Reduced N cycling in response to elevated CO2, warming, and drought in a Danish heathland: Synthesizing results of the CLIMAITE project after two years of treatments

Abstract

Field-scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a seminatural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO 2 ), warming, and extended summer drought in all combinations. Here, we report on the short-term responses of the nitrogen (N) cycle after 2 years of treatments. Elevated CO 2 significantly affected aboveground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co-dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Belowground, elevated CO 2 had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH 4 1 consumption, gross mineralization, potential nitrification, denitrification and N 2 O emissions. Drought reduced belowground gross N mineralization and decreased fauna N mass and fauna N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue in response to elevated CO 2 , as well as drought-induced decreases in litter N production and fauna N mineralization prevailed in the full treatment combination. Overall, the simulated future climate scenario therefore lead to reduced N turnover, which could act to reduce the potential growth response of plants to elevated atmospheric CO 2 concentration.

Root growth and N dynamics in response to multi-year experimental warming, summer drought and elevated CO2 in a mixed heathland-grass ecosystem

“…The site had a relatively low atmospheric N bulk deposition of 1.35 AE 0.04 g N m -2 year -1 in 2007 (Larsen et al 2011).…”

“…The treatments were initiated in October 2005. See Mikkelsen et al (2008) and Larsen et al (2011) for further details of the experimental design and set-up. Fig.…”

“…Drought was expected to slow down root growth, whereas the temperature treatment was expected to increase root growth, depending on soil moisture content. We hypothesised that nitrogen uptake would increase in response to elevated CO 2 , demonstrating a larger nutrient deficiency, but it would decrease under warming due to higher mineralisation and nutrient availability (Andresen et al 2010b;Larsen et al 2011). Drought, by contrast, was expected to decrease N acquisition.…”

Functional Relationships with N Deposition Differ According to Stand Maturity in Calluna-Dominated Heathland

“…Climatic influences on the strength of N deposition relationships have also been identified in modeling studies by Britton et al (2001) as well as in previous surveys of lowland and upland heathlands (Jones and Power 2012;Southon et al 2013), although rainfall, rather than temperature, was found to be important in previous studies. Temperature has a strong influence on vegetation productivity (Larsen et al 2011) and will also affect soil microbial activity (Selsted et al 2012) and thus rates of organic matter turnover and nutrient cycling within soils. A positive effect of higher temperatures on Calluna shoot growth has been shown in a gradient study of heathland vegetation in northern Europe (Prieto et al 2009) and also indirectly in an altitudinal gradient study by Hicks et al (2000).…”

Nitrification kinetics and ammonia‐oxidizing community respond to warming and altered precipitation

“…Soil nitrification rates are sensitive to warming (Larsen et al 2011, Ma et al 2011) and changes in precipitation or soil moisture (Lamersdorf et al 1998, Emmett et al 2004; however, the mechanisms underlying these changes are poorly understood. Nitrification influences soil ammov www.esajournals.org nium (NH 4 þ ) and nitrate (NO 3 À ) content, NO 3 À leaching from the soil, and denitrification, which results in the production of nitrous oxide (N 2 O).…”