Mechanism of action of nitrification inhibitors based on dimethylpyrazole: A matter of chelation

Predicting Toxicity toward Nitrifiers by Attention-Enhanced Graph Neural Networks and Transfer Learning from Baseline Toxicity

Assessing chemical environmental impacts is critical but challenging due to the time-consuming nature of experimental testing. Graph neural networks (GNNs) support superior prediction performance and mechanistic interpretation of (eco-)toxicity data, but face the risk of overfitting on the typically small experimental data sets. In contrast to purely data-driven approaches, we propose a mechanism-guided transfer learning strategy that is highly efficient and provides key insights into the underlying drivers of (eco-)toxicity. By leveraging the mechanistic link between baseline toxicity and toxicity toward nitrifiers, we pretrained a GNN on lipophilicity data (log P) and subsequently fine-tuned it on the limited data set of toxicity toward nitrifiers, achieving prediction performance comparable with pretraining on much larger but mechanistically less relevant data sets. Additionally, we enhanced GNN interpretability by adjusting multihead attentions after convolutional layers to identify key substructures, and quantified their contributions using a Shapley Value method adapted for graph-structured data with improved computational efficiency. The highlighted substructures aligned well with and effectively distinguished known structural alerts for baseline toxicity and specific modes of toxic action in nitrifiers. The proposed strategy will allow uncovering new structural alerts in other (eco)toxicity data, and thus foster new mechanistic insights to support chemical risk assessment and safe-by-design principles.

The regulatory mechanism controlling nitrification inhibitors-induced mitigation of nitrification and NO3--N leaching in alkaline purple soil

Nitrification inhibitors (NIs) are critical to reduce nitrogen (N) leaching losses. However, the efficacy of different NIs can be highly variable across soils and crop types, and a deeper understanding of the mechanistic basis of this efficiency variation, especially in purple soil under vegetable production, is lacking. To enrich this knowledge gap, the impact of different NIs amendment (3,4-dimethylpyrazole phosphate, DMPP; dicyandiamide, DCD; nitrapyrin, NP) on nitrification and the microbial mechanistic basis of controlling nitrate (NO3--N) leaching of vegetable purple soil was explored in southwest China. The results showed that DCD and NP effect is dose-dependent, with 10% DCD, 1% DMPP and 1% NP were found to be optimal for nitrification inhibition in vegetable purple soil. When compared with the control treatment without NIs amendments, DCD, DMPP and NP reduced NO3--N leaching by 26.3%, 30.6% and 19.2%, respectively. It was noteworthy that NO3--N leaching inhibition was mediated predominantly by inhibiting ammonia-oxidizing bacteria (AOB) abundance. DCD, NP and DMPP incorporation decreased the AOB abundance by 39.8%, 73.2% and 51.4% and suppressed the ammonia monooxygenase (AMO) activity by 22.2%, 36.8% and 28.7%, respectively, in comparison with the control treatment without NIs amendments. DMPP inhibited AOB abundance and AMO activity much more than DCD and NP. DMPP also significantly decreased AOB alpha diversity and altered their community structure, whereas DCD and NP had no significant effect. The mantel test indicated that AOB abundance and AMO activity are strongly correlated with NO3--N leaching rate. These results show that soil application of 1% DMPP effectively mitigates NO3--N leaching from sub-tropical alkaline purple vegetable soil. This study also expanded our mechanistic understanding of NO3--N leaching and its regulators in an alkaline soil vegetable production system with N fertilizer and NI inputs.

Effective strategy to improve nitrification inhibitor efficiency and minimize environmental risk with microenvironments created by ecofriendly biocomposites

Over half the global population depends on food grown with synthetic nitrogen fertilizers, but much of this nitrogen is lost as nitrates, N2O and NH3, harming the environment and health and incurring substantial environmental costs. Practical technologies aimed at enhancing nitrogen efficiency to reduce these losses promised considerable societal benefits. Nitrification inhibitors (NIs) can help reduce these losses, but their effectiveness varies, often lasting only weeks or days, for the strategy to improve NIs efficiency reducing environmental pollution that are still poorly contrived. Therefore, this study developed a novel approach by ecofriendly alginate and polyphenols to create a microenvironment (SANMP), which increased NIs based on DMPP stability at temperatures between 70 and 125 °C (47%-77% increase), in compound fertilizers (1.4%-11% increase), and in soils with a wide pH range of 5.6-7.9 (21%-27% increase). Enhanced stability can significantly increase environmental benefits in agriculture. SANMP reduces N2O emissions by 89% relative to nitrogen fertilizer-only treatments and a further 26% decrease compared to traditional DMPP formulations. Analysis of the chemical structure of alginate-metal-polyphenol hybrid materials demonstrated that DMPP immobilization, achieved through pore filling, chelation, and electrostatic attraction, significantly reduced its degradation from high temperatures, pH fluctuations, environmental ions, and soil microbial activities. The novel microenvironment offers an effective solution to the problems of high cost and unstable inhibition efficiency of DMPP, thus improving its environmental and agricultural benefits. This technology promises to offer solutions for nutrient management strategies that are efficient, highly beneficial to the environment and cost-effective.

