Genetic mitigation strategies to tackle agricultural GHG emissions: The case for biological nitrification inhibition technology

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.

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.

Interaction of ammonium nutrition with essential mineral cations

Plant growth and development depend on sufficient nutrient availability in soils. Agricultural soils are generally nitrogen (N) deficient, and thus soils need to be supplemented with fertilizers. Ammonium (NH4+) is a major inorganic N source. However, at high concentrations, NH4+ becomes a stressor that inhibits plant growth. The cause of NH4+ stress or toxicity is multifactorial, but the interaction of NH4+ with other nutrients is among the main determinants of plants' sensitivity towards high NH4+ supply. In addition, NH4+ uptake and assimilation provoke the acidification of the cell external medium (apoplast/rhizosphere), which has a clear impact on nutrient availability. This review summarizes current knowledge, at both the physiological and the molecular level, of the interaction of NH4+ nutrition with essential mineral elements that are absorbed as cations, both macronutrients (K+, Ca2+, Mg2+) and micronutrients (Fe2+/3+, Mn2+, Cu+/2+, Zn2+, Ni2+). We hypothesize that considering these nutritional interactions, and soil pH, when formulating fertilizers may be key in order to boost the use of NH4+-based fertilizers, which have less environmental impact compared with nitrate-based ones. In addition, we are convinced that better understanding of these interactions will help to identify novel targets with the potential to improve crop productivity.

An In Silico Analysis of Synthetic and Natural Compounds as Inhibitors of Nitrous Oxide Reductase (N2OR) and Nitrite Reductase (NIR)

Nitrification inhibitors are recognized as a key approach that decreases the denitrification process to inhibit the loss of nitrogen to the atmosphere in the form of N2O. Targeting denitrification microbes directly could be one of the mitigation approaches. However, minimal attempts have been devoted towards the development of denitrification inhibitors. In this study, we aimed to investigate the molecular docking behavior of the nitrous oxide reductase (N2OR) and nitrite reductase (NIR) involved in the microbial denitrification pathway. Specifically, in silico screening was performed to detect the inhibitors of nitrous oxide reductase (N2OR) and nitrite reductase (NIR) using the PatchDock tool. Additionally, a toxicity analysis based on insecticide-likeness, Bee-Tox screening, and a STITCH analysis were performed using the SwissADME, Bee-Tox, and pkCSM free online servers, respectively. Among the twenty-two compounds tested, nine ligands were predicted to comply well with the TICE rule. Furthermore, the Bee-Tox screening revealed that none of the selected 22 ligands exhibited toxicity on honey bees. The STITCH analysis showed that two ligands, namely procyanidin B2 and thiocyanate, have interactions with both the Paracoccus denitrificans and Hyphomicrobium denitrificans microbial proteins. The molecular docking results indicated that ammonia exhibited the second least atomic contact energy (ACE) of -15.83 kcal/mol with Paracoccus denitrificans nitrous oxide reductase (N2OR) and an ACE of -15.20 kcal/mol with Hyphomicrobium denitrificans nitrite reductase (NIR). The inhibition of both the target enzymes (N2OR and NIR) supports the view of a low denitrification property and suggests the potential future applications of natural/synthetic compounds as significant nitrification inhibitors.

Optimizing sowing patterns in winter wheat can reduce N2O emissions and improve grain yield and NUE by enhancing N uptake

