Biological Nitrification Inhibition Research Articles

Inhibition profile of three biological nitrification inhibitors and their response to soil pH modification in two contrasting soils.

Up to 70% of the nitrogen (N) fertilizer applied to agricultural soils is lost through microbially mediated processes, such as nitrification. This can be counteracted by synthetic and biological compounds that inhibit nitrification. However, for many biological nitrification inhibitors (BNIs), the interaction with soil properties, nitrifier specificity, and effective concentrations are unclear. Here, we investigated three synthetic nitrification inhibitors (SNIs) (DCD, DMPP, and nitrapyrin) and three BNIs (methyl 3(4-hydroxyphenyl) propionate (MHPP), methyl 3(4-hydroxyphenyl) acrylate (MHPA), and limonene) in two agricultural soils differing in pH and nitrifier communities. The efficacies of SNIs and BNIs were resilient to short-term pH changes in the neutral pH soil, whereas the efficacy of some BNIs increased by neutralizing the alkaline soil. Among the BNIs, MHPA showed the highest inhibition and was, together with MHPP, identified as a putative AOB/comammox-selective inhibitor. Additionally, MHPA and limonene effectively inhibited nitrification at concentrations comparable to those used for DCD. Moreover, we identified the effective concentrations at which 50 and 80% of inhibition is observed (EC50 and EC80) for the BNIs, and similar EC80 values were observed in both soils. Overall, our results show that these BNIs could potentially serve as effective alternatives to SNIs currently used.

Sugarcane exudates as nitrification inhibitors; improvement of soybean nitrogen recovery and yield by reducing soil nitrification and N2O emission using 15N tracing techniques

BNI (biological nitrification inhibition) derived from root exudates is typically used in rotation cropping systems to increase the crop nitrogen efficiency (NUE). Recently, the root exudates secreted by three sugarcane varieties in Thailand have been found to inhibit the nitrification and accumulation. The objective of the study was to evaluate the effectiveness of the sugarcane root exudate on (i) soil nitrification rate and N dynamics (soil mineral N and nitrous oxide (N2O) emission) and (ii) growth, yield, and 15N recovery by soybean under greenhouse conditions. The experiment was laid out as a randomized complete block design (RCBD), with five treatments and four replications: (i) N fertilizer (Control); (ii) Control + Uthong 13 sugarcane root exudate (UT13); (iii) Control + Khon Kaen 3 sugarcane root exudate (KK3); (iv) Control + wild cane sugarcane root exudate (Spone) and (v) Control + dicyandiamide (DCD). 15N fertilizer was applied to a micro-plot for 15N recovery. Soybean was planted and fertilized at 75 kg ha−1 N as (NH4)2SO4. Soil samples were collected at 1, 3, 7, 14, 21 and 28 days after N fertilization (DANF) for pH, potential nitrification rate (PNR), NH4+, NO3− and MBN measurement at 0–15 and 15–30 cm soil depths. A gas sample was collected by the soil chamber technique for an N2O emissions study on the soil sampling date. The soybean growth was monitored at 30 and 60 DAP (days after planting) for height, leaf area, leaf area index (LAI), SCMR and dry weight (DW). At soybean harvest, seed yield, yield components and N uptake were recorded. Our study revealed that the sugarcane root exudates significantly decreased PNR up to 14 DANF and increased NH4+ content in soil and reduced commutative N2O–N fluxes after 14 DANF, compared to N treatment (p < 005). The root exudate treatments not only increased growth (total DW and LAI at 60 DAP), yield (number of pods and seeds per plant) and N uptake (17–19%) but also 15N recovery in the total plant. This confirms the capacity of BNIs from sugarcane root exudates. In conclusion, sugarcane root exudates can effectively mitigate N2O emissions and enhance soybean N uptake under greenhouse conditions.

Altitudinal patterns of alpine soil ammonia-oxidizing community structure and potential nitrification rate.

