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The effect of grazing on the abundance, composition, and methane (CH4) uptake of methanotrophs in grasslands has been well documented in the past few decades, but the dominant communities of active methanotrophs responsible for CH4 oxidation activity in grazed soils are still poorly understood. In this study, we characterized the metabolically active, aerobic methanotrophs in grasslands with three different levels of grazing (light, medium, and heavy) by combining DNA-stable isotope probing (SIP) and quantitative PCR (qPCR) for methane monooxygenase (pmoA) gene– and 16S rRNA gene–based amplicon sequencing. The CH4 oxidation potential was as low as 0.51 μmol g dry weight−1 day−1 in the ungrazed control, while it decreased as grazing intensity increased in grazed fields, ranging from 2.25 μmol g dry weight−1 day−1 in light grazed fields to 1.59 in heavily grazed fields. Increased CH4 oxidation activity was paralleled by twofold increases in abundance of pmoA genes and relative abundance of methanotroph-affiliated 16S rRNA genes in the total microbial community in grazed soils. SIP and sequencing revealed that the genera Methylobacter and Methylosarcina (type I; Gammaproteobacteria) and Methylocystis (type II; Alphaproteobacteria) were active methanotrophs responsible for CH4 oxidation in grazed soils. Light and intermediate grazing stimulated the growth and activity of methanotrophs, while heavy grazing decreased the abundance and diversity of the active methanotrophs in the typical steppe. Redundancy and correlation analysis further indicated that the variation of bulk density and soil C and N induced by grazing determined the abundance, diversity of active methanotrophs, and methane oxidation activity in the long-term grazed grassland soil. [ABSTRACT FROM AUTHOR]

Background and aims: Different grazing management practices have a significant impact on the sustainability of grassland ecosystems. This study invested the vertical distribution of soil nitrogen (N) forms and soil microbial community structures in a semiarid grassland ecosystem under different grazing management practices in Inner Mongolia. Methods: Soil samples were collected from three semiarid grassland plots subjected to different long-term management practices namely, free grazing (FG) and two different periods of enclosure (E83, enclosed since 1983 and E97, enclosed since 1997). The soil organic nitrogen (N) pools were analyzed by classical methods, and the bacterial community was assessed by PCR-DGGE and high-throughput sequencing. Results: The surface soil N-supplying capacity was in the order of E97 ≥ E83 ≥ FG. The soil ammonium N, amino N, and N-supplying capacity were greater in the enclosed plots than in the FG plot. Additionally, the 0–40-cm soil layer showed the influence of different management practices on the soil properties. The structure and diversity of the soil microbial community also varied with the management type. The soil organic N composition was significantly related to the soil bacterial community structure and microbial categories. Conclusions: An appropriate number of years of fencing helps to improve the soil surface nutrient status, whereas overgrazing and prolonged enclosure are not conducive to the restoration of soil nutrients. Different grazing management practices can affect the microbial community structure and turnover of soil N in grasslands. [ABSTRACT FROM AUTHOR]

To investigate the effects of stocking rates on nitrification activity and active nitrifying communities in a typical steppe grazing system, we conducted a laboratory incubation study using soil from a 10-year-old grassland gradient grazing experiment with sheep. A combination of molecular methods, such as DNA-based stable-isotope probing (DNA-SIP), real-time quantitative PCR, and high-throughput sequencing, was used to identify changes of nitrification activity and active nitrifying communities under different stocking rates (0 (SR0), 3 (SR3), 6 (SR6), and 9 (SR9) sheep per ha). The nitrification activity of soils was significantly increased by light grazing (SR3), while it was significantly decreased by heavy grazing (SR9). Nitrososphaera viennensis lineage of ammonia-oxidizing archaea (AOA) functionally predominated over ammonia-oxidizing bacteria (AOB) in nitrification in the SR3 soil, while the Nitrosospira cluster 3 of AOB was the major player in the SR9 soil. Therefore, stocking rates altered the distribution of active nitrifying communities by affecting soil chemical and physical conditions. [ABSTRACT FROM AUTHOR]