Increasing nitrogen (N) input is essential to satisfy the rising global wheat demand, but this increases nitrous oxide (N₂O) emissions, thereby exacerbating global climate change. Higher yields accompanied by reduced N₂O emissions are essential to synergistically reduce greenhouse warming and ensure global food security. In this study, we conducted a trial using two sowing patterns (conventional drilling sowing [CD] and wide belt sowing [WB], with seedling belt widths of 2-3 and 8-10 cm, respectively) with four N rates (0, 168, 240, and 312 kg ha^-1, hereafter N0, N168, N240, and N312, respectively) during the 2019-2020 and 2020-2021 growing seasons. We investigated the impacts of growing season, sowing pattern, and N rate on N₂O emissions, N₂O emissions factors (EFs), global warming potential (GWP), yield-scaled N₂O emissions, grain yield, N use efficiency (NUE), plant N uptake and soil inorganic N concentrations at jointing, anthesis, and maturity. The results showed that sowing pattern and N rate interactions influenced the N₂O emissions markedly. Compared to CD, WB significantly reduced cumulative N₂O emissions, N₂O EFs, GWP, and yield-scaled N₂O emissions for N168, N240, and N312, with the largest reduction seen at N312. Furthermore, WB markedly improved plant N uptake and reduced soil inorganic N compared to CD at each N rate. Correlation analyses indicated that WB mitigated the N₂O emissions at various N rates mainly through efficient N uptake and reduced soil inorganic N. The highest grain yield occurred under a combination of WB and N312, under which the yield-scaled N₂O emissions were equal to the local management (sowing with CD at N240). In conclusion, WB sowing could synergistically decrease N₂O emissions and obtain high grain yields and NUEs, especially at higher N rates.

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.

Syringic acid from rice roots inhibits soil nitrification and N2O emission under red and paddy soils but not a calcareous soil

Syringic acid (SA) is a novel biological nitrification inhibitor (BNIs) discovered in rice root exudates with significant inhibition of Nitrosomonas strains. However, the inhibitory effect of SA on nitrification and nitrous oxide (N₂O) emissions in different soils and the environmental factors controlling the degree of inhibition have not been studied. Using 14-day microcosm incubation, we investigated the effects of different concentrations of SA on nitrification activity, abundance of ammonia-oxidizing microorganisms, and N₂O emissions in three typical agricultural soils. The nitrification inhibitory efficacy of SA was strongest in acidic red soil, followed by weakly acidic paddy soil, with no significant effect in an alkaline calcareous soil. Potential nitrification activity (PNA) were also greatly reduced by SA additions in paddy and red soil. Pearson correlation analysis showed that the inhibitory efficacy of SA might be negatively correlated with soil pH and positively correlated with clay percentage. SA treatments significantly reduced N₂O emissions by 69.1-79.3% from paddy soil and by 40.8%-46.4% from red soil, respectively, but no effect was recorded in the calcareous soil. SA addition possessed dual inhibition of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) abundance in paddy and red soil. Structural equation modelling revealed that soil ammonium (NH₄ ⁺) and dissolved organic carbon content (DOC) were the key variables explaining AOA and AOB abundance and subsequent N₂O emissions. Our results support the potential for the use of the BNI SA in mitigating N₂O emissions and enhancing N utilization in red and paddy soils.

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 soil NO 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.

Soil carbon sequestration, greenhouse gas emissions, and water pollution under different tillage practices

Tillage is a common agricultural practice and a critical component of agricultural systems that is frequently employed worldwide in croplands to reduce climatic and soil restrictions while also sustaining various ecosystem services. Tillage can affect a variety of soil-mediated processes, e.g., soil carbon sequestration (SCS) or depletion, greenhouse gas (GHG) (CO₂, CH_4, and N₂O) emission, and water pollution. Several tillage practices are in vogue globally, and they exhibit varied impacts on these processes. Hence, there is a dire need to synthesize, collate and comprehensively present these interlinked phenomena to facilitate future researches. This study deals with the co-benefits and trade-offs produced by several tillage practices on SCS and related soil properties, GHG emissions, and water quality. We hypothesized that improved tillage practices could enable agriculture to contribute to SCS and mitigate GHG emissions and leaching of nutrients and pesticides. Based on our current understanding, we conclude that sustainable soil moisture level and soil temperature management is crucial under different tillage practices to offset leaching loss of soil stored nutrients/pesticides, GHG emissions and ensuring SCS. For instance, higher carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions from conventional tillage (CT) and no-tillage (NT) could be attributed to the fluctuations in soil moisture and temperature regimes. In addition, NT may enhance nitrate (NO₃^-) leaching over CT because of improved soil structure, infiltration capacity, and greater water flux, however, suggesting that the eutrophication potential of NT is high. Our study indicates that the evaluation of the eutrophication potential of different tillage practices is still overlooked. Our study suggests that improving tillage practices in terms of mitigation of N₂O emission and preventing NO₃^- pollution may be sustainable if nitrification inhibitors are applied.

