The responses of alpine grassland to four seasons of CO 2 enrichment: a synthesis

Abstract

Alpine grassland at 2 470 m altitude in the Swiss Central alps was exposed to elevated CO 2 by using open top chambers (16 ambient, 16 elevated CO 2). Some plots received mineral fertilizer at a rate of N-deposition commonly measured in low altitude parts of Europe. Here we present a summary of results and data from the final harvest. Above-ground biomass measured after the completion of growth in the fourth season of treatment was not affected by CO 2 enrichment as was found by previous biometric estimates, but mean below-ground biomass was slightly stimulated (+ 12%, n.s.). In contrast, net CO 2 uptake per unit land area was strongly stimulated by CO 2 enrichment at the beginning of the experiment, and during the early part of each season. However, the CO 2 stimulation decreased during the later part of each growing season. By year four, also mid-season differences in CO 2 uptake per unit land area had disappeared. Neither microbial biomass, soil respiration in the laboratory, nor in situ land-area-based CO 2 evolution during the 10 week growing season increased under elevated CO 2. The total biomass N-pool and free soil nitrate and ammonium (capture by ion exchange resin bags) remained unaffected, whereas leaf nitrogen concentration was reduced and nonstructural carbohydrate concentration increased under elevated CO 2 in forbs. These differences in tissue composition largely disappeared during senescence and litter formation. Despite low CO 2 responsiveness at ecosystem level, species responses differed in terms of nitrogen, carbohydrates, tillering and flowering, suggesting the possibility for long-term changes in community structure. Addition of NPK equivalent to 40 kg N ha −1 a −1 had massive effects on all plant traits studied, but did not enable stimulated growth under CO 2 enrichment. However, when fertilizer and CO 2 enrichment were provided jointly, soil microbes were stimulated, indicating a co-limitation by carbon and nutrients (most likely nitrogen). Since responses to elevated CO 2 were absent in both warm and cold growing seasons, we conclude that this late successional plant community is carbon saturated at current atmospheric CO 2 concentrations for reasons not directly related to nutrient supply and climate. Perhaps, contrary to our expectation, evolutionary adjustments of this “old” ecosystem to the life conditions at high altitudes caused carbon to become a surplus resource today.