Copper pyrazole addition regulates soil mineral nitrogen turnover by mediating microbial traits

The huge amount of urea applied has necessitated best-developed practices to slow down the release of nitrogen (N) fertilizer while minimizing nitrate loss. However, the impact of nitrification inhibitors on mineral-N turnover and the associated microbial mechanisms at different stages remains unknown. A 60-day incubation experiment was conducted with four treatments: no fertilizer (CK), urea (U), urea with copper pyrazole (UC), and urea coated with copper pyrazole (SUC), to evaluate the changes about soil ammonia N (N H 4 + -N) and nitrate N ( NO 3 - -N) levels as well as in soil microbial community throughout the whole incubation period. The results showed that copper pyrazole exhibited significantly higher inhibition rates on urease compared to other metal-pyrazole coordination compounds. The soilN H 4 + -N content peaked on the 10th day and was significantly greater in UC compared to U, while the NO 3 - -N content was significantly greater in U compared to UC on the 60th day. Copper pyrazole mainly decreased the expression of nitrifying (AOB-amoA) and denitrifying (nirK) genes, impacting the soil microbial community. Co-occurrence network suggested that Mycobacterium and Cronobacter sakazakii-driven Cluster 4 community potentially affected the nitrification process in the initial phase, convertingN H 4 + -N to NO 3 - -N. Fusarium-driven Cluster 3 community likely facilitated the denitrification of NO 3 - -N and caused N loss to the atmosphere in the late stage. The application of copper pyrazole may influence the process of nitrification and denitrification by regulating soil microbial traits (module community and functional genes). Our research indicates that the addition of copper pyrazole alters the community function driven by keystone taxa, altering mineral-N turnover and supporting the use of nitrification inhibitors in sustainable agriculture.

Impact of diversified cropping systems and fertilization strategies on soil microbial abundance and functional potentials for nitrogen cycling

Diversified cropping systems and fertilization strategies were proposed to enhance the abundance and diversity of the soil microbiome, thereby stabilizing their beneficial services for maintaining soil fertility and supporting plant growth. Here, we assessed across three different long-term field experiments in Europe (Netherlands, Belgium, Northern Germany) whether diversified cropping systems and fertilization strategies also affect their functional gene abundance. Soil DNA was analyzed by quantitative PCR for quantifying bacteria, archaea and fungi as well as functional genes related to nitrogen (N) transformations; including bacterial and archaeal nitrification (amoA-bac,arch), three steps of the denitrification process (nirK, nirS and nosZ-cladeI,II) and N2 assimilation (nifH), respectively. Crop diversification and fertilization strategies generally enhanced soil total carbon (C), N and microbial abundance, but with variation between sites. Overall effects of diversified cropping systems and fertilization strategies on functional genes were much stronger than on the abundance of bacteria, archaea and fungi. The legume-based cropping systems showed great potential not only in stimulating the growth of N-fixing microorganisms but also in boosting downstream functional potentials for N cycling. The sorghum-based intercropping system suppressed soil ammonia oxidizing prokaryotes. N fertilization reduced the abundance of nitrifiers and denitrifiers except for ammonia-oxidizing bacteria, while the application of the synthetic nitrification inhibitor DMPP combined with mineral N reduced growth of both ammonia-oxidizing bacteria and archaea. In conclusion, this study demonstrates a strong impact of diversified agricultural practices on the soil microbiome and their functional potentials mediating N transformations.

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.

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.