Factors Influencing Soil Nitrification Process and the Effect on Environment and Health

To meet the global demand for food, several factors have been deployed by agriculturists to supply plants with nitrogen. These factors have been observed to influence the soil nitrification process. Understanding the aftermath effect on the environment and health would provoke efficient management. We review literature on these factors, their aftermath effect on the environment and suggest strategies for better management. Synthetic fertilizers and chemical nitrification inhibitors are the most emphasized factors that influence the nitrification process. The process ceases when pH is &lt;5.0. The range of temperature suitable for the proliferation of ammonia oxidizing archaea is within 30 to 37oC while that of ammonia oxidizing bacteria is within 16 to 23oC. Some of the influencing factors excessively speed up the rate of the nitrification process. This leads to excess production of nitrate, accumulation of nitrite as a result of decoupling between nitritation process and nitratation process. The inhibition mechanism of chemical nitrification inhibitors either causes a reduction in the nitrifying micro-organisms or impedes the amoA gene's function. The effects on the environment are soil acidification, global warming, and eutrophication. Some of the health effects attributed to the influence are methemoglobinemia, neurotoxicity, phytotoxicity and cancer. Biomagnification of the chemicals along the food chain is also a major concern. The use of well-researched and scientifically formulated organic fertilizers consisting of microbial inoculum, well-treated organic manure and good soil conditioner are eco-friendly. They are encouraged to be used to efficiently manage the process. Urban agriculture could promote food production, but environmental sustainability should be ensured.

On (Dis)Connections and Transformations: The Role of the Agricultural Innovation System in the Adoption of Improved Forages in Colombia

Feeding improvement strategies are key in increasing cattle productivity and reducing its environmental footprint. Nevertheless, Colombian tropical cattle systems still feature serious deficiencies in both forage quality and availability. As a result of past and on-going forage Research and Development (R&amp;D) processes, institutions have released 23 grass and legume cultivars of superior characteristics in terms of forage quality, supply, or adaptation to different soil and climate conditions, while providing numerous environmental benefits. However, low levels of adoption are observed: although R&amp;D processes are a necessary condition for adoption, they are still not sufficient to guarantee agricultural technification in Colombia. The ultimate success occurs only when end-users make effective use of a technology–a link constantly interrupted. Agricultural innovation requires complex processes of interaction in which knowledge is shared amongst organizations involved in the Agricultural Innovation System (AIS), namely: suitable links, attitudes, practices, governance structures, and policies. The objective of this study is to identify limitations and opportunities in R&amp;D, adoption, and diffusion of forage technologies in Colombia from an AIS perspective. Particularly, we present a study case pertaining to research institutions only, to (a) map the involved actors and describe their roles and links, and (b) identify the events that marked the evolution of the AIS and the course of forage R&amp;D in its research-related components. We applied a qualitative methodology based on focus group discussions, in-depth interviews, literature review, and historical analysis. Results show that the complex nature of institutions and the interactions between them determine the historical transformation of diffusion of forage technologies. The lack of connection between institutions and the weak intensity of the relationships, prevent the convergence of interests and objectives, leading to vicious cycles that hamper technology adoption. Insufficient synchronization between institutions of different nature (and even between those that share similar objectives) results in efficiency losses due to an unnecessary repetition of activities and processes. We provide recommendations for policy- and decision-makers that will help in both a restructuration of the AIS and a better allocation of funds for R&amp;D, and thus support the development of more effective pathways for forage adoption and scaling.

