Interdependencies between temperature and moisture sensitivities of CO<sub>2</sub> 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 under different land-use and climate (soil moisture and temperature) we conducted a laboratory incubation experiment. Emission measurements of carbon dioxide under controlled conditions were conducted using soil monoliths of nine sites from the ÉCLAIRE flux network. Sites are located all over Europe; from the UK in the west to the Ukraine in the east; Italy in the south to Finland in the north and can be separated according to four land-uses (forests, grasslands, arable lands and one peatland). Intact soil cores were incubated in the laboratory at the temperatures 5, 10, 15, 20, and 25 °C in a two factorial design of five soil moisture levels (5, 20, 40, 60, 80 (100)% water filled pore space, WFPS), before analysed for CO2 fluxes 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-uses 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 40–70% 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 0.86–10.85 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. We found at both coniferous forest sites a strong increase of the temperature sensitivity at a moisture range between 20–40% WFPS. In our study moisture sensitivity (MS) of CO2 efflux 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.