Synthesis of a Six‐year Study of Calcareous Grassland Responses to in Situ Co₂enrichment

We exposed species‐rich temperate calcareous grassland to elevated CO 2 (600 μL/L) for six growing seasons and studied carbon (C) and nutrient (nitrogen [N] and phosphorus [P]) cycling, water relations, and plant community structure and diversity. CO 2 enrichment stimulated leaf‐ and ecosystem‐level daytime CO 2 uptake and increased plant community productivity; relative CO 2 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 CO 2 , 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 [CO 2 ] were found except for the subdominant sedges Carex flacca and C. caryophyllea , which responded positively to elevated CO 2 , mainly due to increased soil moisture. Bryophytes also responded to CO 2 enrichment for the same reason. At a more aggregate level, elevated CO 2 increased species evenness (but not richness). Community productivity was N‐limited; legumes derived >90% of their N from atmospheric N 2 , but did not respond to elevated CO 2 because they were limited by low P availability. This ultimately also prevented larger community‐level responses to elevated CO 2 because eventually extra legume N would have been transferred to nonlegumes, resulting in extra biomass. Higher biomass in elevated CO 2 was attained by increasing C:N and C:P ratios; total N and P in plant biomass remained unaffected by [CO 2 ]. N retention, legume dinitrogen fixation (both assessed with 15 N), and microbial net N immobilization did not change. Based on our findings, our main conclusions are: (1) The effect of elevated CO 2 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 CO 2 , 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 CO 2 responses, which contrasts with greenhouse studies and emphasizes the importance of field studies in predicting long‐term effects of increasing [CO 2 ] on natural ecosystems.