An ex ante life cycle assessment of wheat with high biological nitrification inhibition capacity

It is essential to increase food production to meet the projected population increase while reducing environmental loads. Biological nitrification inhibition (BNI)-enabled wheat genetic stocks are under development through chromosome engineering by transferring chromosomal regions carrying the BNI trait from a wild relative (Leymus racemosus (Lam.) Tzvelev) into elite wheat varieties; field evaluation of these newly developed BNI-wheat varieties has started. Ten years from now, BNI-enabled elite wheat varieties are expected to be deployed in wheat production systems. This study aims to evaluate the impacts of introducing these novel genetic solutions on life cycle greenhouse gas (LC-GHG) emissions, nitrogen (N) fertilizer application rates and N-use efficiency (NUE). Scenarios were developed based on evidence of nitrification inhibition and nitrous oxide (N₂O) emission reduction by BNI crops and by synthetic nitrification inhibitors (SNIs), as both BNI-wheat and SNIs slow the nitrification process. Scenarios including BNI-wheat will inhibit nitrification by 30% by 2030 and 40% by 2050. It was assumed that N fertilizer application rates can potentially be reduced, as N losses through N₂O emissions, leaching and runoff are expected to be lower. The results show that the impacts from BNI-wheat with 40% nitrification inhibition by 2050 are assessed to be positive: a 15.0% reduction in N fertilization, a 15.9% reduction in LC-GHG emissions, and a 16.7% improvement in NUE at the farm level. An increase in ammonia volatilization had little influence on the reduction in LC-GHG emissions. The GHG emissions associated with N fertilizer production and soil N₂O emissions can be reduced between 7.3 and 9.5% across the wheat-harvested area worldwide by BNI-wheat with 30% and 40% nitrification inhibition, respectively. However, the present study recommends further technological developments (e.g. further developments in BNI-wheat and the development of more powerful SNIs) to reduce environmental impacts while improving wheat production to meet the increasing worldwide demand.

CH4 and N2O Emissions From Cattle Excreta: A Review of Main Drivers and Mitigation Strategies in Grazing Systems

Cattle production systems are an important source of greenhouse gases (GHG) emitted to the atmosphere. Animal manure and managed soils are the most important sources of emissions from livestock after enteric methane. It is estimated that the N2O and CH4 produced in grasslands and manure management systems can contribute up to 25% of the emissions generated at the farm level, and therefore it is important to identify strategies to reduce the fluxes of these gases, especially in grazing systems where mitigation strategies have received less attention. This review describes the main factors that affect the emission of GHG from manure in bovine systems and the main strategies for their mitigation with emphasis on grazing production systems. The emissions of N2O and CH4 are highly variable and depend on multiple factors, which makes it difficult to use strategies that mitigate both gases simultaneously. We found that strategies such as the optimization of the diet, the implementation of silvopastoral systems and other practices with the capacity to improve soil quality and cover, and the use of nitrogen fixing plants are among the practices with more potential to reduce emissions from manure and at the same time contribute to increase carbon capture and improve food production. These strategies can be implemented to reduce the emissions of both gases and, depending on the method used and the production system, the reductions can reach up to 50% of CH4 or N2O emissions from manure according to different studies. However, many research gaps should be addressed in order to obtain such reductions at a larger scale.

Risk Reduction and Productivity Increase Through Integrating Arachis pintoi in Cattle Production Systems in the Colombian Orinoquía

