¹⁵N immobilization in forest soil: a sterilization experiment coupled with ¹⁵CPMAS NMR spectroscopy

Abstract

In temperate forests, soils are the main sink for atmospheric N deposition. The main processes proposed for N retention are microbial and abiotic immobilization in soil organic matter. The relative importance of these processes as well as the kind of resulting chemical compounds are not totally understood. We carried out a laboratory incubation of Hg-sterilized and non-sterilized organic and organo-mineral soil horizons, labelled with either 15 NO 3 À or 15 NH 4 þ . The labelled samples were incubated for 1 hour, 1 day, or 6 days, then subjected to K 2 SO 4 extraction and analysed with 15 N CPMAS NMR spectroscopy. N immobilization was already effective in all samples and treatments after 1 hour. The corresponding NMR spectra showed that part of the immobilized 15 N was already incorporated into an amide structure. In the sterilized soils labelled with 15 NH 4 þ , the tracer was rapidly and largely immobilized by an unknown process related to the presence of Hg. In the sterilized soils labelled with 15 NO 3 À , between one-third and one-half of the added tracer was immobilized during the first hour and only 10% more over the 6 days. These results suggest that the sterilization was incomplete at first, allowing relatively great microbial immobilization during the first hour. By contrast, over a longer time, NO 3 À immobilization was significantly reduced to a level corresponding to an abiotic process as Hg sterilization became more effective. Even if the low signal-to-noise ratio precluded quantitative 15 N NMR measurements, we showed that the amide-peptide signal, considered as a biotic signature, was dominant in all cases. RésuméDans les foreˆts tempe´re´es, les sols constituent le puits principal vis-a`-vis des apports atmosphe´riques azote´s. Deux processus sont a`l'origine de la re´tention de l'azote dans le sol, l'immobilisation microbienne et l'immobilisation abiotique sur la matie`re organique. L'importance relative de ces deux processus ainsi que les formes chimiques qui en re´sultent ne sont pas e´lucide´es. Nous avons mene´au laboratoire une expe´rience d'incubation qui porte sur des horizons organiques, organo-mine´raux, ste´rilise´s ou non a`Hg et marque´s ou non a`1 5 NO 3 À ou 15 NH 4 þ . Suite aù ne incubation d'une heure, d'un jour et de 6 jours, des extractions chimiques a`K 2 SO 4 et de la N CPMAS spectroscopie de Re´sonance Magne´tique Nucle´aire ont e´te´re´alise´es sur les e´chantillons marque´s 15 NO 3 À , 15 NH 4 þ . Apre`s une heure, on note une immobilisation de l'azote dans tous les e´chantillons de sol, quelque soit le traitement effectue´. Les spectres RMN correspondant indiquent qu'une partie du 15 N immobilise´est sous forme d'amides. Dans les e´chantillons ste´rilise´s marque´s a`1 5 NH 4 þ , le traceur est rapidement et massivement immobilise´par un processus non identifie´et lie´a`la pre´sence de Hg. Dans les sols ste´rilise´s marque´s a`1 5 NO 3 À , entre un tiers et la moitie´de la quantite´du traceur sont immobilise´s au cours de la premie`re heure. Au cours des 6 jours suivants...

Abiotic immobilization of nitrate in two soils of relic Abies pinsapo-fir forests under Mediterranean climate

“…Mercuric chloride (HgCl 2 ) is a sterilant that produces very few changes in soil chemical and physical properties, but the addition of this reagent could interfere with the proposed mechanism, the 'ferrous wheel hypothesis', as this is a process that depends on redox chemistry. There are also doubts about how quickly HgCl 2 sterilizes the sample (Morier et al 2008). Although autoclaving can change concentrations of extractable Fe and dissolved organic matter, it is an eYcient method to eliminate viable microorganisms and their exoenzymes while other soil properties, such as exchange capacity, surface area and pH, are less aVected (Wolf and Skipper 1994).…”

Forest succession, soil carbon accumulation, and rapid nitrogen storage in poorly remineralized soil organic matter

“…We have no data on the molecular forms of the stable tracer N we observed, but some of it was perhaps stored in amides, functional groups that serve as peptide bonds. Tracer 15 N is transferred to amides from quite different soil amendments, including mineral N (Clinton et al 1995, Morier et al 2008), plant residue N (DiCosty et al 2003, and charred organic matter N (de la Rosa and Knicker 2011), so this transfer appears to be pervasive. Moreover, the amide signature appears stable, as it is present in the organic matter of all soil physical fractions, and remains constant through time even while the N concentrations of those fractions are changing (DiCosty et al 2003).…”

“…Nitrogen storage in SOM is rapid. Within just hours to days of being introduced to soil in reactive mineral forms, N is incorporated in soil organic molecules (Zogg et al 2000, Morier et al 2008). Furthermore, this N storage in SOM is stable.…”

Characterization of fecal nitrogen forms produced by a sheep fed with 15N labeled ryegrass

“…Powerful methods (as solid state cross polarization/magic angle spinning 15 N nuclear magnetic resonance [SS CP/MAS 15 N NMR], or Curie-point pyrolysis-gas chromatography/mass spectrometry [Cp Py-GC/MS]) are available to resolve the chemical structures in which N is found in feces. Whereas Cp Py-GC/MS has been used to characterize carbon (C) and N forms in forage, soil organic matter, composts, municipal solid wastes (Ralph and Hatfield 1991;Reeves and Francis 1997;Leinweber and Schulten 2000;Gonzalez-Vila et al 2001;Franke et al 2006Franke et al , 2007 and more recently in manure (Calderon et al 2006;Schnitzer et al 2007), fewer works used NMR to characterize N forms in these organic compounds because of the low concentration of 15 N at natural abundance in these samples (Morier et al 2008). To our knowledge animal feces have not yet been characterized by SS CP/MAS 15 N NMR.…”