PLANT-GROWTH AND SOIL PROCESSES IN TEMPERATE GRASSLAND COMMUNITIES ATELEVATED CO2

Turves of a Mollic Psammaquent soil were used in controlled environmen t rooms to examine the response of managed temperate pasture communiti es to 350, 525 or 700 p.p.m. CO2. Yield of herbage (regrowth over 3-we ek intervals) increased only slightly with higher CO2; however, the bo tanical composition was markedly different. At elevated CO2 Paspalum d ilatatum (C4) and Lolium perenne (C3) declined as a proportion of harv ested yield despite a stimulation of single leaf photosynthesis that w as comparable to that found in Trifolium repens, a species that increa sed in abundance. Changes in species composition were largely a conseq uence of CO2-induced differences in axillary bud activity. Net primary productivity below-ground was stimulated by CO2. Soil CO2-C productio n was greater in elevated CO2 treatments, and was consistent with a gr eater input of herbage and root mass and/or metabolites and of more re adily decomposable material. Levels of microbial biomass were unchange d, but enchytraeids were more abundant at elevated CO2. Tracking of (C O2)-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 effec ts on net mineralization of N were observed.