『Abstract
Human alteration of the nitrogen cycle has stimulated research
on nitrogen cycling in many aquatic and terrestrial ecosystems,
where analyses of nitrate (NO3-)
by standard laboratory methods are common. A recent study by Colman
et al. (Biogeochemistry 84: 161-169, 2007) identified a potential
analytical interference of soluble iron (Fe) with NO3-
quantification by standard flow-injection analysis of soil extracts,
and suggested that this interference may have led Dail et al.
(Biogeochemistry 54: 131-146, 2001) to make an erroneous assessment
of abiotic nitrate immobilization in prior 15N pool
dilution studies of Harvard Forest soils. In this paper, we reproduce
the Fe interference problem systematically and show that it is
likely related to dissolved, complexed-Fe interfering with the
calorimetric analysis of NO2-.
We also show how standard additions of NO3-
and NO2- to soil extracts at native
dissolved Fe concentrations reveal when the Fe interference problem
occurs, and permit the assessment of its significance for past,
present, and future analyses. We demonstrate low soluble Fe concentrations
and good recovery of standard additions of NO3-
and NO2- in extracts of sterilized
Harvard Forest soils. Hence, we maintain that rapid NO3-
immobilization occurred in sterilized samples of the Harvard Forest
O horizon in the study by Dail et al. (2001). Furthermore, additional
evidence is accumulating in the literature for rapid disappearance
of NO3- added to soils, suggesting
that our observations were not the result of an isolated analytical
artifact. The conditions for NO3-
reduction are likely to be highly dependent on microsite properties,
both in situ and in the laboratory. The so-called “ferrous wheel
hypothesis” (Davidson et al., Glob Chang Biol 9: 228-235, 2003)
remains an unproven, viable explanation for published observations.
Keywords: Iron; Nitrate; Nitrite; Nitrogen; Soil extracts; Abiotic
immobilization; Ferrous wheel hypothesis』
Introduction
Iron in nitrate/nitrite standard solutions
Iron in soil extracts
Why wasn't NO3- immobilization
observed by Colman et al. (2007) ?
Status of the ferrous wheel hypothesis and nitrate quantification
Conclusions
Acknowledgments
References