Uptake of organic nitrogen by coastal wetland plants under elevated CO₂

Aims: 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. Methods: 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: 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. Conclusions: 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.