Net mineralization of N at deeper soil depths as a potential mechanism for sustained forest production under elevated [CO2]

Elevated atmospheric carbon dioxide concentrations [CO2] is projected to increase forest production, which could increase ecosystem carbon (C) storage. This study contributes to our broad goal of understanding the causes and consequences of increased fine-root production and mortality under elevated [CO2] by examining potential gross nitrogen (N) cycling rates throughout the soil profile. Our study was conducted in a CO2-enriched sweetgum (Liquidambar styraciflua L.) plantation in Oak Ridge, TN, USA. We used N isotope pool dilution methodology to measure potential gross N cycling rates in laboratory incubations of soil from four depth increments to 60 cm. Our objectives were twofold: (1) to determine whether N is available for root acquisition in deeper soil and (2) to determine whether elevated [CO2], which has increased inputs of labile C resulting from greater fine-root mortality at depth, has altered N cycling rates. Although gross N fluxes declined with soil depth, we found that N is potentially available for roots to access, especially below 15 cm depth where rates of microbial consumption of mineral N were reduced relative to production. Overall, up to 60% of potential gross N mineralization and 100% of potential net N mineralization occurred below 15 cm depth at this site. This finding was supported by in situ measurements from ion-exchange resins, where total inorganic N availability at 55 cm depth was equal to or greater than N availability at 15 cm depth. While it is likely that trees grown under elevated [CO2] are accessing a larger pool of inorganic N by mining deeper soil, we found no effect of elevated [CO2] on potential gross or net N cycling rates. Thus, increased root exploration of the soil volume under elevated [CO2] may be more important than changes in potential gross N cycling rates in sustaining forest responses to rising atmospheric CO2.