| Although the researches about the soil nutrients, the immobilization of C and N in microbes and N mineralization have been investigated in many terrestrial environments, the similar research on the Tibetan Plateau is scarce. To better understand the dynamics of the soil nutrients and microbial biomass on the Tibetan plateau, we analyzed the effect of land-use management (natural grassland, abandoned old-field and crop field) on soil physiochemical properties and microbial biomass (SMB); investigated the effects of fencing and grazing on the microbial biomass carbon (Cmic) and nitrogen (Nmic), and mineralized N in soils at four sites with different grazing intensity:no-grazing (fencing), light-, moderate-and heavy-grazing of an alpine meadow on the northeastern Tibetan plateau. At the same time, we analyzed the distribution of Cmic and Nmic from sites at 5 different altitudes (3050, 3180,3570,3600,3910 m), the relationships among them and soil properties, and compared the net N mineralization rate both from indoor-and in-situ incubation. The results showed:1. The losses of SOC and TN were about 45% and 43%, respectively, due to cultivation after more than 50 years comparing with natural grassland. Because of the abandonment of cultivation for about a decade, SOC and TN were increased by 27% and 23%, respectively, in comparison with the crop field. Microbial carbon (ranging from 357.5 to 761.6 mg kg-1 soil) in the old-field was intermediate between the crop field and grassland. Microbial nitrogen (ranging from 29.9 to 106.7 mg kg-1 soil) and respiration (ranging from 60.4 to 96.4 mg CO2-Cg-1Cmic d-1) were not significantly lower in the old-field than those in the grassland. Thus it could be concluded that cultivation decreased the soil organic matter and SMB, while the adoption of abandonment has achieved some targets of grassland restoration in the alpine region of Gansu Province on the northeastern Tibetan plateau.2. The effect of grazing on soil nutrients was not statistically significant, while moderate grazing markedly stimulated the accumulation and immobilization of C and N in microbes. As a good measure of the efficiency of soil nutrients conversion into microbial C and N, Cmic/SOC and Nmic/TN were higher in the moderate-grazing site, which demonstrated that proper management of grassland promoted enrichment of the available soil C and N pool in the study area.3. As a measure of soil quality, N mineralization potential was higher in the fencing site and lower in the heavy-grazing site, showing the similar trend with SOC and TN, which demonstrated that the fencing is beneficial to the restoration and/or maintenance of soil quality, while the heavy grazing reduces soil quality.4. There was no significant difference for Cmic among different altitudes; but significant differences for Nmic, Cmic/SOC and Nmic/TN were detected among different altitudes, and the latter three had a significantly negative correlation with elevation, which demonstrated that the microbes fixed more and effective potential nutrition in the low altitude.5. There was a significantly positive correlation between soil pH and microbial biomass (p<0.01):The bacteria adapt to the neutral or slightly alkaline environment (pH in the low-altitude ranges from 6.7 to 7.1, the neutral soil), while the acidic soil environment of high-elevation (pH ranges from 5.2 to 5.7) maybe suppress the activity of bacteria-dominated microbial communities, which caused less C and N fixed by microbes in high-altitude. There was a negative correlation between soil moisture and microbial biomass (p>0.05), which was because too much water led to reduced oxygen, thus inhibited microbial activity in the soil.6. The net N mineralization from the indoor incubation was larger than that from the in-situ incubation, which indicated that there would be higher N mineralization rates under the conditions with appropriate temperature and moisture.
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