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

Soil CO2 efflux ( Fsoil) 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 Fsoil 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 Fsoil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual Fsoil increased under elevated [ CO2] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [ CO2] reduced Fsoil 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 Fsoil 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 Fsoil, showing a decrease of ca. 114 g C m−2 yr−1 per 1 g m−2 increase in soil N availability, with consistently higher Fsoil in elevated [ CO2] plots ca. 127 g C per 100 ppm [ CO2] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [ CO2] and N fertilization on Fsoil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production. [ABSTRACT FROM AUTHOR]