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Abstract Urease and nitrification inhibitors are designed to mitigate ammonia (NH 3) volatilization and nitrous oxide (N 2 O) emission, but uncertainties on the agronomic and economic benefits of these inhibitors prevent their widespread adoption in pasture systems, particularly in subtropical regions where no such information is available. Here we report a field experiment that was conducted in a subtropical pasture in Queensland, Australia to examine whether the use of the urease inhibitor N -(n -butyl) thiophosphoric triamide (NBPT, applied as Green UreaNV®) and the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP, applied as Urea with ENTEC®) is environmentally, agronomically and economically viable. We found that Green UreaNV® and Urea with ENTEC® decreased NH 3 volatilization and N 2 O emission by 44 and 15%, respectively, compared to granular urea. Pasture biomass and nitrogen (N) uptake were increased by 22–36% and 23–32%, respectively, with application of the inhibitors compared to granular urea. A simple economic assessment indicates that the fertilizer cost for pasture production was 5.4, 4.4 and 6.0 Australian cents per kg dry matter for urea, Green UreaNV® and Urea with ENTEC®, respectively, during the experimental period. The mitigation of N loss using the inhibitors can reduce the environmental cost associated with pasture production. These results suggest that the use of these inhibitors can provide environmental, agronomic and economic benefits to a subtropical pasture. Graphical abstract Unlabelled Image Highlights • Effects of urease and nitrification inhibitors on N dynamics of a subtropical pasture were examined. • NBPT and DMPP decreased NH 3 volatilization by 44% and N 2 O emission by 15%, respectively. • The use of these inhibitors increased pasture biomass and N uptake by ~20–30%. • These inhibitors can provide environmental, agronomic and economic benefits. [ABSTRACT FROM AUTHOR]

Nitrification inhibitors show promise in decreasing nitrous oxide (N2O) emission from agricultural systems worldwide, but they may be much less effective than previously thought when both direct and indirect emissions are taken into account. Whilst nitrification inhibitors are effective at decreasing direct N2O emission and nitrate ( NO3-) leaching, limited studies suggest that they may increase ammonia ( NH3) volatilization and, subsequently, indirect N2O emission. These dual effects are typically not considered when evaluating the inhibitors as a climate change mitigation tool. Here, we collate results from the literature that simultaneously examined the effects of nitrification inhibitors on N2O and NH3 emissions. We found that nitrification inhibitors decreased direct N2O emission by 0.2-4.5 kg N2O-N ha−1 (8-57%), but generally increased NH3 emission by 0.2-18.7 kg NH3-N ha−1 (3-65%). Taking into account the estimated indirect N2O emission from deposited NH3, the overall impact of nitrification inhibitors ranged from −4.5 (reduction) to +0.5 (increase) kg N2O-N ha−1. Our results suggest that the beneficial effect of nitrification inhibitors in decreasing direct N2O emission can be undermined or even outweighed by an increase in NH3 volatilization. [ABSTRACT FROM AUTHOR]

Abstract: Laboratory incubation experiments were conducted to compare the effects of the nitrification inhibitors 3,4-dimethylpyrazole phosphate (DMPP) and 2-Chloro-6-(trichloromethyl)-pyridine (N-serve) on nitrification and nitrous oxide (N2O) emission from a Vertosol from southern Australia, under controlled moisture and temperature. Nitrification rates in the control soil were strongly influenced by the temperature and moisture, increasing by a factor of 3.6 for each 10 °C increase between 5 and 25 °C. DMPP inhibited nitrification effectively for 42 days at 5–15 °C and 40–60% water filled pore space (WFPS). DMPP also slowed nitrification appreciably at 25 °C when the soil was at 40% WFPS, but was less effective at 60% water filled pore space. N-serve inhibited nitrification effectively for 42 days under all test conditions. Emissions of N2O from the urea treatment (no inhibitors) significantly increased with increasing temperature and moisture. The ratio of total N2O emission to total nitrification was not constant and varied from around 0.03% at 5 °C and 40% WFPS to 0.12% at 25 °C and 60% WFPS. DMPP and N-serve reduced cumulative N2O emission over 42 days by more than 65% under all the imposed conditions. [Copyright &y& Elsevier]

