不同作物对旱地农田残留硝态氮的利用差异

A comparison of the use of residual soil nitrate by winter wheat and alfalfa in the drylands of China's Loess Plateau

在黄土高原南部娄土上,通过2a田间试验研究了小麦和苜蓿对土壤中不同累积量的残留硝态氮的利用差异。研究包括0—3 m土壤残留硝态氮累积量(设N1、N2、N3、N4、N5和N6共6个水平,残留硝态氮量依次增加)和作物种类(冬小麦和苜蓿)2个因素,分别采用冬小麦-夏休闲-冬小麦和苜蓿连作种植方式。结果表明,不施用氮肥条件下,冬小麦-休闲-冬小麦轮作周期与苜蓿连作2a内,土壤残留硝态氮的消长有明显差异。在第1季小麦生长期间,小麦的氮素携出量(63.9—130.3 kg/hm2)、氮素携出量占播前残留硝态氮量的比例(18%—27%)及氮素携出量占该生长季硝态氮减少量的比例(29%—62%)均显著高于同期的苜蓿处理。在第2个生长季内,苜蓿的氮素携出量是小麦当季氮素携出量的近6倍,但由于苜蓿固氮作用强烈,至第2生长季结束后,0—3 m土壤硝态氮量与苜蓿播前相比平均只减少了72.4 kg/hm2,而麦田0—3 m土壤硝态氮量与小麦播前相比减少了158.3 kg/hm2。在短期内如果通过种植作物消耗土壤剖面的残留硝态氮,冬小麦比苜蓿更有优势。第1季小麦氮素携出量与小麦播前0—2 m(r=0.920**)和0—3 m(r=0.857*)土层残留硝态氮量呈显著或极显著正相关,与0—1 m土层残留硝态氮量没有显著相关性;第1生长季苜蓿氮素携出量与播前0—1 m土壤硝态氮累积量呈显著正相关关系(r=0.846*),而与0—2 m和0—3 m土壤硝态氮累积量的相关性并不显著。小麦比苜蓿能利用更深土层中的硝态氮。随着播前0—3 m土壤残留硝态氮的增加,小麦和苜蓿地上部氮素携出量呈增加的趋势,硝态氮表观损失也显著增加。

The application of excessive amounts of nitrogen fertilizer in dryland habitats continues to result in an increase in the amount of residual nitrate found in dryland soil profiles in the southern Loess Plateau of north central China. Efforts to use this resource have attracted considerable attention. A two-year field experiment was conducted in the southern Loess Plateau to investigate and compare how residual soil nitrate is used in winter wheat and alfalfa fields. Crops of alfalfa and winter wheat were grown in fields with six levels of residual soil nitrate,measured as the accumulated nitrate at soil depths of 0—3 m,with continuous alfalfa and wheat-summer fallow-wheat systems. Both crops exhibited similar yet different changes in the levels of residual soil nitrate during the two year study without the application of additional nitrogen fertilizer.During the first wheat growing season,soil nitrate at a depth of 0—3 m decreased by 42% —66% in the wheat fields while it decreased by 41% —73% in the alfalfa fields,compared with N levels before sowing. N uptake（ 63. 9—130. 3 kg /hm2）,the proportion of N uptake from the total residual soil nitrate prior to crop sowing（ 18%—27%）,and the proportion of N uptake from soil nitrate reduction（ 29% —62%） was much higher for winter wheat than for alfalfa. The apparent reduction of soil nitrate in the upper 0—3 m of soil in the wheat field（ 62. 1—317. 1 kg /hm2） was lower than that in the alfalfa field（ 95. 0—614. 1 kg /hm2）. During the second wheat growing season,N uptake of alfalfa was 593.8 kg/hm2,six times higher than the N uptake of wheat. However,residual soil nitrate in the alfalfa treatments increased as a result of strong N fixation by alfalfa. Compared with the N levels prior to crop sowing,the residual soil nitrate in the alfalfa field declined by 72. 4 kg /hm2 while it declined by 158. 3 kg /hm2 in the wheat field after two growing seasons. These results indicate that winter wheat used more residual soil nitrate than alfalfa during the two year study. If the goal is to reduce residual soil nitrate levels,winter wheat has the advantage of reducing nitrate levels faster than does alfalfa in a short period of time.During the first wheat growth season,N uptake by wheat had a significant positive correlation to the levels of residual soil nitrate at a depth of 0—2 m（ r = 0. 920＊＊） and 0—3 m（ r=0.857＊） while it had no significant correlation to residual soil nitrate at depths of 0—1 m. On the contrary,N uptake by alfalfa had a significant positive correlation to residual soil nitrate at a depth of 0—1 m（ r = 0. 846＊） prior to alfalfa sowing while it was not significantly correlated to residual soil nitrate at depths of 0—2 m and 0—3 m. These results indicate that residual nitrate in deeper soil contributed more to winter wheat N uptake when compared to N uptake by alfalfa.The uptake of N by both wheat and alfalfa was significantly positively correlated to the levels of residual nitrate in the soil at depths of 0—3 m prior to the first season of wheat sowing. However,with higher initial soil nitrate levels,the apparent loss of N also increased. These results suggest that with increased levels of residual nitrate in the soil,crops were able to increase their uptake of soil N,resulting in the removal of higher amounts of N by the crops while higher amounts of N are lost from soil profile.