Effects of nitrogen availability on microbial activities,densities and functional diversities involved in the degradation of a Mediterranean evergreen oak litter (Quercus ilex L.)

The effect of available nitrogen N ((NH₄)₂S0₄) amendments on various microbial variables in three different layers (OhLn, OhLv, OhLf) of a Mediterranean litter profile under an evergreen oak forest (Quercus ilex L.) were studied. Since Mediterranean litters are generally N limiting, the goal of the study was to understand how low (0.1 and 1%) and high (5 and 10%) N amendments impact specific biological variables such as hyphal length, community-level-catabolic-profiles (CLCPs) in ECO and FF Biolog? plates, basal respiration, enzymatic activities (i.e. alkaline phosphatases (AlP), laccases, peroxidases and cellulases), and laccase and cellulase isoforms from three different litter layers. Results indicated that the effects of N amendments occurred over very short incubation time (3 d), and varied depending on N concentration and litter organic matter (OM) quality (i.e. depth). Thus, it appeared that the more active layer was the intermediate (OhLv) layer, which probably contained the most labile and available C pools. As a consequence, OhLv was also the layer showing globally the more intensive microbial responses following low N amendments. Indeed, in this layer, low N supplies caused several marked increases in enzymes activities (i.e. laccases, cellulases and alkaline phosphatases), hyphal length and isoenzyme patterns, suggesting a microbial reallocation of C to biomass and enzyme production. On the contrary, high N supplies resulted in adverse effects on almost all the variables, suggesting repression or cytotoxic phenomena.