Throughfall chemistry in a loblolly pine plantation under elevated atmospheric CO2 concentrations

Accelerated tree growth under elevated atmospheric CO2 concentrations may influence nutrient cycling in forests by (i) increasing the total leaf area, (ii) increasing the supply of soluble carbohydrate in leaf tissue, and (iii) increasing nutrient-use efficiency. Here we report the results of intensive sampling and laboratory analyses of NH4+, NO3-, PO43-, H+, K+, Na+, Ca2+, Mg2+, Cl-, SO42-, and dissolved organic carbon (DOC) in throughfall precipitation during the first 2.5+ years of the Duke University Free-Air CO2 Enrichment (FACE) experiment. After two growing seasons, a large increase (i.e., 48%) in throughfall deposition of DOC and significant trends in throughfall volume and in the deposition of NH4+, NO3-, H+, and K+ can be attributed to the elevated CO2 treatment. The substantial increase in deposition of DOC is most likely associated with increased availability of soluble C in plant foliage, whereas accelerated canopy growth may account for significant trends toward decreasing throughfall volume, decreasing deposition of NH4+, NO3-, and H+, and increasing deposition of K+ under elevated CO2. Despite considerable year-to-year variability, there were seasonal trends in net deposition of NO3-, H+, cations, and DOC associated with plant growth and leaf senescence. The altered chemical fluxes in throughfall suggest that soil solution chemistry may also be substantially altered with continued increases in atmospheric CO2 concentrations in the future.