| In the loess hilly regions of China,large-scale ecological restoration had been carried out to turn the ecological environment around and there are certain differences in soil nutrients of different terrains and different land use patterns.In this paper,the Nangou watershed in Ansai County,Shaanxi Province was used as the study area.The geostatistics and the three-dimensional interpolation methods were used to analyze the spatial structure and the distribution of soil organic matter,total nitrogen and available nutrients based on ground monitoring and remote sensing technology.And we studied the soil nutrient trends of slopes in different land use combinations and the relationship between soil organic carbon and topography and direction of returning farmland.The main results were as follows:?1?The nutrients in topsoil were significantly higher than that in deep,the average content in the depth of 0-100 cm was not high as a whole,which the organic matter,total nitrogen and available nitrogen were at a low level,and available phosphorus and available potassium were at a medium level.They were dominated by structural factors and all had moderate spatial variability.Among them,the spatial variation of available potassium was small and the spatial autocorrelation was strong,followed by organic matter and total nitrogen,and the spatial variation of available nitrogen and available phosphorus was larger and the spatial autocorrelation was weaker.The correlation between organic matter and total nitrogen was extremely strong,their fractal dimension was smaller than available nutrients,proving that the spatial complexity of organic matter and total nitrogen was weaker.Elevation,slope gradient,aspect and NDVI can explain about 40%variation of organic matter,total nitrogen and about 20%variation of available nutrients.?2?The three-dimensional method was superior to the two-dimensional method in simulating the spatial distribution of soil nutrients.Among them,the three-dimensional inverse distance weighting method was more suitable for the spatial simulation of organic matter,total nitrogen,available nitrogen and available phosphorus,and the prediction accuracy was improved by 10.06%,7.60%,7.92%and 1.78%compared with the three-dimensional kriging method;the three-dimensional kriging method was more suitable for the available potassium,and the prediction accuracy was improved by 7.47%compared with the three-dimensional inverse distance weighting method.In the three-dimensional simulation results,the soil nutrients all decreased with the increase of soil depth,they changed significantly in the topsoil and stabilized in the deep.The organic matter,total nitrogen and available potassium were more abundant in the northeast of this watershed,followed by the southwest and less in the middle;the available nitrogen was higher in the fruit tree planting area in the southeast corner and the available phosphorus was higher in the reforestation protection area in the southwest corner.?3?According to the order of land use from top to bottom of the slope,the organic matter and total nitrogen were higher in the top of the slopes of"forest/shrub-grass"and"forest/shrub-crop"than that in the bottom,and the available nutrients in the top were lower than that in the bottom.The organic matter and total nitrogen were lower in the top of the slopes of"fruiter-forest/shrub-grass"and"fruiter-forest/shrub-crop"than that in the bottom,and the available nutrients was higher in the top of the slope of"fruiter-forest/shrub-grass"than that in the bottom,the available nutrients of the slope of"fruiter-forest/shrub-crop"were both higher in the top and the bottom.Among the four typical land use combination slopes,the organic matter and total nitrogen were highest in the slope of"forest/shrub-grass"and lowest in the slope of"fruiter-forest/shrub-crop",and the available nitrogen were highest in the slope of"fruiter-forest/shrub-crop",while the differences of available phosphorus and available potassium among the slopes with different land use combinations were not obvious.?4?The range of soil organic carbon density in the study area was 0.32-2.08 kg·m-2,with the average value of 0.74 kg·m-2.In slopes where the land use were all forests,the soil organic carbon density was ranked as concave slope>straight slope>convex slope and shady slope>sunny slope,and its complexity was ranked as concave slope>convex slope>straight slope and shady slope>sunny slope.The soils of concave slope on the shady side had better carbon sequestration capacity.The soil organic carbon storage at the depth of0-100 cm in the study area was 4211.26 t.With the idea of replacing time with space,the soil organic carbon of farm land converted into forest,grassland and orchard increased by57.04%,34.82%and 12.15%respectively,and the carbon sequestration capacity was expressed as forest>grassland>orchard.In the process of ecological restoration,we suggest the slope can be reconstructed by taking advantage of the dominant topography and the dominant direction of returning farmland. |
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