Study On The Mineralization Of Soil Organic Matter And Its Microbial Mechanism Under Different Use Patterns In Inner Mongolia

In the natural grassland, soil organic matter ?SOM? mineralization is the key pathway for providing available nutrients to plants. Mowing, enclosure and grazing are three common grassland utilization and management practices, which might have impacts on SOM mineralization. To date, scholars have been mainly focused on studying the effects of different utilization and management practices on plant species composition, aboveground biomass and soil nutrients, while few reports are available on how these different management prctices impact on SOM mineralization. SOM mineralization is driven by microbes, and the complex fraction and structure of SOM probably lead to different microbial mechanisms of its mineralization. It is critically important to deeply understand the processes of SOM mineralization by coupling SOM structure with microbial community under different grassland utilization and management practices. Based on two long-term field trials ?mowing trial with different mowing frequencies and enclosure trail with different enclosure periods? in Xilinhot region in Inner Mongolia, and combined incubation experiments, this study was aimed at exploring the relationships among SOM mineralization, microbial communities and SOM functional groups in a semiarid grassland and clarifying the impacts of temperature and moisture on grassland SOM mineralization by applying advanced techniques of solid nuclear magnetic resonance ?NMR?, fluorescent quantitation PCR?qPCR? and high-throughput sequencing. The primary foundlings were summarized as follows:?1? In the 13-year mowing trial, soil basic properties, net soil organic nitrogen mineralization rate?R_m?, SOM functional groups, microbial biomass carbon ?MBC? and bacterial and fungal community abundance and diversity were measured and analyzed.The results showed that soil pH, moisture and available phosphorus ?AP? content ranged from 7.24 to 7.49, from 7.0% to 8.5% and from 1.0 mg kg^-1 to 1.3 mg kg^-1,respectively, and there were no significant differences among different mowing treatments ?P > 0.05?. The SOM content in the mowing once a year treatment ?M1? was significantly higher than that in the Control ?CK, unmown? and mowing twice a year treatment ?M2?. Meanwhile, R_m, MBC content, Alkyl C/O-alkyl C ?A/OA? and fungal community abundance and diversity in M1 were significantly higher than those in M2 or CK while these indexes in M2 were significantly lower than those in CK ?P < 0.05?.A positive linear correlation was observed between R_m and fungal community abundance ?r = 0.688, P = 0.005?. Ml significantly improved the relative abundance of Mortierella, a fungal specie characterized by degrading lignin and cellulose.However, M2 inhibited degrading fungus and significantly enhanced the relative abundance of some pathogenic fungus.?2? In both long-term field trials, the seasonal dynamics of SOM mineralization and bacterial,fungal and archaea community were studied and the results showed that in the mowing trial, the soil R_m, and bacterial and fungal abundance in M1 were significantly higher than those in CK while these indices in M2 were significantly lower than that in CK ?P < 0.05?. Bacterial and fungal abundance increased gradually from May to Oct., which might resulte in the increase of MBC and R_m. However, archaea abundance in Oct. was significantly lower than that in May and Aug. ?P < 0.05?. In the enclosure trial, R_m and different microbial community abundance in the soil with enclosure from 1997 ?E97? were similar to those in the soil with enclosure from 1983?E83? ?P > 0.05? while they were significantly higher than those in the overgrazing ?G?treatment and significantly lower than those in Ml ?P < 0.05?.?3? According to the results of the two long-term field trials, the treatments of enclosure from 1983 ?E83,E?,moderate mowing ?mowing once a year,M? and overgrazing ?G? were chosen to systematically study the correlation among R_m, SOM functional groups and bacterial and fungal abundance and diversity under different grassland utilization and management practices. The results showed that the contents of all soil nutrients ?except for rapidly available potassium,RK?,microbial indices and A/OA ?Alkyl C/O-alkyl C, an index measured by NMR and indicating the degrading degree of SOM?, ranged M > E > G and the differences were significant ?P < 0.05?.Soil pH in G was significantly higher than that in E and M, but soil pH in all treatments were neutral. Soil RK content in E was significantly higher than that in G and M, and M was significantly higher than G. However, soil RK was rich in the study site and its content was above 210 mg kg^-1 in all treatments. Multivariate regression tree conducted by R using the data of microbial abundance and diversity and SOM functional group content showed that SOM functional groups had little influence on bacterial community composition while Aryl C and Carbonyl C explained 51.1% and 25.1% of total microbial community composition variance, respectively.?4? In the incubation experiment, the temperature and precipitation of different months in Inner Mongolia were simulated to study the response of SOM functional groups and microbial community to temperature and moisture after the soil experienced freezing-thawing-drying-rewetting,and the results showed that when the soil was thawed after the freezing at low temperature ?-20?, -15??, different freezing temperatures had little impacts on R_m ?P > 0.05?. However, compared with the soil before incubation, mineral nitrogen content in soils with the treatments VT1 which experienced different freezing and thawing temperatures, and VT2 which experienced a sudden thawing at a relative high temperature ?15?? after a lower freezing temperature ?-20?? significantly increased. After freezing-thawing and drying-rewetting, bacterial community abundance significantly decreased by 32.8% and 49.7%while fungal community abundance significantly increased by 6.8% and 33.3%,respectively ?P < 0.05?. After freezing-thawing-drying-rewetting, Substituted alkyl C content in soil with VT1 treatment was significantly lower than that in the soils with constant temperature and moisture ?P < 0.05?. In addition, Carbonyls and Aromatics were also decreased after drying-rewetting.