| Loess hilly region,which suffers concentrated rainfall and erosion-prone soils,is one of the key areas of water and soil conservation and ecological restoration in China.In the past few decades,in order to efficiently reduce the soil-erosion,a series of ecological construction measures have been implemented.Now,the ecological environment and soil nutrient status have been significantly improved,and gradually formed relatively stable patterns of land-use.Carbon and nitrogen are important components of soil nutrients and also the main nutrient sources of vegetation growth.They are often used as an essential index to evaluate the management of land-use sustainability and play an irreplaceable role in balancing the global carbon and nitrogen cycle and the impact on climate change.The patterns of land-use shift the balance of carbon and nitrogen by affecting the decomposition and sequestration of carbon and nitrogen in soil.In addition,it indirectly affects terrestrial biocarbon and nitrogen pools by affecting soil fertility and plant growth.Exploring the changes of soil carbon and nitrogen storage in response to land-use change in the loess hilly region has important scientific significance for improving soil quality and carbon and nitrogen sequestration in soil,and evaluating the sustainable development of vegetation restoration.In this study,the Shanghuang watershed where land-use changes rapidly,was taken as the study area.Based on the visual interpretation of remote sensing images,this paper studied the pattern of land-use changing from 1982 to 2020.Soil bulk density,soil moisture content and carbon and nitrogen content were measured among different land use types.By implementing of Gs+geostatistics software and ArcGIS Kriging interpolation,explore the spatial distribution characteristics of basic physical and chemical properties of soil.Soil organic carbon(SOC)and total nitrogen(TN)storage were estimated and the response of them to land-use change was analyzed,so as to provide a data basis for discussing the influence of land-use change on the dynamic development of soil carbon and nitrogen.The main findings are as follows:(1)With the extension of the time of turning farmland into forests,the area of farmland gradually decreased,while the shrubland and natural grassland increased continuously.Natural grassland was the main landscape type in this watershed from 1982 to 1990.From 1990 to 2005,the area of natural grassland shrank sharply,and the cultivated land area owing to human factors.Between 2005 and 2020,shrubland was the main land-use type due to ecological restoration policies such as returning farmland to forest and grassland.(2)There are significant differences in soil bulk density among different land use types.The soil bulk density of cultivated land and abandoned land are greater than that of natural grassland and shrubland significantly,and it decreases with the deepening of soil layer.The soil bulk density showed moderate spatial heterogeneity.On the surface of the soil,the areas with high and low bulk density are mainly concentrated in sloping farmland and natural grassland,respectively.And the high values of soil bulk density are concentrated in farmland in the middle of the catchment in the lower layer.There was no significant difference in soil moisture content among different land-uses.The soil moisture content of natural grassland and shrubland decreased gradually,while that of cultivated land increased significantly.Besides,the soil moisture content showed strong spatial heterogeneity.And the soil moisture content presents a trend of high in the east and low in the west which the high value area was mainly distributed in natural grassland and shrubland,while the low value area was mainly distributed in cultivated land.(3)There are significant differences in soil carbon and nitrogen content under different land use types.The soil organic carbon content in shrubs and natural grassland is significantly higher than in abandoned land.The soil total nitrogen content of cultivated land was the highest,and that of shrubland,grassland and abandoned land was low.Soil C/N distribution was similar in different soil layers among different land use types,which natural grassland>artificial grassland>shrubland>abandoned land>farmland.(4)According to the spatial distribution interpolation graph,the spatial distribution pattern of soil carbon showing a high values in the east and a low values in the west in the each layer of soil.The high values were distributed in the shrubland and natural grassland,while the low values were distributed in farmland.There are significant differences in soil nitrogen content in each soil layer.In the 0-10 cm,the high value of soil total nitrogen was distributed in natural grassland and shrubland,while the low value was mainly distributed in terraced farmland and the high value of soil nitrate nitrogen was concentrated in the farmland.In the 10-60 cm,the high value of soil total nitrogen was concentrated in the cultivated,and the nitrate nitrogen tends to be homogenized.In the contrast,in 30-60 cm,the ammonium nitrogen decreased gradually from south to north,and the highest value was distributed in the shrubland and natural grassland.(5)As the changing the land-use patterns between 2010 and 2020,the storage of soil carbon and nitrogen in Shanghuang changed obviously.In this decade,the storage of soil organic carbon and nitrogen increased significantly.Implementation of the policy"Grain for Green" make the carbon source from litter increased,improve microbial activity,and improve soil nutrient conditions.Eventually,it can contribute to contribute to doing the storage of soil organic carbon and nitrogen storage.In summary,the area of farmland decreased,while the area of shrubland and natural grassland increased continuously in the Shanghuang watershed from 1982 to 2020.A stable land-use pattern dominated by shrubland and natural grassland was gradually formed after 2010.Result from the shifting of land-use change,soil carbon and nitrogen storage increased significantly from 2010 to 2020.This study provides a scientific basis for clarifying the response of soil carbon and nitrogen fixation potential to land-use change,and is of great significance for the sustainable development of vegetation restoration in the Loess Plateau and some arid and semi-arid regions. |
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