Topographic and ungulate regulation of soil C turnover in a temperate grassland ecosystem

A major obstacle for predicting the effects of climate and land use changes on global soil carbon (C) stores is the very limited knowledge about the long timescale dynamics of the relatively stable fraction of soil C, which represents the bulk of soil C and the primary determinant of the long-term C balance of terrestrial ecosystems. In this study, we examined how variable topo-edaphic conditions and herds of native migratory ungulates influenced turnover of the stable pool (total minus active fraction) of soil C in grasslands of Yellowstone National Park (YNP). Soil C properties were determined for grasslands located inside and outside long-term ungulate exclosures established 1958-1962 at seven variable topographic positions. Active C pool sizes, estimated with soil laboratory incubations, and soil radiocarbon measures were used to parameterize a process-based model to determine turnover of the stable C pool at the sites. Stable C turnover ranged 37-653 and 89-869 years for 0-10 and 0-20 cm soils, respectively. Among ungrazed communities, there was a trend for stable soil C turnover to slow along topographic gradients of increasing soil moisture, soil C content, and shoot biomass from hilltop to slope-bottom positions. This was likely a result of an increasing amount of support tissue resulting in greater concentrations of lignin and cellulose as shoot biomass increased down slope. In contrast, across the grazed landscape, stable C turnover sped up from hilltop to slope-bottom positions, which was likely a consequence of grazer effects on plant species composition along the topographic gradient. These findings indicated that despite topography playing the primary role in controlling such important site characteristics as soil moisture, soil C content, and plant production in YNP grassland, the long-term turnover of the stable C pool was determined by herbivores. The results demonstrate the important regulatory role of herbivores in controlling the C balance of this semiarid grassland ecosystem.