AMMONIUM AND NITRATE NUTRITION OF PLANTS

Tipping the plant-microbe competition for nitrogen in agricultural soils

Nitrogen (N) is the most limiting nutrient in agroecosystems, and its indiscriminate application is at the center of the environmental challenges facing agriculture. To solve this dilemma, crops' nitrogen use efficiency (NUE) needs to increase - in other words, more of the applied nitrogen needs to reach humans. Microbes are the key to cracking this problem. Microbes use nitrogen as an energy source, an electron acceptor, or incorporate it in their biomass. These activities change the form and availability of nitrogen for crops' uptake, impacting its NUE, yields and produce quality. Plants (and microbes) have, however, evolved many mechanisms to compete for soil nitrogen. Understanding and harnessing these competitive mechanisms would enable us to tip the nitrogen balance to the advantage of crops. We will review these competitive mechanisms and highlight some approaches that were applied to reduce microbial competition for N in an agricultural context.

Nitric oxide-mediated regulation of macronutrients in plants

In plant physiology, nitric oxide (NO) is a widely used signaling molecule. It is a free radical and an important component of the N-cycle. NO is produced endogenously inside plant cells, where it participates in multiple functions and provides protection against several abiotic and biotic stresses. NO and its interplay with macronutrients had remarkable effects on plant growth and development, the signaling pathway, and defense mechanisms. Its chemical properties, synthetic pathways, physiological effects, antioxidant action, signal transduction, and regulation of transporter genes and proteins have been studied. NO emerges as a key regulator under macronutrient deficiency. In plants, NO also affects reactive oxygen species (ROS), reactive nitrogen species (RNS), and post-translational modifications (PTMs). The function of NO and its significant control in the functions and adjustments of macronutrients under macronutrient deficit were summed up in this review. NO regulate functions of macronutrients and associated signaling events involved with macronutrient transporters in different plants.

Implementing a soil ammonium N fertilizer management for synchronizing potato N demands

Potatoes, as a high-nitrogen (N)-demand crop, are strongly influenced by both the quantity and form of N supply. Previous studies have demonstrated that applying nitrate N prior to tuber formation and ammonium N post-tuber formation can substantially enhance potato yields and improve N fertilizer use efficiency. However, the ammonium N introduced into the soil undergoes nitrification, creating challenges in aligning the N supply form with the needs of potatoes. This study explored novel N regulation strategies aimed at augmenting potato yields and improving N fertilizer use efficiency. Two field experiments were conducted from 2020 to 2022. Experiment 1 involved four N gradients, namely no N, 150 kg N ha-1, 300 kg N ha-1, and 450 kg N ha-1. Soil samples were collected regularly to determine the transformation patterns of soil ammonium N during potato growth. Experiment 2 included three N management practices: farmer practice (Con), "nitrate followed by ammonium" with nitrification inhibitor (N-NI), and optimization (the soil ammonium N transformation-based split application of N fertilizer, Opt). The potato yield and N fertilizer use efficiency were compared to assess the performance of the optimized strategy. The results showed that 90 % of the ammonium N transformed 20 days after the basal dressing of N. When N fertilizer was applied as top dressing during the tuber formation and bulking stages, more than 90 % of ammonium N was transformed after 10 days. The optimized strategy resulted in a 20 % increase in potato yield, a 20 % increase in N fertilizer partial factor productivity, and a 12-20 % reduction in residual inorganic N in the 0-60 cm soil layer. This suggests that ammonium N applied as base fertilizer exhibits a relatively slow transformation rate, while applying ammonium N as top dressing during the tuber formation and bulking stages accelerates the transformation rate. The split application of ammonium N based on soil ammonium N transformation patterns can improve the alignment between the N supply form with the specific demands of potatoes.

Boosting cadmium tolerance in Phoebe zhennan: the synergistic effects of exogenous nitrogen and phosphorus treatments promoting antioxidant defense and root development

