Abstract
Volatile nitrogen oxides (N2O, NO, NO2, HONO, …) can negatively impact climate, air quality, and human health. Using soils collected from temperate forests across the eastern United States, we show microbial communities involved in nitrogen (N) cycling are structured, in large part, by the composition of overstory trees, leading to predictable N‐cycling syndromes, with consequences for emissions of volatile nitrogen oxides to air. Trees associating with arbuscular mycorrhizal (AM) fungi promote soil microbial communities with higher N‐cycle potential and activity, relative to microbial communities in soils dominated by trees associating with ectomycorrhizal (ECM) fungi. Metagenomic analysis and gene expression studies reveal a 5 and 3.5 times greater estimated N‐cycle gene and transcript copy numbers, respectively, in AM relative to ECM soil. Furthermore, we observe a 60% linear decrease in volatile reactive nitrogen gas flux (NOy ≡ NO, NO2, HONO) as ECM tree abundance increases. Compared to oxic conditions, gas flux potential of N2O and NO increase significantly under anoxic conditions for AM soil (30‐ and 120‐fold increase), but not ECM soil—likely owing to small concentrations of available substrate (NO3‐) in ECM soil. Linear mixed effects modeling shows that ECM tree abundance, microbial process rates, and geographic location are primarily responsible for variation in peak potential NOy flux. Given that nearly all tree species associate with either AM or ECM fungi, our results indicate that the consequences of tree species shifts associated with global change may have predictable consequences for soil N cycling.
Highlights
We found that peak NOy flux from soil microcosms and soil pH were linked to higher abundances of genes in the amo, nap, nor, and nrf operons
Analysis of N-cycle gene distribution for Moores Creek soil is consistent with the gradient plot soil, showing the relative proportion of genes associated with hydroxylamine oxidation, nitrite reduction, nitrate reduction, nitric oxide reduction, and nitrous oxide reduction increased from ECM to arbuscular mycorrhizal (AM) (Figure 6)
The higher abundance of N-cycle genes such as amoABC, hao, and nxrAB in AM-dominated soils are in agreement with previous studies showing that under oxic conditions, AM soils have greater rates of net nitrification (Midgley & Phillips, 2016; Mushinski et al, 2019; Phillips et al, 2013), which likely explains high rates of oxic NOy flux shown in AM soils (Figure 7)
Summary
Moores Creek sieved soil was used to determine the relative influence of oxic and anoxic conditions on AM and ECM soil; the N-cycle microbial response, N-cycle rates, and fluxes of N gases. To determine microbial N-cycle gene expression under different levels of oxygen availability, AM- and ECM-dominated soils from Moores Creek were incubated under different headspace atmospheres [oxic: ultra-pure air (20% O2, 80% N2) or anoxic: helium (100% He)] for 8 and 24 hr.
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