COS 55-9
Accounting for C and N used in pasture grass biomass production: effects of climate change above- and belowground

Wednesday, August 13, 2014: 10:50 AM
302/303, Sacramento Convention Center
Astrid Volder, Plant Sciences, University of California -Davis, Davis, CA
John Evans, Research School of Biology, Australian National University, Canberra, Australia
Roger M. Gifford, CSIRO Plant Industry, Canberra, Australia

Fully understanding the combined effect of elevated CO2 and climate warming on nitrogen (N) cycling in pastures requires an understanding of changes in tissue N and C:N ratios in response to climate and management treatments. In addition, it requires a full accounting of above- and belowground biomass production. We examined the effects of elevated atmospheric CO2 concentration, atmospheric warming and clipping frequency on tissue C and N concentrations and biomass responses of the pasture grass Phalaris aquatica. Phalaris aquatica swards were grown in the field within six transparent temperature gradient tunnels - three at ambient atmospheric CO2 concentrations and three at 750 ppm CO2 concentration. Within each tunnel there were three air warming treatments; no warming (ambient control), +2.2 / +4.0 oC above ambient day/night warming, and +3.0 oC continuous warming. Within each warming treatment there were two clipping frequencies, low and high. Treatments were maintained from September 2001 until May 2003. We used sequential above- and belowground harvests to track tissue C and N concentrations through time and used minirhizotron tubes to estimate root production.


On an annual basis, total N used for new herbage production was unaffected by elevated atmospheric CO2 or warming, but increased with increased clipping frequency, while carbon used for herbage production was increased by elevated atmospheric CO2, decreased by increased clipping frequency and unaffected by warming. C and N allocated to new root production was 26-27 % on average. Total C and N used for new root production was unaffected by elevated atmospheric CO2 concentration, decreased by increased defoliation frequency, and unaffected by warming. Overall, N used for new biomass production (above and below ground combined) was not affected by elevated atmospheric CO2 concentrations or clipping frequency, in spite of strong effects of both treatments on tissue N concentrations. Total C used for biomass production was strongly reduced by increased clipping frequency. We did find a CO2 x warming effect on total N and C used for production that was not apparent from either above- or belowground data. Our data show the importance of interpreting tissue nutrient changes in combination with treatment effects on biomass production in both above- and belowground compartments.