Evaluation of a crop rotation with biological inhibition potential to avoid N2O emissions in comparison with synthetic nitrification inhibition

Agriculture has increased the release of reactive nitrogen to the environment due to crops' low nitrogen-use efficiency (NUE) after the application of nitrogen-fertilisers. Practices like the use of stabilized-fertilisers with nitrification inhibitors such as DMPP (3,4-dimethylpyrazole phosphate) have been adopted to reduce nitrogen losses. Otherwise, cover crops can be used in crop-rotation-strategies to reduce soil nitrogen pollution and benefit the following culture. Sorghum (Sorghum bicolor) could be a good candidate as it is drought tolerant and its culture can reduce nitrogen losses derived from nitrification because it exudates biological nitrification inhibitors (BNIs). This work aimed to evaluate the effect of fallow-wheat and sorghum cover crop-wheat rotations on N₂O emissions and the grain yield of winter wheat crop. In addition, the suitability of DMPP addition was also analyzed. The use of sorghum as a cover crop might not be a suitable option to mitigate nitrogen losses in the subsequent crop. Although sorghum-wheat rotation was able to reduce 22% the abundance of amoA, it presented an increment of 77% in cumulative N₂O emissions compared to fallow-wheat rotation, which was probably related to a greater abundance of heterotrophic-denitrification genes. On the other hand, the application of DMPP avoided the growth of ammonia-oxidizing bacteria and maintained the N₂O emissions at the levels of unfertilized-soils in both rotations. As a conclusion, the use of DMPP would be recommendable regardless of the rotation since it maintains NH₄⁺ in the soil for longer and mitigates the impact of the crop residues on nitrogen soil dynamics.

Nitrification rate in dairy cattle urine patches can be inhibited by changing soil bioavailable Cu concentration

Ammonia oxidation to hydroxylamine is catalyzed by the ammonia monooxygenase enzyme and copper (Cu) is a key element for this process. We investigated the effect of soil bioavailable Cu changes induced through the application of Cu-complexing compounds on nitrification rate, ammonia-oxidizing bacteria (AOB) and archaea (AOA) amoA gene abundance, and mineral nitrogen (N) leaching in urine patches using the Manawatu Recent soil. Further, evaluated the combination of organic compound calcium lignosulphonate (LS) with a growth stimulant Gibberellic acid (GA). Treatments were applied in May 2021 as late-autumn treatments: control (no urine), urine-only at 600 kg N ha^-1, urine + dicyandiamide (DCD), urine + co-poly-acrylic-maleic acid (PA-MA), urine + LS, urine + split-application of LS (2LS), and urine + combination of GA plus LS (GA + LS). In addition, another four treatments were applied in July 2021 as mid-winter treatments: control, urine-only at 600 kg N ha^-1, urine + GA, and urine + GA + LS. Soil bioavailable Cu and mineral N leaching were examined during the experimental period. The AOB/AOA amoA genes were quantified using quantitative polymerase chain reaction. Changes in soil bioavailable Cu across treatments correlated with nitrification rate and AOB amoA abundance in late-autumn while the AOA amoA abundance did not change. The reduction in soil bioavailable Cu induced by the PA-MA and 2LS was linked to significant (P < 0.05) reduction in mineral N leaching of 16 and 30%, respectively, relative to the urine-only. The LS did not induce a significant effect on either bioavailable Cu or mineral N leaching relative to urine-only. The GA + LS reduced mineral N leaching by 10% relative to LS in late-autumn, however, there was no significant effect in mid-winter. This study demonstrated that reducing soil bioavailable Cu can be a potential strategy to reduce N leaching from urine patches.

Effect of nitrification inhibitor (DMPP) on nitrous oxide emissions from agricultural fields: Automated and manual measurements

Nitrogen fertilisation contributes significantly to the atmospheric increase of nitrous oxide (N₂O). Application of nitrification inhibitors (NIs) is a promising strategy to mitigate N₂O emissions and improve N-use efficiency in agricultural systems. This study investigated the effect of NI, 3,4-dimethylpyrazol phosphate (DMPP) on N₂O mitigation from spring barley and spring oilseed rape. Manual and automatic chamber methodologies were used to capture spatial and temporal variability in N₂O emissions. In a second experiment, we study the effect of N fertiliser levels without NI (0 %, 50 %, 100 %, 150 % and 200 % of recommended amount of N fertiliser), as well as 100 % of N with NI on N₂O emissions in spring barley. The automated chamber measurements showed dynamics of N₂O changes throughout the season, including positive and negative peaks that were unobservable with manual chambers due to low temporal resolution. Although not significant, application of NI tended to reduce N₂O emissions. The reduction was on average 16 % in spring barley and 58 % in spring oilseed rape in manual chamber measurements. However, N₂O reduction was 108 % in continuous automatic chamber measurements in spring barley. The N₂O EFs for the growing season were very low (0.025 % to 0.148 %), with a greater reduction in EF in spring oilseed rape (76 %) than in spring barley (32 %) with NI application. A positive correlation (R = 80 %) was observed between N fertiliser levels and N₂O emissions. Crop yield and crop N uptake were not significantly affected by the use of NI. This study highlighted that NI can reduce N₂O emissions, but the reduction effects are plot, crop and microclimate specific. Long-term experiments with continuous plot-scale measurements are needed to capture and optimise N₂O mitigation effect of NIs across wide variability in soils and microclimates in agroecosystems.

