Exploratory Graph

To visualize the global distribution of research sites for inhibitor applications, we utilized R software (version R-4.5.0). Most study sites were located in Asia and Europe (Fig. 1).

For each variable, the figure presents the pooled effect size as the natural logarithm of the response ratio (lnRR), accompanied by 95% confidence intervals. Negative values indicate a reduction due to nitrification inhibitors (NIs) application, while positive values reflect an increase. The results reveal that soil nitrate (NO₃⁻–N) concentrations significantly increased by 16.67%, suggesting effective suppression of nitrification and enhanced nitrate retention. Soil ammonium (NH₄⁺–N) levels increased by 21.21%, indicating the accumulation of ammonium due to reduced microbial oxidation. Ammonia (NH₃) emissions increased significantly by 17.99%, possibly driven by higher ammonium concentrations and subsequent volatilization. Nitrous oxide (N₂O) emissions were significantly reduced by 67.31%, demonstrating the strong potential of NIs in mitigating agricultural greenhouse gas emissions. Crop yield exhibited a modest but positive response, with an average increase of 6.1%, highlighting potential agronomic benefits. Nitrogen use efficiency (NUE) improved significantly by 16.55%, reflecting more effective nitrogen retention in the soil and uptake by crops (Fig. 2).

Fig. 2.  The forest plot summarizing the meta-analysis of the effects of nitrification inhibitors (NIs) application versus no NIs treatment on six key indicators: soil nitrate (NN), soil ammonium (AN), ammonia (NH₃) emissions, nitrous oxide (N₂O) emissions, crop yield, and nitrogen use efficiency (NUE). “ * ” indicated P < 0.05

In this meta-analysis, five types of nitrification inhibitors (NIs) were selected for evaluation based on their sufficient representation in the literature. Overall, all five NIs significantly improved both crop yield and nitrogen use efficiency (NUE). For crop yield, DMPP (3,4-dimethylpyrazole phosphate) showed the greatest enhancement, leading to a significant increase of 7.36% compared to the control. In terms of NUE, DCD (dicyandiamide) was the most effective, resulting in a significant improvement of 35.85%. Regarding gaseous nitrogen losses, the combination of DCD and NBPT (N-(n-butyl) thiophosphoric triamide) was the most effective in suppressing ammonia (NH₃) emissions, achieving a reduction of 84.14%. For nitrous oxide (N₂O) emissions, DMPP again proved most effective, reducing emissions by an impressive 98.64%. As for soil ammonium (NH₄⁺–N) concentration, all inhibitors significantly increased its contentration, with DCD having the strongest effect (+49.3%). In contrast, NBPT significantly reduced soil NH₄⁺–N by 34.89%, likely due to its specific urease inhibition mechanism affecting nitrogen transformation pathways (Fig. 3).

In summary, DMPP appears to be the most promising nitrification inhibitor, offering both agronomic benefits through increased yield and environmental benefits via strong suppression of N₂O emissions.

Given that the dataset included studies conducted under various cropping systems, I evaluated the effects of nitrification inhibitors (NIs) application across different plant types. The analysis revealed that NIs application did not significantly alter soil ammonium (NH₄⁺–N) concentrations under most crops. However, in soils planted with straw-based systems, NH₄⁺–N levels were reduced following NIs treatment, possibly due to specific soil-plant interactions or microbial activity under high-residue conditions. NIs application significantly increased crop yield in both wheat and maize, by 7.70% and 8.25%, respectively, demonstrating consistent agronomic benefits in cereal cropping systems. Across most crop types, NIs application led to a notable reduction in soil nitrous oxide (N₂O) emissions, with the strongest effect observed in maize fields, where emissions decreased by 61.68%. In terms of nitrogen use efficiency (NUE), improvements were observed for nearly all crops except oats. The greatest enhancement occurred in cotton, where NUE increased by 25.02%, suggesting that NIs can be particularly effective in high nitrogen-demand crops (Fig. 4).

Overall, the application of nitrification inhibitors consistently enhanced crop yield and NUEwhile simultaneously mitigating N₂O emissions across a variety of crop production systems.

Soil mineralized nitrogen, including nitrate (NO₃⁻–N) and ammonium (NH₄⁺–N), is a key indicator of soil nutrient availability. As shown in previous sections, the application of nitrification inhibitors (NIs) significantly alters the concentrations of both NO₃⁻ and NH₄⁺ in soil, and also affects crop yield and N₂O emissions. However, the interrelationship among these variables following NIs application remained unclear. To address this, we modeled the relationships between soil mineralized nitrogen and both crop yield and N₂O emissions under NI treatment. The results revealed a significant positive correlation between mineralized nitrogen and crop yield (R = 0.58, P = 0.017), indicating that higher levels of mineralized N contribute to improved crop productivity when NIs are applied.

