title: Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux.
abstract: 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.
authors.affiliation: Division of Environmental Science & Policy, Nicholas School of the Environment, Duke University, Durham, 27708-0328, NC, USA; Coweeta Hyrdologic Laboratory, Southern Research Station, USDA Forest Service, 3160 Coweeta Lab Road, Otto, 28763, NC, USA.
authors.forename: A Christopher
authors.initials: AC
authors.lastname: Oishi
authors.forename: Sari
authors.initials: S
authors.lastname: Palmroth
authors.forename: Kurt H
authors.initials: KH
authors.lastname: Johnsen
authors.forename: Heather R
authors.initials: HR
authors.lastname: McCarthy
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
mesh_terms: D002245:Carbon Dioxide / Q000494:pharmacology; D009584:Nitrogen; D009657:North Carolina; D041603:Pinus taeda; D018515:Plant Leaves; D012987:Soil / Q000737:chemistry; D013696:Temperature; D014197:Trees; D014867:Water
chemical_list: D012987:Soil; D014867:Water; D002245:Carbon Dioxide; D009584:Nitrogen
keywords: FACE; Pinus taeda; automated soil respiration measurements; nitrogen fertilization; primary productivity; soil water content