PS 40-114 - Soil type constrains grazing effects on microbial diversity and nitrogen cycling processes across the Great Plains

Abstract: Soils teem with life, most of it too small to see– these microorganisms release plant-available nutrients from soil organic matter (SOM), and can remove and retain soil carbon (C) and nitrogen (N) through their feeding, growth, and death. Like microorganisms, large ungulate grazers are an essential component of grassland ecosystem nutrient cycling, through their digestion of organic N in plant tissues and returning N to the soil in a more bioavailable form. Because many ecological studies on grassland nutrient cycling are conducted in the absence of grazers, there is a need to define the strength of grazing effects on N cycling across diverse grassland ecosystems. We collected soils from eight locations across the Flint Hills of KS and OK, and at five more sites in western KS, NE, and MN, USA, and measured SOM, microbial activity and diversity in actively and minimally grazed (bison and cattle) areas. SOM level, thus microbial activity, depends on soil mineral chemistry, so we expected soil type to predict C-cycling activities better than grazing status. In contrast, we expected grazing to consistently increase N availability and N-cycling rates. An index of soil chemistry, pH, predicted most of the seven assayed extracellular enzyme activities, except the indicator of microbial N limitation, which was reduced on average by grazing (ANOVA: F,P=5.7, 0.004), independent of the expected among-site variation (Site=8.3, < 0.001, Site*Graze=1.2, 0.28). This result reflects a broad tendency for grazing to increase available N, as expected. SOM varied widely among sites and soil orders and series, but there was no consistent effect, even in direction, of grazing on SOM concentrations (Site*Graze=6.1, < 0.0001). Variation in soil microbial composition was also explained primarily by site and soil type, but there was a small effect of grazing (PERMANOVA: R²= 0.334, 0.155, 0.105, 0.033 for soil unit, site, site*grazing, and grazing, respectively; all P< 0.01). Beyond the strong and expected soil effect on microbial biodiversity, only a few, but identifiable, microbial taxa respond consistently to grazing. The next challenge, important for both basic and applied understanding of grassland ecosystems, is to define whether these microbial populations are reliable indicators of increased N-cycling rates across a range of locations. Differences in soil chemistry have developed over millenia, but bison have been absent from North American grasslands for only hundreds of years: Studying grasslands, with grazers present, is critical for understanding and conserving grassland biodiversity and ecosystem function globally.