NUTRIENT RELATIONS IN CALCAREOUS GRASSLAND UNDER ELEVATED CO2

Plant nutrient responses to 4 years of CO2 enrichment were investigate d in situ in calcareous grassland. Beginning in year 2, plant abovegro und C:N ratios were increased by 9% to 22% at elevated CO2 (P < 0.01), depending on year. Total amounts of N removed in biomass harvests dur ing the first 4 years were not affected by elevated CO2 (19.9 +/- 1.3 and 21.1 +/- 1.3 g N m(-2) at ambient and elevated CO2), indicating th at the observed plant biomass increases were solely attained by diluti on of nutrients. Total aboveground P and tissue N:P ratios also were n ot altered by CO2 enrichment (12.5 +/- 2 g N g(-1) P in both treatment s). In contrast to non-legumes (>98% of community aboveground biomass) , legume C/N was not reduced at elevated CO2 and legume N:P was slight ly increased. We attribute the less reduced N concentration in legumes at elevated CO2 to the fact that virtually all legume N originated fr om symbiotic N-2 fixation (%N-dfa approximate to 90%), and thus legume growth was not limited by soil N. While total plant N was not affecte d by elevated CO2, microbial N pools increased by +18% under CO2 enric hment (P = 0.04) and plant available soil N decreased. Hence, there wa s a net increase in the overall biotic N pool, largely due increases i n the microbial N pool. In order to assess the effects of legumes for ecosystem CO2 responses and to estimate the degree to which plant grow th was P-limited, two greenhouse experiments were conducted, using fir stly undisturbed grassland monoliths from the field site, and secondly designed 'microcosm' communities on natural soil. Half the microcosms were planted with legumes and half were planted without. Both monolit hs and microcosms were exposed to elevated CO2 and P fertilization in a factored design. After two seasons, plant N pools in both unfertiliz ed monoliths and microcosm communities were unaffected by CO2 enrichme nt, similar to what was found in the field. However, when P was added total plant N pools increased at elevated CO2. This community-level ef fect originated almost solely from legume stimulation. The results sug gest a complex interaction between atmospheric CO2 concentrations, N a nd P supply. Overall ecosystem productivity is N-limited, whereas CO2 effects on legume growth and their N-2 fixation are limited by P.