Nitrogen availability limits the net primary productivity in alpine meadows on the Qinghai-Tibetan Plateau, which is regulated by ammonia-oxidizing microorganisms. However, little is known about the elevational patterns of soil ammonia oxidizers in alpine meadows. Here, we investigated the potential nitrification rate (PNR), abundance, and community diversity of soil ammonia-oxidizing microorganisms along the altitudinal gradient between 3,200 and 4,200 m in Qinghai-Tibetan alpine meadows. We found that both PNR and amoA gene abundance declined from 3,400 to 4,200 m but lowered at 3,200 m, possibly due to intense substrate competition and biological nitrification inhibition from grasses. The primary contributors to soil nitrification were ammonia-oxidizing archaea (AOA), and their proportionate share of soil nitrification increased with altitude in comparison to ammonia-oxidizing bacteria (AOB). The alpha diversity of AOA increased by higher temperature and plant richness at low elevations, while decreased by higher moisture and low legume biomass at middle elevations. In contrast, the alpha diversity of AOB increased along elevation. The elevational patterns of AOA and AOB communities were primarily driven by temperature, soil moisture, and vegetation. These findings suggest that elevation-induced climate changes, such as shifts in temperature and water conditions, could potentially alter the soil nitrification process in alpine meadows through changes in vegetation and soil properties, which provide new insights into how soil ammonia oxidizers respond to climate change in alpine meadows.IMPORTANCEThe importance of this study is revealing that elevational patterns and nitrification contributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities were primarily driven by temperature, soil moisture, and vegetation. Compared to AOB, the relative contribution of AOA to soil nitrification increased at higher elevations. The research highlights the potential impact of elevation-induced climate change on nitrification processes in alpine meadows, mediated by alterations in vegetation and soil properties. By providing new insights into how ammonia oxidizers respond to climate change, this study contributes valuable knowledge to the field of microbial ecology and helps predict ecological responses to environmental changes in alpine meadows.

Root exudate fingerprint of Brachiaria humidicola reveals vanillin as a novel and effective nitrification inhibitor.

Introduction: Biological Nitrification Inhibition (BNI) is defined as the plant-mediated control of soil nitrification via the release of nitrification inhibitors. BNI of Brachiaria humidicola (syn. Urochloa humidicola) has been mainly attributed to root-exuded fusicoccane-type diterpenes, e.g., 3-epi-brachialactone. We hypothesized, however, that BNI of B. humidicola is caused by an assemblage of bioactive secondary metabolites. Methods: B. humidicola root exudates were collected hydroponically, and metabolites were isolated by semi-preparative HPLC. Chemical structures were elucidated by HRMS as well as 1D and 2D NMR spectroscopy. Nitrification inhibiting potential of isolated metabolites was evaluated by a Nitrosomonas europaea based bioassay. Results and discussion: Besides previously described brachialactone isomers and derivatives, five phenol and cinnamic acid derivatives were identified in the root exudates of B. humidicola: 2-hydroxy-3-(hydroxymethyl)benzaldehyde, vanillin, umbelliferone and both trans- and cis-2,6-dimethoxycinnamic acid. Notably, vanillin revealed a substantially higher nitrification inhibiting activity than 3-epi-brachialactone (ED50 ∼ 12.5μg·ml-1, ED80 ∼ 20μg·ml-1), identifying this phenolic aldehyde as novel nitrification inhibitor (NI). Furthermore, vanillin exudation rates were in the same range as 3-epi-brachialactone (1-4μg·h-1·g-1 root DM), suggesting a substantial contribution to the overall inhibitory activity of B. humidicola root exudates. In relation to the verification of the encountered effects within soils and considering the exclusion of any detrimental impact on the soil microbiome, the biosynthetic pathway of vanillin via the precursor phenylalanine and the intermediates p-coumaric acid/ferulic acid (precursors of further phenolic NI) might constitute a promising BNI breeding target. This applies not only to Brachiaria spp., but also to crops in general, owing to the highly conserved nature of these metabolites.

Assessing the activity of different plant-derived molecules and potential biological nitrification inhibitors on a range of soil ammonia- and nitrite-oxidizing strains.

