Effects Of Crop-mulberry System On Soil Nitrogen And Phosphorus Loss In Purple Soil Sloping Farmland And Its Economic Benefit Analysis

Nitrogen and phosphorus are macronutrients necessary for crop growth,and are also the main factors causing agricultural non-point source pollution and water pollution.Sloping farmland is an important source and place of agricultural nitrogen and phosphorus loss.Surface runoff and interflow are the two main ways of nitrogen and phosphorus migration and loss in sloping farmland.The purple soil with weak corrosion resistance and poor water and soil conservation capacity is the main agricultural soil type in the Three Gorges Reservoir,accounting for more than 70%of the total cultivated land area in the Three Gorges Reservoir area.The high dependence on chemical fertilizers,coupled with the large slope of the reservoir area and the same season of rain and heat,lead to serious water and soil loss and nitrogen and phosphorus nutrient loss of the purple soil slope farmland in the reservoir area,which directly threatens the safety of water environment in the reservoir area.Therefore,using reasonable measures to control the loss of nitrogen and phosphorus nutrients in purple soil slope farmland is of great significance for the prevention and control of agricultural non-point source pollution in the Three Gorges Reservoir area.Study on the Crop-mulberry system（corn-mustard rotation）of purple soil slope farmland in the Three Gorges Reservoir area,a set of 15°runoff plots（9.0 m×3.0 m）were established in the opening field.Four treatments,i.e.one-belt mulberry（T₁）,two-belt mulberry（T₂）,three-belt mulberry（T₃）and non-mulberry hedge（CK）were set up.Open ditches and hidden ditches were set at the bottom of slope surface and 60cm in depth to collect surface runoff and soil middle flow,so as to continuously monitor the runoff,sediment,and nitrogen and phosphorus losses in different Crop-mulberry configuration runoff plots from August 2020 to August 2021.In order to explore the effect of mulberry planting on soil nitrogen and phosphorus loss in purple soil slope farmland in the Three Gorges Reservoir area,and provide more data support and theoretical basis for optimizing farming and mulberry configuration.The main results are as follows:（1）The surface runoff and interflow of each treatment were as follows:CK>one-belt mulberry>two-belt mulberry>three-belt mulberry.Compared with CK（286.6 mm）,the total runoff of three-belt mulberry,two-belt mulberry and one-belt mulberry was significantly reduced by 61.8%,33.1%and 21.1%,respectively,and the total amount of interflow was significantly reduced by 69.5%and 37.1%,respectively,compared with CK（226.3 mm）,the surface runoff decreased by 32.9%and 18.0%,respectively;and the one-belt mulberry had no significant interception effect on surface runoff.Interflow was the main runoff mode of purple soil slope farmland,accounting for 63.0%-79.0%of the total runoff.In addition,the annual soil loss of mulberry in the three-belt,two-belt and one-belt was significantly lower than that of CK(158.27 kg·hm^-2)by 56.3%,10.4%and 19.8%,respectively.The mulberry hedge had a significant interception effect on soil loss.More runoff and less sediment was the main characteristics of soil erosion in purple soil slope farmland during the experiment period.The maize planting season（March to July）was the main period for runoff and soil loss of purple soil slope farmland,accounting for 40.9%-45.1%and 50.3%-88.7%of the total annual runoff and soil loss,respectively.