Effects of Long-Term Cattle Grazing and Woody Plant Encroachment On Soil Microbial Communities At the Santa Rita Experimental Range, Arizona.

Livestock grazing is considered a key driver of woody plant encroachment in dryland ecosystems worldwide.  Woody plant establishment in these systems creates “islands of fertility,” in part by modifying erosional processes such that soil and plant litter are deposited beneath the shrub canopy, creating a nutrient rich soil/litter matrix that supports enhanced soil microbial biomass pools.  In this study, we utilized a long-term grazing exclosure (>80 yrs) at the Santa Rita Experimental Range south of Tucson, Arizona and phopsholipid fatty acid analysis to quantify livestock grazing effects on soil microbial communities associated with the complex vegetative mosaic that characterizes these dryland systems (i.e., bare soil, grass patches, and trees). Consistent with previous studies, we found that total microbial biomass increased from bare soil, to grass patches, and was greatest near the bole under large, live trees.  Ordination analysis of the most abundant fatty acids indicated that bare soil microbial communities differed from that of tree and grass-dominated areas, and these differences were more pronounced under large, live trees than small or dead trees. In the few instances where direct cattle grazing effects were observed, they occurred in soils associated with small, live trees.  The fungal biomarker 18:2n6 was the only fatty acid with a significant effect of grazing: long-term grazing resulted in lower relative abundances of this fungal biomarker across all vegetation types.  Our results suggest that direct effects of long-term grazing on soil microbial communities are less dramatic than the indirect effects of grazing.  Alterations in the abundance of vegetation types and soil redistribution that often accompany grazing and woody plant encroachment are important drivers of soil microbial communities in these dryland systems.