Resource stoichiometry and the biogeochemical consequences of nitrogen deposition in a mixed deciduous forest

Abstract

Ecosystems often show differential sensitivity to chronic nitrogen (N) deposition; hence, a critical challenge is to improve our understanding of how and why site-specific factors mediate biogeochemical responses to N enrichment. We examined the extent to which N impacts on soil carbon (C) and N dynamics depend on microbial resource stoichiometry. We added N to forest plots dominated by ectomycorrhizal (ECM) trees, which have litter and soil pools rich in organic N and relatively wide C:N ratios, and adjacent forest plots dominated by arbuscular mycorrhizal (AM) trees, which have litter and soil pools rich in inorganic N and relatively narrow C:N ratios. While microbes in both plot types exhibited fairly strict biomass homeostasis, microbes in AM- and ECM-dominated plots differed in their physiological responses to N addition. Microbes in ECM plots responded to N enrichment by decreasing their investment in N-acquisition enzymes (relative to C-acquisition enzymes) and increasing N mineralization rates (relative to C mineralization rates), suggesting that N addition alleviated microbial N demand. In contrast, heterotrophic microbial activities in AM plots were unaffected by N addition, most likely as a result of N-induced increases in net nitrification (60% increase relative to control plots) and nitrate mobilization (e.g., sixfold increases in mobilization relative to control plots). Combined, our findings suggest the stoichiometric differences between AM and ECM soils are the primary drivers of the observed responses. Plant and microbial communities characterized by wide C:N are more susceptible to N-induced changes in decomposition and soil C dynamics, whereas communities characterized by narrow C:N are more susceptible to N-induced nitrate leaching losses. Hence, the biogeochemical consequences of N deposition in temperate forests may be driven by the stoichiometry of the dominant trees and their associated microbes.

Feedbacks between plant N demand and rhizosphere priming depend on type of mycorrhizal association

“…14 for AM stands and 28 for ECM stands. This relationship was consistent with Midgley and Phillips (), who observed a two‐fold difference in the ratio of C mineralisation to N mineralisation between AM and ECM plots. If the majority of additional plant growth went to woody tissue with a typical C : N ratio of 300 or more, priming effects would likely drive a net increase in total ecosystem C storage, although tree mortality or depletion of soil N over time could eventually limit this increase.…”

Carbon:Nitrogen:Phosphorus Stoichiometry in Fungi: A Meta-Analysis

“…Indeed, Cleveland and Liptzin (2007) suggested that an average soil microbial N:P ratio might be a more appropriate index of ecosystem nutrient limitation than plant N:P ratios. A recent study showed that the biogeochemical consequences of N deposition in temperate forests may be driven by the stoichiometry of the dominant trees and their associated microbes (Midgley and Phillips, 2016) while van Diepen et al (2017) found that fungi exposed to chronic nitrogen enrichment are less able to decay leaf litter. All of these studies point to the importance of multi-resource interactions in nutrient cycling in soils.…”

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

“…Plant control over organolytic activities is relatively stronger under EcM trees in temperate forest soils (Brzostek & Finzi, ). In particular, activities of proteases, chitinases, polyphenol oxidases and acid phosphatases are relatively greater in EcM‐dominated habitats (Phillips & Fahey, ; Brzostek & Finzi, ; Phillips et al ., ; Yin, Wheeler & Phillips, ; Midgley & Phillips, ), whereas cellulase activity is greater in AM‐dominated habitats (Cheeke et al ., ) in temperate forests. Brzostek et al .…”