Effects of pasture management on soil fertility and microbial communities in the semi-arid grasslands of Inner Mongolia

Grasslands are an important ecosystem covering about a quarter of the earth's surface. Different management practices of grassland ecosystems can have a major impact on the sustainability of these ecosystems. The objective of this study was to determine the impact of different pasture management practices on soil properties and microbial communities in the semi-arid grassland ecosystem in Inner Mongolia. Long-term experimental plots were established in the semi-arid grasslands of Inner Mongolia to study the effect of different grazing practices on soil properties and microbial communities. The treatments included (1) enclosure from grazing since 1983 (E83), (2) enclosure from grazing since 1996 (E96), and (3) continuous free grazing (FG). We collected the soil samples from these treatments to study soil properties and microbial biomass abundance and diversity. An incubation study was also conducted using soils from E96 and FG treatments to determine the growth responses of ammonia oxidizers to urea addition. Soil organic matter and total N increased when the grassland was enclosed from grazing, but soil fertility did not increase further with continued enclosure extending from 1996 to 1983. Enclosure also increased microbial biomass but did not significantly affect the microbial diversity. Both ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) grew when supplied with urea-N, but the growth rate was higher in the soil from FG than in the soils from enclosed areas. The phospholipid fatty acids (PLFAs) of bacteria i15:0, 16:1 omega 7c, 16:1 omega 5c, 16:0, 18:1 omega 7c, and actinomycetes 10-Me-16:0 used the most of the C-13-urea in both the E83 and FG soils. There was higher incorporation of C-13 in PLFA 16:0 in the E83 soil after 3 and 7 days incubation, compared with the FG soil, suggesting higher metabolic activity in the E83 soil than the soil from the FG treatment. Most of the effects by the different pasture management practices were confined to the surface soil (0-20 cm), and there was minimal effect in the subsoils (below 20 cm). These results suggest that enclosure of grassland from grazing not only affects soil fertility but also microbial biomass and ammonia-oxidizing populations. Microbial communities are sensitive to pasture management changes, and these have implications to nutrient cycling and management in these grassland ecosystems.