Spatial trade-offs and synergies of ecosystem services for grazing-prohibited grassland in agro-pasture ecotone

Sustainable management of landscapes determines the relationship between each ecosystem service on different spatial scales. Taking Yanchi County of Ningxia in China as the research area, this paper aims to clarify the interaction between the main ecosystem services of grazing-prohibited grassland in agro-pasture ecotone, using various techniques, such as data extraction of grassland, remote sensing inversion, and spatial analysis. After local correction to InVEST model, three key ecosystem services, including the carbon storage, the water conservation and soil conservation after grazing prohibition during 2000－2015, were measured to define the features of their spatial-temporal changes. The spatial trade-offs were represented quantitatively for three services, and the partial correlation analysis at a pixel scale was used to analyze the spatial synergies at the township scale. The results show that three indicators of grassland ecosystem, including the functions of carbon storage, water and soil conservation, were significantly enhanced since the grazing prohibition. Specifically, the total carbon storage and density increased from 2 174 600 tons and 5.78 t/hm2 to 2 631 200 tons and 6.19 t/hm2, respectively; the quantity and density of water conservation increased from 28.84 million tons and 76.69 t/hm2 to 61.18 million tons and 143.92 t/hm2, respectively; and the total quantity and density of soil conservation increased from 13.83 million tons and 36.78 t/hm2 to 34.74 million tons and 81.71 t/hm2, respectively. However, there was a significant "N" type fluctuation in the increasing process. During the whole period, the range of carbon storage in grassland ecosystem varied slowly with continuous increase or decrease; while the water and soil conservation showed a remarkable increase or decrease. In terms of regional differences, the increases of carbon storage and water conservation in the north were higher than that in the south, whereas, the increase of soil conservation in the south was higher than that in the north. The function of carbon storage was closely related to the grassland type, indicating small interannual fluctuation and stable growth. The water and soil conservation can depend significantly on precipitation and terrain, leading to large interannual fluctuations and significant spatial differences. There was a spatial synergistic relationship between the soil and water conservation (86.34%) of grassland. The relationship between carbon storage and water conservation was complex, where the spatial trade-off and synergetic relationship were not correlation (47.66% and 52.34% respectively), while the carbon storage and soil conservation showed a spatial synergistic relationship (69.55%). Terrains dominated the synergies and trade-offs relationship between the carbon storage and water conservation, as well the carbon storage and soil conservation. Specifically, the synergetic relationship in areas revealed a large topographic relief, while there was most trade-offs relationship in flat areas. Therefore, the quantitative visualization can be used to evaluate the spatial trade-offs and synergies among the services of grassland ecosystem. The proposed method can further provide a scientific basis for the optimization of the grassland ecosystem, and thereby serve as a sound support for the sustainable management of grassland resources.