（2）Mulberry hedgerow intercropping significantly reduced soil nitrogen loss in purple soil slope farmland.Compared with CK(72.60 kg·hm^-2),the total nitrogen loss of the three-belt,two-belt and one-belt mulberry decreased significantly by 63.4%,33.0%and 20.5%respectively.Among them,the nitrogen loss in interflow decreased by 66.2%,34.6%and 22.0%,and the nitrogen loss in surface runoff decreased by38.6%,18.9%and 8.2%,compared with CK(0.11 kg·hm^-2),the nitrogen loss by sediment was significantly reduced by 72.2%,49.8%and 10.7%respectively,but one-belt mulberry had no significant interception effect on nitrogen loss in surface runoff and sediment.Interflow is the main way of soil nitrogen loss,accounting for82.7%～89.6%of the total annual nitrogen loss.Dissolved nitrogen is the main form of nitrogen loss in runoff of sloping farmland in purple soil,accounting for 72.3%-82.9%of total nitrogen loss in surface runoff and 72.2%-85.9%of total nitrogen loss in interflow.Corn planting season and August of 2021 fallow period were the main periods of soil nitrogen loss,accounting for 77.9%-89.2%of the total annual nitrogen loss.Corn season was the main period of soil nitrogen loss with sediment,accounting for 56.8%-83.9%of the total nitrogen loss in sediment.（3）The carrier of soil nitrogen and phosphorus loss is very different.The carrier of nitrogen loss is runoff,while the suspended sediment is the main carrier of phosphorus loss.In this study,the average total phosphorus concentration of surface runoff in each treatment was 3.1-4.4 times of total phosphorus concentration in soil flow.Therefore,surface was the main way of phosphorus loss in purple soil slope farmland,accounting for 52.7%-73.2%of the total annual phosphorus loss;except for three-belt mulberry（47.4%）,particulate phosphorus was the main form of soil phosphorus loss,accounting for 51.6%-59.9%of total phosphorus loss in runoff.The difference of interception effect of mulberry hedgerow intercropping on surface runoff and interflow also led to its weaker interception effect on soil phosphorus loss than that of nitrogen.Among the three mulberry configuration modes,only three-belt mulberry had significant interception effect on soil phosphorus loss.The total annual phosphorus loss was significantly reduced by 62.0%compared with CK(1.06 kg N·hm^-2),and the phosphorus loss in runoff and sediment decreased by 61.3%and 65.9%,respectively.There was no significant interception effect on phosphorus loss in runoff,but the phosphorus loss in sediment was 27.0%and 29.0%lower than that of CK(0.16 kg N·hm^-2),respectively.Corn season was the main period of soil phosphorus loss with runoff and sediment,accounting for 53.8%-62.5%and 54.9%-91.4%respectively.Nitrogen interception is the focus of controlling nutrient loss in purple soil sloping farmland and reducing agricultural non-point source pollution.（4）The competition between mulberry hedges and crops affected crop yield,resulting in a decrease in the economic yield of each treatment with the increase in the number of mulberry hedges planted.There was no significant difference in the economic yield of mustard among the treatments,but the economic yield of maize in two-belt and three-belt mulberry decreased by 9.6%and 13.3%,respectively,compared with CK.However,the total biomass of plants in the crop-mulberry system was still increased by 3.5%-8.9%compared with CK,and the nitrogen and phosphorus absorption of plants was significantly higher than that in CK by 11.1%-16.7%.Estimating the annual economic benefits of the three planting patterns of mulberry showed that the annual economic benefits of mulberry planting system were 1.1-1.3times that of CK treatment.Even excluding the benefits of silkworm breeding,the annual economic benefits of the three crop-mulberry configuration modes of the one-belt,two-belt and three-belt mulberry have reached 14.05,15.25 and 16.62 million yuan/hm²,respectively,which were 9.3%,18.7%and 29.3%higher than those of the traditional agricultural mode（1,285 million yuan/hm²）,respectively.The agricultural and mulberry system conforms to the farmers’pursuit of economic benefits.Considering the characteristics of runoff and sediment yield,soil nitrogen and phosphorus nutrient loss,crop yield and economic benefits,three-belt mulberry and two-belt mulberry are both worthy of promotion.However,when mulberry hedges are used to prevent and control agricultural non-point source pollution,mulberry branches should be pruned in time in spring and summer to control them at an appropriate height,weaken the competition between hedgerows and crops for light,and reduce the impact of mulberry hedgerows on crop yield.