Synthetic nitrification inhibitors are routinely used with nitrogen fertilizers to reduce nitrogen losses from agroecosystems, despite having drawbacks like poor efficiency, cost, and entry into the food chain. Plant-derived BNIs constitute a more environmentally conducive alternative. Knowledge on the activity of BNIs to soil nitrifiers is largely based on bioassays with a single Nitrosomonas europaea strain which does not constitute a dominant member of the community of ammonia-oxidizing microorganisms (AOM) in soil. We determined the activity of several plant-derived molecules reported as having activity, including the recently discovered maize-isolated BNI, zeanone, and its natural analog, 2-methoxy-1,4-naphthoquinone, on a range of ecologically relevant AOM and one nitrite-oxidizing bacterial culture, expanding our knowledge on the intrinsic inhibition potential of BNIs toward AOM and highlighting the necessity for a deeper understanding of the effect of BNIs on the overall soil microbiome integrity before their further use in agricultural settings.

Biological and chemical nitrification inhibitors exhibited different effects on soil gross N nitrification rate and N2O production: a 15N microcosm study.

Nitrification inhibitors (NIs) are considered as an effective strategy for reducing nitrification rate and related environmental nitrogen (N) loss. However, whether plant-derived biological NIs had an advantage over chemical NIs in simultaneously inhibiting nitrification rate and N2O production remains unclear. Here, we conducted an aerobic 15N microcosmic incubation experiment to compare the effects of a biological NI (methyl 3-(4-hydroxyphenyl) propionate, MHPP) with three chemical NIs, 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin), dicyandiamide (DCD), and 3,4-dimethylpyrazole phosphate (DMPP) on (i) gross N mineralization and nitrification rate and (ii) the relative importance of nitrification and denitrification in N2O emission in a calcareous soil. The results showed that DMPP significantly inhibited m_gross rate (P < 0.05), whereas DCD, nitrapyrin, and MHPP only numerically inhibited it. Gross N nitrification (n_gross) rates were inhibited by 9.48% in the DCD treatment to 51.5% in the nitrapyrin treatment. Chemical NIs primarily affected the amoA gene abundance of ammonia-oxidizing bacteria (AOB), whereas biological NIs affected the amoA gene abundance of ammonia-oxidizing archaea (AOA) and AOB. AOB's community composition was more susceptible to NIs than AOA, and NIs mainly targeted Nitrosospira clusters of AOB. Chemical NIs of DCD, DMPP, and nitrapyrin proportionally reduced N2O production from nitrification and denitrification. However, the biological NI MHPP stimulated short-term N2O emission and increased the proportion of N2O from denitrification. Our findings showed that the influence of NIs on gross N mineralization rate (m_gross) was dependent on the NI type. MHPP exhibited a moderate n_gross inhibitory capacity compared with the three chemical NIs. The mechanisms of chemical and biological NIs inhibiting n_gross can be partly attributed to changes in the abundance and community of ammonia oxidizers. A more comprehensive evaluation is needed to determine whether biological NIs have advantages over chemical NIs in inhibiting greenhouse gas emissions.

Agritech to Tame the Nitrogen Cycle.

While the Haber-Bosch process for N-fixation has enabled a steady food supply for half of humanity, substantial use of synthetic fertilizers has caused a radical unevenness in the global N-cycle. The resulting increases in nitrate production and greenhouse gas (GHG) emissions have contributed to eutrophication of both ground and surface waters, the growth of oxygen minimum zones in coastal regions, ozone depletion, and rising global temperatures. As stated by the Food and Agriculture Organization of the United Nations, agriculture releases ∼9.3 Gt CO2 equivalents per year, of which methane (CH4) and nitrous oxide (N2O) account for 5.3 Gt CO2 equivalents. N-pollution and slowing the runaway N-cycle requires a combined effort to replace chemical fertilizers with biological alternatives, which after a 10-yr span of usage could eliminate a minimum of 30% of ag-related GHG emissions (∼1.59 Gt), protect waterways from nitrate pollution, and protect soils from further deterioration. Agritech solutions include bringing biological fertilizers and biological nitrification inhibitors to the marketplace to reduce the microbial conversion of fertilizer nitrogen into GHGs and other toxic intermediates. Worldwide adoption of these plant-derived molecules will substantially elevate nitrogen use efficiency by crops while blocking the dominant source of N2O to the atmosphere and simultaneously protecting the biological CH4 sink. Additional agritech solutions to curtail N-pollution, soil erosion, and deterioration of freshwater supplies include soil-free aquaponics systems that utilize improved microbial inocula to enhance nitrogen use efficiency without GHG production. With adequate and timely investment and scale-up, microbe-based agritech solutions emphasizing N-cycling processes can dramatically reduce GHG emissions on short time lines.

