Inhibitors are widely considered an efficient tool for reducing nitrogen (N) loss and improving N use efficiency, but their effectiveness is highly variable across agroecosystems. In this study, we synthesized 182 studies (222 sites) worldwide to evaluate the impacts of inhibitors (urease inhibitors [UI], nitrification inhibitors [NI] and combined inhibitors) on crop yields and gaseous N loss (ammonia [NH₃] and nitrous oxide [N₂O] emissions) and explored their responses to different management and environmental factors including inhibitor application timing, fertilization regime, cropping system, water management, soil properties and climatic conditions using subgroup meta‐analysis, meta‐regression and multivariate analyses. The UI were most effective in enhancing crop yields (by 5%) and reducing NH₃ volatilization (by 51%), whereas NI were most effective at reducing N₂O emissions (by 49%). The application of UI mitigates NH₃ loss and increases crop yields especially in high NH₃‐N loss scenarios, whereas NI application would minimize the net N₂O emissions and the resultant environmental impacts especially in low NH₃‐N loss scenarios. Alternatively, the combined application of UI and NI enables producers to balance crop production and environmental conservation goals without pollution tradeoffs. The inhibitor efficacy for decreasing gaseous N loss was dependent upon soil and climatic conditions and management practices. Notably, both meta‐regression and multivariate analyses suggest that inhibitors provide a greater opportunity for reducing fertilizer N inputs in high‐N‐surplus systems and presumably favor crop yield enhancement under soil N deficiency situations. The pursuit of an improved understanding of the interactions between plant‐soil‐climate‐management systems and different types of inhibitors should continue to optimize the effectiveness of inhibitors for reducing environmental losses while increasing productivity.

Soybean ( Glycine max L.) is known to contribute to soil N reserves when grown in rotation with other high‐value crops such as corn ( Zea mays L.) and winter wheat ( Triticum aestivum L.). However, continuous soybean and “short” soybean rotations (e.g., corn–soybean, wheat–soybean) may cause declining soybean yields and degrading soil health over time. In this long‐term field study, we determined crop rotation effects on soybean yield and soil health indicators for a cool, humid, clay loam soil in southwestern Ontario. The study used nine soybean rotations, which included continuous soybean (S), corn–soybean (C–S), corn–soybean–soybean (C–S–S), and six rotations where winter wheat (WW) was grown with red clover ( Trifolium pratense L.) (+RC) and without underseeded red clover (i.e., winter wheat–soybean [WW–S, WW+RC–S], corn–soybean–winter wheat [C–S–WW, C–S–WW+RC], and winter wheat–soybean–soybean [WW–S–S, WW+RC–S–S]). Ten soil health indicators during the first and second soybean phase of each rotation were measured in 2018, whereas soybean yield was measured from 2002 to 2018. Soybean yields in 2‐ and 3‐yr rotations were 39–44% and 48–52% greater, respectively, relative to continuous soybean excluding the rotations with 2 of 3 yr of soybean (C–S–S, WW–S–S, WW+RC–S–S), which were only 22–35% greater than continuous soybean. Partial least squares regression indicated that inorganic N, particulate organic matter N, particulate organic matter C, potentially mineralizable N, total C, soil respiration rate, and water extractable organic soil C were the most important soil health indicators, explaining 34% of the total variation in soybean yields. It was concluded that soybean grown in 3‐yr rotations with corn and winter wheat produced the largest soybean yields and the greatest positive impacts on soil health indicators likely owing to cereal crops enhancing C inputs into soil.

Demand for food is rising and the ability of any particular soil to support sustainable food production is dependent upon a variety of soil biochemical, chemical and physical soil parameters. However, the challenge is that the impacts of a new management practice on soil properties may take years to assess. In this field study, we investigated the effects of long‐term cropping rotation treatments (established in 2001) on soil health indicators from soils under monoculture, 2‐yr and 3‐yr crop rotations with and without a cover crop. In particular, we compared soil heath indicators under eight cropping sequences including continuous winter wheat ( Triticum aestivum L.) (WW), soybean ( Glycine max L.)–winter wheat (S–WW), corn ( Zea mays L.)–soybean–winter wheat (C–S–WW), and winter wheat–soybean–soybean (WW–S–S) all with and without red clover ( Trifolium pretense L.) (RC) in the wheat phase of the rotation. We measured ten soil health indicators collected from the wheat phase of the rotation. Crop rotation had greater effects on soil health indicators than cover crop. After 17 years, crop yields were 23–28% greater for the 2‐yr and 3‐yr rotations than monoculture WW in the presence of red clover. In the absence of red clover, yields were 32–39% greater for the 2‐yr and 3‐yr rotations than monoculture WW. However, the soil health indicators were significantly greater for monoculture WW than the 2‐yr and 3‐yr rotations. These results suggest WW enhanced soil health while crop rotations with soybean negatively impacted most biochemical soil health parameters.

Ensuring sufficient fertilizer nitrogen (N) for crops while minimizing N losses requires best management practices optimized for climate, crop, soil, and root zone hydrology. In Ontario, pre‐plant N fertilization of corn ( Zea mays L.) is common; however, this practice extends the time between application and significant root interception of N by the plant, potentially increasing the risk of N loss through soil nitrous oxide emissions, ammonia (NH ₃ ) volatilization, and nitrate leaching. These losses contribute to greenhouse gas emissions, affect air quality (NH ₃ ), and are a substantial financial loss. This study compared three N placement methods (broadcast urea [BrUrea], broadcast incorporated urea [BrIncUrea], and injected urea ammonium nitrate [InjUAN]) and the presence or absence of N metabolite inhibitors (urease inhibitor [UI], urease plus nitrification inhibitor [UI+NI]). Fertilizer N was applied immediately before planting (150 kg N ha ⁻¹ ) to all treatments except for the control. Averaged over 3 yr (2015–2017), NH ₃ losses were reduced by 34% from BrIncUrea, by 42–55% from BrUrea+UI+NI and BrIncUrea+UI+NI, and by 99% from InjUAN relative to BrUrea (21 kg N ha ⁻¹ ). On average, N application increased corn grain yields by 83% relative to the control (6 t ha ⁻¹ ). There were no annual yield differences among N placement methods. It was concluded that incorporation or injection of N in soil and use of urease and nitrification inhibitors reduced NH ₃ emissions when N fertilizer was applied pre‐plant.