Soil-specific C and N responses to changing atmospheric CO2 concentrations in a mesic grassland ecosystem

Background/Question/Methods 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-21^st 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.
Results/Conclusions

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.