Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux

Soil CO2 efflux (F-soil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO2] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long-term effects of these factors on F-soil are less clear. Expanding on previous studies at the Duke Free-Air CO2 Enrichment (FACE) site, we quantified the effects of elevated [CO2] and N fertilization on F-soil using daily measurements from automated chambers over 10years. Consistent with previous results, compared to ambient unfertilized plots, annual F-soil increased under elevated [CO2] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO2] reduced F-soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment-induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual F-soil from potential values - estimated based on temperature alone assuming nonlimiting soil water content - by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO2]. Variability in soil N availability among plots accounted for the spatial variability in F-soil, showing a decrease of ca. 114gCm(-2)yr(-1) per 1gm(-2) increase in soil N availability, with consistently higher F-soil in elevated [CO2] plots ca. 127gC per 100ppm [CO2] over the +200ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO2] and N fertilization on F-soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.