Fungal denitrification: Bipolaris sorokiniana exclusively denitrifies inorganic nitrogen in the presence and absence of oxygen

Fungi may play an important role in the production of the greenhouse gas nitrous oxide (N2O). Bipolaris sorokiniana is a ubiquitous saprobe found in soils worldwide, yet denitrification by this fungal strain has not previously been reported. We aimed to test if B. sorokiniana would produce N2O and CO2 in the presence of organic and inorganic forms of nitrogen (N) under microaerobic and anaerobic conditions. Nitrogen source (organic-N, inorganic-N, no-N control) significantly affected N2O and CO2 production both in the presence and absence of oxygen, which contrasts with bacterial denitrification. Inorganic N addition increased denitrification of N2O (from 0 to 0.3 mu g N(2)0-N h(-1) g(-1) biomass) and reduced respiration of CO2 (from 0.1 to 0.02 mg CO2 h(-1) g(-1) biomass). Isotope analyses indicated that nitrite, rather than ammonium or glutamine, was transformed to N2O. Results suggest the source of N may play a larger role in fungal N2O production than oxygen status

Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question

Removal of excess nitrogen (N) can best be achieved through denitrification processes that transform N in water and terrestrial ecosystems to di-nitrogen (N-2) gas. The greenhouse gas nitrous oxide (N2O) is considered an intermediate or end-product in denitrification pathways. Both abiotic and biotic denitrification processes use a single N source to form N2O. However, N-2 can be formed from two distinct N sources (known as hybrid N-2) through biologically mediated processes of anammox and codenitrification. We questioned if hybrid N-2 produced during fungal incubation at neutral pH could be attributed to abiotic nitrosation and if N2O was consumed during N-2 formation. Experiments with gas chromatography indicated N-2 was formed in the presence of live and dead fungi and in the absence of fungi, while N2O steadily increased. We used isotope pairing techniques and confirmed abiotic production of hybrid N-2 under both anoxic and 20% O-2 atmosphere conditions. Our findings question the assumptions that (1) N2O is an intermediate required for N-2 formation, (2) production of N-2 and N2O requires anaerobiosis, and (3) hybrid N-2 is evidence of codenitrification and/ or anammox. The N cycle framework should include abiotic production of N-2

Significance of inhibitor volume in on-farm mitigation of nitrous oxide emission from dairy cattle urine patches

Technologies are being developed for the targeted mitigation of nitrogen (N) losses from livestock urine patches using urease and nitrification inhibitors (UIs and Nls). In our earlier study, we identified a major limitation for inhibitor efficiency, specifically, the application of a 40 mL volume of inhibitor solution to a 2L of urine patch (i.e., 1:50, based on New Zealand recommended dicyandiamide [DCD] application rate of 10 kg DCD dissolved in 800 L water ha-1).This ongoing research evaluates the effect of inhibitor treatments by varying the inhibitor: urine volume ratio from 1:50 to 1:10 (200 mL of inhibitor to the 2L of urine patch) on nitrous oxide (N2O) mitigation of five nitrification inhibitors: DCD, 3,4-dimethylpyrazole phosphate (DMPP), 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin), and two confidential compounds (named A and C, provided by AgResearch). These inhibitors were applied 24 hours after creating 2L simulated urine patches (within 0.5 m2 chambers) in two dairy-grazed pasture soils with contrasting drainage (poorly vs well drained). Results showed that the N2O emissions reduction efficiency from urine patches was the highest (35.8%–46.7%) with DCD followed by inhibitor C (26.9%–27.9%). The reductions in emission from the other inhibitors were not significant (11.0%–23.0% with DMPP and nitrapyrin, respectively; and 1.5%–15.6% with inhibitor A). In this study, diluting the inhibitor solutions resulted in retention of only 3% to 18% of the NIs by the pasture canopy compared with up to 59% (with 1:50) in our previous study. This dilution increases the amount of inhibitor reaching the soil, offering a potential option for effectively reducing N2O emissions from cattle urine patches. However, dilution may result in concentrations below threshold levels of DMPP, nitrapyrin and inhibitor A, compromising their effectiveness. These results warrant further research to optimise inhibitor application rate and volume and measure inhibitor residues for developing best practice for targeted application of inhibitors to urine patches while addressing unintended food and human health risks.</div