Potential Secretory Transporters and Biosynthetic Precursors of Biological Nitrification Inhibitor 1,9-Decanediol in Rice as Revealed by Transcriptome and Metabolome Analyses

Biological nitrification inhibitors (BNIs) are released from plant roots and inhibit the nitrification activity of microorganisms in soils, reducing NO3‒ leaching and N2O emissions, and increasing nitrogen- use efficiency (NUE). Several recent studies have focused on the identification of new BNIs, yet little is known about the genetic loci that govern their biosynthesis and secretion. We applied a combined transcriptomic and metabolomic analysis to investigate possible biosynthetic pathways and transporters involved in the biosynthesis and release of BNI 1,9-decanediol (1,9-D), which was previously identified in rice root exudates. Our results linked four fatty acids, icosapentaenoic acid, linoleate, norlinolenic acid, and polyhydroxy-α,ω-divarboxylic acid, with 1,9-D biosynthesis and three transporter families, namely the ATP-binding cassette protein family, the multidrug and toxic compound extrusion family, and the major facilitator superfamily, with 1,9-D release from roots into the soil medium. Our finding provided candidates for further work on the genes implicated in the biosynthesis and secretion of 1,9-D and pinpoint genetic loci for crop breeding to improve NUE by enhancing 1,9-D secretion, with the potential to reduce NO3‒ leaching and N2O emissions from agricultural soils.

Will crops with biological nitrification inhibition capacity be favored under future atmospheric CO2?

Will crops with biological nitrification inhibition capacity be favored under future atmospheric CO2?

Genetic variation among elite inbred lines suggests potential to breed for BNI-capacity in maize

Biological nitrification inhibition (BNI) is a plant function where root systems release antibiotic compounds (BNIs) specifically aimed at suppressing nitrifiers to limit soil-nitrate formation in the root zone. Little is known about BNI-activity in maize (Zea mays L.), the most important food, feed, and energy crop. Two categories of BNIs are released from maize roots; hydrophobic and hydrophilic BNIs, that determine BNI-capacity in root systems. Zeanone is a recently discovered hydrophobic compound with BNI-activity, released from maize roots. The objectives of this study were to understand/quantify the relationship between zeanone activity and hydrophobic BNI-capacity. We assessed genetic variability among 250 CIMMYT maize lines (CMLs) characterized for hydrophobic BNI-capacity and zeanone activity, towards developing genetic markers linked to this trait in maize. CMLs with high BNI-capacity and ability to release zeanone from roots were identified. GWAS was performed using 27,085 SNPs (with unique positions on the B73v.4 reference genome, and false discovery rate = 10), and phenotypic information for BNI-capacity and zeanone production from root systems. Eighteen significant markers were identified; three associated with specific BNI-activity (SBNI), four with BNI-activity per plant (BNIPP), another ten were common between SBNI and BNIPP, and one with zeanone release. Further, 30 annotated genes were associated with the significant SNPs; most of these genes are involved in pathways of “biological process”, and one (AMT5) in ammonium regulation in maize roots. Although the inbred lines in this study were not developed for BNI-traits, the identification of markers associated with BNI-capacity suggests the possibility of using these genomic tools in marker-assisted selection to improve hydrophobic BNI-capacity in maize.

Changes in soil pore structure generated by the root systems of maize, sorghum and switchgrass affect in situ N2O emissions and bacterial denitrification

