The temperature dependence of Ecosystem Respiration (ER) is often assessed based on the temperature of one specific layer. Air temperature or temperatures in the first ten centimetres of the soil profile are the most frequently used temperatures in models. However, previous studies showed that the relationship between ER and temperature is depth dependent, making depth selection for temperature measurements an important issue, especially at short timescales. The present study explores one possible way to assess this relationship by synchronising the ER and temperature signals and to test if the relationship between ER and temperature differs between daytime and nighttime. To do so, ER measurements were undertaken in 2013 in four Sphagnum-peatlands across France using the closed chamber method. The ER fluxes were measured hourly during 72 hours in each of four replicates in each site. Synchronisations between ER and T signal were determined for each depth (from surface to 30 cm depth) by selecting the time-delay leading to the best correlation between ER and soil temperatures and ER was then modelled. Our results showed that: (i) the delay between ER and soil temperature is greater in peat than in mineral soils; (ii) at a daily timescale synchronisation can improve the model representation using soil temperatures.

To cite this version:Benoît D'angelo, Ary Bruand, Jiangtao Qin, Xihnua Peng, Christian Hartmann, et al.. Origin of the high sensitivity of Chinese red clay soils to drought: significance of the clay characteristics. Geoderma, Elsevier, 2014, 223-225, pp.46-53. 10 AbstractThe red clay soils which are widespread in China are known to be highly sensitive to drought during the dry season but the origin of this high sensitivity to drought remains unclear.Several red clay soils were selected in the Hunan province for study. We studied their basic physico-chemical properties and clay mineralogy, their structure and shrinkage properties, as well as their water retention properties. Results show that the amount of water available between -330 and -15 000 hPa water potential is consistent with that recorded in many other clay soils from different parts of the world and thus cannot explain the high sensitivity of the red clay soils to drought. This high sensitivity to drought might be related to the high proportion of poorly available water which was characterized by the amount of available water between -3300 and -15 000 hPa water potential. Comparison with clay soils located in different parts of the world and for which the sensitivity to drought was not identified, showed that this proportion of poorly available water is indeed much higher in the red clay soils 2 studied than in clay soils representing a large range of both clay content and mineralogy. This specific behaviour of the red clay soils studied is thought to be related to the history of their parent materials: these materials are continental sediments which may have been submitted to great hydric stress, thus leading to strongly consolidated soils with consequences such as a high proportion of poorly available water, strong aggregation and weak shrinkage properties.

Peatlands are a highly effective natural carbon sink. However, the future of the carbon stored in these ecosystems is still uncertain because of the pressure they undergo. As estimation of the peatland carbon balance shows whether the system functions as carbon sink or source. La Guette peatland is a temperate Sphagnum-dominated peatland invaded by vascular plants, mainly Molinia caerulea. The studied site was hydrologically disturbed for years by a road crossing its southern part and draining water out of the system. Our aim was to estimate the main carbon fluxes and to calculate the carbon balance at the ecosystem scale. To reach this goal, CO 2 and CH 4 fluxes, DOC content as well as environmental variables were measured monthly for 2 years on 20 plots spread across the site to taking into account spatial variability. The peatland carbon balance was estimated using empirical models. Results showed that the CO2 fluxes were above 1000 gC m -2 yr -1 . In 2013 and 2014 the peatland was a net C source to the atmosphere with an emission of 220±33 gC m -2 yr -1 . These results provided evidence that restoration should be performed in order to reduce the water losses and favour the Sphagnum-dominance of this peatland.