Effects Of Cover Crop On Water,carbon And Nitrogen Distribution In Deep Soil Of Apple Orchard In Loess Plateau

Longdong dry plateau area is a vital apple-producing area in Gansu Province.Apple farming has become a pillar for local farmers to increase their income and consolidate their poverty alleviation achievements.Traditional orchards are mainly managed by clean tillage,which will lead to ecological and production problems,such as degradation of the orchard,reduction of biodiversity,and loss of fruit yield and quality.Orchard with cover crop is a mode of orchard soil cultivation management,in which herbaceous plants are planted between rows of fruit trees or throughout the orchard.Orchard with cover crop has many ecological benefits,such as improving soil structure and fertility,increasing carbon sequestration in the agricultural system,suppressing weeds,and reducing pesticide use.Optimizing soil water,carbon and nitrogen pools in the deep Loess Plateau soils is an essential contribution to enhancing productive ecological functions and sustainable development of the agricultural system.Yet,there is a lack of research assessing the effect of long-term apple orchard grass mulching on deep soil water,carbon,and nitrogen stocks.Therefore,this study conducted a monitoring experiment on soil water,carbon and nitrogen in a deep layer of orchard grass mulching system in the apple orchard of Qingyang National Field Observation and Research Station in Gansu province from 2020 to 2021.With farmland as the control,we compared the effect of grass mulching and clean tillage management on soil water,carbon and nitrogen distribution in 0-500 cm layer.The main research results are as follows:（1）In the 0-300 cm soil layer,soil moisture content（19.6%）was significantly lower than in the control（21.3%）.However,there was no difference in the 300-500cm soil layer（P>0.05）.After cover crop,the average soil moisture content in 0-500cm layer in the apple orchard with cover crop was 17.9%,lower than that in clean tillage orchard（20.4%）.The soil moisture content in the shallow layer decreased more than that in the deep layer.The decrease in soil water content in grass-mulched orchards in 2021 was more significant than that in the orchard with clean tillage in2020.Cover crop increased the water deficit degree of the orchard,and the water deficit of grass orchard in 0-500 cm soil layer was 151.8-176.2 mm,which was3.2-4.2 times that of clear tillage orchard.（2）The nitrate-nitrogen content in the apple orchard’s 0-100 cm soil layer was not significantly different from control farmland.However,a significant accumulation of nitrate-nitrogen occurred in the 100-500 cm soil layer,and the content was significantly higher than that in the control（P<0.01）.The average nitrate-nitrogen content of 0-30 cm soil layer in clean tillage orchard was 100.2 kg·hm^-2,1.7 times the one with cover crop.The consumption of nitrate-nitrogen in shallow soil in the orchard with cover crop is more than that in the orchard with clean tillage.In the400-500 cm soil layer,the accumulated nitrate-nitrogen in the orchard with clean tillage was 920.1 kg·hm^-2,and the one with cover crop was 630.3 kg·hm^-2,which was31.5%lower than the orchard with clean tillage.Therefore,cover crop management measures reduced the leaching of nitrate-nitrogen to the deep layer and promoted the fruit trees to absorb and utilize nitrate-nitrogen in the deep layer.The total nitrogen content of the soil 0-500 cm soil profile was not significantly different between treatments（P>0.05）.（3）The increase in soil organic carbon was insignificant（P>0.05）in the apple orchard with clean tillage compared to the reference field.In contrast,the average soil organic carbon content in the 0-500 cm soil layer of the orchard with cover crop reached 5.9 g·kg^-1,which is an increase of 20.6%compared to the orchard with clean tillage(4.9 g·kg^-1),and the deeper soil organic carbon content increased more than the shallow layer.Cover crop mainly promoted the organic carbon content in the 100-400cm soil layer between the rows of the orchard.The organic carbon content of the orchard is significantly affected by the number of years of living mulch growing,with an increase in organic carbon content in 2021 compared to 2020.The total carbon stock in 0-500 cm soil layer in orchards with cover crop was 24.7%-33.7%higher than that in orchards with clean tillage.In the 0-500 cm soil layer,the soil carbon to nitrogen ratio is more significant in orchards with cover crop than in orchards with clean tillage.The soil carbon to nitrogen ratio in the orchard with cover crop is closer to the optimal soil carbon to nitrogen ratio.（4）The crop mulching treatment changed the distribution pattern of the root system of the fruit trees and promoted the distribution of the root system in the shallow layer（0-150 cm）.The average root length density in 0-150 cm soil layer of cover crop treatment was 0.214 and 0.140 cm·cm^-3 in 2020 and 2021,with an increase of 89.2%and 6.0%,respectively,compared to the orchard with clean tillage.The cover crop treatment had no significant effect on roots distribution of fruit tree in150-500 cm deep soil.The root length density of the orchard with cover crop was0.095 and 0.064 cm·cm^-3 in 2020 and 2021,respectively.The root length density of the orchard with clean tillage was 0.061 cm·cm^-3 in two years.Soil moisture content in 0-500 cm soil layer of orchards with and without cover crop decreased with the increase in long density of fine root of fruit trees.The correlation was higher after cover crop and in drought years.Moreover,the correlation between soil water content and total nitrogen content and the fine root length density increased after crop cover.In conclusion,there is water competition between fruit and grass in apple orchards’long-term cover crop system in the Longdong dry plateau area.Cover crop increases the degree of soil water deficit in orchards.However,orchards with cover crop reduce the accumulation of nitrate-nitrogen in the deep soil,improve nitrogen utilization,increase soil carbon sinks,especially in the deep layer,and improve the soil carbon to nitrogen ratio and the soil quality.Therefore,it is recommended to strengthen the agronomic management of the orchard,such as planting density,bandwidth,mowing,and planting duration,to minimize the adverse effects on the soil moisture environment and give full play to the ecological service functions of the cover crop orchard model such as drug reduction and carbon sequestration.The research has specific guiding significance for optimizing orchard farming management and the joint promotion of production ecological functions in the Loess Plateau region.