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.