Study On Interspecific Water Relationship Of Walnut-wheat/Soybean Agroforestry System

In recent years,the abnormal changes of global climate aggravated the uneven spatial and temporal distribution of precipitation in the region,which has brought new challenges to the high yield of agriculture and forestry in North China.Agroforestry system has unique significance in water and soil conservation,water conservation,regional microclimate regulation and increase of gross output value per unit area.Therefore,in this paper,the method of stable hydrogen and oxygen isotope was used to study the interspecific water utilization strategy of walnut/wheat/soybean agroforestry system with plant spacing of 4m*5m,which provides scientific guidance and basis for efficient management of agroforestry system.The main results were as follows:(1)The coronal leaf area index of walnut showed a trend of increased first and then decreased from April to September,reaching the maximum in August(2.79).(2)In the vertical direction of walnut-wheat and walnut-soybean agroforestry system,root competition area mainly concentrated in 0-30 cm soil layer and 0-20 cm soil layer.Compared with the control wheat,the root distribution center of gravity depth of wheat in the agroforestry system to niche overlap.The closer the walnut root system of the agroforestry system is to the tree row,the greater the root length density is,and the root system is densely distributed within the range of0.5m-1.5m from the tree row.The root length density on both sides of the tree row was significantly different from that in the middle of the tree row(P<0.01).The horizontal spatial distribution of root length density of wheat and soybean in agroforestry system is opposite.The root length density of wheat and soybean reached the maximum at flowering-filling period and grain filling stage,respectively,during which the water competition between crops and walnuts was the most intense.(3)With the increase of soil depth,the soil water content of walnut-wheat agroforestry system and control system both showed a trend of increase.Soil water content of walnut-soybean complex system showed a decreasing trend,while that of control soybean showed an increasing trend.Due to the shade of walnut trees and the retention of soil moisture by the roots,the soil moisture content of the walnut-wheat-wheat-soybean agroforestry system increased by 4% and0.7%,respectively,compared with the control.In the horizontal direction,the distribution of soil water content in walnut-wheat agroforestry system presented an "M" type distribution,that is,the minimum value of soil water content appears at M,and the maximum value of soil water contentis at W1.5m.Walnut-soybean system has an " inverted V" distribution,that is,the maximum value of soil water content is at W1.5m,and there is no significant difference in soil water content at each horizontal measuring point in the agroforestry system.(4)In the agroforestry system of walnut and wheat,walnut mainly used deep soil water(60%)before the jointing stage of wheat and shallow water(72.15% and 68.85%)during wheat flowering-filling stage to maturity stage.In the compound system of walnut and soybean,walnut mainly used shallow soil water(68.44%),during the whole phenological period,and the competition between flowering-filling period and maturity period is obvious.In the walnut-soybean complex system,walnut mainly uses shallow soil water(68.44%)and there is no significant difference among different periods(P >0.05).Shallow soil water(70.11%)was mainly utilized by soybean in the complex system during the whole phenological period.During this period,rainfall supplemented the shallow soil moisture,which to some extent alleviated the competition between the two on the shallow soil moisture.(5)In the agroforestry system,both wheat and soybean have negative avoidance reactions,which are manifested by thinning of stems and elongation of internodes.In the agroforestry system,wheat SPAD showed a V-type distribution of " high on both sides and low in the middle",while soybean SPAD showed an opposite trend.Compared with the control,SPAD of wheat and soybean in the agroforestry system decreased by 2.3% and 6.73% respectively.The wheat/soybean yield of the agroforestry system decreases from the middle to both sides of the tree row.Compared with the control,the yield of wheat and soybean in the agroforestry system decreased by 50% and70% respectively.In summary,in the walnut-wheat complex system,appropriate irrigation should be applied before the maximum root length density period(flowering-filling period)arrives to reduce wheat yield reduction caused by interspecific water competition,or appropriately increase the spacing between tree rows to reduce interspecific niche overlap and improve wheat yield per unit area.In the walnut-soybean agroforestry model,the walnut leaf area index approaches the maximum value,which seriously affects the soybean yield under shade.It is suggested to plant Chinese herbal medicine crops such as Radix Isatidis and Semen Cassia with strong negative tolerance instead.