Soil aggregates organic carbon change and its influence in Tibetan alpine steppe

Abstract: The alpine steppes, mainly distributed in cold and drought terrains in the Northern Tibetan Plateau at over 4,500 meters above mean sea level, is a grassland ecosystem with the largest area, most important ecological state, and most characteristic soil-forming process. In recent decades, its natural degenerative process of the steppe is taking a turn for the worse due to desertification. Thus, it is of great significance to explore the changing process of soil aggregates organic carbon (SAOC) and its impacts on soil structure, and to seek methods for maintenance and stability of soil structure in the alpine steppe. Three alpine steppes were selected randomly as research areas in this study, and each research area was spaced at 50-100 kilometers, and included three sampling areas, namely, normal steppe (vegetation coverage of 45%-65% and light surface layer desertification), light degraded steppe (vegetation coverage>20%-<45%, and significant surface layer desertification), and severely degraded steppe (vegetation coverage<20% and severe surface layer desertification). Three sampling micro-areas were set randomly in each sampling area, and each micro-area contained three sampling points. The soil samples were collected separately by surface layer (0-10 cm) and the subsurface layer (10-20 cm) at each sampling point. Three samples at the same layers in the same micro-area made up one mixing soil sample (the undisturbed sample was hard to collect because the tested soil was sandy soil). The mass fraction of soil aggregates in a different particle size was tested by the wet screening; the soil organic carbon (SOC) and soil aggregates organic carbon (SAOC) were determined by the potassium dichromate volumetric method - external heating method. The results show as follows: 1) The mass fraction is not significant for SAOC in the same grain diameter in different soil layers of the normal steppe, and the mass fractions for SAOC in the same grain diameter for the degraded steppe present an obvious increase with soil layer deepening. Except light degraded steppe surface layer, the mass fraction for organic carbon of microaggregates (<0.25 mm) is greater than that of macroaggregates (>0.25 mm) in different soil layers in different states of steppes. 2) The SAOC change for alpine steppe is special, and mainly shows SAOC mass fractions go down at 0-10 cm and 10-20 cm soil layers in the degraded steppe, but its decreasing amplitude becomes low with the aggravation of the grassland degradation. 3) Compared with the light degraded steppe, the loss of macroaggregates organic carbon increases by 2.87 g/kg, and the loss of microaggregates organic carbon decreases by 2.90 g/kg in the severely degraded steppe surface layer; the losses of macroaggregates and microaggregates organic carbon reduce by 1.40 and 0.34 g/kg respectively in the subsurface layer. It is seen obviously that the soil anti-corrosion in the severely degraded steppe is lower than that of the light degraded steppe, and soil environment in a severely degraded steppe becomes more unstable because of the bigger loss of its macroaggregates organic carbon. 4) The SOC mass fraction is extremely significant (p<0.01) and has a positive correlation with that of SAOC in different particle sizes; soil moisture content shows significant (p<0.05) and has a negative correlation with the mass fraction of SAOC in different particle sizes; and the volume weight and temperature of soil have a very slight impact on SAOC.