Interaction of soil moisture and elevated CO2on the above-ground growth rate, root length density and gas exchange of turves from temperate pasture

Abstract

Interactions between water availability and elevated atmospheric CO 2 concentrations have the potential to be important factors in determining future forage supply from temperate pastures. Using large turves from an established pasture, the response of these communities at 350 or 700 μl l -1 CO 2 to a soil moisture deficit and to recovery from the deficit in comparison to turves that were well-watered throughout was measured. Prior to this experiment the turves had been exposed to the CO 2 treatments for 324 d. Net CO 2 exchange continued at elevated CO 2 even when the volumetric soil moisture content was less than 0.10 m 3 m -3 soil ; at the same moisture deficit gas exchange at ambient CO 2 was zero. The additional carbon fixed by the elevated CO 2 turves was primarily allocated below-ground as shown by the maintenance of root length density at the same level as in well-watered turves. When the dry turves were rewatered there was compensatory growth at ambient CO 2 so that the above-ground growth rate exceeded that of turves that had not experienced a moisture deficit. At the start of this experiment, the turves that were growing at 700 μl l -1 CO 2 had a greater proportion of legume (principally white clover, Trifolium repens L.) in the harvested herbage. There was a trend for the legume content at elevated CO 2 to be reduced under a soil moisture deficit. The results indicate different strategies in response to soil moisture deficits depending on the CO 2 concentration. At ambient CO 2 , growth stopped, but plants were able to respond strongly on rewatering ; while at elevated CO 2 growth continued (particularly below-ground), but no additional growth was evident on rewatering. Ecosystem gas exchange measurements taken at the end of the experiment (after 429 d of exposure to CO 2 ) showed 33% more CO 2 was fixed at elevated CO 2 with only a small (12%) and nonsignificant downward regulation.