Impact of dimethylpyrazole-based nitrification inhibitors on soil-borne bacteria

Enhancing agroecosystem nitrogen management: microbial insights for improved nitrification inhibition

Nitrification is a key microbial process in the nitrogen (N) cycle that converts ammonia to nitrate. Excessive nitrification, typically occurring in agroecosystems, has negative environmental impacts, including eutrophication and greenhouse gas emissions. Nitrification inhibitors (NIs) are widely used to manage N in agricultural systems by reducing nitrification rates and improving N use efficiency. However, the effectiveness of NIs can vary depending on the soil conditions, which, in turn, affect the microbial community and the balance between different functional groups of nitrifying microorganisms. Understanding the mechanisms underlying the effectiveness of NIs, and how this is affected by the soil microbial communities or abiotic factors, is crucial for promoting sustainable fertilizer practices. Therefore, this review examines the different types of NIs and how abiotic parameters can influence the nitrifying community, and, therefore, the efficacy of NIs. By discussing the latest research in this field, we provide insights that could facilitate the development of more targeted, efficient, or complementary NIs that improve the application of NIs for sustainable management practices in agroecosystems.

Effectiveness of three nitrification inhibitors on mitigating trace gas emissions from different soil textures under surface and subsurface drip irrigation

Nitrification inhibitors (NIs) and drip irrigation are recommended to mitigate trace gas emissions from agricultural soils. However, studies comparing the effect of different NIs on the release of trace gases from soils with contrasting textures under subsurface (SBD) and surface (SD) drip irrigation are lacking. Therefore, this study aimed to assess the effectiveness of three NIs in mitigating nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) emissions from two soils with different textures under SBD, with pipe buried in 10 cm depth, and SD. Two greenhouse experiments were carried out with silt loam and loamy sand soil textures cultivated with wheat under SBD and SD to assess the effectiveness of the NIs Dicyandiamide (DCD), 3,4-Dimethylpyrazole phosphate (DMPP), and 3-Methylpyrazol combined with Triazol (MP + TZ). Ammonium sulfate was applied at a rate of 0.18 g N kg soil-1. The measured variables were daily and cumulative N2O-N, CO2-C, and CH4-C emissions, as well as soil NH4+-N and NO3--N concentrations. The NIs and SBD had additive effects on reducing N2O-N emissions in the silt loam, but not in the loamy sand soil texture. Under SBD, total N2O-N emissions were 44% and 52% lower than under SD in the silt loam and loamy sand soil textures, respectively. Moreover, DMPP kept the highest NH4+-N concentrations and promoted the lowest N2O-N release. CO2-C and CH4-C total emissions were not affected by the treatments. Our findings supported the hypothesis that SBD decreases N2O-N emissions relative to SD. Among the investigated NIs, DMPP has the highest effectiveness in retarding nitrification and mitigating N2O-N release under the studied treatments. Finally, in coarse-textured soils, the use of NIs could be sufficient to significantly abate N2O-N emissions.

Core Soil Microorganisms and Abiotic Properties as Key Mechanisms of Complementary Nanoscale Zerovalent Iron and Nitrification Inhibitors in Decreasing Paclobutrazol Residues and Nitrous Oxide Emissions

Agrochemical residues and nitrous oxide (N2O) emissions have caused considerable threats to agricultural soil ecology. Nanoscale zerovalent iron (nZVI) and nitrification inhibitors might be complementary to each other to diminish soil agrochemical residues and N2O emissions and enhance soil bacterial community diversities. Compared to the control, the nZVI application declined soil paclobutrazol residues by 5.9% but also decreased the bacterial community co-occurrence network node. Combined nZVI and Dicyandiamide applications significantly decreased soil N2O emission rates and paclobutrazol residues but promoted Shannon diversity of the bacterial community. The increased soil pH, ammonium nitrogen, and Actinobacteriota could promote soil paclobutrazol dissipation. The nZVI generated double-edged sword effects of positively decreasing paclobutrazol residues and N2O emissions but negatively influencing soil multifunctionalities. The nZVI and Dicyandiamide could be complementary to each other in diminishing soil agrochemical residues and N2O emission rates but promoting soil bacterial community diversities simultaneously.

