Increased soil moisture content increases plant N uptake and the abundance of 15N in plant biomass

Abstract

The natural abundance of 15 N in plant biomass has been used to infer how N dynamics change with elevated atmospheric CO2 and changing water availability. However, it remains unclear if atmospheric CO2 effects on plant biomass 15 N are driven by CO2-induced changes in soil moisture. We tested whether 15 N abundance (expressed as δ 15 N) in plant biomass would increase with increasing soil moisture content at two atmospheric CO2 levels. In a greenhouse experiment we grew sunflower (Helian- thus annuus) at ambient and elevated CO2 (760 ppm) with three soil moisture levels maintained at 45, 65, and 85% of field capacity, thereby eliminating potential CO2-induced soil moisture effects. The δ 15 N value of total plant biomass increased signifi- cantly with increased soil moisture content at both CO2 levels, possibly due to increased uptake of 15 N- rich organic N. Although not adequately replicated, plant biomass δ 15 N was lower under elevated than under ambient CO2 after adjusting for plant N uptake effects. Thus, increases in soil moisture can increase plant biomass δ 15 N, while elevated CO2 can decrease plant biomass δ 15 N other than by modifying soil moisture.