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

Background/Question/Methods

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.

Results/Conclusions

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.