Impact of a low level of CO2 enrichment on soil carbon and nitrogen pools and mineralization rates over ten years in a seasonally dry, grazed pasture

Abstract We have followed the responses of the properties of a sandy soil (0–50 mm), including carbon (C) and nitrogen (N) concentrations, to 10 years of ambient and elevated (475 μl l −1 ) CO 2 in a FACE (Free Air CO 2 Enrichment) experiment in a grazed pasture in North Island, New Zealand. We have previously reported results for the first 5 years where, at this relatively low level of CO 2 enrichment, most soil properties changed gradually and non-significantly. Here we add data from a further 5 years of enrichment and find that most soil properties had changed significantly. Soil moisture was greater under elevated CO 2 but we could find no evidence that this was the cause of changes in other properties, suggesting that increased inputs of C and N in the high CO 2 regime were driving soil differences. When soil total C and N were adjusted to account for differences in starting concentrations there was evidence of increased pool sizes and increased C/N under elevated CO 2 . Microbial C and N pools also increased with CO 2 as did heterotrophic respiration and labile C and N fractions. There were few changes in N mineralization but no evidence for reduced rates under elevated CO 2 . There was little evidence for a direct effect of changes in soil properties on plant biomass responses to CO 2 but the changes we observed relate to pools and fluxes that are known to be important in both production and environmental outcomes in grassland systems.