| Citrus industry is the pillar and characteristic industry of the rural economy in the Three Gorges Reservoir Area,and its cultivated soil resources are mainly purple soil.The weak resistance of purple soil to erosion,coupled with adverse factors such as multiple sloping fields and concentrated rainfall in the reservoir area,as well as the traditional farming method of surface clearing by fruit farmers,has made the soil erosion problem of citrus orchards in the reservoir area very serious.At the same time,due to the massive application of pesticides and fertilizers,the problem of heavy metal pollution in orchard soil has become increasingly prominent.Green manure plants have a good performance in improving soil,controlling soil erosion,and repairing soil heavy metal pollution.Therefore,planting green manure is becoming a vigorously promoted ground management technology for citrus orchards in the reservoir area.Based on previous studies,this study selected typical gramineous green manure,Lolium multiflorum L.,and typical leguminous green manure,Vicia villosa Roth,and Trifolium repen,which had good growth performance on purple soil slope farmland,as the research objects.Four treatments,namely,Lolium multiflorum L.mulching,Vicia villosa Roth mulching,Trifolium repen mulching,and clearing tillage,were provided in purple soil slope runoff plots planted with citrus fruit trees,From October 2021 to September 2022,under natural rainfall conditions to investigate the effects of different green manure coverage on soil erodibility reduction,soil erosion control,and heavy metal pollution prevention in purple soil citrus slope gardens,with a view to providing theoretical and practical basis for the prevention and control of soil erosion and soil heavy metal pollution in purple soil citrus slope gardens in the Three Gorges Reservoir Area.The main conclusions are as follows:(1)Compared with the clear tillage plot,the physical properties of the soil in the three green manure plant coverage plots of Lolium multiflorum L.,Vicia villosa Roth,and Trifolium repen have improved in different directions,mainly as follows:Vicia villosa Roth has the best performance in reducing soil bulk density,improving soil infiltration rate,improving soil quality,water content,and saturated soil water storage,while Trifolium repen has the best effect in increasing total soil porosity.The interception effect of Lolium multiflorum L.and Vicia villosa Roth on the migration of clay particles towards the downhill direction is better than that of Trifolium repen.The improvement effect on soil erodibility is as follows:Vicia villosa Roth>Lolium multiflorum L.>Trifolium repen.The soil erodibility factor was significantly negatively correlated with monthly rainfall erosivity Rm,non capillary porosity,average infiltration rate in the first 30 minutes,and clay content(p<0.01),while it was significantly positively correlated with capillary porosity and silt content(p<0.01).The increase in soil infiltration rate is beneficial for reducing soil erodibility.Increasing the content of clay particles can enhance the adsorption between soil particles,making it easier to condense with cementitious substances such as humus into microaggregates,thereby forming a soil aggregate structure,which plays a role in improving soil conditions.Compared to sand particles,an increase in silt content will increase soil erodibility in the study area.In May and June,when rainfall erosion is the strongest,the reducing effect of green manure on soil erodibility is greater than the negative impact of increased rainfall erosion on soil.(2)The P-PQ relationship for the study area is PQ=0.01906P~2-2.37914P+107.68349(R~2=0.95641,p<0.01).When the soil erosion ratio PQ=90%,the erosion rainfall standard for the study area P=7.9mm is obtained.From October 2021 to September 2022,there were 36 erosive rainfall events,accounting for 43.37%of the total 83 rainfall events in the year.The cumulative erosive rainfall throughout the year was 840.5mm,accounting for 83.72%of the total annual rainfall of 1004.0mm.The main force of runoff and sediment production on the exposed surface of the study area is the subsurface flow.When the rainfall intensity level is moderate rain or heavy rain,the water and soil conservation work in the study area should focus on taking appropriate measures to conserve water and soil for the subsurface flow.When the rainfall reaches the intensity level of rainstorm and above(May and June),in addition to paying attention to geological disasters that may be caused by excessive flow in the soil,attention should also be paid to intercepting the erosion sediment in the surface runoff.Planting green manure can prevent and control water and soil loss in purple soil citrus slope gardens to varying degrees.Lolium multiflorum L.is a green fertilizer variety that has the best effect on preventing and controlling soil erosion in the study area during erosive rainfall events.In 21erosive rainfall events throughout the year,the total flow yield and total sediment yield decreased by 29.75%and 61.46%compared to the tillage area under Lolium multiflorum L.coverage.The runoff and sediment yield of citrus orchards on purple soil slopes are mainly affected by sub-rainfall P and sub-rainfall erosivity R,therefore,during the period of maximum sub-rainfall P and sub-rainfall erosivity R(June),efforts should be made to strengthen soil and water conservation in orchard soils.Compared with the clear tillage plots,the three green manure mulching treatments all weakened the response of the plots sediment yield to various rainfall factors,thereby effectively intercepting the soil erosion effect of rainfall on the purple soil slope citrus orchard.The specific weakening effect was shown as follows:Lolium multiflorum L.>Vicia villosa Roth>Trifolium repen,The results showed that Lolium multiflorum L.mulching was more suitable for soil erosion control in purple soil citrus slope gardens than villosa Roth and Trifolium repen mulching.(3)After applying organic fertilizer,the content of Pb,Cr,Mn,and Zn in the soil of each treatment plot reached the annual peak,indicating that the content and effectiveness of heavy metals in organic fertilizer from poultry and livestock manure were higher than those of chemical fertilizers.The control of Pb and Cr contents in soil and eroded sediment showed that Lolium multiflorum L.was the best;The control of Mn content in Vicia villosa Roth was the best,followed by Lolium multiflorum L.;The control of Zn content in soil and eroded sediment is best performed by Trifolium repen,followed by Lolium multiflorum L.,That is,the comprehensive control effect of Pb,Cr,Mn,and Zn content in soil and eroded sediment is best performed by Lolium multiflorum L..The contents of Pb,Cr,Mn,and Zn in soil are correlated with vegetation coverage,soil loss,soil erodibility,and soil particle composition,but do not exhibit significant correlation with rainfall factors.Pb,Cr,and Zn elements in soil may be easily adsorbed on surface soil particles that are prone to erosion,and transported downstream by runoff sediment.In clearing plots,as the soil erodibility increases,the soil’s ability to resist erosion becomes weaker,and large soil particles are broken by external forces.Mn in the soil may be further released,which shows a trend of increasing soil Mn content.The increase in the content of Pb and Cr in soil will have a toxic effect on citrus fruit trees and inhibit their growth and development,with the greatest impact being on the content of Pb in soil.To sum up,Vicia villosa Roth has the best improvement effect on reducing soil erodibility,while Lolium multiflorum L.mulch has the best soil and water conservation effect on purple soil citrus slope gardens,and Lolium multiflorum L.mulch has the best comprehensive control effect on heavy metal pollution in citrus orchard soil.Based on the above three aspects,planting Lolium multiflorum L.in purple soil citrus slope gardens in the Three Gorges Reservoir Area is conducive to the prevention and control of soil erosion and soil heavy metal pollution. |
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