In this paper, 1 Tables 2 and3 should have appeared as below: TABLE 2 Interlaboratory comparison of USGS51 and USGS52 N 2 O isotopic measurements.Individual laboratory uncertainties are standard deviations of minimum of three repeated measurements against the laboratory reference gas and do not include any calibration uncertainties of the laboratory reference gas with respect to Air-N 2 and VSMOW Laboratory code Name δ 15 N Air-N2 (mUr) δ 18 O VSMOW (mUr) δ 15 N α Air-N2 (mUr) δ 15 N β Air-N2 (mUr) S P (mUr) Methodology
Nitrous oxide (N2O) is a potent greenhouse gas with a 100-year global warming potential approximately 300 times that of CO2. Because microbes account for over 75% of the N2O released in the U.S., understanding the biochemical processes by which N2O is produced is critical to our efforts to mitigate climate change. In the current study, we used gas chromatography–isotope ratio mass spectrometry (GC–IRMS) to measure the δ15N, δ18O, δ15Nα, and δ15Nβ of N2O generated by purified fungal nitric oxide reductase (P450nor) from Histoplasma capsulatum. The isotope values were used to calculate site preference (SP) values (difference in δ15N between the central (α) and terminal (β) N atoms in N2O), enrichment factors (ε), and kinetic isotope effects (KIEs). Both oxygen and Nα displayed normal isotope effects during enzymatic NO reduction with ε values of −25.7‰ (KIE = 1.0264) and −12.6‰ (KIE = 1.0127), respectively. However, bulk nitrogen (average δ15N of Nα and Nβ) and Nβ exhibited inverse isotope effects with ε values of 14.0‰ (KIE = 0.9862) and 36.1‰ (KIE = 0.9651), respectively. The observed inverse isotope effect in δ15Nβ is consistent with reversible binding of the first NO in the P450nor reaction mechanism. In contrast to the constant SP observed during NO reduction in microbial cultures, the site preference measured for purified H. capsulatum P450nor was not constant, increasing from ∼15‰ to ∼29‰ during the course of the reaction. This indicates that SP for microbial cultures can vary depending on the growth conditions, which may complicate source tracing during microbial denitrification.
Summary 1. We estimated uptake of stream water dissolved organic carbon (DOC) through a whole‐stream addition of a 13 C‐DOC tracer coupled with laboratory measurements of bioavailability of the tracer and stream water DOC. 2. The tracer, a leachate of 13 C‐labelled tree tissues, was added to the head waters of White Clay Creek, Pennsylvania, U.S.A., over a 2‐h period and followed 1.27 km downstream to generate mass transfer coefficients for DOC lability classes within the tracer. 3. From the longitudinal 13 C uptake curve, we resolved labile and semi‐labile DOC classes within the 13 C‐DOC tracer comprising 82% and 18% of the tracer respectively. 4. Plug‐flow laboratory bioreactors colonized and maintained with stream water were used to determine the concentration of stream water DOC fractions that had a similar lability to the labile and semi‐labile classes within the tracer and we assumed that stream water DOC and tracer DOC with comparable lability fractions in the bioreactors behaved similarly in the stream, i.e. they had the same mass transfer coefficients. 5. A small fraction (8.6%) of the stream water DOC was labile, travelling 238 m downstream before being taken up. The remaining bioavailable stream water DOC was semi‐labile and transported 4.5 km downstream before being taken up. These uptake lengths suggest that the labile DOC is an energy source within a stream reach, while the semi‐labile DOC is exported out of the reach to larger rivers and the downstream estuary, where it may provide energy for marine microbial communities or simply be exported to the oceans.
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