Root litter decomposition rates and impacts of drought are regulated by ecosystem legacy

Nicholas Glass, Eduardo Dias de Oliveira, Christopher J. Whelan and Miquel A. Gonzalez-Meler, Biological Sciences, University of Illinois at Chicago, Chicago, IL, Christopher J. Whelan, Cancer Physiology, Moffitt Cancer Center, Tampa, FL, Beth Drewniak and Roser Matamala, Argonne National Laboratory

Background/Question/Methods
Simulation of soil organic matter accrual (SOM) in Earth system models involves the estimation of root decomposition across many landscapes, including pastures, crop fields, and native ecosystems such as tallgrass prairie. However, many models assume 50% of root carbon is transferred into SOM with little empirical evidence. In fact, root decomposition involves complex interactions between climate and biogeochemical cycles, which vary by ecosystem legacy. To further investigate the impacts of ecosystem legacy on decomposition in soil, we measured root decomposition along with soil moisture and temperature across five land use types at USDA Forest Service Midewin National Tallgrass Prairie: 1) pasture, 2) row crop fields, 3) remnant prairie, 4) prairie restored from pasture, and 5) prairie restored from crop fields. Root bags, filled with a standard root material constituted by Panicum virgatum roots, were buried in the soil and collected monthly to determine the root weight lost/decomposition versus time. In addition, the onset of a long droughty period during the decomposition experiment allowed us to determine how the soil environment affected root decomposition rates across the land use treatments. We hypothesized that root decomposition rates in soil with a legacy of agriculture would be greater but less impacted by drought than those in other land use types.
Results/Conclusions
Soil moisture content and land use type had larger effects on root litter decomposition than soil temperature in our experiment. Drought effects on decomposition varied by current land use and by land use legacy. Decomposition rates in pastures were accelerated during drought but were negligible post-drought. Row crop field decomposition rates were consistently high until drought conditions occurred, during which decomposition immediately declined. Remnant prairies were not immediately affected by drought, but post-drought remnant prairies experienced nearly a 20% reduction in root litter dry weight, which represents the greatest increase in decomposition that we observed. For restored prairies, before the onset of drought decomposition rates were fairly steady and similar. During drought, the amount of root dry weight lost in prairie restored from row crop fields was 26% greater than that from prairie restored from pasture. However, post-drought decomposition was 17% less in prairie restored from row crop fields than in prairie restored from pasture. Our results indicate that ecosystem legacy exerts strong control over root decomposition for many years after land use has changed, and that drought effects on decomposition vary temporally and in magnitude according to site-specific legacies.