Atmospheric Nitrate Deposition, Microbial Community Composition, and Enzyme Activity in Northern Hardwood Forests

On a global scale, human activity has increased the atmospheric input of NO − 3 to many terrestrial ecosystems. Anthropogenic NO − 3 may be a potent modifier of ecosystem function, especially in temperate forests that are sometimes N limited. However, the impact of chronic N deposition on soil microorganisms is still poorly understood. Nitrate entering Lake States forests is rapidly assimilated by the microbial community and it is subsequently released as NH + 4 Because high levels of NH + 4 inhibit the activity of lignin‐degrading soil fungi, we reasoned that chronic N additions could alter the composition and function of heterotrophic microbial communities in soil, and hence the ecosystem‐level processes they mediate. We tested our hypothesis in four northern hardwood ecosystems in northern Michigan, which received experimental N additions (30 kg NO − 3 –N ha −1 yr −1 ) during the past 8 yr. We quantified microbial community function by measuring the activity of extracellular enzymes involved in plant litter degradation and described microbial community composition using phospholipid fatty acid (PLFA) analysis. Chronic N additions significantly suppressed β‐glucosidase activity by 24% in mineral soil and suppressed phenol oxidase activity by 35% in surface litter. We found no evidence that chronic N additions altered microbial community composition; NO − 3 addition did not alter the relative abundance of bacterial, actinomycetal, fungal, or protozoan PLFAs. However, NO − 3 additions significantly reduced microbial biomass by 18% relative to the control treatment. Results indicate that N additions broadly suppressed all microbial groups, not just the activity and abundance of lignin‐degrading fungi.