Current climate change models predict increasing drought conditions in the American Southwest and Central Plains regions for the 21st Century. The Extreme Drought in Grasslands Experiment (EDGE) is assessing the impacts of extreme rainfall reduction (66% of growing season precipitation) on key ecological processes such as plant productivity and biogeochemical cycles across six central US grasslands (New Mexico (2 sites), Colorado, Kansas (2 sites), and Wyoming). In this study, we determined the effect of chronic growing season drought on rates of litter decomposition at all six EDGE sites. To do so, we used the Tea Bag Index Method (TBI) as a method to compare rates of decomposition among sites and treatments. This method standardizes litter of known quality, allowing for a uniform cross-site comparison. Following the TBI protocols, five bags of Lipton green and rooibos tea were buried at 8-cm depth for 90 days in each of 10 ambient and 10 reduced rainfall plots in each experimental grassland. After sample collection, samples were weighed, and decomposition rates were calculated using the TBI standard protocol. Analysis of Variance with mixed effects models were used to test the effect of summer rainfall reduction on belowground decomposition rates across and within experimental sites.
We found a negative effect of drought (event size reduction) in the tea bag decomposition rates at the Colorado Short Grass Steppe (SGS) and Konza tallgrass prairie (KNZ) in Kansas. In SGS, day 90 mean decomposition under drought conditions was significantly lower than ambient conditions (Ambient, M=0.0053, SD=0.0011); (Drought, M=0.004, SD=0.0013). Similarly, KNZ experienced less decomposition under drought treatment than ambient (Ambient: M=0.014, SD=0.007; Drought: M=0.0083, SD=0.004). Decomposition in ambient treatments between the more arid SGS and mesic KNZ sites was significantly different. A ratio of chronic drought and ambient conditions between KNZ and SGS indicates that KNZ is most sensitive (proportionately lower rate of decomposition) to drought. Results suggest that decreasing precipitation during the growing season leads to a decrease in litter decomposition. This may be due to a reduction in soil moisture, soil litter mixing, and responses of saprotrophic communities. However, further analysis will test how summer precipitation and/or soil moisture regulate the effects of chronic drought on the decomposition rates within and across sites. It may well be that future drought conditions could lead to a reduction of the soil organic matter pool, creating nutrient limitations which may impact the productivity of North American grassland communities.