N-driven changes in a plant community affect leaf-litter traits and may delay organic matter decomposition in a Mediterranean maquis

Organic matter (OM) decomposition is typically controlled by climate, soil properties, litter quality and soil microorganisms. Availability of nitrogen (N) also influences decomposition, but its effects on decomposition are controversial and most studies have only addressed decomposition of individual plant species grown under high N availability. We experimentally manipulated the dose of available N in a Mediterranean Basin maquis in south-western Europe, with low ambient N deposition (5.2 kg N ha-1 yr-1) and low soil N content (0.1%). N availability was modified by the addition of 40 and 80 kg N ha-1 yr-1 as NH4NO3. Control plots were not fertilized. After 2.5 years of N additions, we accounted for the integrated effects of N enrichment on litter decomposability taking into consideration the N-driven changes in the whole plant community (changes in plant species composition and litter quality). We collected soil from the no N addition treatment (control) and three types of leaf-litter (from three N addition treatments - 0, 40 and 80 kg N ha-1 yr-1) from the N-manipulation field experiment and performed a microcosms controlled decomposition study. Distinct leaf-litter traits were quantified (N and lignin concentration and C/N and lignin/N ratios) and related with decomposition and soil microbial biomass and activity. The leaf-litter consisted mostly of leaves from summer semi-deciduous shrubs, but relative to the control (no N addition), the treatment receiving 80 kg N ha-1 yr-1 had twice the amount of evergreen sclerophyll leaf-litter and higher lignin and N concentrations giving lower C/N and lignin/N ratios. As a result, OM decomposition in the microcosms containing 80 kg N ha-1 yr-1 litter was slower (with concomitant reduction in soil microbial biomass and activity) than in those containing 40 kg N ha-1 yr-1 litter. At the ecosystem level, N-driven changes in plant community altered leaf-litter traits (e.g. increased litter lignin and N content and decreased lignin/N ratio), which were powerful determinants of litter decomposition rates under controlled conditions. The results suggest that increasing N availability in this nutrient poor Mediterranean maquis may select species with litter traits that could delay decomposition and increase soil OM accumulation