Carbon input control over soil organic matter dynamics in a temperate grassland exposed to elevated CO2 and warming

Elevated CO2 generally increases soil C pools. However, greater available C concentrations can potentially stimulate soil organic matter (SOM) decomposition. The effects of climate warming on C storage can also be positive or negative. There is a high 5 degree of uncertainty on the combined effects of climate warming and atmospheric CO2 increase on SOM dynamics and its potential feedbacks to climate change. Semiarid systems are predicted to show strong ecosystem responses to both factors. Global change factors can have contrasting effects for different SOM pools, thus, to understand the mechanisms underlying the combined effects of multiple factors on soil C storage, 10 effects on individual C pools and their kinetics should be evaluated. We assessed SOM dynamics by conducting long-term laboratory incubations of soils from PHACE (Prairie Heating and CO2 Enrichment experiment), an elevated CO2 and warming field experiment in semi-arid, native northern mixed grass prairie, Wyoming, USA. We measured total C mineralization and estimated the size of the labile pool and the decomposition 15 rates of the labile and resistant SOM pools. To examine the role of plant inputs on SOM dynamics we measured aboveground biomass, root biomass, and soil dissolved organic C (DOC). Greater aboveground productivity under elevated CO2 translated into enlarged pools of readily available C (measured as total mineralized C, labile C pool and DOC). The effects of warming on the labile C only occurred in the first year 20 of warming suggesting a transient effect of the microbial response to increased temperature. Experimental climate change affected the intrinsic decomposability of both the labile and resistant C pools. Positive relationships of the rate of decomposition of the resistant C with aboveground and belowground biomass and dissolved organic C suggested that plant inputs mediated the response by enhancing the degradability of 25 the resistant C. Our results contribute to a growing body of literature suggesting that priming is a ubiquitous phenomenon that should be included in C cycle models.