Co-Occurring Anammox, Denitrification, and Codenitrification in Agricultural Soils

Abstract

dAnammox and denitrification mediated by bacteria are known to be the major microbial processes converting fixed N to N 2 gas in various ecosystems. Codenitrification and denitrification by fungi are additional pathways producing N 2 in soils. However, fungal codenitrification and denitrification have not been well investigated in agricultural soils. To evaluate bacterial and fungal processes contributing to N 2 production, molecular and 15 N isotope analyses were conducted with soil samples collected at six different agricultural fields in the United States. Denitrifying and anammox bacterial abundances were measured based on quantitative PCR (qPCR) of nitrous oxide reductase (nosZ) and hydrazine oxidase (hzo) genes, respectively, while the internal transcribed spacer (ITS) of Fusarium oxysporum was quantified to estimate the abundance of codenitrifying and denitrifying fungi.15 N tracer incubation experiments with 15 NO 3 ؊ or 15 NH 4 ؉ addition were conducted to measure the N 2 production rates from anammox, denitrification, and codenitrification. Soil incubation experiments with antibiotic treatments were also used to differentiate between fungal and bacterial N 2 production rates in soil samples. Denitrifying bacteria were found to be the most abundant, followed by F. oxysporum based on the qPCR assays. The potential denitrification rates by bacteria and fungi ranged from 4.118 to 42.121 nmol N 2 -N g ؊1 day ؊1 , while the combined potential rates of anammox and codenitrification ranged from 2.796 to 147.711 nmol N 2 -N g ؊1 day ؊1 . Soil incubation experiments with antibiotics indicated that fungal codenitrification was the primary process contributing to N 2 production in the North Carolina soil. This study clearly demonstrates the importance of fungal processes in the agricultural N cycle. The application of inorganic nitrogen fertilizers in agricultural fields causes various environmental problems, including eutrophication and habitat degradation. Accurate quantification of the processes removing fixed nitrogen is increasingly important to gain a better understanding of the fate of nitrogen in agricultural soils. Three microbial processes, denitrification, codenitrification, and anaerobic ammonium oxidation (anammox), are involved in the removal of nitrogen from soils through the production of nitrous oxide (N 2 O) or dinitrogen gas (N 2 ) (1, 2). Denitrification is a microbial process in which nitrate (NO 3 Ϫ ) and nitrite (NO 2 Ϫ ) are converted to N 2 O and N 2 in aerobic and anaerobic soils. It is a well-studied process in bacteria and has recently been found to occur in archaea and fungi (3). Codenitrification produces N 2 O and N 2 through the reduction of nitrite (NO 2 Ϫ ) by other nitrogen compounds, including azide, ammonium (NH 4 ϩ ), salicylhydroxamic acid, and hydroxylamine (4, 5). Codenitrification can occur in both fungi (e.g., Fusarium oxysporum) and bacteria (e.g., Streptomyces antibioticus) (5, 6) and has been measured in grassland and agricultural soils (4, 7). Fungal codenitrification...

An overlooked nitrogen loss linked to anaerobic ammonium oxidation in estuarine sediments in China

“…Finally, co-denitrification may be an additional pathway for N 2 production in sediments. Co-denitrification can generate 29 N 2 by reducing 45 N 2 O, which is produced by using 14 NH 4 + and 15 NO 3 − / 15 NO 2 − in 15 N isotope pairing experiments (Long et al 2013). Nevertheless, the ubiquitous detection of anammox bacteria indicated that anammox must play an important role in N 2 production.…”

Electron Shuttles Enhance Anaerobic Ammonium Oxidation Coupled to Iron(III) Reduction

“…However, for the in situ ferrihydrite enrichments, the NO 3 − and NO 2 − concentrations were decreased (1.5−6.7 μmol L −1 for NO 3 − and 30−32 μmol L −1 for NO 2 − ) in the AQDS amendment but increased (1−15 μmol L −1 for NO 3 − and 15−22 μmol L −1 for NO 2 − ) in the biochar and biochar + AQDS amendments ( Figure S1). …”

“…2,4−6 Several anaerobic pathways contribute to N loss in terrestrial ecosystems, including denitrification, codenitrification, and anaerobic ammonium oxidation (anammox). 2,3 It was recently reported that anammox could be coupled to iron(III) reduction (termed Feammox) with N 2 , NO 2 − , or NO 3 − as the endproducts. 4−7 The following three equations and accompanying free energies summarize these three end routes of Feammox (Δ r G m represents the energy yielded from the balanced redox reaction).…”

Activity, abundance and community structure of anammox bacteria along depth profiles in three different paddy soils

“…To measure the rates of anammox and denitrification in paddy soils, slurry incubation experiments with 15 NO 3 À were carried out using the protocol as previously described (Long et al, 2013). Briefly, 2 g of soil was added into 12-mL Exetainer tubes (Labco, High Wycombe, UK) and saturated with sterilized distilled water, and the tubes were vacuumed for 1 min and flushed with ultrahigh pure N 2 for 2 min to create anoxic conditions.…”

“…Prevalence of anammox in terrestrial ecosystem such as freshwater wetlands was later reported (Humbert et al, 2010;Zhu et al, 2011a;He et al, 2012). As anammox bacteria are responsible for N loss from farmlands which receive large amount of N fertilizers, studies that focus on anammox in agricultural soils are highly desirable (Long et al, 2013;Shen et al, 2013). Paddy soils are frequently flooded, making these habitats quite similar to freshwater wetlands.…”