Micro-scale spatial heterogeneity and the loss of carbon, nitrogen and phosphorus in degraded grassland in Ordos Plateau, northwestern China

The micro-scale spatial distribution and loss of carbon, nitrogen and phosphorus were examined in degraded grassland near Ordos, in the Mu Us Sand-land, northwestern China. Five communities that represented a series of successionally degraded stages in desertification were chosen for the work. The dominant plant of Community 1 was the steppe grass Stipa bungeana; Community 2 was dominated by a mix of S. bungeana and the shrub Artemisia ordosia; Community 3 was A. ordosia; Community 4 was a mix of A. ordosia and the desert grass Cynanchum komorovii; and Community 5 was C. komorovii. The soils in root-spheres and in the bare openings between plants in five successionally degraded plant communities were analyzed for total organic carbon (TOC), total nitrogen (TN), inorganic nitrogen ( IN), total phosphorus (TP), and available phosphorus (AP). The results showed that the heterogeneity process of the soil chemistry was characterized first by TOC heterogeneity and later by TN heterogeneity. The heterogeneity process of TP was only characterized in the community 3. No significant heterogeneity was present for AP in the five community stages. At the beginning of degradation, invasion by the shrub A. ordosia of S. bungeana grassland was found to lead to competition for soil elements between S. bungeana and A. ordosia and made the Community 2 soil environment temporarily homogeneous. In Community 3, however, the soil elements became spatially heterogeneous, and this led to the development of 'islands of fertility'. The concentrations of soil elements ( TOC, TN and IN) were greatest in the shrub root-spheres. With further desertification ( from Community 4 to Community 5), the islands of fertility began to collapse, and the concentrations of the main soil elements declined rapidly. The dynamics of soil phosphorus under progressive desertification were different from those of the other soil elements. TP decreased from form Community 1 to Community 5, while the AP concentration did not change in mid-level desertification ( Community 3), but increased with serious desertification (Community 5).