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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]

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]