Article pubs.acs.org/est
Low δ18O Values of Nitrate Produced from Nitrification in Temperate Forest Soils Yunting Fang,†,‡ Keisuke Koba,†,* Akiko Makabe,† Feifei Zhu,†,§ Shaoyan Fan,† Xueyan Liu,† and Muneoki Yoh† †
Graduate School of Agriculture, Tokyo University of Agriculture and Technology, Saiwai-cho 3-5-8, Fuchu, Tokyo 183-8509, Japan State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China § Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China ‡
S Supporting Information *
ABSTRACT: Analyses of δ18O of nitrate (NO3−) have been widely used in partitioning NO3− sources. However the δ18O value of NO3− produced from nitrification (microbial NO3−) is commonly estimated using the δ18O of environmental water and molecular oxygen in a 2:1 ratio. Here our laboratory incubation of nine temperate forest soils across a 1500 m elevation gradient demonstrates that microbial NO3− has lower δ18O values than the predicted using the 2:1 ratio (by 5.2− 9.5‰ at low elevation sites), in contrast to previous reports showing higher δ18O values (up to +15‰) than their predicted values. Elevated δ18O values of microbial NO3− were observed at high elevation sites where soil was more acidic, perhaps due to accelerated O-exchange between nitrite, an intermediate product of nitrification, and water. Lower δ18O of microbial NO3− than the predicted and from previous observations suggests that the contribution of anthropogenic N inputs, such as fertilizer and atmospheric deposition, to a given ecosystem and the progress of denitrification in nitrogen removal are greater than we know. More than half of the δ18O of stream NO3− lower than the predicted value along the elevation gradient also indicate the impropriety using the 2:1 ratio for differentiating NO3− sources.
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INTRODUCTION The global nitrogen (N) cycle has been disrupted by the N release associated with human activities including fossil fuel combustion, agricultural fertilization, and N-fixing plant cultivation.1−3 Consequently, it is important to constrain N sources and sinks in the environment. Analysis of stable isotope ratios (δ15N and δ18O) of nitrate (NO3−) has been widely used to differentiate sources of NO3− in groundwater (e.g., ref 4) and surface water (e.g., refs 5−8), and to constrain the oceanic N budget.9 However, the δ18O values of NO3− produced from microbiologically mediated nitrification (microbial NO3−) were often not measured, and instead were commonly estimated using biochemical stoichiometry, that is, reflecting the isotopic composition of the environmental water (H2O) and atmospheric molecular oxygen (O2) in a 2:1 ratio, in most of these studies (e.g., refs 5, 10, and 11). Such estimates are based upon knowledge of the fractional contributions of O sources during chemolithoautotrophic nitrification.12−14 With δ18O values for waters in the normal range of −25‰ (relative to Vienna Standard Mean Ocean Water (VSMOW)) to +4‰, and with the δ18O value for soil O2 of about +23.5‰ (atmospheric O2), the δ18O value of soil microbial NO3− © 2012 American Chemical Society
produced from in situ nitrification is predicted to be between −10‰ and +10‰.15 A few studies have measured δ18O values of soil microbial NO3−, but actually found that the measured values are even more positive (up to +15‰) than the values predicted using biochemical stoichiometry (in the 2:1 ratio). Moreover, the measured values are often higher than the δ18O values of NO3− in ground- or surface waters (see ref 16). The utilization of such a simple 2:1 ratio above for calculation of the δ18O involves an assumption that exchange and fractionation of oxygen isotopes during nitrification are minimal. This assumption contrasts to the recent works showing that large kinetic 18O isotope effects are associated with the incorporation of O2 and H2O in laboratory pure culture studies using marine nitrifying bacteria.17,18 In most cases, kinetic isotopic fractionation engenders preferential reaction or incorporation of molecules containing light isotopes, which will lower the δ18O value of NO3− produced Received: Revised: Accepted: Published: 8723
February 10, 2012 June 17, 2012 July 19, 2012 July 19, 2012 dx.doi.org/10.1021/es300510r | Environ. Sci. Technol. 2012, 46, 8723−8730
Environmental Science & Technology
Article
Table 1. Soil Properties (0−5 cm Mineral Soil) and Stream NO3− Concentration from Nine Temperate Forests along the Elevation Gradient in Central Japan
sites A B C D E F G H I Pc
elevation (m, a.s.l) 223 249 299 722 758 900 1271 1289 1668
bulk soil density (g cm−3)
