| Shrub encroachment is an important ecological problem in the arid and semi-arid regions of the world,and has a great impact on surface habitats,ecosystem functions and structures.In order to recover the fragile ecosystem of degraded desert steppe in Ningxia,the Caragana korshinskii cultivar is widely introduced for vegetation restoration in this area,accompanied by soil drought,herb diversity decreases and resulting in the weakening of grassland functions.Therefore,the research based on the study of the soil properties of shrubland and enclosed grassland and grazing land during the over 30 years of shrub introduction in community scale(ie:grassland-grazing-shrub introduction process),was continuous to conducted to explore and analysis the spatial evolution pattern of soil water(SW),soil organic carbon(SOC),soil total nitrogen(STN)of grassland-shrub mosaics formed by the introduction of shrubs and threshold effect of aboveground biomass on soil moisture in study area scale(grassland-shrub mosaic:grassland,grassland-shrub edge,shrubland)by using the classic Statistics,geostatistics,and GIS methods.The objective of this paper is to provide a scientific insight and data support for the restoration of artificial vegetation in desert steppe in Ningxia area.The main conclusions are as follows:(1)In the processof grassland-grazing-shrub introduction with different years(3 y,12 y,22 y)and spacing(40 m,6 m,2 m),due to deep SW consumption intensified the shrublands had no significant increase of SW in the 0?200 cm soil deep layer and no obvious spring SW regain period,compared with the significant vertical dynamics of SW and significant seasonal dynamics(spring regain,summer growth depletion and autumn rainfall supplementation)in enclosed grassland and grazing land.And SW of shrublands was significantly lower than that of enclosed grassland and the highest in grazed land(P<0.05).Except for the enclosed grassland,there existed SW losses in other lands,and the relative SW deficit is 6.69?97.16 mm,the compared SW deficit index(CSWDI)value fluctuated from 0.03 to 12.10,and The plot compared SW deficit index(PCSWDI)of the shrublands increased with the increase of shrub introduction age and density.(2)The process of grassland transformation into shrub(grassland,grassland-shrub edge,shrubland)in large-scale spatial analysis shows that 0?200cm deep SW in grassland-shrub edge was more than that in grassland and in shrubland in dry(July)and wet(October)seasons of SW.However,and the difference between dry and wet seasons was not significant(P>0.05),the overall study area was relatively dry,and the spatial variation coefficient of SW was 32.59%?101.97%,showing moderate variation and no seasonal dynamics of SW in the study area scale.In the dry season(July),the A0 of SW in 0?200 cm soil layer is 1.42?133.87 km,and the spatial heterogeneity(MSH)[C/(C0+C)]is 0.104?1.000.The changes of MSH in the three treatments were increased,decreased and increased with the depth of the soil layer,respectively.In the wet season(October),the A0 of SW in 0?200cm soil layer is 3.52?78.81 km,and the MSH is 0.179-1.000.The changes of MSH of the three treatments showed a decreasing trend with the depth of the soil layer.The overall spatial heterogeneity of SW in the conversion of grassland(0.866)to shrubland(0.796)showed an increasing trend.(3)SOC in the 0-10 cm soil layer was gradually increased during the grassland-grazing-shrub introduction process,and SOC of the shrublands was significantly higher than those of the grazing and enclosed grassland,ranging from 0.506 to 3.229 g kg-1.But in the 0?100cm soil layer they had significantly increases compared with the grazing land(P<0.05)and no significant difference with the SOC of the enclosed grassland(P>0.05).The spatial analysis in study area scale showed that the SOC in the 0?100cm soil layer appeared that of shrublands more than that of grassland-shrub edge and the less of grassland,and the SOC content vertically increased with the depth of the soil layer,and the variation coefficient is moderate variation(54.44%?78.82%).The A0 of SOC of the 0-100 cm soil layer is 4.11-56.50 km,and the MSH is 0.203?1.000.The spatial heterogeneity of SOC in the three treatments were increases,increases and decreases with the depth of the soil layer,respectively.The overall spatial heterogeneity of SOC of the conversion of grassland(0.836)to shrubland(0.871)decreased in study area scale.(4)In the grassland-grazing-shrub introduction process,STN in the 0?10 cm soil layer showed a gradual increase trend,ranging from 0.050 to 0.209 g kg-1,and in the 0?100cm soil layer STN was the same trend as SOC.The spatial analysis in study area scale showed that the vertical dynamics of STN in 0?100cm soil layer increased,and appeared that of grassland-shrub edge more than that of grassland and far more than that of shrubland,but the difference was not significant(P>0.05).The variation coefficient is 42.23%?55.42%,showing moderate variation.The A0 of STN range of the 0?100cm soil layer is 1.39?42.22 km,and the MSH is 0.320?1.000.The MSH of the three treatment were decreases,does not change and increases with the the depth of the soil layer,respectively.The overall spatial heterogeneity of 0-100 cm STN in the grassland(0.970)transformation into shrubland(0.864)increased.(5)The aboveground biomass of shrublands were much larger than that of grasslands in study area.The ecological thresholds of SW for aboveground biomass in the dry(July)and wet(October)seasons in the study area were[8.11%,0.07 kg·m-2]and[14.19%,0.06 kg·m-2],the SW threshold range is[8.11%?14.19%]. |
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