Grassland degradation typically characterized by the invasion of shrubs and semi-shrubs is a critical issue in arid and semi-arid desert regions throughout the world, which was broadly believed to eventually result in vegetation cover loss and the induced severe change of ecohydrological processes. The aim of this paper was to understand the response of runoff generation and the induced nutrient loss to shrub encroachment in desert grassland in arid regions since they are much closely related in drylands.
We took the grasslands and shrublands in the southeast fringe of Tengger Desert, north China, for the in situ case study. Simulated rainfall experiments were conducted on grassland plots (Grassland treatment, GT), intershrub bare soil plots (Bare soil treatment, BT) as well as the plots consisting of both bare soil patch and shrub patch (Shrub treatment, ST) in shrubland. The time to pond and to generate runoff, and the volume of rainfall to start runoff, the time series of runoff and nitrogen concentration, runoff coefficients, total nitrogen loss, and average concentration of total nitrogen were investigated simultaneously. By using these data, we compared soil water infiltration, runoff generation and nitrogen production from different habitats and elucidated the influences of the replacement of grassland by shrubland on the hydrological processes and the induced biogeochemistry cycling.
Results showed that: the time to pond and to generated runoff and the amount of rainfall to start runoff for GT were slightly greater than those from ST, but significantly greater than those from intershrub bare soil plots, indicating the higher soil water infiltration rates in grassland than in shrubland. Weighted runoff coefficient in shrubland habitats was 34.46%, which was significantly less than that in intershrub plots, slightly larger than that in ST, and it was 2.26 times greater than that in GT. Time series concentration of nitrogen in ST was greater than that in intershrub plots but less than that in GT. For all the samples collected from each treatment, negative linear correlations were found between the nitrogen concentration and the amount of runoff discharge. Total nitrogen loss from shrubland habitat was 0.23 g/m², which was slightly smaller than that from intershrub plots but remarkably greater than that from shrub plots, and it was 2.09 times greater than the nitrogen loss from grassland; the volume-weighted mean concentration of nitrogen in the runoff from shrub plots, intershrub plots and shrubland habitat were 0.009, 0.007 and 0.011 g/L, respectively, they were less than the corresponding value in grassland (i.e., 0.012 g/L), all which confirmed the increased nitrogen loss induced by the vegetation replacement. The results indicate that vegetation cover decline/loss derived from the encroachment of woody plants in original grassland inevitably leads to the increase of runoff water and nutrients losses. If the general trend continued without interference, rainwater, soil material and nutrients would no longer be efficiently captured and stored within the ecosystem, then the landscape would become a degraded non-conservable system that can be termed dysfunction, these processes would expectably lead to continuous desertification.