Effects of chronic experimental N deposition on northern hardwood forests fade after cessation of N additions

Anthropogenic nitrogen (N) deposition has the potential to affect forest ecosystem carbon (C) storage and sequestration both positively and negatively. Effects on aboveground biomass increment seem to be largely positive, but the impact of elevated N inputs on soil microbial systems in forests is often negative, reducing microbial biomass and soil respiration rates. Twenty-four years of experimental N deposition at a rate of 3 g N m⁻² y⁻¹ broadly suppressed soil microorganisms across four northern hardwood forest sites. To assess responses to reductions in chronic N inputs, soil and sugar maple root samples from the study sites were collected and examined for phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) indicators of microbial groups at one- and three-years after the cessation of the N deposition treatment, and tree growth was measured annually for six years after treatments stopped.

In the first year post-treatment, all soil and root microbial indicators remained suppressed, but by three years post-cessation of N additions, indicators for soil fungi and arbuscular mycorrhizal fungi (AMF) showed much less suppression, with several bacterial groups and measures of the total microbial community showing full recovery. By year three, significant differences also no longer existed between control and previously N-amended plots for PLFA and NLFA biomarkers indicative of AMF in tree fine roots. During active N loading, trees receiving the N deposition treatment increased their annual aboveground growth increment, in concert with reduced carbon allocation to AMF, but this growth enhancement faded during the six years after cessation of N additions.

Indicators of suppressed microbial community abundance showed gradual recovery from one to three years after cessation of long-term experimental N loading. This recovery indicates the potential for microbially-mediated ecosystem C sequestration responses to high anthropogenic N deposition, such as enhanced tree growth, reduced decomposition rates, and increased surface soil carbon content, to also fade when N inputs are reduced.