These findings suggest that after NI application, monitoring soil mineralized nitrogen levels is essential to ensure optimal nitrogen availability for plant uptake and to guide nitrogen management strategies effectively (Fig. 5).

Ammonia (NH₃) volatilization and nitrous oxide (N₂O) emissions are two major pathways of nitrogen loss from agricultural soils. These losses not only reduce the availability of nitrogen for plant uptake but also contribute to environmental pollution. One of the key objectives of applying nitrification inhibitors (NIs) is to mitigate such nitrogen losses and improve nitrogen use efficiency (NUE). In this analysis, we evaluated how NH₃ and N₂O emissions following NI application relate to plant NUE. The results showed that N₂O emissions had no significant relationship with NUE under NI treatment. However, a strong positive correlation was observed between NH₃ emissions and NUE (R = 0.96, P < 0.05), indicating that higher NH₃ emissions were associated with greater nitrogen use efficiency. This counterintuitive result may be attributed to the fact that increased NH₃ volatilization could reflect elevated levels of available soil ammonium (NH₄⁺), which in turn enhances nitrogen uptake by crops. Additionally, some degree of NH₃ loss may occur under conditions where nitrogen supply exceeds crop demand, yet still supports vigorous plant growth and efficient nitrogen utilization (Fig. 6).

According to the results, no significant correlation was observed between soil NH₃ and N₂O emissions following the application of nitrification inhibitors (Fig. 7)

Fig. 7. The relationships between NH3 emissions and soil N₂O emissions under nitrification inhibitor (NIs) application. The 95 per cent confidence intervals are shaded in the figure.

In addition to evaluating the effects of different types of nitrification inhibitors (NIs), the duration of their effectiveness has also been widely considered in previous studies. Some reports have suggested that a longer duration of NI activity is associated with greater reductions in soil N₂O emissions (Fig. 8).

However, our meta-analysis found that the duration of NI application did not significantly affect soil mineralized nitrogen dynamics, N₂O emissions, NH₃ volatilization, or nitrogen use efficiency (NUE) at the global scale. These findings suggest that the effectiveness of NIs may be more strongly influenced by other factors such as soil properties, crop type, or environmental conditions, rather than the duration alone (Fig. 8).

Fig. 8. Relationships between the duration of nitrification inhibitor (NI) effectiveness (in months) and soil mineralized nitrogen, ammonia (NH₃) emissions, nitrous oxide (N₂O) emissions, and nitrogen use efficiency (NUE). The 95 per cent confidence intervals are shaded in the figure.

The results indicate that, following the application of nitrification inhibitors, the key environmental factor influencing soil NH₃ emissions is mean annual precipitation, while mean annual temperature plays a dominant role in regulating plant nitrogen use efficiency (NUE). Additionally, soil pH was identified as a critical factor affecting the behavior of nitrification inhibitors in soils (Fig. 9).

Fig. 9. Random forest analysis identifying key predictors of soil NH₃ emissions, plant nitrogen use efficiency (NUE), and soil N₂O emissions under nitrification inhibitor (NI) application. “*” indicates P < 0.05, and “**” indicates P < 0.01. Mean of annual temperature (MAT), mean of annual precipitation (MAP), organic carbon (OC), total nitrogen (TN), and available phosphorus (AP).

Based on the variable importance identified through random forest analysis, we further modeled the relationships between MAP and NH₃ emissions, MAT and NUE, as well as soil pH and N₂O emissions under nitrification inhibitors (NIs) application (Fig. 10).

Globally, NH₃ emissions from soils treated with NIs were positively correlated with mean annual precipitation (MAP). This trend may be attributed to higher soil moisture under increased rainfall, which promotes ammonium accumulation near the soil surface and enhances conditions for ammonia volatilization, especially when NIs reduce nitrification rates (Fig. 10) 

Nitrogen use efficiency (NUE) increased with rising mean annual temperature (MAT). This is likely because higher temperatures can stimulate microbial activity and root metabolism, thereby enhancing nitrogen uptake and assimilation in plants when nitrification is inhibited and nitrogen is retained longer in ammonium form (Fig. 10).

Fig. 10. Relationships observed under nitrification inhibitor (NI) application: mean annual precipitation (MAP) vs. soil NH₃ emissions; mean annual temperature (MAT) vs. plant nitrogen use efficiency (NUE); and soil pH vs. N₂O emissions. The 95 per cent confidence intervals are shaded in the figure.