Legacy and Belowground Resource Availability Effects on Plant-Soil Interactions in Tallgrass Prairies

In the U.S.A. Midwest, less than 10% of tallgrass prairies remain, and soils have lost 25‒40 million grams of carbon per hectare. Tallgrass prairie restorations can mitigate global climate change by regaining soil carbon, but it is unclear if legacy effects left by past land use impact rates of soil carbon accrual. Also, prairie plants' belowground responses to water and nutrient availability remain difficult to predict due to uncertainties surrounding root architectural and morphological associations to resource acquisition. Because root systems comprise ~66% of plant productivity in grasslands, understanding both legacy effects on soil and root responses to soil resources is vital for accurate predictions of soil carbon dynamics in tallgrass prairies. We assessed legacy effects on soil carbon and nitrogen accrual from 2008 to 2018 and root litter decomposition rates at Midewin National Tallgrass Prairie. Also, we measured Panicum virgatum root responses to partitioned water and nutrients in split-root laboratory experiments using root scans and traces. Because moderate drought occurred at Midewin in late 2020, we were able to determine drought impacts on decomposition rates in different land use histories. Tallgrass prairies restored directly from croplands accrued soil nitrogen more slowly than prairies restored from pastures largely due to reductions of soil bulk density in crop-restorations. However, soil carbon accrual rates were more similar. For Panicum virgatum, root length, surface area, and number of tips were progressively reduced as root (developmental) order increased in split-root compartments with water but no nutrients. Reduced development of lateral roots in the presence of water suggests that absorptive root proliferation is not necessary for optimal water uptake in tallgrass prairie grasses. Finally, root litter decomposition rates were impacted by land use history, but soil moisture and temperature were better predictors of decomposition rates in normal conditions. During drought, time since disturbance (grazing or row cropping) was most strongly correlated with decomposition rates. Our results show that legacies of croplands facilitate increased decomposition and soil carbon loss compared to pastoral land use history. Also, grasses' increased absorptive root development in response to nutrients facilitates increased root turnover, whereas water availability does not.