Long-term enhancement of N availability and plant growth under elevated CO2 in a semi-arid grassland

While rising atmospheric CO2 has the potential to enhance plant growth and biomass accumulation, rates of these processes may be constrained by soil nitrogen (N) availability. Despite much effort, it is still uncertain how elevated CO2 affects long-term soil N dynamics. We used open-top chambers to examine the effect of 5 years of elevated atmospheric CO2 concentration (720 vs. 368 p.p.m.) on N dynamics in a semi-arid grassland ecosystem in north-eastern Colorado, USA. In the first year 0.5 g m(-2) of ammonium nitrate-N, 99.9 atom% N-15, was added to each plot. We examined the effect of elevated CO2 on N mineralization and plant N uptake by tracking the labelled and total N in plant and soil over the following 5 years. Plant growth and plant N uptake remained significantly higher under elevated than under ambient CO2. The fraction of labelled N (expressed per unit of total N) in above-ground biomass declined over time, and this decline was greater under elevated CO2. The amount and fraction of labelled N in the soil did not change with time and was unaffected by elevated CO2. These results suggest that with time, N released from mineralization in the soil diluted the labelled N in above-ground biomass and that this dilution effect caused by N mineralization was greater under elevated CO2. More of the mineralized N ended up in the above-ground biomass of Stipa comata and forbs (C-3) than in Bouteloua gracilis (C-4) under elevated CO2. Increased soil moisture under elevated CO2 likely supported higher rates of N mineralization, thereby reducing N constraints on plant growth. Therefore, in semi-arid systems, plant growth and species composition responses to elevated CO2 may be more persistent than in mesic systems where N mineralization is less constrained by soil moisture.