Precipitation Induced Changes in Decomposition Processes and Soil Carbon Stabilization

Climate change is expected to alter the amount and timing of precipitation (PPT) in semiarid ecosystems and ultimately soil carbon (C) storage, which is determined by the C input of plant production and C output through decomposition. The net effect of changes in productivity and litter decomposition on soil C stabilization following prolonged changes in PPT are unclear. It is also uncertain how litter quality and changes in soil moisture will interact to alter litter decomposition processes. In order to make more accurate predictions of how changes in PPT may alter ecosystem C dynamics, we need to quantify how long-term shifts in PPT impact soil C contents and improve our understanding of what mechanistically drives changes in decomposition under PPT shifts. My aims were to investigate: (1) how changes in the amount and timing of PPT affect C stabilization in semiarid ecosystems and how they are mediated by vegetation type, (2) the importance of direct (microbial decomposition) and indirect (litter quality) effects of PPT shifts on aboveground litter decomposition, and (3) how litter quality and soil moisture interact at a mechanistic level to affect litter and soil organic C (SOC) decomposition. To quantify changes in soil C stabilization and vegetation mediation thereof under long-term PPT shifts, soils were collected from a long-term PPT experiment, then fractionated and analyzed for SOC contents. PPT treatments delivered during the previous 18 years consisted of three regimes: ambient (no additional PPT); summer (200 mm irrigation added during the growing season); and winter (200 mm irrigation added during the cold dormant season). To assess the importance of direct versus indirect effects of PPT on aboveground decomposition, we conducted a year-long litterbag study at the long-term PPT manipulation experimental field site. To assess the interaction between litter quality and soil moisture and its impact on decomposition, we conducted an incubation of three litter types and five soil moisture treatments. The three litter types (basin big sagebrush, Artemisia tridentate subsp. tridentata; crested wheatgrass, agropyron cristatum; Utah milkvetch, Hedysarum boreale) had average δ13C values of -27.47±0.18‰ and were decomposed in soil with a δ13C value of -16.01±0.01‰, which allowed us to partition respired C into litter-derived or SOC-derived. The data from the long-term PPT manipulation experiment indicate that long-term increases in PPT amount can promote soil C stabilization rates in semiarid ecosystems if the increases happen during times of the year when plant C inputs rates exceed soil C decomposition rates. The data from the incubation experiment indicate that high soil moisture levels may override any litter quality effects on decomposition that promote different decomposition and nitrogen (N) mineralization rates at low soil moisture levels. Overall, we found that changes in timing of PPT matter for aboveground litter decomposition and soil C storage and that increases in soil moisture do not affect litter decomposition equally across species. Future research should focus on quantifying impacts of shifts in the timing of PPT on ecosystem function and consider the limitations of litter quality at high soil moisture levels for predicting soil C respiration