Uptake of organic nitrogen by coastal wetland plants under elevated CO2

Abstract

This study was conducted to answer the question of whether elevated CO2 and the presence or absence of inorganic nitrogen (NH4+) affect the uptake of different forms of organic nitrogen in two dominant saltmarsh species Schoenoplectus americanus and Spartina patens. S. americanus and S. patens were grown under elevated and ambient CO2 conditions and a series of hydroponic assays were conducted using dual labelled 13C15N– glycine, glutamic acid and urea supplied in both the absence and presence of NH4+. Results show rates of glycine and urea uptake were lower under elevated CO2 conditions for both species. Ratios of 13C and 15N in S. patens roots showed that at least 68 and 79% of glycine under ambient and elevated CO2, respectively, was taken up intact. Provision of NH4+ with organic N caused organic N uptake rates to decline by up to 75% in S. americanus and up to 50% in S. patens compared with plants that only received organic N. The reduction in organic N uptake in the presence of NH4+ suggests that plants rely primarily on mineral N in the field. In addition, we can deduce that organic N uptake is not likely to supply plants with the additional N required under elevated CO2, and that the repressive effects of elevated CO2 on organic N uptake may have negative consequences for ecosystem productivity and carbon sequestration.