Biological nitrification inhibitor-trait enhances nitrogen uptake by suppressing nitrifier activity and improves ammonium assimilation in two elite wheat varieties

Synthetic nitrification inhibitors (SNI) and biological nitrification inhibitors (BNI) are promising tools to limit nitrogen (N) pollution derived from agriculture. Modern wheat cultivars lack sufficient capacity to exude BNIs, but, fortunately, the chromosome region (Lr#n-SA) controlling BNI production in Leymus racemosus, a wild relative of wheat, was introduced into two elite wheat cultivars, ROELFS and MUNAL. Using BNI-isogenic-lines could become a cost-effective, farmer-friendly, and globally scalable technology that incentivizes more sustainable and environmentally friendly agronomic practices. We studied how BNI-trait improves N-uptake, and N-use, both with ammonium and nitrate fertilization, analysing representative indicators of soil nitrification inhibition, and plant metabolism. Synthesizing BNI molecules did not mean a metabolic cost since Control and BNI-isogenic-lines from ROELFS and MUNAL presented similar agronomic performance and plant development. In the soil, ROELFS-BNI and MUNAL-BNI plants decreased ammonia-oxidizing bacteria (AOB) abundance by 60% and 45% respectively, delaying ammonium oxidation without reducing the total abundance of bacteria or archaea. Interestingly, BNI-trait presented a synergistic effect with SNIs since made it also possible to decrease the AOA abundance. ROELFS-BNI and MUNAL-BNI plants showed a reduced leaf nitrate reductase (NR) activity as a consequence of lower soilNO 3 - formation and a higher amino acid content compared to BNI-trait lacking lines, indicating that the transfer of Lr#-SA was able to induce a higher capacity to assimilate ammonium. Moreover, the impact of the BNI-trait in wheat cultivars was also noticeable for nitrate fertilization, with improved N absorption, and therefore, reducing soil nitrate content.

Quantitative Evaluation of Noncovalent Interactions between 3,4-Dimethyl-1H-pyrazole and Dissolved Humic Substances by NMR Spectroscopy

Nitrification inhibitors (NI) represent a valid chemical strategy to retard nitrogen oxidation in soil and limit nitrate leaching or nitrogen oxide emission. We hypothesized that humic substances can complex NI, thus affecting their activity, mobility, and persistence in soil. Therefore, we focused on 3,4-dimethylpyrazole phosphate (DMPP) by placing it in contact with increasing concentrations of model fulvic (FA) and humic (HA) acids. The complex formation was assessed through advanced and composite NMR techniques (chemical shift drift, line-broadening effect, relaxation times, saturation transfer difference (STD), and diffusion ordered spectroscopy (DOSY)). Our results showed that both humic substances interacted with DMPP, with HA exhibiting a significantly greater affinity than FA. STD emphasized the pivotal role of the aromatic signal, for HA-DMPP association, and both alkyl methyl groups, for FA-DMPP association. The fractions of complexed DMPP were determined on the basis of self-diffusion coefficients, which were then exploited to calculate both the humo-complex affinity constants and the free Gibbs energy (K_d and ΔG for HA were 0.5169 M and -1636 kJ mol^-1, respectively). We concluded that DMPP-based NI efficiency may be altered by soil organic matter, characterized by a pronounced hydrophobic nature. This is relevant to improve nitrogen management and lower its environmental impact.

