Yunting Fang^a,^b, Keisuke Koba^b,¹, Akiko Makabe^b, Chieko Takahashi^b, Weixing Zhu^c, Takahiro Hayashi^b, Azusa A. Hokari^b, Rieko Urakawa^d, Edith Bai^a, Benjamin Z. Houlton^e, Dan Xi^a, Shasha Zhang^a, Kayo Matsushita^b, Ying Tu^a, Dongwei Liu^a, Feifei Zhu^a, Zhenyu Wang^a, Guoyi Zhou^f, Dexiang Chen^g, Tomoko Makita^b, Hiroto Toda^b, Xueyan Liu^b, Quansheng Chen^a,^h, Deqiang Zhang^f, Yide Li^g, and Muneoki Yoh^b

Significance

Nitrogen (N) losses from terrestrial ecosystems can occur as inert forms or heat-trapping greenhouse gases, and via nitrate (NO₃⁻) leaching to drainage waters, which can contribute to eutrophication and anoxia in downstream ecosystems. Here, we use natural isotopes to demonstrate that microbial gaseous N production via denitrification is the dominant pathway of NO₃⁻ removal from forest ecosystems, with gaseous N losses that are up to ∼60-fold higher than those based on traditional techniques. Denitrification becomes less efficient compared with NO₃⁻ leaching in more N-polluted ecosystems, which has important implications for assessing the connections between terrestrial soils and downstream ecosystems under rising anthropogenic N deposition.

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 (N₂) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO₃⁻) 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 NO₃⁻ leaching, pointing to widespread dominance of denitrification in removing NO₃⁻ from forest ecosystems across a range of conditions. Further, we report that the loss of NO₃⁻ to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

See: http://www.pnas.org/content/112/5/1470.abstract.html?etoc

PNAS February 3, 2015; vol.112; no.5: 1470-1474