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

Soil CO₂efflux (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 Fₛₒᵢₗare less clear. Expanding on previous studies at the Duke Free‐Air CO₂Enrichment (FACE) site, we quantified the effects of elevated [CO₂] and N fertilization on Fₛₒᵢₗusing daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual Fₛₒᵢₗincreased under elevated [CO₂] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO₂] reduced Fₛₒᵢₗto 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 Fₛₒᵢₗfrom 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⁻¹per 1 g m⁻²increase in soil N availability, with consistently higher Fₛₒᵢₗin 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 Fₛₒᵢₗin this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.