Destabilization of SOM Following Overgrazing and Aggregate Deterioration in a Steppe Ecosystem in Inner Mongolia (China).
Markus Steffens, Angelika Koelbl, and Ingrid Koegel-Knabner. Lehrstuhl für Bodenkunde TU Muenchen, Am Hochanger 2, Freising, Germany
Intensive land use and especially overgrazing in semi-arid grasslands result in degradation of steppe vegetation associated with effects on the amount, composition, and turnover of Soil Organic matter (SOM). The concurrent degradation of soil structure and destruction of aggregation leads to enhanced soil erosion and dust storms. Our study focusses on the effect of overgrazing on the destabilization and changing composition of functional SOM pools and the potential of regeneration through improved grazing management. We sampled five sites with long-term different grazing intensities in calcic chernozem steppe soils of Inner Mongolia, each with one statistical grid and three representative soil pits. Each point of the statistical grid was analyzed for bulk density, C-, N- and S-concentration, pH-value, d13C and thickness of the humic horizon. The spatial structure of these parameters was analyzed using statistical and geostatistical tools as analyses of variance, theoretical and experimental variograms and special kriging methods. Four horizons were sampled in each pit and sieved in different aggregate size classes. A sonication method was used as indicator for aggregate stability. Bulk soil organic matter was separated into distinct fractions by density and particle size fractionation. CN measurements, solid-state 13C NMR spectroscopy and radiocarbon dating were used for the analyses of the chemical quantity and quality of these soil organic matter fractions. The statistical and geostatistical analyses of the grid-data show significant differences between the five grazing intensities concerning total organic carbon, total nitrogen, and bulk density. While low grazing intensities correlate with high OC and N concentrations and low bulk densities, high grazing pressure is associated with low OC and N concentrations and high bulk densities. The C and N stocks of the topmost 4 cm as calculated using an equivalent mass show a loss of C and N as a result of reduced organic matter input in the overgrazed plots. Analyses of the pit samples emphasize these findings. The tests for aggregate stability show differences between the different grazing intensities. Plots with high grazing intensities have less structured soils with smaller and less stable aggregates than ungrazed areas. Density and particle size fractionation reveal that ungrazed plots contain more occluded and free particulate organic matter (POM) than grazed plots. Furthermore solid-state 13C NMR spectroscopy of bulk soils show that plots with higher grazing intensities are characterized by higher proportions of Carbonyl C and Aryl C and ungrazed plots by more O/N Alkyl C and Alkyl C. This indicates differences in litter composition and decomposition depending on the grazing intensities and lead to the assumption that ungrazed areas contain less decomposed organic C probably deriving from grass residues. These results show that high grazing intensities reduce the input of organic matter to the soils and cause loss of C and N stocks and decline in soil aggregation. Improved grazing management might be a method to stop and in longer time scales invert these processes, and at the same time offer the possibility of an increased carbon sequestration.