SYNTHESIS OF A SIX‐YEAR STUDY OF CALCAREOUS GRASSLAND RESPONSES TO IN SITU CO₂ ENRICHMENT

Abstract

We exposed species-rich temperate calcareous grassland to elevated CO 2 (600 L/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 ϽϪ0.98 (r 2 Ͼ 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 Ͼ90% 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 waterlimited 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.

The Potential Impact of Climate Change on Soil Properties and Processes and Corresponding Influence on Food Security

“…Nitrogen limitations may negatively affect plant growth [57], and modeling of C dynamics as influenced by N indicates less C sequestration by soil than originally expected given CO 2 fertilization [56]. A long-term elevated CO 2 experiment in a grasslands ecosystem indicated that N and P became limiting within two years, again limiting plant biomass response to elevated CO 2 [58]. Niklaus and Körner [58] concluded that the increases in plant productivity they did see were due primarily to soil moisture status as opposed to a CO 2 fertilization effect.…”

“…A long-term elevated CO 2 experiment in a grasslands ecosystem indicated that N and P became limiting within two years, again limiting plant biomass response to elevated CO 2 [58]. Niklaus and Körner [58] concluded that the increases in plant productivity they did see were due primarily to soil moisture status as opposed to a CO 2 fertilization effect. Experiments looking at the decomposition of plant tissues grown under elevated atmospheric CO 2 also indicate that increased levels of CO 2 are emitted during that decomposition [59], and research by Carney et al [60] observed soil organic C levels declining under increased atmospheric CO 2 levels due to increased microbial activity.…”

Will Elevated Carbon Dioxide Concentration Amplify the Benefits of Nitrogen Fixation in Legumes?

“…In other cases, stimulation of N 2 fixation by elevated [CO 2 ] is limited by the availability of other essential elements (Reich et al, 2006;van Groenigen et al, 2006). For example, in a 6-year study of a N-poor calcareous grassland, phosphorus (P) limited the response of legumes to elevated [CO 2 ], which in turn limited N transfer to nonleguminous species in the ecosystem (Niklaus and Kö rner, 2004). A study of a Florida scruboak ecosystem reported that the initial stimulation of N 2 fixation in Galactia elliottii disappeared over time as molybdenum (Mo) became limiting in elevated [CO 2 ] (Hungate et al, 2004).…”

Identifying and prioritising services in European terrestrial and freshwater ecosystems

“…However, in Europe, estimates of soil organic carbon loss of 15% from arable and rotational grass soils, 16% from soils under permanent managed grassland and 23% from soils on agriculturally managed, semi-natural land are cited in EASAC (2009). Semi-natural grasslands and heathlands/shrublands make some contribution to regulating the climate, but biomass production and carbon sequestration tends to be modest due to nitrogen and phosphorus limitation (Rogers et al 1988;Niklaus and Körner 2004). At present, agro-ecosystems contribute negatively to climate regulation through the emission of methane (NH 4 , around a quarter to one-third of emissions in Europe stem from livestock) and nitrous oxide (N 2 O, from use of fertiliser and denitrification) (EEA 1995).…”