冬小麦秸秆覆盖下根区温度变化对根系吸收能力影响机制

In the North China Plain (NCP), the double cropping of winter-wheat and maize consumed 800-850 mm water annually, in which 25-30% was from soil evaporation. Reducing soil water evaporation would be an effective water-saving measure in this serious water shortage region. Straw mulch is one of the effective methods to reduce soil evaporation. The double cropping of winter wheat and maize in the NCP produced abundant straw that provide the materials for straw mulch. With the mechanization in harvesting winter wheat, the straw of winter wheat has been successfully used to mulch maize that significantly reduced crop water use. However, straw mulch to winter wheat had negative effects on crop yield that prevented its application in practice. Winter wheat grows in cool season, the existence of straw on the soil surface reduced soil temperature during the day and increased soil temperature during the night to the top 20 cm soil layer, where most of the root system was located. Root activity is very sensitive to soil temperature. Would this variation in soil temperature under straw mulch be responsible for the yield reduction of winter wheat? The purposes of this research are to examine the root growth, activities of water and nutrients uptake by root system, leaf photosynthetic ability, above-ground growth and dry matter accumulation and allocation to winter wheat under straw mulch and without mulch, to qualify the effects of variation in soil temperature under straw mulch on yield of winter wheat under mulch. The results of this application might provide technical solutions to reduce the negative effects of straw mulch on winter wheat to facilitate the application of this practice to conserve irrigation water in the NCP.

华北是我国重要的小麦玉米生产区，冬小麦和夏玉米一年两作年耗水量800-850mm，其中25-30%农田耗水是土壤棵间蒸发失水，减少这部分水分损失对农田节水具有重要意义。秸秆覆盖是减少农田棵间蒸发的有效措施之一，全程利用冬小麦夏玉米秸秆实现对两季作物的覆盖对华北农田节水有重要意义。但夏玉米秸秆覆盖冬小麦后对冬小麦的生长发育产生了一些不利影响，造成了冬小麦不增产或减产，限制了该项技术的推广。大部分研究推测秸秆覆盖引起的土壤温度变化是减产的主要因素之一，但减产的机制仍不清楚。本项研究以对作物生长发育和代谢活动较敏感的耕层土壤温度的变化对小麦根系生长、土壤水氮供应能力、根系吸收活性和地上部分生理生态的影响研究，明确秸秆覆盖引起的土壤温度变化对冬小麦生长和产量的影响机制，对消除秸秆覆盖负效应，在生产上大面积推广冬小麦秸秆覆盖技术，在实现节水同时保证作物高产高效有重要意义。

针对秸秆覆盖麦田后对冬小麦的生长发育产生不利影响，以及造成冬小麦不增产或减产问题，开展了秸秆覆盖耕层土壤温度的变化对小麦根系生长、土壤水氮供应能力、內源激素和地上部分生理生态的影响研究，明确了秸秆覆盖引起的土壤温度变化对冬小麦生长和产量的影响机制，为生产上大面积推广提供理论依据。利用不同秸秆覆盖量对土壤温度产生的差异影响，覆盖量设2450 kg/hm2（LM）、3675 kg/hm2（MM）、7350 kg/hm2（HM）和不覆盖（CK）。结果表明，秸秆覆盖对冬小麦整个生育期土壤温度的日均值影响不大，但不同生育期秸秆覆盖对土壤温度的影响不相同。在小麦三叶期-返青期气温比较低的冬季，秸秆覆盖具有提升土壤温度的作用，并且随着秸秆覆盖量的增加，提升作用增大。以2013-2014年为例，三叶期至返青期，2.5cm、5cm、10cm、20cm、40cm各层覆盖处理土壤温度日均比对照提高0.65℃、0.60℃、0.57℃、0.84℃和 0.92℃。返青期-成熟期气温较高的夏季，覆盖具有降低土壤温度作用。返青期-拔节期降温最大，拔节期后随着小麦冠层覆盖度的增大，覆盖对土壤温度的影响减弱。不同覆盖量处理土壤温度的差异不显著。秸秆覆盖对土壤耕层温度的影响，引起了小麦內源激素（生长素IAA、脱落酸ABA、玉米素核甘ZR和赤霉素GA）的变化，研究表明，IAA/(ZR+Z)值低时, 有利于分蘖发生，反之则不利于分蘖发生。本研究中的越冬期和返青期IAA/ZR比值CK最低，覆盖处理较高，LM、MM和HM的比值分别为CK的2.0、2.3和3.5倍，拔节期覆盖处理的IAA/ZR值与CK基本一致，扬花灌浆期覆盖处理的值低于对照。因此，覆盖处理内源激素的变化，抑制了小麦分蘖，导致了覆盖小麦成熟有效穗数低于CK。秸秆覆盖后冬季增温效应促进了越冬前根系生长，根长密度大于CK；返青后秸秆覆盖降温效应减弱了根系生长。冬季由于根系生长快于CK，消耗了更多的土壤氮，造成返青-拔节期土壤全氮含量低于CK。秸秆覆盖对冬小麦产量的影响，三叶期覆盖产量高于播种后覆盖，随着覆盖量的增加，小麦产量有降低的趋势，但差异不显著，覆盖处理减产的主要原因是有效穗数的降低。因此，冬小麦-夏玉米一年两熟灌溉区，为了降低秸秆覆盖对冬小麦产量的不利作用，秸秆覆盖应在三叶期后实施。