Elevated Carbon Dioxide Alters Soil Nitrogen Dynamics in an Intact Mojave Desert Ecosystem

Arid ecosystems are predicted to be among the most responsive to elevated carbon dioxide because of projected increases in plant water-use efficiency. Increases in net primary production will also be regulated by available nitrogen because nitrogen is the primary resource limiting net production once water is available. Plant-available nitrogen has decreased 40 to 50 % under elevated carbon dioxide. We hypothesize this is due to an increase in microbial activity caused by increased carbon input into the soil, because the decrease in inorganic nitrogen is accompanied by significant increases in soil respiration and plant nitrogen isotope composition. The increase in plant nitrogen isotope composition is likely the result of increased microbial fractionation of inorganic nitrogen during immobilization. Experimental manipulation of soil labile carbon also increased soil respiration and decreased gaseous nitrogen loss and plant-available nitrogen. This study suggests that while elevated carbon dioxide may increase available water in arid ecosystems, it might also decrease available nitrogen.