Moss‐associated cyanobacteria nitrogen (N2‐) fixation can contribute to support moss growth and constitutes a major source of new N in boreal forest ecosystems. The biomass of moss‐colonizing cyanobacteria and their N2‐fixation are usually considered linearly correlated. Yet, recent evidence has shown that cyanobacterial biomass and N2‐fixation can be decoupled, suggesting that they are not necessary affected by the same environmental and ecological drivers. Climate and nutrients were reported as affecting moss‐associated N2‐fixation, with equivocal results, whereas drivers of moss cyanobacterial biomass remain unclear. In addition, these drivers are often determined through manipulative experiments (e.g. fertilization and incubation) and remain to be validated with complementary observational studies to help us better understand future impacts of global change on the moss–cyanobacteria symbiosis. We hypothesized that moss‐associated cyanobacterial biomass is controlled in situ by factors affecting bacterial growth, whereas N2‐fixation is controlled by factors affecting enzymatic reactions. Using random forests, Spearman correlations and linear mixed‐effects models, we determined the main drivers of cyanobacterial biomass and N2‐fixation of two feather moss species, which were collected over 3 years along a 1000‐km latitudinal transect in the eastern Canadian boreal forest. We found that temperature, precipitation and phosphorus were the main positive drivers of moss cyanobacterial biomass and that temperature and molybdenum were the main positive drivers of N2‐fixation. Vanadium was a negative driver of N2‐fixation, suggesting the use of alternative nitrogenases by cyanobacteria. Both cyanobacterial biomass and N2‐fixation were strongly influenced by the moss species and were negatively correlated with moss C:N stoichiometry, highlighting the role of N2‐fixation in moss N enrichment. Synthesis. We identified for the first time some environmental drivers of moss‐associated cyanobacterial biomass and showed that they contrast with the drivers of N2‐fixation, which should be considered in further research and confirmed in other experimental settings. This is an important advance in our knowledge of moss–cyanobacteria associations, which would greatly help in better predicting the impacts of global change on this symbiosis and on nitrogen inputs in boreal forest ecosystems.

Chlorinated paraffins (CPs) are high-volume chemicals used in numerous industrial applications. Their quantitative analysis is extremely challenging and this work presents the optimization of an analytical method based on gas chromatography hyphenated with electron capture negative ionization timeof-flight high-resolution mass spectrometry (GC-ECNI-TOF HRMS) for the simultaneous determination of short-chain and medium-chain CPs (SCCPs and MCCPs, respectively) in fish tissues (i.e. dorsal muscle). The resolution of the TOF-MS analyzer reduced or eliminated isobaric interferences and the CP response was optimized through Design of Experiment. A simple clean-up 2 procedure based on adsorption chromatography further removed some potentially interfering organochlorines. Good selectivity, linearity and accuracy were achieved; method detection limits or limits of reporting were compatible with expected levels in wild fish (0.03-0.35 ng g-1 wet weight, ww, depending on the congener). This method was proven suitable for the analysis of CPs in tissues of common barbel Barbus barbus, a fish species frequently used for water quality monitoring purposes in Europe. SCCPs and MCCPs were found to be widespread within the Rhône river basin (France). At all locations, MCCP concentrations (1.3-72.7 ng g-1 ww) were higher than those of SCCPs (0.3-10.6 ng g-1 ww) and levels were systematically lower than the proposed Predicted No Effect Concentrations (PNECs). Spatial variations of SCCP composition profiles largely surpassed those of MCCPs, suggesting the influence of local sources.

Some steps of the soil nitrogen cycle are sensitive to environmental pressures like soil moisture or contamination, which are expected to evolve during the next decades but such a double stress is not yet documented. This study aimed at assessing the importance of the soil moisture on the impact of copper (Cu) contaminations on the N cycling soil function using the potential nitrifying activities (PNA) as bioindicator. A loamy soil was first incubated 6 weeks in either 30, 60, or 90% of its water holding capacity (WHC) or alternating drought and rewetting periods. Thereafter, soil samples were exposed to a gradient of Cu concentrations through a bioassay. The dose-response curves of PNA in function of added Cu were modelled and we compared the different effective Cu concentrations (ECx) producing x % of PNA inhibition to highlight differences in threshold values. The preincubation moisture treatments significantly affected the PNA responses to the secondary Cu stress with, for instance, hormetic responses in all cases except for the dry-rewetting treatment. Small PNA inhibitions were estimated for high Cu doses in the soils with low water contents (30% WHC) or submitted to dry-rewetting cycles, contrarily to the patterns observed for the soils with high water contents (90% WHC) or submitted to a single period of drought. Overall, significant differences were found in estimated ECx values between moisture treatments.