| The unreasonable composition,type,and configuration of landscape vegetation in the small watershed land use system are the main causes of soil erosion.The optimization of vegetation structure to improve ecological functions is to fundamentally curb regional soil erosion,non-point source pollution,and achieve regional sustainable development.In this study,the small watershed of the Shizikou reservoir area in the red soil hilly area of Xinyu City,Jiangxi Province is selected as the research object.Under the premise of the development of the fruit industry in the small watershed,the spatial information processing function of GIS is applied,and based on the"source-sink"theory,comprehensive consideration is given to the soil erosion and the total nitrogen/total phosphorus non-point source pollution load in the small watershed,using revised universal soil loss equation and grid spatial load contrast index to calculate quantitatively,and the impact of landscape quantitative structure characteristics is discussed.The ecological risk analysis was carried out at the three scale levels of small watershed,water catchment area and landscape type,and a strategy for optimizing the vegetation structure based on the protection of the water source of Shizikou Reservoir was proposed.The results show that:?1?overall,the land use cover type of the small watershed is mainly forest land,followed by citrus orchards.The proportion of sink landscape in small watershed is much higher than that of source landscape.The landscape fragmentation and heterogeneity in the catchment area of the small watershed show an increasing trend,and the upper area has the best landscape aggregation degree.In small watersheds,the“sink”landscape arbor forests and waters have high dominance and fragmentation,followed by citrus orchards and dry lands in the“source”landscape.The aggregation degree of forest land is the highest,followed by paddy fields and citrus orchards.?2?The small watershed is divided into 19 catchment areas.Among them,the proportion of"source"landscape is higher than that of"sink"landscape in No.8,12,16 and 17 catchment area,and citrus orchards are the dominant landscape;the proportion of"source"and"sink"landscape in 11,15 and 19 catchment areas is relatively equal,of which the dry land in No.11 catchment area is the dominant landscape;the proportion of"sink"landscape is higher than that of"source"landscape in other all catchment area.In terms of landscape pattern characteristics,the“source”landscape of No.8,11,12,15,16,17,19 has low fragmentation,and the“sink”landscape has high fragmentation,No.5,6,8,12,and 19 catchment areas have high“source”landscape aggregation and low“sink”landscape aggregation,which is conducive to the generation and concentration of non-point source pollution.?3?The results of erosion modulus analysis show that the small watershed is at a moderate erosion level,with the slightest erosion area having the largest area,and moderate and above erosion areas accounting for 19.45%,and the overall erosion degree shows that the lower part of the small watershed is higher than the upper.On the scale of catchment area,No.16 and No.17 catchment areas are at the level of intensive erosion,No.6,9,15,13,14,12,10,2 and 11 catchment areas are at the moderate erosion level,and the rest are at the level of light erosion.At the land level,in the"source"landscape,the average soil erosion modulus of bare land?citrus garden?oil tea garden and dry land are at or above the level of intensity erosion,and the paddy field is slightly eroded;the average soil erosion modulus of grassland and shrub land in the"sink"landscape is at the level of intensive erosion,while that of forest land is at the level of light erosion.Among them,catchment areas and land types with moderate or above erosion level are the key areas to be regulated.?4?The landscape spatial load contrast index of total nitrogen/total phosphorus non-point source pollution shows that the intensity of phosphorus"source"in small watershed is lower than that of nitrogen"source",and the distribution of nitrogen and phosphorus“source”and“sink”is not much different.Strong“sources”of nitrogen and phosphorus in the non-point source pollution are concentrated in the reservoir zone.The landscape"sink"effect of small watershed and 19 catchment areas is greater than"source"effect in general.Among the land types,paddy fields,residences,camellia gardens and citrus orchards are phosphorus non-point source pollution sources.Camellia orchards,citrus orchards,paddy fields,dry land,bare land and residences are nitrogen non-point source pollution sources.These types of non-point source pollution sources are the key land use cover types that need to be regulated.?5?Spearman correlation analysis shows that the proportion of"source"landscapes and citrus orchards is an extremely important source of soil erosion and nitrogen and phosphorus non-point source pollution.Transportation land is also a source of nitrogen and phosphorus non-point source pollution,and the higher the degree of aggregation and dominance of citrus orchards and transportation land,the more favorable it is for erosion and non-point source pollution of nitrogen and phosphorus load;and the larger the arbor forest area,the lower the degree of fragmentation,the higher the degree of agglomeration and dominance,the greater the reduction of erosion and pollution.?6?Based on the correlation between landscape characteristics and soil erosion,total nitrogen/total phosphorus non-point source load,combined with the identified key"source"landscape and catchment area,this paper proposes vegetation structure optimization measures based on the regulation of soil erosion and non-point source pollution in small watershed,and analyzes the optimization results:Among the three measures,the first method is to mix the tall fescue with arrow peas in the surface of the citrus orchard,and the effect of optimizing soil erosion is the best.The average soil erosion modulus of the small watershed is reduced to1730.23 t·km-2·a-1,switch to light erosion level,and the catchment area of medium and intensity erosion turn to light erosion;measure three lay a 30-meter-wide waterfront vegetation buffer zone along the reservoir,which has the best effect on controlling the total nitrogen/phosphorus load of non-point source pollution.The landscape spatial load contrast index of total nitrogen and total phosphorus of non-point source pollution in small watershed decreased from-0.555 and-0.506 to-0.579 and-0.524 respectively,and 11 catchment areas also decreased by 0.002-0.081,with an average decrease of about 0.030;the second measure is to convert bare land into arbor woodland,and the effect of optimizing nitrogen and phosphorus load on soil erosion and non-point source pollution is medium.Based on the above three measures,the vegetation structure of the small watershed was optimized.The forest area of the small watershed increased by about65.3 hectares,the bare land disappeared,and the area of citrus orchards,camellia gardens and other areas were reduced to varying degrees.The optimization effect is the best.The average soil erosion modulus of the small watershed is reduced to1638.50 t·km-2·a-1,the erosion level is reduced to mild,and the medium and strong erosion catchment area is also reduced to mild.The total nitrogen/total phosphorus spatial load contrast index of small watersheds decreased to-0.725 and-0.634,respectively,and the 12 key catchment areas also all decreased by 0.022 to 0.474,with an average decrease of about 0.197. |
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