Evaluation of effects of sustained decadal precipitation manipulations on soil carbon stocks

Throughout a 13 year period, the Throughfall Displacement Experiment sustained both increased (+33; wet) and decreased (-33%; dry) throughfall into an upland oak forest in Tennessee. Organic (O) horizon carbon (C) stocks were measured at several occasions before, during and after the experiment and mineral soil C stocks before and after the experiment. In the O horizon, higher C stocks were observed in the dry treatment compared to the ambient and wet, attributable to a combination of enhanced litter inputs and reduced decomposition. No precipitation treatment effects on mineral soil C stocks were found to a depth of 60 cm. Conversely, long-term reductions in surface mineral soil C stocks were surprisingly high for all treatments (3.5-2.7% C in the 0-15 cm layer and from 0.6 to 0.5% in the 15-30 cm layer) over the duration of the experiment. A clear explanation for this temporal trend in C storage was not readily apparent.