A review and meta-analysis of mitigation measures for nitrous oxide emissions from crop residues

Crop residues are of crucial importance to maintain or even increase soil carbon stocks and fertility, and thereby to address the global challenge of climate change mitigation. However, crop residues can also potentially stimulate emissions of the greenhouse gas nitrous oxide (N₂O) from soils. A better understanding of how to mitigate N₂O emissions due to crop residue management while promoting positive effects on soil carbon is needed to reconcile the opposing effects of crop residues on the greenhouse gas balance of agroecosystems. Here, we combine a literature review and a meta-analysis to identify and assess measures for mitigating N₂O emissions due to crop residue application to agricultural fields. Our study shows that crop residue removal, shallow incorporation, incorporation of residues with C:N ratio > 30 and avoiding incorporation of residues from crops terminated at an immature physiological stage, are measures leading to significantly lower N₂O emissions. Other practices such as incorporation timing and interactions with fertilisers are less conclusive. Several of the evaluated N₂O mitigation measures implied negative side-effects on yield, soil organic carbon storage, nitrate leaching and/or ammonia volatilization. We identified additional strategies with potential to reduce crop residue N₂O emissions without strong negative side-effects, which require further research. These are: a) treatment of crop residues before field application, e.g., conversion of residues into biochar or anaerobic digestate, b) co-application with nitrification inhibitors or N-immobilizing materials such as compost with a high C:N ratio, paper waste or sawdust, and c) use of residues obtained from crop mixtures. Our study provides a scientific basis to be developed over the coming years on how to increase the sustainability of agroecosystems though adequate crop residue management.

DMPP and Polymer-Coated Urea Promoted Growth and Increased Yield of Greenhouse Tomatoes

Improvements in nitrogen (N) use efficiency reduce stress on the environment and improve tomato production. A two-year trial was conducted in greenhouse tomatoes with a split-plot design, in which one factor was the N application rate (150 kg·ha−1, N1; 200 kg·ha−1, N2; and 250 kg·ha−1, N3) and two other factors were the type of urea applied (urea, T1; slow-release (polymer-coated) urea, T2, and nitrification inhibitors (3,4-dimethylpyrazole phosphate, DMPP) + urea, T3); no N fertilizer was applied in the control. The effects of the nitrogen (N) application rate and type of urea applied on the root morphology indexes, growth indexes, photosynthetic parameters, yield (Y), water use efficiency (WUE), and nitrogen agronomic efficiency (NAE) of greenhouse tomatoes were investigated. The results show that an appropriate N application rate (200 kg·ha−1) can improve tomato growth and net photosynthetic rate (Pn). With T3, the Y and WUE of greenhouse tomatoes first increased and then decreased as the N application rate increased, but with T1 and T2, the Y and WUE increased as the N application rate increased. The NAE of greenhouse tomatoes was significantly lower with N3 than with N2. The root growth, plant growth, Pn, Y, WUE, and NAE of the tomatoes were improved with T2 and T3 compared to T1. These findings can be used to promote N conservation and increase the Y of facility agriculture crops.

DMPP reduces nitrogen fertilizer application rate, improves fruit quality, and reduces environmental cost of intensive apple production in China

In China, excessive application of nitrogen (N) fertilizer is common in intensive apple production. To resolve issues of benefit reduction and environmental pollution caused by excessive N, a two-year trial was conducted in an apple orchard with a split-plot design, in which the main factor was the N level (500, 400, 300, and 200 kg N ha^-1 year^-1, expressed as TN, TN80%, TN60%, and TN40%, respectively) and the deputy factor was whether or not to add 3,4-dimethylpyrazole phosphate (DMPP, expressed as +D). The effects of N reduction combined with DMPP on soil N transformation, fruit quality, economic benefits, and environmental effects were investigated. The results showed that DMPP reduced the production of nitrate and its vertical migration by inhibiting the abundance of AOB amoA and decreased N₂O emission by reducing nirKC1 levels. Moreover, N reduction combined with DMPP improved N use efficiency (26.67-49.35%) and reduced N loss rate (15.25-38.76%). Compared with TN, TN60% + D increased the content of anthocyanin and soluble sugar by 21.15% and 13.09%, respectively, and decreased environmental costs caused by NH₃ volatilization and N₂O emission by 33.84%, while maintaining yield and N utilization rate at relatively high levels. Considering the agronomic, economic and environmental benefits, on the basis of traditional N application rate, 40% N reduction combined with DMPP (TN60% + D) could ensure target yield, corresponding quality and economic benefits, maintain soil N fertility, and reduce the risk of N losses to the environment. The present research could provide references for green, efficient, and sustainable development of apple production.

