PARENT SESSION
Oral Session # 32: Herbivory II: Grasslands.
Presiding: K Suttle
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.

Consumer-decomposer interactions in a semi-arid grazing ecosystem.

Sankaran, Mahesh ^{1, 2}, Augustine, David³, ¹ NERC Centre for Population Biology, Ascot, Berkshire, UK² NREL, Colorado State University, Fort Collins, CO, USA³ Syracuse University, Syracuse, NY, USA

ABSTRACT- Ecosystem-level studies of producer-decomposer interactions have focused primarily on plant production and soil texture as regulators of decomposer abundance, but have rarely considered the role of grazers in mediating such interactions. Here, we conducted replicated exclosure experiments at both high and low levels of soil fertility to investigate the effects of large, mammalian grazers on decomposer biomass and activity patterns in a semi-arid grazing ecosystem in Kenya. Within only 2 years of grazer exclusion, microbial biomass increased in fenced grassland across all levels of soil fertility, indicating that grazers regulate decomposer populations in this system by depressing the quantity and/or quality of plant inputs to soils. However, across all study sites, microbial biomass was highly correlated with soil carbon content, suggesting that landscape-scale constraints on soil organic matter content are even more important than grazing in regulating microbial abundance. As soil organic carbon levels increased, both microbial biomass and the fraction of soil carbon accounted for by microbes increased, and this was associated with an increased efficiency of resource utilization by microbes and lowered carbon turnover rates in soils. Our results support previous ecosystem-level studies showing that microbial biomass and growth are constrained by plant production and soil C availability. In addition, our findings also demonstrate that decomposer abundance can be influenced by an ecosystem's trophic structure, with significant reductions in biomass occurring as a result of herbivores diverting plant carbon away from soils.

Key words: microbial biomass, soil organic carbon, grazing, ANPP