AbstractDue to the heterogeneous nature of soil pore structure, processes such as nitrification and denitrification can occur simultaneously at microscopic levels, making prediction of small-scale nitrous oxide (N2O) emissions in the field notoriously difficult. We assessed N2O+N2 emissions from soils under maize (Zea mays L.), switchgrass (Panicum virgatum L.), and energy sorghum (Sorghum bicolor L.), three potential bioenergy crops in order to identify the importance of different N2O sources to microsite production, and relate N2O source differences to crop-associated differences in pore structure formation. The combination of isotopic surveys of N2O in the field during one growing season and X-ray computed tomography (CT) enabled us to link results from isotopic mappings to soil structural properties. Further, our methodology allowed us to evaluate the potential for in situ N2O suppression by biological nitrification inhibition (BNI) in energy sorghum. Our results demonstrated that the fraction of N2O originating from bacterial denitrification and reduction of N2O to N2 is largely determined by the volume of particulate organic matter occluded within the soil matrix and the anaerobic soil volume. Bacterial denitrification was greater in switchgrass than in the annual crops, related to changes in pore structure caused by the coarse root system. This led to high N-loses through N2 emissions in the switchgrass system throughout the season a novel finding given the lack of data in the literature for total denitrification. Isotopic mapping indicated no differences in N2O-fluxes or their source processes between maize and energy sorghum that could be associated with the release of BNI by the investigated sorghum variety. The results of this research show how differences in soil pore structures among cropping systems can determine both N2O production via denitrification and total denitrification N losses in situ.

Insight into the functional mechanisms of nitrogen-cycling inhibitors in decreasing yield-scaled ammonia volatilization and nitrous oxide emission: A global meta-analysis

Soil ammonia (NH3) volatilization and nitrous oxide (N2O) emission decrease nitrogen (N) utilization efficiency and cause some environmental problems. The N-cycling inhibitors are suggested to apply to enhance N utilization efficiency. Quantifying effects of N-cycling inhibitors on yield-scaled NH3 volatilization and N2O emission and functional genes could provide support for the optimal selection and application of N-cycling inhibitor. We conducted a meta-analysis to reveal the effects of N-cycling inhibitors on soil abiotic properties, functional genes and yield-scaled NH3 volatilization and N2O emission by extracting data from 166 published articles and linked their comprehensive relationships. The N-cycling inhibitors in this meta-analysis mainly includes nitrification inhibitors 3, 4-dimethyl pyrazole phosphate, dicyandiamide and 2-chloro-6-trichloromethylpyridine, urease inhibitor N-(n-butyl) thiophosphoric triamide and biological nitrification inhibitors methyl 4-hydroxybenzoate and 1, 9-decanediol. The N-cycling inhibitor applications significantly increased alkaline soil pH but significantly decreased acidic soil pH. The N-cycling inhibitors decreased soil AOB amoA gene abundances mostly under the condition of pH 4.5–6 (mean: 212%, 95% confidence intervals (CI): 249% and −176%) and significantly decreased nirS gene (mean: 39%; 95% CI: 72% and −6%). The yield-scaled NH3 volatilization was significantly decreased by the N-cycling inhibitors under the condition of soil pH = 7–8.5 (mean: 45%; 95% CI: 59% and −31%). The yield-scaled N2O emission was also significantly reduced by all N-cycling inhibitors and had negative correlations with the soil nirK and nirS gene abundances. The effects of N-cycling inhibitors on soil pH, ammonium-N, nitrate-N and nitrifying and denitrifying genes and yield-scaled NH3 volatilization and N2O emission were dominated by the inhibitor types, soil textures, crop species and environmental pH. Our study could provide technical support for the optimal selection and application of N-cycling inhibitor under different environmental conditions.

Arbuscular mycorrhizal hyphae selectively suppress soil ammonia oxidizers – but probably not by production of biological nitrification inhibitors

Arbuscular mycorrhizal hyphae selectively suppress soil ammonia oxidizers – but probably not by production of biological nitrification inhibitors

Benefits of biological nitrification inhibition of Leymus chinensis under alkaline stress: the regulatory function of ammonium-N exceeds its nutritional function.

