Variation in small-scale spatial heterogeneity of soil properties and vegetation with different land use in semiarid grassland ecosystem

Soil properties (i.e. soil organic carbon, SOC; soil organic nitrogen, SON; and soil C/N ratio) and vegetation in a semiarid grassland of Inner Mongolia, northern China, were studied with the method of geostatistical analysis. We examined the spatial heterogeneity of soil and plants, and possible impacts of land use on their heterogeneity and on the relationship between soil resources and plant richness. Land use affected small scale spatial heterogeneity in plants and soil. SOC, SON and C/N ratio displayed autocorrelation over a range of ～2 m under most circumstances on sites where livestock grazing had been excluded. The uncontrolled grazing site (UG, i.e. unregulated grazing by excessive livestock) displayed an increased range of spatial autocorrelation and the total amount of variability in soil nitrogen over the other land use types. Plant life forms and plant species exhibited spatial autocorrelation over a range of about 2 m on the grazing exclusion (GE) and mowed (MW) sites, while pattern of spatial autocorrelation for several less common species on the UG site were difficult to predict. Plant species richness was positively related with spatial heterogeneity of SOC, SON and C/N on both GE and MW sites, and with only SOC heterogeneity on the UG site. These suggest that spatial soil heterogeneity plays an active role in maintaining plant species richness. However, we call for caution in generalization of the control of spatial soil heterogeneity over plant richness when multiple modes of disturbances are present, as we found in this study that higher total amount of variation in soil nitrogen and C/N ratio on the over-grazed UG site did not lead to increased plant species richness, and that land use had apparent effects on the patterns of spatial heterogeneity in both vegetation and soil.