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Nitrate reduction pathways and interactions with iron in the drainage water infiltration zone of a riparian wetland soil

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Abstract

Eutrophication of natural water bodies is moderated by transformation of nitrate (NO3) in riparian wetlands, which serve as filters of infiltrating drain water from upland agricultural areas. The present study comprised field observations, laboratory experiments and metagenomic studies to describe NO3 removing transformation pathways and interactions with the cycling of iron (Fe) in a temperate riparian wetland soil profile down to 1 m depth. Water samples from piezometers showed a distinct plume of NO3 in the subsurface soil where agricultural drain water was infiltrating. However, within a distance of few meters in the water flow direction, NO3 was depleted from the percolating water. Sampling and analyses of soil from the active zone of the biogeochemical NO3 removal showed that denitrifying enzyme activity was ~ tenfold higher in the upper 0–25 cm than in the lower 25–100 cm. Yet, net transformation of NO3 was substantial also at 25–100 cm when assayed with relatively undisturbed soil samples and by 15N tracer techniques in soil slurries. Transformation pathways of dissimilatory nitrate reduction to ammonium and anaerobic ammonium oxidation were identified, but were quantitatively minor as compared to denitrification. Heterotrophic denitrification and denitrification mediated by oxidation of ferrous iron, Fe(II), were identified as important processes in the wetland soil. The latter was substantiated by geochemical observations, by rates of NO3 depletion in slurry incubations with added FeCl2, and by identification of microorganisms with known capacity of NO3 reduction coupled to Fe(II) oxidation (Acidovorax sp.). The transformation pathway of iron-mediated NO3 reduction could involve biotic and abiotic reactions, and N2O, which is a potent greenhouse gas, was a major product of the process. It remains to be seen under field conditions if N2O emission hotspots are linked to specific sites of dynamic NO3 reduction coupled to Fe(II) oxidation.

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Fig. 1

Figure adapted from Petersen et al. (2020a)

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source processes of recovered 15N in soil slurries amended with a 15NO3, b 15NO3 + Fe2+, c 15NH4+, and d 14NO3 + 15NH4+. Sources were identified as denitrification to N2 (grey), anammox to N2 (blue), DNRA to NH4+ (white), and production of N2O (red). Nitrate depletion rates (black) were calculated from linear regression of time courses of NO3 concentrations (Supplementary Fig. S3). Not all processes were traceable in all treatments, e.g., no denitrification or DNRA was quantified from treatments with addded 15NH4+ (c, d). Note the 50-fold differences in x-axis scale of transformation rates between upper (a, b) and lower (c, d) panels

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Data availability

Water chemistry data are available from the IGB Freshwater Research and Environmental Database (https://fred.igb-berlin.de/data/package/516, study site Fensholt, ID 136). Metagenomic sequence data has been deposited at MG-RAST under the ID mgm4881181.3 (Peat_soil_Metagenome). Static link = https://www.mg-rast.org/linkin.cgi?metagenome=mgm4881181.3.

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Acknowledgements

We thank Bodil Stensgaard, Marianne Ahrenfeldt Stevenson, Henrik Nørgaard, Margit Paulsen, Mette Søgaard Ejsing-Duun and Joseph Martin for assistance in the field and in the lab. Thanks also to Rasmus Rasmussen for facilitating field work in Fensholt and to two journal reviewers for helpful comments.

Funding

This project was financially supported by Innovation Fund Denmark, grant no. 4106-00027B (TReNDS). The main author was funded by a PhD scholarship from GSST, Aarhus University.

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RJP, CK and LE designed the field studies; RJP and LE designed the laboratory studies, RJP, CP and LE performed the field studies; RJP, ZL and LE performed the laboratory studies; AJ performed the metagenomic studies; BT, RJP and ZL performed the 15N analyzes; RJP and LE performed the data analyzes with assistance from BT, AJ and ZL; RJP and LE wrote the manuscript; all authors contributed to discussion and revision of the manuscript.

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Correspondence to Rasmus Jes Petersen.

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Petersen, R.J., Liang, Z., Prinds, C. et al. Nitrate reduction pathways and interactions with iron in the drainage water infiltration zone of a riparian wetland soil. Biogeochemistry 150, 235–255 (2020). https://doi.org/10.1007/s10533-020-00695-2

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  • DOI: https://doi.org/10.1007/s10533-020-00695-2

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