Contribution of recently fixed carbon to the efflux of carbon dioxide, methane and nitrous oxide in a sub-alpine grassland

The exchange of trace greenhouse gases (GHGs) across the atmosphere-biosphere interface can represent substantial components of site C and N budgets. Pathways for soil GHG production are primarily related to microbial activity and substrate supply proximal to microbial communities. Consequently, variation of GHG flux rates is underpinned by environmental factors which govern microbial metabolism and substrate availability or movement within the rhizosphere. Provision of labile, recently fixed carbon into the rhizosphere through root exudates is a dynamic process that may prime soil microbial activity, resulting in enhanced rates of GHG transfer and possible perturbations to the SOM pool. In this study we examined the contribution of recently fixed carbon to rates of CO2, CH4 and N2O flux utilising a unique, automated system to continuously sample experimental defoliation treatments in a sub-alpine grassland in NSW, Australia. Our objective was to determine what affect both immediate and gradual removal of above ground photosynthetic capability would have on GHG budgets in this system. Rates of CO2 were not affected by the elimination of labile carbon into the rhizosphere. Fastest mean rate of CO2 flux was measured in control plots (104 mg CO2 m-2 h-1) however this did not differ significantly from herbicide treated (99 mg CO2 m-2 h-1) and clipped (80 mg CO2 m-2 h-1) plots. Similarly, there was no treatment effect on rates of atmospheric methane oxidation, which ranged from -107 μg CH4 m-2 h-1 to -96 μg CH4 m-2 h-1. Fluxes of CO2 and CH4 varied significantly with treatment over time. Fluxes of N2O in sprayed plots (2.5 μg N2O m-2 h-1) were significantly higher than in control (1 μg N2O m-2 h-1) or clipped plots (-0.07 μg N2O m-2 h-1). Site NEP data from a local eddy covariance station on this site confirm that the lack of response of GHG fluxes to reduced photosynthetic inputs is likely due to a very low overall site productivity. Microbial biomass figures similarly suggest little perturbation to the soil microbial community from experimental above ground manipulations. These data contrasts in vivo CO2 studies from additional near-by sites which suggests a possible site nutrient limitation. Future studies are discussed.