Subhumid pasture soil microbial communities affected by presence of grazing, but not grazing management

Management-intensive rotational grazing has been shown to stimulate net primary production in subhumid pastures compared to extensive grazing strategies. The degree to which this represents a direct effect on plant developmental stage and community composition, and/or indirect effects of grazing on microbial community structure and nutrient feedback to production is an open question. To better understand whether the effects of pasture management on microbial communities led to indirect nutrient feedbacks, we analyzed microbial lipids and environmental and physical factors of surface soils under four pasture management treatments - management-intensive rotational grazing, continuous grazing, harvesting for hay, and an unmanaged control. Principal component analysis indicated microbial community structure under our two grazed treatments was different than that of ungrazed treatments. While there was no effect of treatments on total microbial biomass, guild comparisons revealed lower fungal-to-bacterial ratios, lower arbuscular mycorrhizal fungi concentrations, and higher actinomycete biomarkers in soils under grazing, irrespective of grazing management treatments. Gram-positive and anaerobic bacteria biomarker abundance was greater in treatments that included grazing and no management when compared to the treatment where biomass was harvested mechanically. There was no evidence of treatment effects on either Gram-negative bacteria or saprotrophic fungi. Regression tree analysis identified litter quality as the main correlate of differences in the microbial community between grazed and ungrazed treatments. Lower root C:N predicted lower F:B and biomarkers of the bacterial guilds Gm+ and actinomycetes, which were dominant in the grazing treatments, while higher root C:N predicted higher F:B and the guild biomarkers of AMF, which dominated in the ungrazed treatments. These results indicate that greater forage production observed in humid and subhumid regions under management-intensive rotational grazing likely stems from the direct effects on plant developmental stage and plant community composition rather than microbial mediated nutrient feedbacks from altered quantity and quality of plant litter. If the potential for a more favorable C balance in subhumid pastures is realized, greater biomass production and therefore greater litter inputs as a result of the direct effects of management-intensive rotational grazing on plant communities is the most likely mechanism. (c) 2012 Elsevier B.V. All rights reserved.