Interdependencies between temperature and moisture sensitivities of CO₂ emissions in European land ecosystems

Abstract. Soil respiration is one of the largest terrestrial fluxes of carbon dioxide (CO₂) to the atmosphere. Hence, small changes in soil respiration rates could have large effects on atmospheric CO₂. In order to assess CO₂ emissions from diverse European soils with different land-use types and climate (soil moisture and temperature), we conducted a laboratory incubation experiment.

Emission measurements of CO₂ under controlled conditions were conducted using soil monoliths of nine sites from a European flux network (ÉCLAIRE). The sites are located all over Europe – from the United Kingdom in the west to Ukraine in the east, and from Italy in the south to Finland in the north – and can be separated according to four land-use types (forests, grasslands, arable lands and one peatland). Intact soil cores were incubated in the laboratory in a two-way factorial design, with temperature (5, 10, 15, 20 and 25 °C) and water-filled pore space (WFPS; 5, 20, 40, 60 and 80 %) as the independent variables, while CO₂ flux was the response variable. The latter was measured with an automated laboratory incubation measurement system.

Land use generally had a substantial influence on carbon dioxide fluxes, with the order of CO₂ emission rates of the different land-use types being grassland > peatland > forest/arable land (P < 0.001). CO₂ efflux responded strongly to varying temperature and moisture content with optimum moisture contents for CO₂ emissions between 40 and 70 % WFPS and a positive relationship between CO₂ emissions and temperature. The relationship between temperature and CO₂ emissions could be well described by a Gaussian model. Q₁₀ values ranged between 0.86 and 10.85 and were negatively related to temperature for most of the moisture contents and sites investigated. At higher temperatures the effect of water and temperature on Q₁₀ was very low. In addition, under cold temperatures Q₁₀ varied with moisture contents, indicating a stronger prospective effect of rain events in cold areas on temperature sensitivity. At both coniferous forest sites we found a strong increase in the temperature sensitivity at a moisture range between 20 and 40 % WFPS.

We developed a new approach to calculate moisture sensitivity (MS) of CO₂ efflux. MS was calculated as the slope of a polynomial function of second degree. Moisture sensitivities were highest under dry and wet conditions. In addition we found a positive relationship between MS of CO₂ efflux and temperature for both arable lands.