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Title: Carbon Input Control Over Soil Organic Matter Dynamics in a Temperate Grassland Exposed to Elevated CO2 and Warming

Author
item CARRILLO, YOLIMA - University Of Wyoming
item PENDALL, ELISE - University Of Wyoming
item Dijkstra, Feike
item Morgan, Jack
item NEWCOMB, JOANNE - University Of Wyoming

Submitted to: Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2010
Publication Date: 9/30/2010
Citation: Carillo, Y., Pendall, E., Dijkstra, F.A., Morgan, J.A., Newcomb, J.M. 2010. Carbon Input Control Over Soil Organic Matter Dynamics in a Temperate Grassland Exposed to Elevated CO2 and Warming. Biogeosciences. 7:1575-1602.

Interpretive Summary: The global atmospheric CO2 concentration and temperature are increasing due to anthropogenic greenhouse gas emissions. Both climate change factors can have large impacts on C storage in rangelands. Most of the C in rangelands resides in soil, and even small changes in soil C storage could lead to a marked change in the atmospheric CO2 concentration. Yet, there is a high uncertainty how combined effects of climate warming and atmospheric CO2 increase will affect C storage in soils. Here we studied the effect of elevated CO2 concentration (from 390 to 600 ppm) and warming (1.5/3° C above ambient temperature during the day/night) in a semi-arid grassland in Wyoming. Soils were taken after 2 and 3 years of elevated CO2 and after 1 and 2 years of warming. Labile C pools (with fast turnover) and decomposition rates of resistant C pools (with slow turnover) were determined in laboratory incubations. Labile C increased with elevated CO2, but only during the first year of warming, indicating that the warming effect on labile C was transient. No effects of elevated CO2 and warming on resistant C decomposition rates were observed. However, a positive relationship was found between the rate of decomposition of the resistant C and plant biomass suggesting that loss of resistant soil C through decomposition (the largest C pool fraction in the soil) can be enhanced with greater plant inputs. Thus, these results show that increased plant C inputs into the soil, caused by elevated CO2 and warming, do not necessarily result in increased soil C storage. Results from this study will help to predict long-term soil C storage with computer simulation models.

Technical Abstract: 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 degree of uncertainty on the combined effects of climate warming and atmospheric CO2 increase on SOM dynamics and its potential feedbacks to climate change. Semi-arid 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, 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 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 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 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.