Tree species influence on microbial functional gene abundance and N cycling in riparian soil

Abstract

Little is known about how above-ground vegetation may differentially influence the below-ground microbial community structure, abundance, and function. Abundance of soil microbial N cycling genes responsible for nitrification (amoA), denitrification (nirK and nirS), and nitrous oxide reduction (nosZ) may vary with tree species and increasing N availability, and these variables may be used to predict production of NO3- and N2O from soil. Variability of nitrification and denitrification rates have also been linked to tree mycorrhizal associations, with soil beneath tree species associated with arbuscular mycorrhizal (AM) exhibiting greater denitrification rates than tree species forming ectomycorrhizal- (ECM) associations. In this study, we integrate observed N microbial functional gene abundance in the soil as influenced by six different tree species in two sub-catchments receiving either high or low N inputs. Soils beneath two ECM-associated tree species (Northern red oak (Quercus rubra) and American beech (Fagus grandifolia)) and beneath four AM-associated tree species (sugar maple (Acer saccharum), tulip poplar (Liriodendron tulipifera), American sycamore (Platanus occidentalis), and black cherry (Prunus serotina)) were analyzed for inorganic N content, potential N2O flux, and microbial gene abundance (amoA, nirK, nirS, and nosZ) was quantified using qPCR techniques. Other soil parameters monitored, including soil pH, moisture, and organic matter. These analyses were used to predict potential NO3- and N2O production as influenced by tree species to provide a greater understanding of soil microbial community response to elevated N levels with tree species as a primary influence. We found that increasing N enrichment contributes to fluctuations in NH4+ and NO3- concentrations and subsequent soil acidification. pH was a driving influence on nirK and nosZ gene abundance, possibly due to reduced functionality. We also found greater concentrations of NH4+ beneath ECM associated tree species with implications of greater N-retention that may be helpful in the remediation of N polluted riparian buffer zones.