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

Nitrogen (N) input from fertilizers modifies the properties of agricultural soils as well as bacterial community diversity, composition and relationships. This can lead to negative impacts such as the deterioration of system multifunctionality, whose maintenance is critical to normal nutrient cycling. Synthetic nitrification inhibitors (NIs) can be combined with fertilizers to improve the efficiency of N use by reducing N losses. However, analysis of their effects on non-target bacteria are scarce. This study aimed to analyze the effect of applying the NIs DMPP and DMPSA on the whole bacterial community. Through 16S rRNA amplicon sequencing we determined the differences between samples in terms of microbial diversity, composition and co-occurrence networks. The application of DMPP and DMPSA exerted little impact on the abundance of the dominant phyla. Nevertheless, several significant shifts were detected in bacterial diversity, co-occurrence networks, and the abundance of particular taxa, where soil water content played a key role. For instance, the application of NIs intensified the negative impact of N fertilization on bacterial diversity under high water-filled pore spaces (WFPS) (>64%), reducing community diversity, whereas alpha-diversity was not affected at low WFPS (<55%). Interestingly, despite NIs are known to inhibit ammonia monooxygenase (AMO) enzyme, both NIs almost exclusively inhibited Nitrosomonas genera among AMO holding nitrifiers. Thus, Nitrosomonas showed abundance reductions of up to 47% (DMPP) and 66% (DMPSA). Nonetheless, non-target bacterial abundances also shifted with NI application. Notably, DMPSA application partially alleviated the negative effect of fertilization on soil multifunctionality. A remarkable increase in populations related to system multifunctionality, such as Armatimonadetes (up to +21%), Cyanobacteria (up to +30%) and Fibrobacteres (up to +25%) was observed when DMPSA was applied. NI application substantially influenced microbial associations by decreasing the complexity of co-occurrence networks, decreasing the total edges and node connectivity, and increasing path distances.

Differential transformation mechanisms of exotic Cr(VI) in agricultural soils with contrasting physio-chemical and biological properties

The transformation mechanisms of Cr(VI) in agricultural soils at the molecular level remain largely unknown due to the multitude of abiotic and biotic factors. In this study, the different speciation and distribution of Cr in two types of agricultural soil (Ultisol and Fluvo-aquic soils) after two weeks of aging was investigated using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy, microfocused X-ray fluorescence (μ-XRF) and X-ray transmission microscopy (STXM). The microbial community structure of the two soils was also analyzed via high-throughput sequencing of 16S rRNA. Cr(VI) availability was relatively lower in the Ultisol than in the Fluvo-aquic soil after aging. Cr K-edge bulk XANES and STXM analysis indicated that Cr(VI) was reduced to Cr(III) in both soils. μ-XRF analysis and STXM analysis indicated the predominant association of Cr with Mn/Fe oxides and/or organo-Fe oxides in both soils. Additionally, STXM-coupled imaging and multiedge XANES analyses demonstrated that carboxylic groups were involved in the reduction of Cr(VI) and subsequent retention of Cr(III). 16S rRNA analysis showed considerably different bacterial communities across the two soils. Redundancy analysis (RDA) suggested that soil properties, including the total carbon content, Fe oxide component and pH, were closely linked to Cr(VI)-reducing functional bacteria in the Ultisol, including chromium-reducing bacteria (CRB) (e.g., Bacillus sp.) and dissimilatory iron-reducing (DIRB) (e.g., Shewanella sp.) bacteria, which possibly promoted Cr(VI) reduction. These findings shed light on the molecular-level transformation mechanisms of Cr(VI) in agricultural soils, which facilitates the effective management of Cr-enriched farmland.

Substituted 1,2,3-triazoles: a new class of nitrification inhibitors

Nitrogen (N) fertilisers amended with nitrification inhibitors can increase nitrogen use efficiencies in agricultural systems but the effectiveness of existing commercial inhibitor products, including 3,4-dimethylpyrazole phosphate (DMPP), is strongly influenced by climatic and edaphic factors. With increasing pressure to reduce the environmental impact of large-scale agriculture it is important to develop new nitrogen-stabilising products that can give reliable and consistent results, particularly for warmer climatic conditions. We synthesised a library of 17 compounds featuring a substituted 1,2,3-triazole motif and performed laboratory incubations in two south-eastern Australian soils. In the neutral (pH 7.3) soil, the compounds N002, N013, N016 and N017, which possess short non-polar alkyl or alkynyl substituents at the triazole ring, retained NH₄⁺-N concentrations at 35 °C soil temperature to a better extent (P < 0.001) than DMPP. In the alkaline soil (pH 8.8) N013 performed better with regards to NH₄⁺-N retention (P = 0.004) than DMPP at 35 °C soil temperature. Overall, our data suggest that substituted 1,2,3-triazoles, which can be synthesized with good yields and excellent atom economy through 1,3-dipolar cycloaddition from readily available starting materials, are promising nitrification inhibitors performing similar to, or better than DMPP, particularly at elevated soil temperatures.