PS 22-13 - Influence of soil carbon availability on N pool size and cycling across Hopkins Memorial Forest

Background/Question/Methods

Microbially-accessible carbon (C) pools are an important predictor of microbial composition and competitive interactions in temperate forests, and also influence the dynamics of microbially-mediated soil nitrogen (N) fluxes. In particular, the relative availability of C and N resources may influence the strength of competitive interactions between autotrophic nitrifying bacteria, archaea, and heterotrophic decomposer organisms. This interaction can mediate the proportion of mineralized N that is nitrified in soils. As nitrate ions are negatively charged and easily leached from soils, the fraction of mineralized N that is nitrified has important implications for ecosystem N retention. We investigated the relationship between soil C availability and N pool size and cycling at Hopkins Memorial Forest (Williams College; Northwestern Massachusetts; 42.7235ºN/73.2227ºW) across two sites of differing elevations (680 m and 260 m respectively) and forest community compositions. In order to quantify the microbially-accessible carbon pool, we incubated plot-specific soil at 20ºC and measured respiration biweekly over seven months. We characterized N pool size using field-incubated resin bags and characterized potential net N mineralization and nitrification fluxes using laboratory incubations.

Results/Conclusions

At Hopkins Forest, plots located at the higher elevation site maintained higher total soil C and N pools relative to the lower site (Higher elevation: 9.9±0.5 %C; Lower elevation: 4.9±0.2 %C), although the soil C-to-N ratio was consistent across sites (CN: 14). While total soil C pools were larger, the fraction of the soil C pool that was microbially-accessible was lower at the higher elevation site. Inorganic N pools were dominated by ammonium across sites, and the ratio of resin-extractable nitrate-to-ammonium decreased with soil C pool size, such that nitrate:ammonium ion ratios were consistently higher at the lower elevation site with low total C pools. The results suggest that a larger fraction of mineralized nitrogen is ultimately nitrified at the lower elevation site maintaining smaller total soil C pools, which may drive spatial variation in soil N retention and watershed N inputs across the forest.