In many parts of the foothills of the Orinoquía region of Colombia, cattle production takes place on poorly drained soils. The region is dominated by extensive grazing systems of Brachiaira humidicola cv. Humidicola, a grass with high adaptation potential under temporal waterlogging conditions. Inadequate management practices and low soil fertility result in degradation, however, with important negative effects on pasture productivity and the quality and provision of (soil) ecosystem services–a situation that is likely to worsen in the near future due to climate change. Against this background, AGROSAVIA (Corporación Colombiana de Investigación Agropecuaria) selected Arachis pintoi CIAT 22160 cv. Centauro (Centauro) as a promising alternative for the sustainable intensification of livestock production and rehabilitation of degraded areas. This study assesses dual-purpose milk production in the foothills of the Colombian Orinoquía from an economic perspective. We compare two production systems: the Centauro–Brachiaira humidicola cv. Humidicola association (new system) and Brachiaira humidicola cv. Humidicola as a monoculture (traditional system). We used cashflow and risk assessment models to estimate economic indicators. The projections for economic returns consider changes in forage characteristics under regional climate change scenarios RCP (2.6, 8.5). The LIFE-SIM model was used to simulate dairy production. Results show that the inclusion of Centauro has the potential to increase animal productivity and profitability under different market scenarios. The impact of climatic variables on forage production is considerable in both climate change scenarios. Both total area and potential distribution of Centauro could change, and biomass production could decline. Brachiaira humidicola cv. Humidicola showed better persistence due to higher nitrogen levels in soil when grown in association with Centauro. The legume also provides a number of ecosystem services, such as improving soil structure and composition, and also contributes to reducing greenhouse gas emissions. This helps to improve the adaptation and mitigation capacity of the system.

Effects of biological nitrification inhibitors on nitrogen use efficiency and greenhouse gas emissions in agricultural soils: A review

To maintain and increase crop yields, large amounts of nitrogen fertilizers have been applied to farmland. However, the nitrogen use efficiency (NUE) of chemical fertilizer remains very low, which may lead to serious environmental problems, including nitrate pollution, air quality degradation and greenhouse gas (GHG) emissions. Nitrification inhibitors can alleviate nitrogen loss by inhibiting nitrification; thus, biological nitrification inhibition by plants has gradually attracted increasing attention due to its low cost and environmental friendliness. Research progress on BNI is reviewed in this article, including the source, mechanisms, influencing factors and application of BNIs. In addition, the impact of BNI on agriculture and GHG emissions is summarized from the perspective of agricultural production and environmental protection, and the key future research prospects of BNIs are also noted.

Enlisting wild grass genes to combat nitrification in wheat farming: A nature-based solution

Globally, wheat farming is a major source of nitrogen pollution. Rapid generation of soil nitrates cause nitrogen leakage and damage ecosystems and human health. Here, we show the 3Ns^bS chromosome arm in wild grass (Leymus racemosus) that controls root nitrification inhibitor production can be transferred into elite wheat cultivars, without disrupting the elite agronomic features. Biological nitrification inhibition (BNI)–enabled wheats can improve soil ammonium levels by slowing down its oxidation and generate significant synergistic benefits from assimilating dual nitrogen forms and improving adaptation to low N systems. Deploying BNI-enabled wheat on a significant proportion of current global wheat area (ca. 225 M ha) could be a powerful nature-based solution for reducing N fertilizer use and nitrogen losses while maintaining productivity.

Active nitrifiers and rapid nitrification are major contributing factors to nitrogen losses in global wheat production. Suppressing nitrifier activity is an effective strategy to limit N losses from agriculture. Production and release of nitrification inhibitors from plant roots is termed “biological nitrification inhibition” (BNI). Here, we report the discovery of a chromosome region that controls BNI production in “wheat grass” Leymus racemosus (Lam.) Tzvelev, located on the short arm of the “Lr#3Ns^b” (Lr#n), which can be transferred to wheat as T3BL.3Ns^bS (denoted Lr#n-SA), where 3BS arm of chromosome 3B of wheat was replaced by 3Ns^bS of L. racemosus. We successfully introduced T3BL.3Ns^bS into the wheat cultivar “Chinese Spring” (CS-Lr#n-SA, referred to as “BNI-CS”), which resulted in the doubling of its BNI capacity. T3BL.3Ns^bS from BNI-CS was then transferred to several elite high-yielding hexaploid wheat cultivars, leading to near doubling of BNI production in “BNI-MUNAL” and “BNI-ROELFS.” Laboratory incubation studies with root-zone soil from field-grown BNI-MUNAL confirmed BNI trait expression, evident from suppression of soil nitrifier activity, reduced nitrification potential, and N₂O emissions. Changes in N metabolism included reductions in both leaf nitrate, nitrate reductase activity, and enhanced glutamine synthetase activity, indicating a shift toward ammonium nutrition. Nitrogen uptake from soil organic matter mineralization improved under low N conditions. Biomass production, grain yields, and N uptake were significantly higher in BNI-MUNAL across N treatments. Grain protein levels and breadmaking attributes were not negatively impacted. Wide use of BNI functions in wheat breeding may combat nitrification in high N input–intensive farming but also can improve adaptation to low N input marginal areas.

