黄土高原旱地夏季休闲期~(15)N标记硝态氮的去向

夏季休闲是黄土高原旱地小麦常见的蓄纳雨水、恢复地力的措施。随着氮肥用量的增加,一季小麦收获后,旱地土壤剖面累积的硝态氮量不断增加,休闲期间降雨量高,残留硝态氮的去向是值得研究的问题。利用~(15)N标记法研究小麦收获后残留肥料氮在黄土高原旱地(陕西长武)夏季休闲期间的去向,即小麦收获后在微区土壤表层(0~15 cm)施入~(15)N标记的硝态氮肥(30 kg hm~(-2)(以纯氮计),约相当于当地小麦一季作物收获后土壤残留肥料氮量),休闲结束后,采集0~200 cm土壤样品,测定了土壤全氮、硝态氮含量及其~(15)N丰度。结果表明,小麦收获(即休闲开始)时0~200 cm土壤剖面硝态氮累积量在205~268 kg hm-2之间(平均244 kg hm~(-2)),累积量较高。夏季休闲期间降水量为157 mm,属欠水年,但休闲结束后,~(15)N标记肥料氮向下迁移已达80 cm土层,下移深度在45~65cm之间,说明,旱地休闲期间硝态氮的淋溶作用不可忽视。夏季休闲结束后,加入的~(15)N标记肥料氮平均损失率为28%,损失机理值得进一步研究。

Fate of 15N Labeled Nitrate in Dryland under Summer Fallow on the Loess Plateau

[Objective]Summer fallow after winter wheat is harvested in dryland on the Loess Plateau is a very common practice to save rain water,and restore soil fertility for the next crop. Nowadays,the content of nitrogen(N)(mainly in nitrate form)in the soil of the dryland as residue of the fertilizer applied to winter wheat is increasing steadily. As summer is the rainy season in this region,whether or not the high rainfall would increase NO3--N leaching loss is an important issue deserving further study. The method of adding 15N labeled nitrate after winter wheat is harvested in dryland is one to quantitativelyexplore fate of the residual fertilizer N in the dryland under summer fallow in the Loess Plateau,and hence to provide a scientific basis for managing Nin dryland.[Method]The 15N labeled fertilization micro-plot(Polyvinylchloridecolumn,25 cm diameter and 25 cm long)method was used to study fate of the residual fertilizer N in the dryland under summer fallow in a field experiment,which hadfive tracts of cropland and two N treatments set up with and without N application(0 and 30 kg N hm-2). 15N labeled nitrate was mixed with the surface soil(0~15 cm)in the micro-plot at a rate equal to the content of the residual fertilizer N in the field after winter wheat was harvested. At the end of the summer fallow,soil samples throughout the soil profile(0~200 cm)were taken fromthe micro-plots,for analysis the contents of soil total nitrogen,NO3--N,and their 15N abundance.[Result]Results show that at the beginning of the summer fallow,the content of NO3--N accumulated in the 0~200 cm soil was quite high,ranging between 205 and 268 kg hm-2 and averaging 244 kg hm-2;At the end of the summer fallow,apparent NO3--N accumulation peaks were found in the soil,40~80 cm in depth,and 15N abundance ranged between 0.408% and 0.482% and averaged 0.429%,being the highest in the soil 60 cm in depth,averaged 0.372% in the soil 80 cm in depth,being higher than the background value of 15N abundance in the soil(0.36715N atom%),and was 0.369 % in the soil 100 cm in depth,more or less the same as the background value,in Treatments N30 in all the five tracts of cropland. In the five tracts of cropland,15N labeled nitrate abundance was the highest in the 40~60 cm soil layer in the soil profiles (0~100 cm),averaging 2.151%;and the lowest abundance was in the 80~100 cm soil layer,averaging 0.407%,both significantly higher than the background value. Although the rainfall during the summer fallow season was only 157 mm,making it a dry year,it still leached 15N labeled nitrate down to 80 cm in depth, through a 45~65 cm thick soil layer,and even 15N labeled nitrate was found in 100 cm depth. At the end of the fallow,the content of residual N averaged 21.6 kg hm-2 or ranged between 20.8 and 22.9 kg hm-2 in the 0~100 cm soil profile,accounting for 71.9% of the applied N fertilizer in all the five tracts of cropland. The content of residual labeled N was 4.5 kg hm-2,2.9% of which was in the form of NO3-- N in the 0~20 cm soil layer;Residual fertilizer N was mainly distributed in the 40~60 cm soil layer,and averaged 17.7 kg hm-2 in content,accounting for 52.8% of the total N fertilizer applied,in which 15N labeled nitrate content was 6.8 kg hm-2. After the summer fallow,the applied 15N labeled N fertilizer was found lost at a rate of 23.8%~30.8%(averaging 28.1%).[Conclusion]Residual fertilizer N accumulated significantly in the 0~200cm soil layer of the dryland after wheat was harvested,posing a potential risk of leaching. In 2014, though it was a dry year,the effect of rain water leaching NO3--N was still quite significant,indicating that the NO3--N leaching in the dryland during the summer fallow season should not be ignored and further studies should be done on mechanisms of its leaching loss.