Residual Nitrogen Fertilizer In Dryland Of The Loess Plateau And Its Fate During Summer Fallow

The application of synthetic nitrogen（N）fertilizer in agriculture has increased rapidly since the 1980s in China.The application of N fertilizer is an effective way to increase crop yields;however,its application also causes dramatic environmental problems.The fates of N fertilizer include being absorbed by crops,remaining in the soil profile,leaching and gaseous loss.The Loess Plateau is an important dryland region in China.Excessive use of N fertilizer is common in this region,and results in high residual N in soil.High residual N in soil is helpful for improving soil fertility,while also increasing N loss risks and causing environmental problems.A summer fallow period from late June to late September is commonly adopted after harvesting wheat to conserve rainwater,and to promote the mineralization of soil organic matter therefore increasing available nutrients in the soil in the Loess Plateau.However,the bare soil and intensive rainfall during summer fallow may increase the loss risk of the residual N（especially nitrate）in soil profiles.The soils on the Loess Plateau have very deep profiles ranged 10 m to 200 m in depth,which favor the leached N accumulation.Therefore,understanding the fates of fertilizer N,especially the leaching of residual N fertilizer in summer fallow and the fate of residual N fertilizer in subsequent crops is the key to improve N use efficiency,and reduce its loss in the Loess Plateau.This study was carried out on two typical dryland agricultural regions of the Loess Plateau:Yangling and Changwu.The fate of N fertilizer was studied by three years studies and ¹⁵N-labeled experiments（including micro-plot and large lysimeter）.Our focus was the leaching of residual N fertilizer in summer fallow and the fate of residual N fertilizer in subsequent crops.The main results were as follows:（1）The ¹⁵N-labeled lysimeter study showed that,after the first wheat（Triticum aestivum L.）season,54%of the ¹⁵N fertilizer was absorbed by wheat,and 38%remained in 0-100 cm soil profile and mainly in 0-40 cm layers.About 40%of the residual ¹⁵N fertilizer was in nitrate form.The NO₃^--N accumulation in 0-200 cm soil profile after wheat harvest on dryland of the Loess Plateau was high.It was averaged 193 kg ha^-1 in 0-200 cm soil profile in Changwu.In Yangling,the NO₃^--N accumulation in 0-200 cm soil profile was significantly increased from 156 kg ha^-1 to 366 kg ha^-1 with the N rate increased from 120 kg ha^-1 to 240 kg ha^-1.It indicated the residual N fertilizer in soil after wheat harvest was high,and was mainly in nitrate form.（2）The soil NO₃^--N content in 0-200 cm soil profiles was determined before and after summer fallow form 2013 to 2015 in Changwu and Yangling.The results showed that the soil NO₃^--N accumulation peak leached 40 cm depth after summer fallow,even 60-80 cm depth when the rainfall was high.The ¹⁵N micro-plot study further proved that the soil residual N fertilizer occurred leaching even if the summer fallow rainfall was low,and the leaching loss was significantly increased with rainfall rates.When the rainfall was lower than the average rainfall,the soil residual NO₃^--N accumulation peak leached 5-25 cm depth,and its loss was less than 25%after summer fallow.When the rainfall was normal,the soil residual NO₃^--N accumulation peak leached 25-45 cm depth,and the loss was 25%-38%.It indicated that the leaching risk of residual N fertilizer was high in summer fallow on dryand of the Loess Plateau.In addition,the leaching of soil residual N fertilizer was less with the increase of soil clay content under similar summer fallow rainfalls.（3）The ¹⁵N-labeled lysimeter（3 m×2.2 m×3 m）study was used to follow the fate of residual N fertilizer in soil of three summer fallows.It was found that the residual N fertilizer leached 20-40 cm depth in each summer fallow,and gradually leached from 0-100 cm soil profile to 100-200 cm soil profile in three summer fallows,and more than 80%of the residual N fertilizer accumulated in 100-200 cm soil profile was in nitrate form.But the total residual N fertilizer content in the 0-200 cm soil profile changed little.It indicated the residual N fertilizer prone to leached in nitrate form and accumulated in the deep soil layers,but the loss was low during summer fallow on dryland of the Loess Plateau.It also indicated the gaseous loss caused by nitrification and denitrification was low.（4）The ¹⁵N-labeled lysimeter（3 m×2.2 m×3 m）study was also used to follow the fate of soil residual N fertilizer in next crops.It was found that the residual ¹⁵N fertilizer uptake by wheat was 6.5%and 5.9%of the applied ¹⁵N fertilizer in the second and third wheat season,respectively.The ¹⁵N-labeled study revealed that the recovery of residual N fertilizer by wheat was significantly affected by the summer fallow rainfalls,higher the summer rainfall,less the residual N was absorbed by next wheat.Therefore,the summer fallow rainfall should be considered in evaluating the fate of residual N fertilizer in the next crop season on dryland of the Loess Plateau.（5）After three years,about 66%of the N fertilizer was taken up by wheat averagely;31%of the N fertilizer remained in 0-200 cm soil profile;and less than 3%of the N fertilizer was lost.It indicated high N fertilizer recovery and low N loss of the applied N fertilizer in winter wheat-summer fallow system on dryland of the Loess Plateau.In summary,a high proportion of the applied N fertilizer remained in soil after wheat harvest;and the accumulation of residual N fertilizer（especially nitrate）in 0-200 cm soil profile was high with long-term N fertilizer application on dryland of the Loess Plateau.Summer fallow increased the leaching risk of soil residual N fertilizer,and the leaching risk was significantly increased with the rainfall rates.The soil residual N fertilizer was leached mainly in nitrate form and accumulated in deep soil layer during summer fallow.The uptake of residual N fertilizer by subsequent crops was high.The total recovery of N fertilizer was high and the loss was low in winter wheat-summer fallow system during three years on dryland of the Loess Plateau.