Drug discovery-based approach identifies new nitrification inhibitors

Nitrogen (N) fertilization is crucial to sustain global food security, but fertilizer N production is energy-demanding and subsequent environmental N losses contribute to biodiversity loss and climate change. N losses can be mitigated be interfering with microbial nitrification, and therefore the use of nitrification inhibitors in enhanced efficiency fertilizers (EEFs) is an important N management strategy to increase N use efficiency and reduce N pollution. However, currently applied nitrification inhibitors have limitations and do not target all nitrifying microorganisms. Here, to identify broad-spectrum nitrification inhibitors, we adopted a drug discovery-based approach and screened 45,400 small molecules on different groups of nitrifying microorganisms. Although a high number of potential nitrification inhibitors were identified, none of them targeted all nitrifier groups. Moreover, a high number of new nitrification inhibitors were shown to be highly effective in culture but did not reduce ammonia consumption in soil. One archaea-targeting inhibitor was not only effective in soil, but even reduced - when co-applied with a bacteria-targeting inhibitor - ammonium consumption and greenhouse gas emissions beyond what is achieved with currently applied nitrification inhibitors. This advocates for combining different types of nitrification inhibitors in EEFs to optimize N management practices and make agriculture more sustainable.

Niche Differentiation Among Canonical Nitrifiers and N2O Reducers Is Linked to Varying Effects of Nitrification Inhibitors DCD and DMPP in Two Arable Soils

The efficacy of nitrification inhibitors (NIs) dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) varies with soil types. Understanding the microbial mechanisms for this variation may lead to better modelling of NI efficacy and therefore on-farm adoption. This study addressed the response patterns of mineral nitrogen, nitrous oxide (N₂O) emission, abundances of N-cycling functional guilds and soil microbiota characteristics, in relation to urea application with or without DCD or DMPP in two arable soils (an alkaline and an acid soil). The inhibition of nitrification rate and N₂O emission by NI application occurred by suppressing ammonia-oxidizing bacteria (AOB) abundances and increasing the abundances of nosZI-N₂O reducers; however, abundances of ammonia-oxidizing archaea (AOA) were also stimulated with NIs-added in these two arable soils. DMPP generally had stronger inhibition efficiency than DCD, and both NIs' addition decreased Nitrobacter, while increased Nitrospira abundance only in alkaline soil. N₂O emissions were positively correlated with AOB and negatively correlated with nosZI in both soils and AOA only in acid soil. Moreover, N₂O emissions were also positively correlated with nirK-type denitrifiers in alkaline soil, and clade A comammox in acid soil. Amendment with DCD or DMPP altered soil microbiota community structure, but had minor effect on community composition. These results highlight a crucial role of the niche differentiation among canonical ammonia oxidizers (AOA/AOB), Nitrobacter and Nitrospira, as well as nosZI- and nosZII-N₂O reducers in determining the varying efficacies of DCD and DMPP in different arable soils.

An optimized hydroponic pipeline for large-scale identification of wheat genotypes with resilient biological nitrification inhibition activity

Several plant species have been reported to inhibit nitrification via their root exudates, the so-called biological nitrification inhibition (BNI). Given the potential of BNI-producing plants to sustainably mitigate N losses in agrosystems, identification of BNI activity in existing germplasms is of paramount importance. A hydroponic system was combined with an optimized Nitrosomonas europaea-based bioassay to determine the BNI activity of root exudates. The pipeline allows collecting and processing hundreds of root exudates simultaneously. An additional assay was established to assess the potential bactericide effect of the root exudates. The pipeline was used to unravel the impact of developmental stage, temperature and osmotic stress on the BNI trait in selected wheat genotypes. Biological nitrification inhibition activity appeared consistently higher in wheat at the pretillering stage as compared to the tillering stage. While low-temperatures did not alter BNI activities in root exudates, osmotic stress appeared to change the BNI activity in a genotype-dependent manner. Further analysis of Nitrosomonas culture after pre-exposure to root exudates suggested that BNI activity has no or limited bactericide effects. The present pipeline will be instrumental to further investigating the dynamics of BNI activity and to uncover the diversity of the BNI trait in plant species.

