Plant growth and soil processes in temperate grassland communities at elevated CO2

Turves of a Mollic Psammaquent soil were used in controlled environment rooms to examine the response of managed temperate pasture communities to 350, 525 or 700 p.p.m. CO2. Yield of herbage (regrowth over 3-week intervals) increased only slightly with higher CO2; however, the botanical composition was markedly different. At elevated CO2 Paspalum dilatatum (C4) and Lolium perenne (C3) declined as a proportion of harvested yield despite a stimulation of single leaf photosynthesis that was comparable to that found in Trifolium repens, a species that increased in abundance. Changes in species composition were largely a consequence of CO2-induced differences in axillary bud activity. Net primary productivity below-ground was stimulated by CO2. Soil CO2-C production was greater in elevated CO2 treatments, and was consistent with a greater input of herbage and root mass and/or metabolites and of more readily decomposable material. Levels of microbial biomass were unchanged, but enchytraeids were more abundant at elevated CO2. Tracking of (CO2)-C-14 into the various C pools also indicated a more rapid turnover of C at elevated CO2 but no change in pool sizes. No consistent effects on net mineralization of N were observed.