Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2008
Publication Date: 11/11/2008
Citation: Dijkstra, F.A., Pendall, E., Mosier, A., King, J., Milchunas, D., Morgan, J.A. 2008. Long-term enhancement of N availability and plant growth under elevated CO2 in a semiarid grassland. Functional Ecology 22:975-982. Interpretive Summary: A global rise in atmospheric [CO2] concentration has the potential to increase plant growth, alter species composition and C sequestration. However, these CO2 effects largely depend on N availability. Throughout five years of elevated [CO2], N availability and plant growth remained significantly higher than under ambient conditions in a semi-arid environment, suggesting that elevated [CO2] can influence ecosystem functioning for a much longer time than in wetter environments. Therefore, climate may be an important factor determining the extent to which ecosystems respond to elevated [CO2].
Technical Abstract: While rising atmospheric [CO2] has the potential to enhance plant growth, it has been suggested that this response may be constrained by soil nitrogen (N) availability. Here we demonstrate in a five-year field study conducted in a semiarid grassland that plant growth, plant N uptake, and soil N availability remained significantly higher under elevated than under ambient [CO2] (720 vs. 368 ppm). A novel 15N tracer method revealed that persistently greater plant N uptake throughout five years of elevated [CO2] was due to greater soil N mineralization. Increased soil moisture and root activity under elevated [CO2] likely enhanced soil N mineralization. These results are in stark contrast to several other grassland field studies in which reduced soil N availability has been observed under elevated [CO2]. Our results suggest that increases in soil moisture due to higher plant water use efficiency under elevated [CO2] may increase N mineralization and thus delay the onset of reduced soil N availability seen in wetter climates where N mineralization is less constrained by soil moisture. Climate may therefore mediate the extent to which ecosystems respond to elevated [CO2].