Print PageClimate warming has the potential to increase plant productivity in grass-dominated systems over the next century, yet it can also promote increased decomposition of plant litter and soil organic matter. Likewise, increased atmospheric nitrogen can increase plant productivity, but can also increase plant litter quality. Therefore, on balance, it is unclear to what extent these global change factors may alter net ecosystem CO2 exchange (NEE) in grass-dominated systems. We measured NEE in response to warming (overhead infrared heaters) and nitrogen addition over two growing seasons in a temperate old field using open-system canopy chambers, which allowed for the measurement of net CO2 flux over diel periods. We also assessed the relationship between NEE and plant biomass responses to the warming and nitrogen treatments monthly.
Results/Conclusions
In both years, our study system was a net source of carbon during the snow free season, with total NEE values of 493 and 480 g CO2 m-2, respectively. Nitrogen addition did not significantly affected NEE in either year, despite a doubling in plant biomass in response to nitrogen addition in the second year. We observed a significant correlation between mean soil temperature and nighttime CO2 exchange across sampling dates. Although this correlation was potentially confounded with seasonal variation in labile carbon pools, it was nevertheless consistent with a potential direct effect of temperature on NEE. There were no significant effects of the warming treatment on NEE or plant biomass. However, because the chamber design required warming to be temporarily halted during NEE measurements, only the indirect effects of our warming treatment could be examined. Overall, our results reveal the capacity for a grass-dominated system to function as a source of carbon decades after being removed from agricultural production. Furthermore, they highlight the potential decoupling of plant and soil responses to global change.
