A mechanism of abiotic immobilization of nitrate in forest ecosystems: the ferrous wheel hypothesis

“Nitrite also readily reacts with dissolved organic compounds to form dissolved organic nitrogen compounds (Davidson et al., 2003), especially in low pH environments where nitrous acid (HNO2) is the key reactant.”

“1999), and reduction of nitrate coupled to abiotic or biotically mediated oxidation of iron (Davidson et al. 2003; Weber et al. 2006).”

“…find strong evidence for a critical threshold within the range tested here, where N addition at rates below this threshold might be unavailable to plants as microbial and abiotic processes likely outcompeted plants for excess N (Johnson et al., 2000; Barrett et al., 2002; Davidson et al., 2003).”

“Davidson et al. (2003) have proposed a mechanism by which this could occur.”

“This phenomenon is explained by the abiotic conversion of NO3 − into dissolved organic N (Davidson et al. 2003).”

“This is based on the assumption that (part of) the additional N is taken up by plants/ trees (or their mycorrhizal symbionts and transported to the plants) to facilitate additional photosynthesis to form new biomass Fixing of additional C does, however, not occur when the additional N deposition is completely immobili…”

“( Davidson et al., 2003 ) Mn Reduction (Luther & al., 1997) Heterotrophic”

“Finally, DON can be formed from the physical and/or biologically mediated association of dissolved organic matter (DOM) with nitrate (NO3 ) in soils (Perakis et al. 2001; Davidson et al. 2003).”

“, sink or source) of the future terrestrial carbon balance (Davidson et al., 2003).”

“N immobilization by promoting organic N formation from inorganic N (Davidson et al., 2003).”

“In this conceptual model, Fe3+ is reduced by SOM to Fe2+, which in turn reduces NO3 − to NO2 −, and NO2 −reacts subsequently with phenolic organic matter to form dissolved organic N compounds (Davidson et al., 2003).”

“A further pathway, which is still subject of a controversy represents the fast abiotic transformation of inorganic N into DON (Dail et al., 2001) via the “ferrous wheel” mechanism (Davidson et al., 2003, 2008).”

“…types is that during decomposition, microbial decomposers synthesize phenolic compounds and/or break down litter lignin and other polyphenolic compounds into compounds that react with inorganic N to form additional compounds that are resistant to decomposition (Fog 1988, Davidson et al. 2003).”

“…recent studies have shed new light on mechanisms of N cycling in general, highlighting the importance of aerobic denitrification, alternative denitrification pathways, and other nitrogen loss mechanisms (Robertson et al. 1995, Silver et al. 2001, Thamdrup and Dalsgaard 2002, Davidson et al. 2003).”

“…through root-uptake (Perakis and Hedin, 2001), leaching (Vitousek and Melillo, 1979; Lovett et al., 2002), abiotic interactions with soil minerals (Davidson et al., 2003), denitrification (Potter et al., 1996; Seitzinger et al., 2006), NH 3 volatilization (Schlesinger and Peterjohn, 1991; Bouwman…”

“may be incorporated into organic matter by abiotic reactions (Azhar et al. 1986, Berntson and Aber 2000, Dail et al. 2001, Davidson et al. 2003, Fitzhugh et al. 2003), but others suggest that this process is unlikely to occur in nature to an ecologically significant degree (Colman et al. 2008,…”

“Although DON can be formed from the biological or physical association of NO 3 − with dissolved organic matte”

“tested the hypothesis of Davidson et al. (2003) using 45 soil samples from a wide range of ecosystems.”

“Microbial immobilization may not be the only process stabilizing soil N, as Davidson et al. (2003) recently detailed soil mineral N immobilization via an abiotic mechanism by the interaction of reduced Fe converting NO3 � to NO2 � with a resulting nitration involving the”

“The reaction of NO2 − with DOM was based upon the rapid reaction of NO2 − with phenolic compounds (Davidson et al. 2003) instead of a rapid incorporation of NO2 − into solid organic matter (Thorn and Mikita 2000; Fitzhugh et al. 2003).”

