. N2O emissions were higher in MW than in other two rotation systems, accounting for 3.2%(Neon) and 3.1%(Nopt) of the total annual N applied, respectively, with more than 60% of the total N2O emissions occurred in the summer maize season. Under RW system, more than 60% of the total N2O emissions concentrated in the winter wheat season due to the flooding condition in most of the rice season. N leaching in MW was higher than that in RW, the leaching losses in Neon and Nopt were accounted for 6.2% and 6.7% of the annual N applied in MW. Other N losses in the three rotation systems may include N losses through surface N runoff and/or N2 via denitrification process. Under conventional N treatment, MW, RW and RF systems will have large N surplus, but under optimal N treatment in our study, these three systems will not get large N surplus except except RF.5) Total N deposition from 2009 to 2013 averaged 38.7 kg N ha-1 yr-1. Wet N deposition, sedimentary (diameter>10 μm) and non-sedimentary (diameter<10μm) dry N deposition accounted for 60.7%,15.7% and 23.6% of annual N deposition, respectively. Reduced N deposition was higher than oxided N deposition, being the main component of the total N deposition. Maximum loadings of all N forms of wet deposition, gaseous NH3, NO2 and particulate NH4+ in dry deposition occurred in summer and minimum loadings in winter. Whether averaged monthly, seasonally and/or annually, dissolved N accounted for more than 70% of the total N deposition. Ammonium, nitrate and organic N accounted for 48.7%,30.3% and 21.0% of TN (total N), respectively. N deposition in the central Sichuan basin increased during the sampling period, especially that of reduced N compounds, and has become a serious threat to local aquatic ecosystems, the surrounding forest and other natural or semi-natural ecosystems in the upper reaches of the Yangtze River. N use and scientific management of N should be paid more attention in order to achieve the target of food security and environmental safety.