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

• Published in: Soil biology & biochemistry 2017-12, Vol.115, p.310-321
• Main Authors: Rappe-George, M.O., Choma, M., Čapek, P., Börjesson, G., Kaštovská, E., Šantrůčková, H., Gärdenäs, A.I.
• Format: Article
• Language: English
• Subjects: Bacteria; Biological Sciences; Biologiska vetenskaper; Biomass; Environmental Sciences; Enzyme activity; Enzymes; Fatty acids; Forestry; Geosciences, Multidisciplinary; Hydrolytic enzyme activities; Leaching; Loading; Microbial biomass; Microbial community composition; Microbiology; Microorganisms; Mikrobiologi; Miljövetenskap; Multidisciplinär geovetenskap; Negibacteria; Nitrogen; Nitrogen addition; Nitrogen additions; Nitrogen deposition; Nitrogen leaching; Phospholipid fatty acids; Phospholipids; Picea abies; Plants (botany); Posibacteria; Soils; Statistical significance
• ISSN: 0038-0717; 1879-3428; 1879-3428
• DOI: 10.1016/j.soilbio.2017.07.015

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 ∼3 kg ha−1 year−1 of N in central Sweden (Stråsan) to ∼15 kg ha−1 year−1 of N in SW Sweden (Skogaby) and Czech Republic (Čertovo). Furthermore, accumulated historical N loading during 1950–2000 (which include experimental N addition performed at Stråsan and Skogaby) ranged ∼200–∼2000 kg ha−1 of N. Across all sites and treatments, current NO3− leaching ranged from low (∼0.1 kg ha−1 year−1 of N) at Stråsan, to high (∼15 kg ha−1 year−1 of N) at Skogaby and Čertovo. 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