Soil carries out functions that are crucial for theenvironment and life on earth and is therefore an essential non renewableresource for mankind. Recently, the European Soil Framework Directive proposalindicated that soil is under increasing environmental pressure mostly due tothe intensification of human activities, which are damaging the capacity ofsoil to continue to perform in full its broad variety of crucial functions.Most of these soil functions are depending on micro-organisms inhabiting the soil.The diversity of soil micro-organisms is the highest on earth with estimates ofseveral thousand to several million different genomes per gram of soil. Howeverfundamental knowledge of the diversity and ecology of microbial communitiescarrying out soil functions is still limited. Understanding the impact ofanthropogenic activities on microbial communities and how this relates to soilfunctioning is therefore a major challenge in soil microbiology. The revolutionin the techniques available to date offer exciting opportunities for a betterunderstanding of the relationships between microbial diversity and soilfunctions. This talk will focus on the novel insights into the impact of humanactivities on microbial communities and potential consequences for ecosystemprocesses.

Tropospheric ozone acts as a phytotoxin which produces an oxidative stress in plants. Two ways of defense are used, either by preventing ozone input through the regulation of stomatal conductance, or by detoxifying ozone and ROS in cells. It is known that stomatal movements are alt ered by ozone. We performed fumigation experiment on three euramerican poplar genotypes ( Po pulus deltoides x Populus Nigra : ‘Carpaccio’, ‘Cima’ and ‘Robusta’ ), cultivated in pots in phytotronic chambers submitted to 120 ppb ozone or filtered air. We explor ed the effects of ozone on stomatal responses to four environmental parameters (blue light, red light, CO 2 and VPD). We also find out using a porometer that the decrease of stomatal conductance due to ozone is earlier on the adaxial face than on abaxial fa ce. Finally, to better understand these impacts, we studied ultrastructure of guard cells by TEM, stomatal density and size of stomata by SEM, and we performed X - ray microanalysis of guard cells mineral content. These approaches are coupled with the study on microdissected stomata of expression of genes involved in regulation of stomatal movements

The tropospheric level of the phytotoxic air pollutant ozone has considerably increased during the last century and is expected to continue to rise. Long-term exposures of higher plants to low ozone concentrations affect biochemical processes prior to any detectable symptoms of visible injury. On the other hand, the current critical level of ozone used to determine the threshold for damaging plants (biomass loss) is still based on the seasonal sum of the external concentrations of the pollutant above 40 nl l-1 (AOT40). Taking into account the stomatal conductance, a more relevant concept is based upon the actual ozone flux in the leaf through the stomata (cumulative uptake of ozone = CUO). CUO however ignores the internal capacity of leaf defense, which led to the concept of “effective ozone flux”, balance between stomatal flux and the intensity of cellular detoxification, with the aim to propose an improved threshold for ozone risk. Although the direct detoxification of ozone (and ROS issued from its decomposition) can primarily be carried out by cell wall ascorbate, the existing level of this antioxidant is not sufficient to indicate the degree of cell sensitivity. The capacity for regeneration of the antioxidant barrier is needed, implying the knowledge of the increased production of reducing power (NAD(P)H), primary supplier for detoxifying processes. It is made possible through the increased participation of the catabolic pathwaysand associated shunts which can provide NAD(P)H. In addition, the large change in the rubisco/PEPcase ratio, due to a huge increase in activity of the latter enzyme, leads to changes in carbon isotopic discrimination, which could be related to water use efficiency.Some results will be presented knowing that the challenge is to integrate the possible indicators in a leaf model to be used, through an upscaling process, in a tree and forest stand model.