Indications that long-term nitrogen loading limits carbon resources for soil microbes

Microbial communities in the organic horizon (O-horizon) of forest soils play key roles in terrestrial nitrogen (N) cycling, but effects on them of long-term high N loading, by N deposition or experimental addition, are not fully understood. Thus, we investigated N-loading effects on soil microbial biomass N, carbon (C) and phosphorus stoichiometry, hydrolytic and oxidative enzymes, community composition (via phospholipid fatty acids, PLFA) and soil chemistry of the O-horizon in study plots of three well studied experimental Norway spruce (Picea abies) forests in Sweden and the Czech Republic. These forests span substantial gradients in current N deposition, experimental N addition and nitrate (NO3-) leaching. Current N deposition ranges from similar to 3 kg ha(-1) year of N in central Sweden (Strasan) to similar to 15 kg ha(-1) year of N in SW Sweden (Skogaby) and Czech Republic (Certovo). Furthermore, accumulated historical N loading during 1950-2000 (which include experimental N addition performed at Strasan and Skogaby) ranged similar to 200-similar to 2000 kg ha(-1) of N. Across all sites and treatments, current NOT leaching ranged from low (similar to 0.1 kg ha(-1) year(-1) of N) at Strasan, to high (similar to 15 kg ha(-1) year(-1) of N) at Skogaby and Certovo. We found significantly lower C/N ratios and greater amounts of extractable inorganic N species in the forest soils' O-horizons at the high N loading plots. Microbial biomass and basal respiration decreased under experimental N addition treatments and tended to decrease with increased N deposition. Similarly, activities of hydrolytic enzyme activity associated with N acquisition were lower, although differences in activities at specific sites with the highest and intermediate historical N deposition levels failed statistical significance. Conversely, activities of soil hydrolytic enzymes associated with C acquisition were greater in study plots exposed high N loading. PLFA profiles indicated shifts in microbial community composition induced by long-term N load, towards higher and lower relative abundance of Gram-positive and Gram-negative bacteria, respectively (but no changes in fungal relative abundance). Taken together, our results suggest that long-term N loading of N-limited Norway spruce forests aggravates limitation of other resources, likely of C, for soil microbial communities. Although microbial variables in the soil O-horizon differed between plots exposed to low and high current N loading, microbial variables in plots that leached small amounts and large amounts of NO3- exposed to high N load were similar. (C) 2017 Published by Elsevier Ltd.