Plants possess intricate defense mechanisms to resist cadmium (Cd) stress, including strategies like metal exclusion, chelation, osmoprotection, and the regulation of photosynthesis, with antioxidants playing a pivotal role. The application of nitrogen (N) and phosphorus (P) fertilizers are reported to bolster these defenses against Cd stress. Several studies investigated the effects of N or P on Cd stress in non-woody plants and crops. However, the relationship between N, P application, and Cd stress resistance in valuable timber trees remains largely unexplored. This study delves into the Cd tolerance mechanisms of Phoebe zhennan, a forest tree species, under various treatments: Cd exposure alone, combined Cd stress with either N or P and Cd stress with both N and P application. Our results revealed that the P application enhanced root biomass and facilitated the translocation of essential nutrients like K, Mn, and Zn. Conversely, N application, especially under Cd stress, significantly inhibited plant growth, with marked reductions in leaf and stem biomass. Additionally, while the application of P resulted in reduced antioxidant enzyme levels, the combined application of N and P markedly amplified the activities of peroxidase by 266.36%, superoxide dismutase by 168.44%, and ascorbate peroxidase by 26.58% under Cd stress. This indicates an amplified capacity of the plant to neutralize reactive oxygen species. The combined treatment also led to effective regulation of nutrient and Cd distribution in roots, shoots, and leaves, illustrating a synergistic effect in mitigating toxic impact of N. The study also highlights a significant alteration in photosynthetic activities under different treatments. The N addition generally reduced chlorophyll content by over 50%, while P and NP treatments enhanced transpiration rates by up to 58.02%. Our findings suggest P and NP fertilization can manage Cd toxicity by facilitating antioxidant production, osmoprotectant, and root development, thus enhancing Cd tolerance processes, and providing novel strategies for managing Cd contamination in the environment.

Rehabilitation of eroded trails and gullies on quartzite rock outcrops with native species in a high-altitude grassland

The quartzite rock outcrops and the native vegetation of grasslands located at the Serra da Calçada Mountain in Minas Gerais State (Brazil) have been severely degraded by extreme sports activities such as motocross and off-road vehicles, greatly damaging the abundant headwaters. The main consequences thereof were hilly and gully erosion processes with soil loss and the deviation of the water from its original paths. However, currently, there is no report of successful restoration efforts in severely eroded outcrops in Brazilian high-altitude grasslands (campo rupestre). Through the Universal Soil Loss Equation (USLE), we found a high general erosion rate in the study site (669.91 t·ha^-1·year^-1), and the specific soil loss provoked by off-road vehicles on trails was significantly greater (49 m³ per 100 m²) than that caused by mountain bikes and trekking (5.8 m³ per 100 m²). We performed the physical reconstruction of eroded outcrops and surface water flow paths by allocating locally available quartzite rocks. These rocks were inoculated with different species of bryophytes and planted with native species under two treatments: un-inoculated and inoculated with arbuscular mycorrhizal fungi (AMF) spores of the Rhizophagus irregularis species. After 2 years, the bryophyte communities showed a similar pattern to the preserved site, and the AMF inoculation favoured plant establishment of most species, especially of the Asteraceae, Cyperaceae, Fabaceae, Malpighiaceae, Orchidaceae and Poaceae families. The AMF also improved the soil fertility, highlighting soil P, SOM, CEC, NH₄⁺-N as well as soil water content and water retention capacity. Poaceae family species showed an outstanding occupation, which was considered a functional indicator of rehabilitation success, functioning as a "hydraulic carpet" for water exportation, conduction and drainage across the outcrops. This study provides an eco-technology to restore severely eroded outcrops over headwaters using native species in the Brazilian high-altitude grasslands.

Effects of combined nitrification inhibitors on soil nitrification, maize yield and nitrogen use efficiency in three agricultural soils

Application of nitrification inhibitors (NIs) with nitrogen (N) fertilizer is one of the most efficient ways to improve nitrogen use efficiency (NUE). To fully understand the efficiency of NIs with N fertilizer on soil nitrification, yield and NUE of maize (Zea mays L.), an outdoor pot experiment with different NIs in three soils with different pH was conducted. Five treatments were established: no fertilizer (Control); ammonium sulfate (AS); ammonium sulfate + 3, 4-dimethyl-pyrazolate phosphate (DMPP) (AD); ammonium sulfate + nitrogen protectant (N-GD) (AN); ammonium sulfate + 3, 4-dimethyl-pyrazolate phosphate + nitrogen protectant (ADN). The results showed that NIs treatments (AD, AN and ADN) significantly reduced soil nitrification in the brown and red soil, especially in AD and ADN, which decreased apparent nitrification rate by 28% - 44% (P < 0.05). All NIs treatments significantly increased yield and NUE of maize in three soils, especially ADN in the cinnamon soil and AD in the red soil were more efficiency, which significantly increased maize yield and apparent nitrogen recovery by 5.07 and 6.81 times, 4.39 and 8.16 times, respectively. No significant difference on maize yield was found in the brown soil, but AN significantly increased apparent nitrogen recovery by 70%. Given that the effect of NIs on both soil nitrification and NUE of maize, DMPP+N-GD was more efficient in the cinnamon soil, while N-GD and DMPP was the most efficiency in the brown and red soil, respectively. In addition, soil pH and soil organic matter play important role in the efficiency of NIs.

