Microbial denitrification dominates nitrate losses from forest ecosystems

Abstract

Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N 2 ) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO 3 − ) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6-30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO 3 − leaching, pointing to widespread dominance of denitrification in removing NO 3 − from forest ecosystems across a range of conditions. Further, we report that the loss of NO 3 − to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.denitrification | nitrate isotopes | nitrogen cycling | forested watersheds

Coupled molecular and isotopic evidence for denitrifier controls over terrestrial nitrogen availability

“…Heterotrophic NO 3 − immobilization also fractionates against O and N isotopes in NO 3 − ; though at a much higher slope, typically between 1.0 and 2.0 (Granger et al, 2010). Our observed 0.55 slope for field sites (R 2 = 0.25, P = 0.00026) is therefore consistent with the imprint of terrestrial denitrifiers on soil NO 3 − consumption (Figure 3), as seen in previous work in tropical and temperate forest soils (Houlton et al, 2006;Fang et al, 2015). We cannot specifically address why the slopes differ across studies, but past work on marine bacteria cultures indicates that chemolithotrophs fractionate a slope closer to 0.5, whereas heterotrophic denitrifiers appear to fractionate O and N isotopes at a higher slope (Frey et al, 2014).…”

“…Moreover, the spatial patterns observed across sites are similarly observed in our short-term incubation experiments (that is, 7 days), thus pointing to the importance of microbial denitrifiers in driving soil NO 3 − availability patterns in both space (Lehmann et al, 2003;Granger et al, 2008) and observations for other terrestrial ecosystems (Houlton et al, 2006;Fang et al, 2015), similar to the spatial relationships observed for all samples across sites (that is,~0.6) ( Figure 6). Not only did we observe a positive relationship between nirS and δ 15 N-NO 3 − mirroring the cross-system results; but there was a significant positive correlation between nirS/16S rRNA and δ 15 N-NO 3 − in our soil incubation experiments as well (R 2 = 0.70, P = 0.001) (Figure 7).…”

“…Over time, such isotopic discrimination ('fractionation') elevates the 15 N/ 14 N of soil nitrate pools relative to the air (Houlton et al, 2006), and ultimately, ecosystem to global-scale isotopic patterns in the terrestrial biosphere (Houlton and Bai, 2009;Mnich and Houlton, 2015 − substrates similar to denitrification (Fang et al, 2015). Denitrifiers have been shown to increase the δ 15 N and δ 18 O of NO 3 − within a small range of characteristic slopes (Lehmann et al, 2003;Houlton et al, 2006;Granger et al, 2008), allowing for dual-isotopic evaluation of denitrification's effect on ecosystem NO 3 − availability (Fang et al, 2015). Natural stable isotope budgets have implied utility of forecasting the effect of the terrestrial N cycle on global climate change Zhu and Riley, 2015); however, the importance of microbialscale denitrification in driving larger 15 N/ 14 N patterns in ecosystems remains unclear.…”

Riparian Corridors: A New Conceptual Framework for Assessing Nitrogen Buffering Across Biomes

“…Yet, the capacity for denitrification can rapidly increase once waterlogged conditions develop in riparian zones (Harms et al, 2009). Indeed, along larger stream orders under these climatic conditions, soil development, including accretion of fine sediment texture and organic matter increase the water residence time and in turn denitrification capacity (Fang et al, 2015;Tiwari et al, 2017;Malone et al, 2018). We recognize that many ecological and climatological dynamics interrupt these broad patterns, which we present only as general predictions to be tested across biomes.…”

“…As with mineralization, the potential for nitrogen retention and removal can vary dramatically between climatic regions, especially because denitrification, one of the main processes responsible for nitrogen removal, is highly dependent on soil moisture conditions and nitrate supply. In tropical and temperate regions, denitrification can account for up to half of the total ecosystem nitrogen losses (Hefting et al, 2004;Houlton et al, 2006;Fang et al, 2015). We hypothesize that denitrification represents a significant part of the N cycling in tropical riparian zones, whatever the stream order (McClain et al, 1994;Décamps et al, 2004;Houlton et al, 2006).…”