Synthesis of a six-year study of calcareous grassland responses to in situ CO2 enrichment

We exposed species-rich temperate calcareous grassland to elevated CO2 (600 muL/L) for six growing seasons and studied carbon (C) and nutrient (nitrogen [N] and phosphorus [PI) cycling, water relations, and plant community structure and diversity. CO2, enrichment stimulated leaf- and ecosystem-level daytime CO2 uptake and increased plant community productivity; relative CO2 effects on aboveground biomass were predicted with r < -0.98 (r(2) < 0.95) by precipitation prior to biomass harvests. The underlying mechanisms were water savings due to reduced leaf conductance under elevated CO2, allowing for more growth in this temporarily water-limited ecosystem; this effect was more important in dry years. At the plant-species level, no effects of [CO2] were found except for the subdominant sedges Carex flacca and C. canlophyllea, which responded positively to elevated CO2, mainly due to increased soil moisture. Bryophytes also responded to CO2 enrichment for the same reason. At a more aggregate level, elevated CO2 increased species evenness (but not richness). Community productivity was N-limited; legumes derived >90% of their N from atmospheric N, but did not respond to elevated CO2 because they were limited by low P availability. This ultimately also prevented larger community-level responses to elevated CO2 because eventually extra legume N would have been transferred to nonlegumes, resulting in extra biomass. Higher biomass in elevated CO2 was attained by increasing C:N and C:P ratios; total N and P in plant biomass remained unaffected by [CO2]. N retention, legume dinitrogen fixation (both assessed with N-15), and microbial net N immobilization did not change. Based on our findings, our main conclusions are: (1) The effect of elevated CO2 is mainly indirect via effects on the hydrological cycle in this water-limited ecosystem; (2) the C cycle responds more than mineral nutrient cycles; (3) low available P ultimately limited community productivity and responses to CO2, and this limitation may not be atypical for many natural ecosystems in which N inventories often are controlled by biologically available P; and (4) that interactions with variable environmental conditions critically co-determine CO2 responses, which contrasts with greenhouse studies and emphasizes the importance of field studies in predicting long-term effects of increasing [CO2] on natural ecosystems.