“24 proposed a method of abiotic nitrate removal called the “ferrous wheel hypothesis” in which reduced iron [Fe(II)] abiotically converts nitrate to nitrite in anaerobic environments; nitrite then reacts with the dissolved organic matter to produce dissolved organic nitrogen.”

“responsible for a rapid NO3 uptake (Konior et al., 1994; Thorn and Mikita, 2000; Davidson et al., 2003; Corre et al., 2006).”

“The only hypothesis currently under consideration for the fast reaction of NO3 to organic N is that of Davidson et al. (2003, 2008): NO3 is reduced to NO2 in soil microsites by reduced metals in the soil (e.g., Fe(II)), followed by reaction of NO2 with organic matter to produce organic N and by…”

“Indeed, there is evidence of direct reaction of NH4NO3 with soluble polyphenols producing chemical compounds more resistant to microbial degradation (Thorn & Mikita, 1992; Davidson et al., 2003).”

“Reduced Fe and Mn species have been suggested as catalysts for abiotic reduction of nitrate to nitrite, with nitrite reacting with dissolved organic compounds via nitration and nitrosation of aromatic ring structures to produce organic N structures (Davidson et al., 2003).”

“The initial retention of labeled nitrate in soils, particularly in low N (fertilized) plots where energetic costs of NO3 assimilation are high, could have resulted from reduction driven by organic or inorganic proton sources (Davidson et al., 2003).”

“Davidson et al. (2003) put forward a hypothesis explaining the abiotic immobilization of NO3 to organic N.”

“However, the ferrous wheel hypothesis [ Davidson et al., 2003 ] predicts that drier, more aerated conditions will reduce the amount of Fe +2 available for abiotic nitrate reduction to nitrite, leading to less DON formation.”

“Davidson et al. (2003) hypothesized a mechanism whereby NO3 is abiotically reduced to NO2 by Fe and subsequently fixed to soluble organic matter.”

“The redox state of DOM in aquatic environments plays an important role in many ecosystem processes including electron transfer, metal binding, and nutrient availability [e.g., Davidson et al., 2003; Ratasuk and Nanny, 2007].”

“For instance, reactions may occur between Fe(II) and NO – 3 in the soil solution, causing oxidation of Fe(II) to Fe(III) oxides and reduction of NO – 3 to NO – 2 [24]: 2Fe2þ þ NO – 3 þ 3H2O↔ 2FeOOHðsÞ þ NO – 2 þ 4Hþ”

“While Davidson et al. (2003) convincingly demonstrate that NO2 can react with DOM to form DON, we suggest that direct reactions between NO2 and solid-phase organic matter may also be important mechanisms of N retention in temperate forest ecosystems.”

“Davidson et al. (2003) demonstrated that Fe2+ acts to promote abiotic denitrification, in which the Fe2+ reduces nitrate to nitrite, then is regenerated by the oxidation of organic carbon.”

“( Davidson et al. 1991, 2003; Johnson et al. 2000).”

“, 2003), and is considered as a possible additional mechanism of N retention in temperate ecosystems (Davidson et al., 2003).”

“However, recent evidence from N fertilization studies in temperate forests indicates that soils, rather than plants, are the dominant long-term sink for applied N, by inference, for N from atmospheric deposition (Nadelhoffer et al., 1999; also see Davidson et al., 2003).”

“Other potential products of NO transformation include nitrogen dioxide (NO2) and nitrite (NO2 ), both of which tend to be rapidly oxidized to NO3 via both biotic and abiotic reactions (Venterea and Rolston, 2002).”

“Most recent evidence indicates that the abiotic incorporation may depend on the quality and quantity of organic carbon (Davidson et al., 2003; Torres-Canabate et al., 2008; Thorn and Mikita, 2000, 1992; Dail et al., 2001; Potthast et al., 1996), aerobic/anaerobic conditions (Davidson et al.”