Enhanced nitrate reductase activity offers Arabidopsis ecotype Landsberg erecta better salt stress resistance than Col-0

The nitrogen utilization efficiency of plants varies depending on the plant species. In modern agriculture, nitrogen fertilizer is used to increase crop production, with the amount of fertilizer addition increasing steadily worldwide. This study included the two most used ecotypes of Arabidopsis thaliana, Landsberg erecta (Ler) and Col-0, which were used to identify differences at the molecular level. We found that the efficiency of nitrogen utilization and salt stress resistance differed between these two ecotypes of the same species. We demonstrated distinct salt stress resistance between Ler and Col-0 depending on the differences in nitrate level, which was explained by different regulation of the NIA2 gene expression in these two ecotypes. Our results demonstrate that the genes and promoters regulate expression of these genes and contribute to trait differences. Further studies are required on genes and promoter elements for an improved understanding of the salinity stress resistance mechanism in plants.

Improved Utilization of Nitrate Nitrogen Through Within-Leaf Nitrogen Allocation Trade-Offs in Leymus chinensis

The Sharply increasing atmospheric nitrogen (N) deposition may substantially impact the N availability and photosynthetic capacity of terrestrial plants. Determining the trade-off relationship between within-leaf N sources and allocation is therefore critical for understanding the photosynthetic response to nitrogen deposition in grassland ecosystems. We conducted field experiments to examine the effects of inorganic nitrogen addition (sole NH₄ ⁺, sole NO₃ ^- and mixed NH₄ ⁺/NO₃ ^-: 50%/50%) on N assimilation and allocation by Leymus chinensis. The leaf N allocated to the photosynthetic apparatus (N_PSN) and chlorophyll content per unit area (Chl_area) were significantly positively correlated with the photosynthetic N-use efficiency (PNUE). The sole NO₃ ^- treatment significantly increased the plant leaf PNUE and biomass by increasing the photosynthetic N allocation and Chl_area. Under the NO₃ treatment, L. chinensis plants devoted more N to their bioenergetics and light-harvesting systems to increase electron transfer. Plants reduced the cell wall N allocation or increased their soluble protein concentrations to balance growth and defense under the NO₃ treatment. In the sole NH₄ ⁺ treatment, however, plants decreased their N allocation to photosynthetic components, but increased their N allocation to the cell wall and elsewhere. Our findings demonstrated that within-leaf N allocation optimization is a key adaptive mechanism by which plants maximize their PNUE and biomass under predicted future global changes.

Stable isotope analyses of nitrogen source and preference for ammonium versus nitrate of riparian plants during the plant growing season in Taihu Lake Basin

Plants are vital components of the nitrogen (N) cycling in the riparian zones. Understanding of N uptake strategies of riparian plants, including N sources and preference in N forms (ammonium (NH₄⁺) vs. nitrate (NO₃^-)), is essential to advance our knowledge on the role that plants play in regulating nutrient biogeochemical cyclings in the riparian areas. In this study, stable N isotopes (δ¹⁵N) of three riparian plants, including Acorus calamus, Canna indica and Phragmites australis, and the δ¹⁵N of NH₄⁺ and NO₃^- in different sources were measured during the plant growing season (June-September) in the Taihu Lake Basin. The dissolved inorganic N (DIN) from river water, groundwater, rainwater and soil were considered as the major N sources for plants in the riparian ecosystem. Our results indicated that soil was the largest source for plant N nutrition, with significantly different (P < 0.05) contributions from soil observed among plant species (80.5 ± 4.1, 73.9 ± 2.8 and 58.7 ± 6.1% for A. calamus, C. indica, and P. australis, respectively). Meanwhile, complex water networks, shallow water tables, and high DIN content in rainwater lead to nonignorable N contributions from river water, groundwater and rainwater to plants. Groundwater contributed more percentage of N to P. australis (12.8 ± 3.2%) than A. calamus (6.1 ± 1.9%) and C. indica (8.0 ± 1.5%), which is likely attributed to the deeper roots of P. australis. All plants showed similar N preference for NO₃^- during the growing season. External environmental conditions and plant characteristics and adaption to more abundant soil NO₃^- content are possible explanations. Our research could provide important information for vegetation selections during the process of riparian ecological restoration. Reasonable choice of vegetation is essential to plant growth and water quality management, especially in agricultural watersheds where N concentrations are relatively high in agricultural runoff due to the wide uses of N fertilizers.

