Nutrient relations in calcareous grassland under elevated CO.

Plant nutrient responses to 4 years of CO enrichment were investigated in situ in calcareous grassland. Beginning in year 2, plant aboveground C:N ratios were increased by 9% to 22% at elevated CO (P < 0.01), depending on year. Total amounts of N removed in biomass harvests during the first 4 years were not affected by elevated CO (19.9 ± 1.3 and 21.1 ± 1.3 g N m at ambient and elevated CO), indicating that the observed plant biomass increases were solely attained by dilution of nutrients. Total aboveground P and tissue N:P ratios also were not altered by CO enrichment (12.5 ± 2 g N g P in both treatments). In contrast to non-legumes (>98% of community aboveground biomass), legume C/N was not reduced at elevated CO and legume N:P was slightly increased. We attribute the less reduced N concentration in legumes at elevated CO to the fact that virtually all legume N originated from symbiotic N fixation (%N ≈ 90%), and thus legume growth was not limited by soil N. While total plant N was not affected by elevated CO, microbial N pools increased by +18% under CO enrichment (P = 0.04) and plant available soil N decreased. Hence, there was a net increase in the overall biotic N pool, largely due increases in the microbial N pool. In order to assess the effects of legumes for ecosystem CO responses and to estimate the degree to which plant growth was P-limited, two greenhouse experiments were conducted, using firstly 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 monoliths and microcosms were exposed to elevated CO and P fertilization in a factored design. After two seasons, plant N pools in both unfertilized monoliths and microcosm communities were unaffected by CO enrichment, similar to what was found in the field. However, when P was added total plant N pools increased at elevated CO. This community-level effect originated almost solely from legume stimulation. The results suggest a complex interaction between atmospheric CO concentrations, N and P supply. Overall ecosystem productivity is N-limited, whereas CO effects on legume growth and their N fixation are limited by P.