Spatial estimation of soil carbon and nitrogen in a grassland shrubland transition

Shrub encroachment via anthropogenic introduction has been used for ecological restoration in degraded desert grassland. However, there have been debates concerning whether this activity can be effective, as contrasting results were frequently reported. The objectives of this study were to address if various Caragana korshinskii planting conditions (planting year and density) had varying influences on soil organic carbon (SOC), soil total nitrogen (STN), and their spatial heterogeneities along the transition from desert grassland to shrubland. We investigated SOC and STN at the community and landscape scales, with the former containing eight land types of enclosed grassland, grazed grassland, and shrubland with varying levels of planting years (3, 12, or 22 years) and planting intervals (2, 6, or 40 m), and the latter consisting of 43 grassland-shrubland mosaic sites (desert grassland, grassland-shrubland edge, and shrubland) covering approximately 2,976 km2 area. At the community scale, the SOC and STN contents from 0 to 100 cm soil depth in shrubland, regardless of varying planting years and densities, were higher than those observed in grazed grassland. Though it appeared that short-term of shrub establishment (3 and 12 years) in shrubland outscored enclosed grassland in SOC accumulation, it was still questionable if this can be consistent for a longer time period as no significant difference was seen for SOC contents between enclosed grassland and shrubland after 22 years of shrub establishment. At the landscape scale, the SOC contents were higher in shrubland than those in desert grassland. However, no significant difference was observed for STN among shrubland and desert grassland. Regarding their spatial heterogeneities, SOC exhibited stronger spatial autocorrelation in grassland-shrubland edge than in desert grassland and shrubland, while STN did not evidently differ in spatial autocorrelation among these transition states. In brief, we have observed differences in soil carbon and nitrogen at community scales, but when crossing over to broad scales do not show the kind of significant spatial heterogeneity that we thought would be present in shrub introductions over the past 30 years. There may be a certain lag in the evolution of soil carbon and nitrogen responses from the introduction of shrubs from the community scale to the landscape scale.