Liu, T., Nan, Z. and Hou, F. 2011. Culturable autotrophic ammonia-oxidizing bacteria population and nitrification potential in a sheep grazing intensity gradient in a grassland on the Loess Plateau of Northwest China. Can. J. Soil Sci. 91: 925-934. Grazing is known to enhance the activity of soil microbial communities in many types of grasslands; however, the potential impacts of rotational grazing activity on soil microbial functional groups remain poorly understood. We investigated the effects of 9 yr of rotational grazing by livestock on culturable autotrophic ammonia-oxidizing bacteria (AOB) population size, nitrification potential and soil properties in a semi-arid grassland of the Loess Plateau in Northwest China. Three stocking rate treatments of 2.7, 5.3 and 8.7 wether lambs ha-1 were evaluated in geographically separated paddocks. Grazing increased nitrification potential and culturable AOB populations compared with ungrazed treatments. Ammonia-oxidizing bacteria populations increased from 155 bacteria g-1 dry soil with 0 sheep ha-1 to 16 218 bacteria g-1 dry soil with 8.7 sheep ha-1. Grazing led to an increase in population of AOB at 0-10 cm soil depth, but had no effect on AOB at 10-20 cm soil depth. Nitrification potential increased from 1.21 mg NO3-N kg-1 soil d-1 in ungrazed treatments to 2.86 mg NO3-N kg-1 soil d-1 at the highest stocking rate. Soil ammonium and nitrate concentrations increased; however, total soil nitrogen and soil moisture content decreased with increased stocking rate for both sampling depths (0-10 cm and 10-20 cm). Soil organic matter was not affected by grazing treatments. Soil nitrification potential and the size of culturable AOB populations were dependent on grazing intensity, soil depth and season. This information is potentially important for the optimal selection of stocking rate for grazed ecosystems.
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