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This position paper summarizes the current understanding of biological nitrification inhibition (BNI) to identify research needs for accelerating the development of BNI as a N2O mitigation strategy for grazed livestock systems. We propose that the initial research focus should be on the systematic screening of agronomically desirable plants for their BNI potency and N2O reduction potential. This requires the development of in situ screening methods that can be combined with reliable N2O emission measurements and microbial and metabolomic analyses to confirm the selective inhibition of nitrification. As BNI-induced reductions in N2O emissions can occur by directly inhibiting nitrification, or via indirect effects on other N transformations, it is also important to measure gross N transformation rates to disentangle these direct and indirect effects. However, an equally important challenge will be to discern the apparent influence of soil N fertility status on the release of BNIs, particularly for more intensively managed grazing systems. [ABSTRACT FROM AUTHOR]

Biological nitrification inhibition is a plant‐mediated rhizosphere process where natural nitrification inhibitors can be produced and released by roots to suppress nitrifier activity in soil. Nitrification is one of the critical soil processes in the nitrogen (N) cycle, but unrestricted and rapid nitrification in agricultural systems can result in major losses of N from the plant–soil system (i.e., by NO3− leaching and gaseous N emissions). In this study, we explored the potential efficacy of biological nitrification inhibitors (linoleic acid [LA] and linolenic acid [LN]) and a proven efficient synthetic (dicyandiamide [DCD]) nitrification inhibitor on N dynamics, nitrous oxide (N2O) and carbon dioxide (CO2) emissions in a highly nitrifying soil. 14C‐labelled LA, LN and DCD mineralization was determined in a parallel experiment to explore the fate of inhibitors after application. We found that LA and LN had no effect on soil NH4+ concentrations, but significantly decreased NO3− concentrations. Soil that received DCD had lower NO3− and higher NH4+ concentrations than the control (soil without nitrification inhibitors). LA and LN increased the cumulative N2O and CO2 emissions when they were applied at high concentrations (635 or 1,270 mg kg−1 dry soil). LA and LN had a much greater mineralization rate than that of DCD: 47–56%, 37–61% and 2.7–5.5%, respectively, after 38 days incubation. We conclude that in contrast to the direct inhibition of nitrification caused by DCD, addition of LA and LN may cause apparent nitrification inhibition by promoting microbial immobilization of soil NH4+ and/or NO3−. Future studies on nitrification inhibitors need to clearly differentiate between the direct and indirect effects that result from addition of these compounds to soil. Highlights: The efficacy and stability of nitrification inhibitors in a highly nitrifying soil were explored.This study supports efforts to mitigate N losses and improve nitrogen use efficiency of inputs.Addition of LA, LN and DCD can decrease NO3− concentration, but their modes of action may be different.The apparent effect of LA and LN on soil NO3− concentration could be indirect. [ABSTRACT FROM AUTHOR]

Nitrification inhibitors, originally proposed for nitrate leaching mitigation, are recommended as effective nitrous oxide (N 2 O) mitigation strategies. Several compounds have been trialled and used in the past including dicyandiamide (DCD) or 3,4-dimethylpyrazole phosphate (DMPP). Yet, little is known about the new nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA). A field experiment using 15 N single-labelled ammonium nitrate ( 15 NH 4 NO 3 or NH 4 15 NO 3 ) was conducted to understand the effectiveness of DMPSA on a biochemical basis in an irrigated maize ( Zea mays L.) crop. Gaseous fluxes, i.e. N 2 O, 15 N 2 O, 15 N 2 , methane (CH 4 ), and carbon dioxide (CO 2 ) were measured, as well as soil mineral N, yield components and 15 N recovery in plant and soil. During the maize cropping period, the use of DMPSA significantly reduced cumulative N 2 O emissions (118 g N ha −1 ) compared to ammonium nitrate without inhibitor (231 g N ha −1 ). The 15 N analyses revealed that most N 2 O losses (particularly during the emission peak) came from 15 NH 4 NO 3 (i.e. nitrification, nitrifier denitrification and/or coupled nitrification denitrification) rather than NH 4 15 NO 3 in this calcareous low C-content soil. As expected, DMPSA decreased N 2 O losses from 15 NH 4 + oxidation, but an effect on non-target microorganisms was noticed, as shown by the significant reduction of respiration rates and N 2 O losses coming from 15 NO 3 − . No significant effect of DMPSA on CH 4 oxidation or 15 N 2 fluxes was observed. The DMPSA did not lead to a significant improvement of the dry weights of grain or biomass, although an increment of root biomass by 64% was found. This compound also tended to increase plant N recovery (average 67.8%) and to decrease soil N recovery (average 18.3%) but differences were not statistically significant. Conversely, the nitrification inhibitor significantly reduced the residual fertilizer-N in the 15–30 cm and 30–45 cm soil layers. The use of DMPSA was confirmed as a highly effective tool to reduce N 2 O emissions from irrigated crops in semi-arid areas. [ABSTRACT FROM AUTHOR]

• Heterotrophic nitrification is an important source of N 2 O at low WFPS. • Similar contributions of nitrification, heterotrophic nitrification and denitrification occurred at medium WFPS level. • High N 2 O emissions result via nitrification coupled denitrification at medium & high WFPS. • Abiotic factors need to be considered as well as biotic factors to understand the cause of emissions. To identify the production and consumption pathways and temporal dynamics of N 2 O emitted from soil, this study uses 15N-labelled substrate-N to quantify the underlying gross N transformation rates using the Ntrace analysis tool and link them to N-emissions. In three experiments twelve soil cores each were incubated in a lab incubation system to measure gaseous emissions, while parallel incubations under the same conditions were set up for destructive soil sampling at 7 time points. Using the triple labelling technique (applying NH 4 NO 3 with either the NH 4 +-N or the NO 3 −-N, or both being 15N labelled), this study investigated the effects of 55, 70 and 85% water filled pore space (deemed to promote nitrification, both nitrification and denitrification, and denitrification, respectively) in a clay soil on gaseous N emissions and investigates the source and processes leading to N 2 O emissions. To assess the utilisation of applied NO 3 − vs. nitrified NO 3 − from applied NH 4 +, the 15N tracing tool Ntrace was used to quantify the rates of immobilisation of NO 3 − and NH 4 +, oxidation of NH 4 +, mineralisation of organic N and subsequent nitrification by the analysis of the 15N in the soil. Gross transformation rates were calculated, indicating the relative importance of added NO 3 − and NO 3 − derived from nitrified added NH 4 +. Results show an important contribution of heterotrophic nitrification (organic N oxidation to NO 3 −) which was highest at the 55% water filled pore space (WFPS), decreasing in its contribution to N-transformation processes with increasing WFPS, while nitrification (NH 4 + oxidation to NO 3 −) was contributing the most at 70% WFPS. The contribution of denitrification increased with increasing WFPS, but only became dominant at 85% WFPS. While denitrification still showed to be most important at high and nitrification at lower WFPS, the actual % WFPS values were not as expected and highlight the fact that WFPS is a contributor, but not the sole/most important parameter determining the type of N-transformation processes taking place. [ABSTRACT FROM AUTHOR]