Availability of Nitrogen in Soil for Irrigated Cotton Following Application of Urea and 3,4-Dimethylpyrazole Phosphate-Coated Urea in Concentrated Bands

Low nitrogen (N) fertilizer use efficiency for irrigated cotton has been attributed to the limited ability of tap roots to access N from concentrated subsurface bands, or the preferential root uptake of microbially-mineralized dissolved organic N. This work investigated how applying high-rate banded urea affects the availability of N in soil and the capacity of cotton roots to take up N. Soil was analyzed for water-extractable total dissolved N and inorganic N species after urea or urea coated with 3,4-dimethylpyrazole phosphate (DMPP) was applied at concentrations of 261, 455, 461, and 597 mg N kg^-1 of (air-dry) soil (mean bulk density: 1.01 g cm^-3). A mass balance was used to compare N applied as fertilizer and in unfertilized soil (supplied N) with the N recovered from soil within the cylinders (recovered N) at five plant growth phases. Root uptake was estimated by comparing ammonium-N (NH₄-N) and nitrate-N (NO₃-N) in soil sampled from within cylinders with soil sampled from immediately outside. Recovered N was up to 100% above supplied N within 30 days of applying urea above 261 mg N kg^-1 of soil. Significantly lower NO₃-N in soil sampled from immediately outside the cylinders suggests urea application stimulates cotton root uptake. The use of DMPP-coated urea prolonged high NH₄-N in soil and inhibited the mineralization of released organic N. These results imply the release of previously sequestered soil organic N within 30 days of applying concentrated urea enhances the availability of NO₃-N in the rhizosphere, reducing N fertilizer use efficiency.

Synergistically mitigating nitric oxide emission by co-applications of biochar and nitrification inhibitor in a tropical agricultural soil

Agricultural soils are the hotspots of nitric oxide (NO) emissions, which are related to atmospheric pollution and greenhouse effect. Biochar application has been recommended as an important countermeasure, however, its mitigation efficiency is limited as biochar, under certain conditions, can stimulate soil nitrification. Therefore, biochar co-applied with nitrification inhibitor could optimize the mitigation potential of biochar. Herein, a laboratory-scale experiment was conducted to investigate the effects of co-application of biochar and nitrification inhibitor on NO emission, nitrogen cycling function and bacterial community in a tropical vegetable soil. Results showed that a single application of biochar or nitrification inhibitor significantly decreased NO emissions, and this mitigation effectiveness was amplified by their co-applications. Soil NO₂^--N intensity, along with abundances of AOB-amoA and nirK were significantly and positively correlated with cumulative NO emissions. The stimulated activity of ammonia monooxygenase and growths of AOB and total comammox Nitrospira by biochar were weakened by nitrification inhibitor, implying decreased nitrification-driven NO production. The nitric oxide reductase activity and related qnorB abundance in nitrification inhibitor-added soils were increased by biochar, indicating promoted NO consumption during denitrification. The nirK abundance and NO₂^--N intensity were decreased more by co-applications of biochar or nitrification inhibitor. Moreover, both biochar and nitrification inhibitor changed bacterial β-diversity, and their co-application synergistically enriched Armatimonadetes and Verrucomicrobia abundances and decreased WPS-2 abundance. This study highlights that co-applications of biochar and nitrification inhibitor can make their respective advantages complementary to each other, thereby achieving a larger mitigation of NO emissions from agricultural soils in tropical regions.

Nitrapyrin Addition Mitigated CO2 Emission from a Calcareous Soil Was Closely Associated with Its Effect on Decreasing Cellulolytic Fungal Community Diversity

Application of nitrification inhibitors (NIs) has been widely used to inhibit nitrification and reduce N₂O emissions. However, the impacts of NI addition on soil carbon transformation and carbon-degrading microbial communities have not been well explored. Here, a microcosm experiment was carried out, and four treatments were designed: (i) unfertilized control, (ii) urea alone, (iii) urea plus cattle manure, and (iv) urea plus cattle manure with nitrapyrin. The influence of nitrapyrin on soil CO₂ emissions, carbon-degrading extracellular enzyme activities, and the abundance and diversity of the cbhI community was investigated. Compared to the treatment of urea plus cattle manure, nitrapyrin significantly decreased cumulative CO₂ emissions by 51.8%. Moreover, cbhI community gene copies and their α-diversities (P < 0.05) were also significantly reduced by nitrapyrin application. A partial least squares path model showed that CO₂ emission was positively associated with cbhI community α-diversity but negatively associated with nitrapyrin addition. We conclude that the mitigation of soil CO₂ emissions by nitrapyrin can be ascribed to its effects on decreasing of cellulose-degrading gene community diversity. Our findings provide new insights into the side-effects of nitrapyrin on abating CO₂ emission.