|Jackson, Robert - DUKE UNIVERSITY|
|Gill, Richard - WASHINGTON STATE UNIV|
Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: May 15, 2008
Publication Date: September 7, 2008
Citation: Jin, V.L., Fay, P.A., Polley, H.W., Jackson, R.B., Gill, R.A. 2008. Soil-specific C and N responses to changing atmospheric CO2 concentrations in a mesic grassland ecosystem. In: Proceedings of the Ecological Society of America, August 3-8, 2008, Milwaukee, Wisconsin. 2008 CDROM. Technical Abstract: Long-term increases in ecosystem productivity under elevated atmospheric CO2 can be expected only when the increased assimilation of carbon (C) is not limited by soil nutrients, namely nitrogen (N). We examined how changes in atmospheric CO2 concentrations affect C and N dynamics in a mesic grassland ecosystem after one year of exposure to a gradient of CO2 ranging from pre-Industrial (285 µmol CO2 mol-1) to mid-21st century concentrations (480 µmol CO2 mol-1). Soil samples (0 to 10 cm) were collected during peak growing season (July 2007) from three contrasting soil series randomly distributed along the CO2 gradient. The soil series used represented three different soil orders: Austin (Mollisol), Bastrop (Alfisol), and Houston (Vertisol). Soil water content (SWC), soil C (total C, total organic C), soil N (total N, NH4+, NO3-), microbial biomass C and N (MBC, MBN), and gross N fluxes (production, consumption) were measured. All measured parameters were significantly affected by soil series (P < 0.05). Across all soils, atmospheric CO2 concentration was positively correlated to SWC and MBN, but negatively correlated with NO3- pool turnover. Changes in gross nitrification rate with CO2 were soil-specific, increasing in Austin and Houston soils, but decreasing in Bastrop soils. Gross NO3- consumption and net nitrification rates were mediated by the interaction between soil type and SWC. Gross NH4+ production (i.e., mineralization) and consumption and soil NH4+ turnover were controlled by interactions between soil type and MBN or soil total C. Net mineralization rates were weakly affected by the interaction between CO2 and the ratio of soil organic C to N (P = 0.0962). Our data indicate that short-term soil microbial responses to changing atmospheric CO2 are dependent on soil type. Nitrogen cycling has been altered directly by CO2, or indirectly via its affect on SWC and/or microbial biomass/activity. Carbon sequestration in surface soils has not changed within this short time frame. Grasslands comprise almost a quarter of the terrestrial biome, so elucidating the mechanisms underlying soil-specific C and N cycling responses to changing atmospheric CO2 is critical for predicting landscape responses to this global change driver.