• Urea application reduced soil pH and increased soil total nitrogen (N), total carbon and nitrate; • Use of DMPP increased soil pH and reduced soil nitrate at a higher application rate of 120 kg Nha−1 relative to N alone; • Addition of N did not influence soil bacterial composition at phylum level but increased AOB abundance at application rate of 80 and 120 kg N ha−1. • DMPP addition did not influence soil bacterial composition at phylum level but reduced AOB and nirK abundance at an application rate of 120 kg N ha−1. Nitrification inhibitors have been reported to reduce nitrous oxide emission and nitrate leaching in agricultural systems. The effects of repeated applications of urea alone or in combination with nitrification inhibitors on nitrogen (N) cycling microbes involved in nitrification and denitrification together with non-targeted microbes are not well understood. Therefore, the objective of this study was to investigate the effects of repeated application of urea and DMPP on soil physio-chemistry, ammonia oxidizers and total bacteria in the soil. We collected soil samples from a 4.5-year field experiment under crop rotation with repeated application of seven treatments, namely control (CK), Urea (U), Urea + DMPP (UE) applied at 40, 80 and 120 kg N ha −1 , each treatment with three replicates. Ammonia-oxidizing bacteria (AOB) gene copy numbers increased as the N application rate increased (from 0 to 120 kg N ha −1). The use of DMPP significantly reduced AOB and nirK gene copy numbers compared to urea alone at an application rate of 120 kg N ha −1. There was no treatment effect on the abundance of ammonia-oxidizing archaea (AOA), Comammox clade A and B, nosZ and bacterial 16S rRNA genes. The community composition of AOB and AOA changed with N addition and use of DMPP but increasing N addition rate changed the composition of AOB only. Addition of N increased potential nitrification rates at 80 and 120 kg N ha −1. There was no significant treatment effect on the relative abundance of bacteria at the phylum level. This experiment demonstrated that the application of N (with or without DMPP) at rates lower than 120 kg N ha−1 would not result in significant impacts on soil archaeal and bacterial ecology. [ABSTRACT FROM AUTHOR]

Acetylene (C 2 H 2 ) is an effective nitrification inhibitor targeting autotrophic ammonia oxidizers, and has shown promise for improving nitrogen use efficiency by mitigating greenhouse gas nitrous oxide (N 2 O) emissions and reducing nitrate leaching. Its efficacy, however, varies considerably with edaphic and environmental conditions and remains largely less studied in dryland agricultural soils. Here we conducted two laboratory microcosm incubations to explore the efficacy of C 2 H 2 across various agricultural soils and under different conditions. The first incubation was with four agricultural soils at 25 °C and 60% water-filled pore space (WFPS), and the second incubation included one cropping soil under a range of conditions (15 °C, 25 °C, 35 °C and 50%, 70% WFPS). Our results showed that incubation of soil with 1% v/v C 2 H 2 resulted in complete or partial inhibition of nitrification, N 2 O emission, and AOA or AOB growth under the experimental conditions. Acetylene can totally inhibit nitrification in acidic cropping and dairy pasture soils through retarding both AOA and AOB growth, while C 2 H 2 partly inhibited nitrification and N 2 O emission in the alkaline vegetable soil through impeding only AOB growth. The highest inhibition effect of C 2 H 2 was achieved at 25 °C and 50% WFPS, while there was no inhibitory effect of C 2 H 2 when soil was incubated at 15 °C and 50% WFPS suggesting soil temperature may have a significant influence on C 2 H 2 effectiveness. The inhibition of C 2 H 2 on cumulative N 2 O emission increased with increasing temperature at 50% WFPS. In contrast, at 70% WFPS, the inhibition of C 2 H 2 on cumulative N 2 O emission decreased with increasing temperature. Since the effect of C 2 H 2 varied with soils and environmental conditions, this highlights the assumption that N 2 O production and nitrification can be affected by low concentrations of C 2 H 2 may be not appropriate in some occasions. [ABSTRACT FROM AUTHOR]

The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is a powerful tool that can be used to promote nitrogen (N) use efficiency and reduce N losses from agricultural systems by slowing nitrification. Mounting evidence has confirmed the functional importance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in nitrification and N2O production; however, their responses to DMPP amendment and the microbial mechanisms underlying the variable efficiencies of DMPP across different soils remain largely unknown. Here we compared the impacts of DMPP on nitrification and the dynamics of ammonia oxidizers between an acidic pasture soil and an alkaline vegetable soil using a 15N tracing and 13CO2-DNA-stableisotope probing (SIP) technique. The results showed that DMPP significantly inhibited nitrification and N2O production in the vegetable soil only, and the transient inhibition was coupled with a significant decrease in AOB abundance. No significant effects on the community structure of ammonia oxidizers or the abundances of total bacteria and denitrifiers were observed in either soil. The 15N tracing experiment revealed that autotrophic nitrification was the predominant form of nitrification in both soils. The 13CO2-DNA-SIP results indicated the involvement of AOB in active nitrification in both soils, but DMPP inhibited the assimilation of 13CO2 into AOB only in the vegetable soil. Our findings provide evidence that DMPP could effectively inhibit nitrification through impeding the abundance and metabolic activity of AOB in the alkaline vegetable soil but not in the acidic pasture soil, possibly due to the low AOB abundance or the adsorption of DMPP by organic matter. [ABSTRACT FROM AUTHOR]