Biological nitrification inhibition in the rhizosphere: determining interactions and impact on microbially mediated processes and potential applications

Nitrification is the microbial conversion of reduced forms of nitrogen (N) to nitrate (NO3-), and in fertilized soils it can lead to substantial N losses via NO3- leaching or nitrous oxide (N2O) production. To limit such problems, synthetic nitrification inhibitors have been applied but their performance differs between soils. In recent years, there has been an increasing interest in the occurrence of biological nitrification inhibition (BNI), a natural phenomenon according to which certain plants can inhibit nitrification through the release of active compounds in root exudates. Here, we synthesize the current state of research but also unravel knowledge gaps in the field. The nitrification process is discussed considering recent discoveries in genomics, biochemistry and ecology of nitrifiers. Secondly, we focus on the 'where' and 'how' of BNI. The N transformations and their interconnections as they occur in, and are affected by, the rhizosphere, are also discussed. The NH4+ and NO3- retention pathways alternative to BNI are reviewed as well. We also provide hypotheses on how plant compounds with putative BNI ability can reach their targets inside the cell and inhibit ammonia oxidation. Finally, we discuss a set of techniques that can be successfully applied to solve unresearched questions in BNI studies.

Nitrous oxide production by ammonia oxidizers: Physiological diversity, niche differentiation and potential mitigation strategies

Oxidation of ammonia to nitrite by bacteria and archaea is responsible for global emissions of nitrous oxide directly and indirectly through provision of nitrite and, after further oxidation, nitrate to denitrifiers. Their contributions to increasing N₂ O emissions are greatest in terrestrial environments, due to the dramatic and continuing increases in use of ammonia-based fertilizers, which have been driven by requirement for increased food production, but which also provide a source of energy for ammonia oxidizers (AO), leading to an imbalance in the terrestrial nitrogen cycle. Direct N₂ O production by AO results from several metabolic processes, sometimes combined with abiotic reactions. Physiological characteristics, including mechanisms for N₂ O production, vary within and between ammonia-oxidizing archaea (AOA) and bacteria (AOB) and comammox bacteria and N₂ O yield of AOB is higher than in the other two groups. There is also strong evidence for niche differentiation between AOA and AOB with respect to environmental conditions in natural and engineered environments. In particular, AOA are favored by low soil pH and AOA and AOB are, respectively, favored by low rates of ammonium supply, equivalent to application of slow-release fertilizer, or high rates of supply, equivalent to addition of high concentrations of inorganic ammonium or urea. These differences between AOA and AOB provide the potential for better fertilization strategies that could both increase fertilizer use efficiency and reduce N₂ O emissions from agricultural soils. This article reviews research on the biochemistry, physiology and ecology of AO and discusses the consequences for AO communities subjected to different agricultural practices and the ways in which this knowledge, coupled with improved methods for characterizing communities, might lead to improved fertilizer use efficiency and mitigation of N₂ O emissions.

