The fate of soil carbon in drylands after the addition of multiple levels of water and nitrogen

Christopher W. Beltz, William K. Lauenroth and Ingrid C. Burke, School of the Environment, Yale University, New Haven, CT, William K. Lauenroth, Department of Botany, University of Wyoming, Laramie, WY

Background/Question/Methods
Drylands, including both semi-arid and arid ecosystems, encompass 40% of the global terrestrial land cover and store 20% of global soil carbon. Global drylands are expected to come under multiple stresses due to global change, including changes to water and nitrogen availability. These changes are likely to result in the alteration of net primary productivity (NPP) and soil respiration, potentially altering soil carbon stocks and the emission of carbon dioxide into the atmosphere. We used water and nitrogen additions to assess their potential impact on soil carbon in three different major dryland ecosystem types in the western United States: shortgrass steppe, mixed-grass prairie, and sagebrush steppe (n=7 within each ecosystem type). We implemented a randomized block design which incorporated water additions (0 or +20% mean monthly precipitation) and N additions (0, 10, and 100 kg N ha-1 yr-1). Soil carbon was estimated in multiple pools. We hypothesized that soil carbon in the pools with the fastest turnover would be most affected and would decrease in soil carbon availability as additions increased. In addition, we expected that N additions alone were unlikely to change soil carbon due to the primary limitation of water in these dryland systems.
Results/Conclusions
Our treatments significantly increased the availability of water and nitrogen for the grassland ecosystems. However, only nitrogen availability was affected by treatments within the sagebrush steppe; water availability was unchanged due to above average precipitation during the study. Of note, the substantial majority of carbon in these systems is organic; across all sites soil inorganic carbon ranged from 0.08-0.10%, with mean total carbon in untreated areas of 819-1827 g C/m2 depending on site and depth. Despite significant changes to the major carbon fluxes of productivity and respiration, soil carbon pools did not respond significantly to our treatments. However, trends suggest a greater response to nitrogen than expected within all ecosystem types regardless of water addition. Alternatively, this lack of change in soil carbon may suggest a lag in change to soil carbon pools or that there are unseen tradeoffs resulting in no actual alteration. Therefore, water and nitrogen addition seem to influence productivity and respiration as expected, but does not greatly impact soil carbon – at least over the two years of this study. This work is ongoing to investigate the response of soil carbon within different dryland ecosystem types to altered water and nitrogen availability.