Widespread Use of the Nitrification Inhibitor Nitrapyrin: Assessing Benefits and Costs to Agriculture, Ecosystems, and Environmental Health

Agricultural production and associated applications of nitrogen (N) fertilizers have increased dramatically in the last century, and current projections to 2050 show that demands will continue to increase as the human population grows. Applied in both organic and inorganic fertilizer forms, N is an essential nutrient in crop productivity. Increased fertilizer applications, however, create the potential for more N loss before plant uptake. One strategy for minimizing N loss is the use of enhanced efficiency fertilizers, fortified with a nitrification inhibitor, such as nitrapyrin. In soils and water, nitrapyrin inhibits the activity of ammonia monooxygenase, a microbial enzyme that catalyzes the first step of nitrification from ammonium to nitrite. Potential benefits of using nitrification inhibitors range from reduced nitrate leaching and nitrous oxide emissions to increased crop yield. The extent of these benefits, however, depends on environmental conditions and management practices. Thus, such benefits are not always realized. Additionally, nitrapyrin has been shown to transport off-field, and it is unknown what effects environmental nitrapyrin could have on nontarget organisms and the ecological nitrogen cycle. Here, we review the agronomic and environmental benefits and costs of nitrapyrin use and present a series of research questions and considerations to be addressed with future nitrification inhibitor research.

Coffee seedlings growth under varied NO3-:NH4+ ratio: Consequences for nitrogen metabolism, amino acids profile, and regulation of plasma membrane H+-ATPase

Root plasma membrane H⁺-ATPase electrochemical equilibrium for optimum coffee plant growth can be modulated by specific ammonium:nitrate (NO₃^-:NH₄⁺) ratio supply. This study aimed to evaluate the coffee seedlings responses to varying ammonium:nitrate (NO₃^-:NH₄⁺) ratio and to depict how much NO₃^- and NH₄⁺ plants can use in terms of growth, nitrogen metabolism, amino acids profile and regulation of root plasma membrane H⁺-ATPase. Coffee plants were grown in nutrient solution with the following NO₃^-:NH₄⁺ ratios (%): 100:0; 87.5:12.5; 50:50; 0:100. Plants were grown in nutrient solution for 90 days and evaluated for growth, nitrate reductase activity as well as the modulation of H⁺-ATPase activity in the plasma membrane of the roots, amino acids profile, chlorophyll a fluorescence parameters and estimated cations and anions taken up by plants. The plants treated with the 87.5:12.5 and 50:50 NO₃^-:NH₄⁺ ratio showed higher ability to absorb nutrients maintaining balanced uptake and as a consequence, 6% and 29%, the highest dry mass yield as compared to the 0:100 NO₃^-:NH₄⁺ ratio. In addition, plants supplied with the 87.5:12.5 and 50:50 NO₃^-:NH₄⁺ ratio had respectively, 58% and 94%, greater photosynthetic capability. Those data suggest that farmers and plant nurseries could implement the 50:50 NO₃^-:NH₄⁺ ratio of nitrogen sources at coffee plantations and seedlings.

Microbial indicators are better predictors of wheat yield and quality than N fertilization

In view of their key roles in many soil- and plant-related processes, we hypothesized that soil microorganisms could play a larger role in determining wheat baking quality than nitrogen fertilization. A field experiment was conducted under bread wheat production conditions, where different fertilization treatments, ranging from 0-120 kg/ha NH4NO3, were applied. Soil samples were taken in May, June and July. Functional genes in the nitrogen cycle were quantified and amplicons of the 16S rRNA gene and the ITS region were sequenced. Wheat yields were measured, and the grain baking quality was analysed for each plot. Fertilisation did not significantly influence the yields and the grain quality. Many bacterial and fungal Amplicon Sequence Variants showed significant positive or negative correlations with yield and grain baking quality parameters. Among the functional gene quantified, the archaeal amoA showed strong negative correlations with the wheat yields and many grain and flour quality parameters. Regression models were able to explain up to 81% of the variability in grain quality based on the microbial data from the May sampling. A better understanding of the microbiology of wheat fields could lead to an optimized management of the N fertilization to maximize yields and grain quality.

