Ferrous Wheel Hypothesis: Abiotic nitrate incorporation into dissolved organic

Published in Geochimica et Cosmochimica Acta (2019), 245, 514-524, doi:

10.1016/j.gca.2018.11.020

Francisco Matus^ab, Svenja Stock^c, Wolfram Eschenbach^d, Jens Dyckmans^d, Carolina Merino^b, Francisco Nájera^b, Moritz Köster^e, Yakov Kuzyakov^cfg, Michaela A. Dippold^eh

aDepartment of Chemical Sciences and Natural Resources, University La Frontera, Temuco, Chile

bLaboratory of Soil Conservation and Dynamic of Volcanic Soils, University La Frontera, Temuco, Chile

cDepartment of Soil Science of Temperate Ecosystems, University of Goettingen, Goettingen, Germany

dCentre for Stable Isotope Research and Analysis, University of Goettingen, Goettingen, Germany

eBiogeochemistry of Agroecosystems, University of Goettingen, 37077 Geottingen, Germany

fAgro-Technological Institute, RUDN University, 117198 Moscow, Russia

gSoil Science Consulting, 37077 Goettingen, Germany

hDepartment of Agricultural Soil Science, University of Goettingen, Goettingen, Germany

Abstract

We evaluated the abiotic formation of dissolved organic nitrogen (DON) by the fast reaction of iron (Fe) with nitrate (NO3−) in the dissolved organic matter (DOM) of volcanic soils in a temperate rainforest (>5000 mm precipitation per year). During five days, the educts and products of abiotic reactions under anoxic conditions were measured in a microcosm experiment depending on the Fe and NO3− concentrations. A control zero-Fe was not used because there was no chemical reaction with nitrate addition. Using a novel technique of automated sample preparation for inorganic N (SPIN) attached to a membrane inlet quadrupole mass spectrometry (MIMS), the ¹⁵N abundances and inorganic N concentrations were determined directly in aqueous solutions. The results were explained in the context of the Ferrous Wheel Hypothesis which states that Fe(II) is utilized to reduce NO3− to nitrite (NO2−) that is incorporated into DOM. Fe(II) is regenerated from Fe(III) in anaerobic soil microsites. Here we tested one part of this hypothesis, the processes occurring in DOM (instead of soil organic matter). Using the SPIN-MIMS technique, we could overcome Ferrous Wheel Hypothesis criticism regarding possible Fe interference during NO3−

analysis. The total recovery of ¹⁵N added as NO3− fluctuated between 63 and 101%, and the remaining ¹⁵N was measured as gaseous N2O. The ¹⁵N-labelled NO3−

added decreased immediately after 15 min of incubation. After five days of incubation, approximately 25% of the labelled NO3−

(e⁻ acceptors) added was transformed to DON in the presence of a high amount of Fe(II) (e⁻ donors). Small amounts of N2O and CO2 provided further evidence of NO3−

reduction

and DOM oxidation, respectively. From these results, we propose a new theoretical model that includes the Ferrous Wheel Hypothesis, where only the transformation of NO3− to DON was proven. The present results explain the high retention of NO3− in DOM from volcanic soils in ecosystems with high precipitation.

2.6.4 Microbial uptake and utilization of inorganic phosphorus in soils of granidioritic origin formed under varying climatic conditions

Manuscript in preparation

Moritz Koester^a,b, Michaela A. Dippold^b, Yue Hue^b,c, Werner Häusler^d, Francisco Matus Baeza^e,f, Francisco Nájera^g,h, Svenja C. Stock^b,i, Sandra Spielvogel^j

aInstiute of Geography, University of Bern, Bern, Switzerland

bBiogeochemistry of Agroecosystems, University of Goettingen, Goettingen, Germany

cGenebank Department, Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Rostock, Germany

dResearch Department Ecology and Ecosystem Management, Chair of Soil Science, Technical University of Munich, Freising, Germany

eLaboratory of Conservation and Dynamic of Volcanic Soils, Universidad de La Frontera, Chile

fNetwork for Extreme Environment Research, NEXER-Chile, Universidad de La Frontera, Chile

gDepartment of Chemical Sciences and Natural Resources, Universidad de La Frontera, Chile

hFaculty of Agricultural Sciences, Universidad de Chile, Santiago, Chile

iInstitute of Geography and Geoecology, Karlsruhe Institute of Technology, Karlsruhe

jInstitute for Plant Nutrition and Soil Science, Christian-Albrechts University Kiel, Kiel, Germany

Abstract

Phosphor (P) sorption in soil is a multivariate process strongly dependent on soil mineralogy, organic carbon (C) content, reactive surface area, and pH, all of which are influenced by climate.

P availability to plants is often limited by strong and fast sorption within soils. Microorganisms can actively desorb P at which they become more effective the more they are limited by this nutrient. The goal of this study was to gain insight in inorganic P sorption to the soil solid phase and, subsequently, microbial P uptake along an aridity gradient across biomes (ecosequence) in the Coastal Cordillera of Chile. To investigate these differences that arose from the different climates under which the soils developed, a combined approach of an ion-exchange kinetics experiment, with subsequent laboratory incubation of the soils was conducted. Rhizosphere conditions for P sorption and microbial uptake are very different from bulk soil and characterized by increased availability of easily degradable C. Therefore, the experimental setup included a subset of mesocosms in the incubation experiment, which received a defined amount of glucose.

