Climate controls on soil respired CO2 in the United States: Implications for 21st century chemical weathering rates in temperate and arid ecosystems

The most recent IPCC (2007) report predicts that regional-scale precipitation patterns will change significantly in the 21st century as a result of increasing atmospheric CO2. The amount of respired CO2 stored in the soil atmosphere is heavily influenced by climate, and the concentration of CO2 in soils controls the concentration of dissolved CO2 ([CO2aq]) in pore water involved in chemical weathering. Therefore, we can expect changes to chemical weathering rates as a result of climate change, which can influence the global carbon cycle through the consumption of CO2 during continental silicate weathering. These changes may even be important on human timescales. To predict changes to the [CO2aq] of soil water, we have produced an extensive literature review of soil respired CO2 measurements to study the spatial variability of soil respired CO2. We show that respired CO2 concentrations and [CO2aq] vary with precipitation. This study focuses on the western United States, where we find a strong relationship between summer average soil-respired CO2 and mean annual precipitation for soils forming in or below 900 mm yr(-1) precipitation (R-2 = 0.91). The correlation breaks down when higher mean annual precipitation rates are considered, restricting the use of this relationship to arid to subhumid precipitation regimes. We estimate the response of [CO2aq] in soil pore water in the western United States to projected anthropogenic CO2 emissions using this new relationship and projected changes in precipitation simulated by the North American Regional Climate Change Assessment Program for the decade 2051-2060. According to our model, [CO2aq] in soil pore water is expected to decrease by up to 50% in the southwestern United States and to increase by up to 50% in areas of the Northern and Central Great Plains. These results have important implications for CO2 consumption and changes to terrestrial carbon cycling in the next century. (C) 2013 Elsevier B. V. All rights reserved.