Expression of N-cycling genes of root microbiomes provides insights for sustaining oilseed crop production

Agricultural production is dependent on inputs of nitrogen (N) whose cycle relies on soil and crop microbiomes. Crop diversification has increased productivity; however, its impact on the expression of microbial genes involved in N-cycling pathways remains unknown. Here, we assessed N-cycling gene expression patterns in the root and rhizosphere microbiomes of five oilseed crops as influenced by three 2-year crop rotations. The first phase consisted of fallow, lentil or wheat, and the second phase consisted of one of five oilseed crops. Expression of bacterial amoA, nirK and nirS genes showed that the microbiome of Ethiopian mustard had the lowest and that of camelina the highest potential for N loss. A preceding rotation phase of lentil significantly increased the expression of nifH gene by 23% compared with wheat and improved nxrA gene expression by 51% with chemical fallow in the following oilseed crops respectively. Lentil substantially increased biological N₂ fixation and reduced denitrification in the following oilseed crops. Our results also revealed that most N-cycling gene transcripts are more abundant in the microbiomes associated with roots than with the rhizosphere. The outcome of our investigation brings a new level of understanding on how crop diversification and rotation sequences are related to N-cycling in annual cropping systems.

Response of soil fertility to 25 years of experimental acidification in a temperate hardwood forest

The effects of enhanced acid deposition from the atmosphere, and associated elevated inputs of N, are widely evident, especially for forests where excess N has led to a variety of deleterious effects. These include declines in biodiversity, a response that will likely require considerable time for recovery. The purpose of this study was to determine responses of plant nutrient availability in surface mineral soil to 25 yr of experimental acidification and N addition in a central Appalachian hardwood forest ecosystem. We hypothesized that chronic additions of (NH₄ )₂ SO₄ will increase mineral N, decrease soil pH, P, and base cations, increase micronutrients (Mn²⁺ and Fe²⁺ ), and increase levels of Al³⁺ . Results supported these predictions, although Mn²⁺ did not vary significantly. Earlier work on these plots found no response of any of the extractable nutrients to 3 yr of treatment, yet after 25 yr, our results suggest that impacts are apparent in the top 5 cm of the A horizon. We surmise that impacts in these soils may have lagged behind the onset of acidification treatments or that several years of treatment were required to overcome preexisting differences in soil ions. Generally, current findings confirm that (NH₄ )₂ SO₄ treatments have lowered the pH, enhanced levels of exchangeable Al³⁺ , and increased stream-water exports of NO₃ ^- and base cations-a process that further acidifies soil. The combination of these changes in surface soils, with their high proportion of fine roots, may contribute to the reduced growth and competitiveness of some hardwood species at the acidified site.

Promoting pepper (Capsicum annuum) photosynthesis via chloroplast ultrastructure and enzyme activities by optimising the ammonium to nitrate ratio

Optimal plant growth in many species is achieved when the two major forms of N are supplied at a particular ratio. In this pot experiment, the effects of five different ammonium:nitrate ratios (ANRs) (0:100, 12.5:87.5, 25:75, 37.5:62.5, and 50:50) on photosynthesis efficiency in chilli pepper (Capsicum annuum L.) plants were evaluated. The results showed that an ANR of 25:75 increased the contents of chl a, leaf area and dry matter, whereas chl b content was not affected by the ANRs. Regarding chlorophyll fluorescence, an ANR of 25:75 also enhanced the actual photochemical efficiency, photochemical quenching and maximum photosynthetic rate. However, the 0:100 and 50:50 ANRs resulted in higher values for nonphotochemical quenching. An inhibition of maximal photochemical efficiency was found when 50% NH4+ was supplied at the later stage of plant growth. The addition of 25% or 37.5% NH4+ was beneficial for gas exchange parameters and the 25% NH4+ optimised the thylakoid of chloroplasts. Compared with nitrate alone, 12.5–50% NH4+ upregulated glutamate dehydrogenase (GDH), the large subunit and the small subunit of Rubisco. It can be concluded that the 25:75 ANR accelerated N assimilation through active GDH, which provides a material basis for chloroplast and Rubisco formation, resulting in the increased photosynthetic rate and enhanced growth in chilli pepper.