Long term beef production in extensive cow-calf systems in the tropical savannas of eastern Colombia

Replacement beef heifers in extensive systems of the neotropical savannas of Colombia are low priority animals and are assigned to native savannas and low-quality pastures, with scarcely researched long-term consequences. The study pooled data from four contemporary grazing experiments to determine the effects of low growth rates of heifers on their lifetime production of liveweight gains (LWGs) per year and per hectare (ha) subject to different management strategies. Three growth rates were imposed on grazing heifers during the growing phase (i.e. nine-36 months of age) on Brachiaria humidicola pastures. Upon the end of the previous phase, animals were allocated to a B. humidicola paddock with a reduced stocking rate or introduced to a B. decumbens sward to allow compensatory LWGs during the breeding phase (i.e. cows plus weaned calves). Severely limited heifers allowed to make compensatory growth did not reach the outputs of better fed animals. Regardless of the strategy followed, high producing years tended to be followed by lower production in the following year. Comparison with reasonably fed breeding cows full-time on well-managed B. decumbens showed important and significant differences in favor of the more intensive system over the lifetime of the animals. Over the nine-year period, the systems’ beef output (i.e. weaned calves plus cows’ LWGs) ranged between 86 and 206 kg/ha/year and demonstrated that a wide array of low input management alternatives is feasible. Beef output increases if the LW of cull cows is considered, particularly if they are allowed a short period of fattening during the rainy reason on a low input pasture.

Controls and Adaptive Management of Nitrification in Agricultural Soils

Agriculture is responsible for over half of the input of reactive nitrogen (N) to terrestrial systems; however improving N availability remains the primary management technique to increase crop yields in most regions. In the majority of agricultural soils, ammonium is rapidly converted to nitrate by nitrification, which increases the mobility of N through the soil matrix, strongly influencing N retention in the system. Decreasing nitrification through management is desirable to decrease N losses and increase N fertilizer use efficiency. We review the controlling factors on the rate and extent of nitrification in agricultural soils from temperate regions including substrate supply, environmental conditions, abundance and diversity of nitrifiers and plant and microbial interactions with nitrifiers. Approaches to the management of nitrification include those that control ammonium substrate availability and those that inhibit nitrifiers directly. Strategies for controlling ammonium substrate availability include timing of fertilization to coincide with rapid plant update, formulation of fertilizers for slow release or with inhibitors, keeping plant growing continuously to assimilate N, and intensify internal N cycling (immobilization). Another effective strategy is to inhibit nitrifiers directly with either synthetic or biological nitrification inhibitors. Commercial nitrification inhibitors are effective but their use is complicated by a changing climate and by organic management requirements. The interactions of the nitrifying organisms with plants or microbes producing biological nitrification inhibitors is a promising approach but just beginning to be critically examined. Climate smart agriculture will need to carefully consider optimized seasonal timing for these strategies to remain effective management tools.

Nitrate reductase activity in leaves as a plant physiological indicator of in vivo biological nitrification inhibition by Brachiaria humidicola

The tropical forage grass Brachiaria humidicola (Bh) controls soil microbial nitrification via biological nitrification inhibition (BNI). The aim of our study was to verify if nitrate reductase activity (NRA) in Bh roots or leaves reflects in vivo performance of BNI in soils. NRA was measured in roots and leaves of contrasting accessions and apomictic hybrids of Bh grown under controlled greenhouse and natural field conditions. Nitrate (NO₃^-) contents were measured in soil solution and in Bh stem sap to validate NRA data. Potential soil nitrification rates (NRs) and leaf δ¹⁵N values were used to verify in vivo BNI by the NRA assay in the field study. NRA was detected in Bh leaves rather than roots, regardless of NO₃^- availability. NRA correlated with NO₃^- contents in soils and stem sap of contrasting Bh genotypes substantiating its reflectance of in vivo BNI performance. Additionally, leaf NRA data from the field study significantly correlated with simultaneously collected NRs and leaf δ¹⁵N data. The leaf NRA assay facilitated a rapid screening of contrasting Bh genotypes for their differences in in vivo performance of BNI under field and greenhouse conditions, but inconsistency of the BNI potential by Bh germplasm was observed. Among Bh genotypes tested, leaf NRA was closely linked with nitrification activity, and consequently with actual BNI performance. It was concluded that NRA in leaves of Bh can serve as an indicator of in vivo BNI activity when complemented with established BNI methodologies (δ¹⁵N, NRs) under greenhouse and field conditions.