P sorption followed a unimodal distribution along the ecosequence. The hyperarid desert soil adsorbed 64.1% and 38.5% of added P in the top- and subsoil, respectively. The forest soils showed strongest sorption capacity of all soils, 98.2% and 99.6% of added P sorbed for in top- and subsoil, respectively. At the two transitional sites, only 31.6% in the topsoil and 26.6% in the subsoil of the arid shrubland, and 35.7% and 33.1% in topsoil and subsoil of/ the Coastal Matorral, of added

P were sorbed to the soil. Microbial P uptake differed significantly among the ecosystems. It was below the detection limit in the hyperarid desert and highest in the in the Coastal Matorral with up to 39.1 µg g-1dry soil. However, a comparison of P in the soil solution of the incubated soils to sterile soils suggested a strong effect of microbes on P in soil solution in the desert soil.

2.7 Additional studies

Nájera, F., Dippold, M.A., Boy, J., Sequel, O., Koester, M., Stock, S., Merino, C., Kuzakov, Y., Matus, F. (2020). Effects of drying/rewetting on soil aggregate dynamics and implications for organic matter turnover. Biology and Fertility of Soils, doi: 10.1007/s00374-020-01469-6

Merino, C., Matus, F., Kuzyakov, Y., Dyckmans, J., Stock, S., Dippold, M.A. Contribution of Fenton reaction and ligninolytic enzymes to soil organic matter mineralization under anoxic conditions. Accepted for publication in Science of the Total Environment.

Acknowledgments

I am thankful for everyone who supported me on this journey in one way or the other and enabled me to complete my thesis. I especially thank Michaela Dippold, for her tireless commitment, support, and assurance during the past years. I am grateful for her mentorship, for giving me direction in times needed and paving my way to become an independent researcher.

I thank Yakov Kuzyakov, for giving me the chance to conduct an exciting research project, the opportunity to develop and contribute my own ideas, and for supporting me with his advice during this journey.

I further thank Christoph Leuschner and Anna Gorbushina for their support as part of my committee and providing valuable input to my work from new perspectives.

I thank the DFG for funding this project within the priority program 1803 “EarthShape – Earth Surface Shaping by Biotia”, and I thank the EarthShape coordination team Todd Ehlers, Friedhelm von Blanckenburg, Kirstin Übernickel, and Leandro Paulino, who always managed to make our project meetings fun happenings with their energy.

Without the help of student assistants and the laboratory technicians in Göttingen this work would not have been possible. I want to thank especially Karin Schmidt and Susann Enzmann, who’s support went beyond mere lab work, as well as the team of the Centre for Stable Isotope Analysis (KOSI) in Göttingen.

Over the past years, I have been lucky to have met and been accompanied by many amazing people. Francisco Nájera, Carolina Merino, Francisco Matus y Roberto Godoy, gracias por recibirme con los brazos abiertos al llegar a Chile y por apoyarme a lo largo de los años. ¡Espero seguir trabajando con ustedes y comenzar nuevos proyectos juntos! Jens Boy, thank you for your insights on the small things and the bigger context. Matthias Gube, thank you for being always a source of advice and for introducing me to the world of fungi, and thank you Johanna Pausch and Richard Phillips, for supporting me in my quest to explore this fascinating world and expand my knowledge. Moritz Köster, you were not only the best colleague I could have wished for to master this project together, but you became a great friend. Thank you for all the support, the many discussions, trusting conversations, and making this journey so much more enjoyable! Nina Hennings, no matter what was going on, you always came to the conclusion that “we can, and we will do this!” Thank you for your trust, your assurance, and simply being there when needed. Andy Breidenbach, you had (and have) the talent to lighten the mood and make me laugh, even when everything went south; thank you for your moral support through all the times. Sara Halicki, thank you for all the evenings to take off our minds from science. Adrienne Keller, you impressed and inspired me with your warmth, positive energy, and scientific curiosity, and I hope to join you on

many more hikes and projects in the coming years. Francisco Nájera, thank you for your immediate friendship, for an amazing journey through Chile, and for introducing me to the best cat I have ever met. Leonie Göbel, you always impressed me with your energy and your “everything is possible” and “ready to help” attitude. Maire Holz, thank you for lending an ear when needed, your advice, and putting things in perspective. And especially, thank you Johannes, for always being by my side.

Legal Statement

Hiermit erkläre ich, dass diese Arbeit weder in gleicher noch in ähnlicher Form bereits anderen Prüfungsbehörden vorgelegen hat und ich mich an keiner anderen Hochschule um einen Doktorgrad beworben habe. Weiter erkläre ich eidesstattlich, dass diese Dissertation selbstständig und ohne unerlaubte Hilfe angefertigt wurde.

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