Effects of elevated temperature and additional growing season precipitation on managed grassland carbon storage and flux

Background/Question/Methods

Managed grasslands and pastures, composed primarily of non-native forage species (perennial grasses and legumes), cover more than 14 million hectares in the eastern half of the United States. These areas are important economically for grazing animal and hay production, but they also play a significant role in regional carbon cycling and the maintenance of overall environmental quality and ecosystem services. In order to evaluate how these systems will respond to predicted changes in climate, we initiated a multi-factor climate change study, elevating air temperature (+3^oC) and increasing growing season precipitation (+30% of long-term mean annual), in a grassland managed for hay production located in central Kentucky.  Treatments began in May 2009 and have run continuously since. We measured the effect of these climate treatments on above- and belowground plant carbon storage and soil respiration fluxes.  Plant carbon pools were measured three times a year corresponding to the timing of hay harvests, and soil respiration rates were measured in the field every two weeks with a photoacoustic gas analyzer.

Results/Conclusions

Elevated temperature initially reduced aboveground plant C pools (93 vs. 189 g C m^-2 in +heat vs. control treatments, respectively, for the July 2009 harvest); however, this effect was reversed or not significant across the other measurement periods. Additional growing season precipitation alone had little effect on aboveground C but significantly stimulated belowground plant C pools.  Elevated temperature reduced soil respiration rates during the growing season (by as much as 40%), but the addition of precipitation eliminated this effect.  Although elevated temperatures stimulated soil respiration during the first fall, winter, and spring months, when summed throughout the project, the +heat treatment lost the least amount of C through this pathway.  Elevated temperature reduced total plant C pools and soil respiration rates in this managed grassland, but the addition of increased growing season precipitation ameliorated these effects: +heat+precipitation treatment was similar (aboveground plant C) to or greater (belowground plant C and soil respiration) than the ambient control. Even in the typically mesic environment of the upper transition zone of the eastern U.S., a 3^oC increase in air temperature is likely to reduce C storage and flux, but this effect may be offset or reversed if increased growing season precipitation occurs concurrently.