The production of root exudates with biological nitrification inhibition (BNI) effects is a strategy adopted by ammonium-N () tolerant plant species that occur in N-limited environments. Most knowledge on BNI comes from plant species that occur in acidic soils. Here, combining field sampling and laboratory culture, we assessed the BNI-capacity of Leymus chinensis, a dominant grass species in alkaline grasslands in eastern Asia, and explored why L. chinensis has BNI ability. The results showed that L. chinensis has strong BNI-capacity. At a concentration of 1 mg mL-1, L. chinensis' root exudates inhibited nitrification in soils influenced by Puccinellia tenuiflora by 72.44%, while DCD only inhibited it by 68.29%. The nitrification potential of the soil of L. chinensis community was only 53% of the P. tenuiflora or 41% of the Suaeda salsa community. We also showed that the supply of driven by L. chinensis' BNI can meet its requirements . In addition, can enhance plant adaptation to alkaline stress by regulating pH, and in turn, the uptake of nitrate-N (). We further demonstrated that the regulatory function of is greater than its nutritional function in alkaline environment. The results offer novel insights into how L. chinensis adapts to high pH and nutrient deficiency stress by secreting BNIs, and reveal, for the first time, differences in the functional roles of and in growth and adaptation under alkaline conditions in a grass species.

Does subjecting plants to water stress enhance biological nitrification inhibition potential of rice?

Biological nitrification inhibition (BNI) is a trait that could improve nitrogen-use efficiency of a crop. We studied varietal differences in BNI potential of rice at the vegetative phase when plants were subjected to water stress. We obtained water-soluble root exudates (RE) and water-extracts of crushed root tissues (RT) from two-week-old seedlings of five rice cultivars grown continuously under adequate water or under water stress and assessed their ability to suppress the activity of Nitrosomonas europaea and the growth of lettuce seedlings. We also investigated how growing rice under continuously saturated (TC) and unsaturated (TU) soil moisture conditions affected potential nitrification rate (PNR) and community structure of ammonia-oxidizing bacteria (AOB) in the rhizosphere compared with bulk soil. We observed that only RT at ≥ 0.50 mg ml−1 suppressed the activity of N. europaea with significant (p < 0.05) differences among rice cultivars. Subjecting rice cultivars PI312777 and PI338046 to water stress significantly (p < 0.05) increased BNI of RT. Inhibition of lettuce root growth by RT was strongly correlated with its BNI (r = 0.83, p < 0.05). In T-RFLP profiles of AOB, the relative abundance of T-RF R205 was lower in TU than in TC in all cultivars except Rexmont. The rice cultivars studied produced compounds that could inhibit ammonia oxidizers, but the degree of BNI differed based on the moisture stress experienced by the crop. Lineages of rice cultivars with known allelopathic potential would be promising candidates to consider for BNI potential.

Intercropping with BNI-sorghum benefits neighbouring maize productivity and mitigates soil nitrification and N2O emission

Sorghum (Sorghum bicolor) roots are well-known for the release of Biological Nitrification Inhibitors (BNIs), such as sorgoleone (SGLN) and methyl 3–4-hydroxyphenyl propionate (MHPP). However, the deployment of sorghums with BNIs in agriculture practices has not been widely investigated and evaluated. In this study, sorghum and maize were intercropped in field and pot cultivation to investigate the Biological Nitrification Inhibition (BNI) effect of sorghum on the productivity of both crops and soil nitrification. Inorganic N content (NO3– and NH4+), N2O emission, and the abundance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in soil were analyzed. The results showed intercropping with sorghum have the potential to improve crop productivity, as indicated by the Land Equivalent Ratio greater than 1.0. Intercropping with sorghum reduced the transformation rate of NH4+ to NO3– in soils and the emission of N2O, as compared to monoculture of maize. Additionally, the abundance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) under intercropping mode were reduced by 41% and 28% in field trials, and reduced by 59% and 23% in pot experiments, respectively. Incubation of the same soils with SGLN and/or MHPP showed that the combined treatment of both BNIs strongly inhibited nitrification and N2O emission, along with lower AOB and AOA abundances, as compared with those under the sole treatment of SGLN or MHPP. Our results indicated that the root exudates of sorghum play a significant role in inhibiting soil nitrification due to the overlapping or amplified effect of various BNIs, which contribute to the nitrogen nutrition of the neighbouring crops under intercropping mode. Therefore, intercropping with BNI crops provides an eco-friendly strategy to improve crop production with reduced N2O emission.

