Photosynthetic responses of temperate species to free air CO2 enrichment (FACE) in a grazed New Zealand pasture

A New Zealand temperate pasture is currently exposed to either ambient air or air enriched to 475 mu bar CO2 using free-air CO2 enrichment (FACE) tech nology. Sheep graze the site regularly, which results in heterogeneity in n utrient return. To investigate leaf photosynthetic responses, leaf gas exch ange characteristics and nitrogen (N) content were measured in two consecut ive years in spring under standard conditions on Lolium perenne L. and Trif olium subterraneum L. and on Trifolium repens L. and Paspalum dilatatum Poi r. in the second year only. Leaves of the three C-3 species growing under F ACE conditions had lower (up to 37% in 1998 and 22% in 1999) photosynthetic rates than leaves growing under ambient conditions, when measured at the s ame standard conditions of high light and 360-380 mu bar CO2. Differences i n photosynthetic rates were correlated with leaf N content and stomatal con ductance when measured under these conditions. There was no difference in p hotosynthetic capacities between ambient or FACE grown P. dilatatum, a C-4 grass. Photosynthetic N use efficiency (A/N) differed among species. For th e C-3 species A/N was on average 25% greater under FACE conditions and L. p erenne had the highest (240 mu mol CO2 mol N-1 s-1) and T. repens the lowes t A/N (142 mu mol CO2 mol N-1 s(-1)) under ambient CO2 partial pressure [p( CO2)]. A/N of L. perenne was similar to that of P. dilatatum measured under ambient p(CO2) but 21% greater under FACE conditions. In the second year, leaf stable carbon isotope compositions (delta C-13) were determined for P. dilatatum, L. perenne and T. repens to assess long-term responses of leaf transpiration efficiency. Using the difference in delta C-13 between ambien t and FACE-grown P. dilatatum as a reference to difference in delta C-13 in ambient and FACE air, we concluded that the ratio of leaf intercellular to ambient p(CO2) (C-i/C-a) was similar between FACE and ambient grown L. per enne and T. repens.