“C interactions in soil (e.g., abiotic reaction of nitrite with dissolved organic matter through nitration and nitrosation of aromatic ring structures) (Davidson et al. 2003).”

“ical sinks for N in the soil and losses of N through denitrification, it is not surprising that plants take up only a small fraction of N inputs via atmospheric deposition (Nadelhoffer et al. 1999; Van Breemen et al. 2002; Davidson et al. 2003; Magill et al. 2004).”

“Those carbon compounds may react with inorganic N to form additional recalcitrant organic compounds that are resistant to decay (Fog,1988; Davidson et al., 2003).”

“…denitrification [Brooks et al., 1996; Burt et al., 1999], (2) plant or microbial assimilation [Hadas et al., 1992; Wetzel, 2001], (3) physical sorption [Triska et al., 1994], and (4) biotic and abiotic retention through iron and sulfur reduction [Brunet and Garcia-Gil, 1996; Davidson et al., 2003].”

“This can be explained by the ferrous wheel hypothesis of (Davidson et al. 2003; Davidson et al. 2008), suggesting that anaerobic microsites promote favorable redox conditions for abiotic NO3 –N reduction.”

“The TOC and DON of the soil were measured following the methods of Davidson, Chorover, and Dail (2003).”

“tion and N-stabilization in non-extractable organic pools (e. g. Davidson et al. 2003 ).”

“dissolved organic nitrogen via reactions with dissolved organic material ( Davidson et al. 2003 ).”

“Other studies have also suggested that even for upland soils that short periods of changing chemical oxidation/reduction conditions can influence NO 3 loss and retention (Davidson et al. 2003; Sickman et al. 2003).”

“can form DON (Davidson et al. 2003), potentially increasing DON production and loss in the enriched N plots.”

“A mechanism that is potentially active during winter is the abiotic immobilization of NO3 − into soil organic matter following reaction with Fe2+ species (Davidson et al., 2003; McPhillips et al., 2015).”

“…at high concentrations (the recommended 5% HgCl2 is 0.18 M Hg2+) that may interfere with a proposed mechanism for abiotic NO3 – incorporation that depends on redox chemistry (Adamson 1952; Hush et al. 1960; Raposo et al. 2000), such as the hypothesized ‘‘ferrous wheel’’ of Davidson et al. (2003).”

“Davidson et al. (2003) described an abiotic mechanism10 producing DON through reaction of nitrite with DOM.”

“Although not directly investigated during any previous field study, the abiotic immobilization of NO3� to DON ( Davidson et al., 2003 ), if functioning within the catchment, could also play a role in NO3� immobilization.”

“As plant materials decay, microbial enzymes depolymerize organic substrates, potentially forming reactive amino and phenol groups that can also condense into nitrogen-rich complexes (Haider et al. 1965; Kelley and Stevenson 1996; Davidson et al. 2003; Berg and McClaugherty 2008).”

“Abiotic processes in soil also play an important role in N retention and supply (Davidson et al. 2003; Moritsuka et al. 2004).”

“The by-products of these processes then react with inorganic N to form more resistant compounds (Davidson et al. 2003).”

“Although speculative, we suspect that the sandy soils and low productivity at our site indicate a low potential for N retention via either biotic or abiotic mechanisms (Aber et al., 1998; Kaye et al., 2002; Davidson et al., 2003).”

“Recently, this fast NO32 reaction is considered as a possible mechanism contributing to N retention in temperate ecosystems ( Davidson et al., 2003 ).”

“Based on long-term N addition experiments, soils, rather than plants, are the dominant long-term N sink from atmospheric deposition (Nadelhoffer et al. 1999; Davidson et al. 2003; Lohse and Matson 2005).”

“It is likely that a large portion of this lost extractable N was due to abiotic processes (Dail et al. 2001; Davidson et al. 2003 ), but further investigation is needed to determine the exact quantities.”