Exploiting crop genotype-specific root-soil interactions to enhance agronomic efficiency

Challenges of soil degradation and changing climate pose major threats to food security in many parts of the world, and new approaches are required to close yield and nutrition gaps through enhanced agronomic efficiency. Combined use of mineral fertilizers, organic inputs, improved germplasm and adaptation of these practices to local contexts through improved agronomy can promote efficiency whilst building stocks of soil organic matter (SOM). Within this framework, recent attention has turned to the nature of plant-soil interactions to increase response to mineral fertilizer inputs through utilisation of nutrients from SOM that are replenished through management. This utilisation has been shown in barley and maize to vary with genotype and to be related to root physiological traits associated with rhizodeposition. The identification of candidate genes associated with rhizodeposition takes this a step closer towards the possibility of breeding for sustainability. Here we discuss this potential and feasibility in the context of maize cropping systems, and explore the potential for a combined approach that optimises utilisation of SOM nutrients together with enhanced biological nitrification inhibition to further improve agronomic efficiency.

Seed treatment with plant-defense elicitors decreases the abundance of ammonia oxidizers associated with winter wheat roots

Priming with elicitors, including phytohormones, is known to induce physiological changes in plants that affect resource allocation and nutrient uptake strategies. However, effects of these changes on belowground microbial communities, particularly those involved in nitrogen (N) transformations, remain poorly characterized. Here, we tested the hypothesis that treating seeds with elicitors would affect N-transforming communities associated to roots and rhizosphere, influencing soil N availability and thus plant performance. We measured the effects of treating winter wheat seeds with cis-jasmone, methyl jasmonate or methyl salicylate on plant traits and on the genetic potential for ammonia oxidation by ammonia-oxidizing bacteria (AOB) and archaea (AOA), the initial inorganic N transformation process resulting in N loss. Elicitors reduced the genetic potential for ammonia oxidation by AOB in root-associated microbial communities and increased traits reflecting N uptake in winter wheat. Proposed mechanisms include increased exploitative competition for ammonium, limiting substrate availability for microorganisms, and induced biological nitrification inhibition capacity actively suppressing ammonia oxidizers. There were minor effects on the composition of the AOB and AOA communities, but an indicator species analysis showed that several AOB and a few AOA genotypes were indicative of elicitor treatments. Seed treatment with elicitors may represent an overlooked strategy to minimize nitrification to mitigate N losses from arable soils.

Non-native Brachiaria humidicola with biological nitrification inhibition capacity stimulates in situ grassland N2O emissions.

Brachiaria humidicola, a tropical grass, could release root exudates with biological nitrification inhibition (BNI) capacity and reduce soil nitrous oxide (N2O) emissions from grasslands. However, evidence of the reduction effect in situ in tropical grasslands in China is lacking. To evaluate the potential effects of B. humidicola on soil N2O emissions, a 2-year (2015-2017) field experiment was established in a Latosol and included eight treatments, consisting of two pastures, non-native B. humidicola and a native grass, Eremochloa ophiuroide, with four nitrogen (N) application rates. The annual urea application rates were 0, 150, 300, and 450 kg N ha-1. The average 2-year E. ophiuroides biomass with and without N fertilization were 9.07-11.45 and 7.34 t ha-1, respectively, and corresponding values for B. humidicola increased to 31.97-39.07 and 29.54 t ha-1, respectively. The N-use efficiencies under E. ophiuroide and B. humidicola cultivation were 9.3-12.0 and 35.5-39.4%, respectively. Annual N2O emissions in the E. ophiuroides and B. humidicola fields were 1.37 and 2.83 kg N2O-N ha-1, respectively, under no N fertilization, and 1.54-3.46 and 4.30-7.19 kg N2O-N ha-1, respectively, under N fertilization. According to the results, B. humidicola cultivation increased soil N2O emissions, especially under N fertilization. This is because B. humidicola exhibited the more effective stimulation effect on N2O production via denitrification primarily due to increased soil organic carbon and exudates than the inhibition effect on N2O production via autotrophic nitrification. Annual yield-scaled N2O emissions in the B. humidicola treatment were 93.02-183.12 mg N2O-N kg-1 biomass, which were significantly lower than those in the E. ophiuroides treatment. Overall, our results suggest that cultivation of the non-native grass, B. humidicola with BNI capacity, increased soil N2O emissions, while decreasing yield-scaled N2O emissions, when compared with native grass cultivation.

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.