Overexpression of the High-Affinity Nitrate Transporter OsNRT2.3b Driven by Different Promoters in Barley Improves Yield and Nutrient Uptake Balance

Improving nitrogen use efficiency (NUE) is very important for crops throughout the world. Rice mainly utilizes ammonium as an N source, but it also has four NRT2 genes involved in nitrate transport. The OsNRT2.3b transporter is important for maintaining cellular pH under mixed N supplies. Overexpression of this transporter driven by a ubiquitin promoter in rice greatly improved yield and NUE. This strategy for improving the NUE of crops may also be important for other cereals such as wheat and barley, which also face the challenges of nutrient uptake balance. To test this idea, we constructed transgenic barley lines overexpressing OsNRT2.3b. These transgenic barley lines overexpressing the rice transporter exhibited improved growth, yield, and NUE. We demonstrated that NRT2 family members and the partner protein HvNAR2.3 were also up-regulated by nitrate treatment (0.2 mM) in the transgenic lines. This suggests that the expression of OsNRT2.3b and other HvNRT2 family members were all up-regulated in the transgenic barley to increase the efficiency of N uptake and usage. We also compared the ubiquitin (Ubi) and a phloem-specific (RSs1) promoter-driven expression of OsNRT2.3b. The Ubi promoter failed to improve nutrient uptake balance, whereas the RSs1 promoter succeed in increasing the N, P, and Fe uptake balance. The nutrient uptake enhancement did not include Mn and Mg. Surprisingly, we found that the choice of promoter influenced the barley phenotype, not only increasing NUE and grain yield, but also improving nutrient uptake balance.

An Efficient Method for In Vitro Shoot-Tip Culture and Sporophyte Production Using Selaginella martensii Spring Sporophyte

Selaginella martensii, an evergreen perennial fern that is native to South America and New Zealand, is named “frosty fern” because of its beautiful white-colored leaves and it is used as an ornamental plant. Efficient propagation methods for this species have not been developed. We aimed to develop an efficient propagation method for S. martensii through in vitro culture. We investigated culture conditions that are suitable for shoot-tip proliferation and growth. The optimum shoot-tip culture conditions were determined while using Murashige and Skoog (MS) medium (quarter, half, full, or double strength) and macronutrients (sucrose and two nitrogen sources) at various concentrations. In MS medium, the shoot tips formed a maximum of 6.77 nodes per explant, and each node formed two new shoot tips (i.e., 2⁶ or 64 shoot tips). When using branching segments containing an angle meristem, the shoot-to-rhizophore formation ratio could be controlled by medium supplementation with plant-growth regulators. Sporophytes that were grown from shoot tips in vitro were acclimated in ex vitro soil conditions and successfully survived in the greenhouse. Numerous shoot tips could be obtained from in vitro-grown sporophytes and be proliferated ex vitro to produce a large number of plants. This method provides a way of shortening the time that is required for producing a large stock of S. martensii planting material.

Expression of a cucumber alanine aminotransferase2 gene improves nitrogen use efficiency in transgenic rice

Background

Rice can absorb less than 40% of applied nitrogen fertilizer, whereas the unabsorbed nitrogen fertilizer may cause environmental problems, such as algal blooms in freshwater and increased production of nitrous oxide, a greenhouse gas which is 300 times more potent than carbon dioxide. Development of nitrogen use efficient (NUE) rice is essential for more environmentally friendly rice production. Recently, NUE rice has been developed by root-specific expression of alanine aminotransferase (AlaAT) gene from barley, a monocot plant. Therefore, we tested the efficacy of AlaAT gene from cucumber in transgenic rice, aiming to provide evidence for the conservation of AlaAT gene function in monocot and dicot.

Results

AlaAT gene from cucumber (CsAlaAT2) has been successfully cloned and constructed on pCAMBIA1300 plant expression vectors under the control of tissue-specific promoter OsAnt1. Agrobacterium tumefaciens-mediated transformation of Indonesian rice cv. Fatmawati using this construct produced 14 transgenic events. Pre-screening of T1 seedlings grown in the agar medium containing low nitrogen concentration identified selected events that were superior in the root dry weight. Southern hybridization confirmed the integration of T-DNA in the selected event genomes, each of them carried 1, 2, or 3 T-DNA insertions. Efficacy assay of three lead events in the greenhouse showed that in general transgenic events had increased biomass, tiller number, nitrogen content, and grain yield compared to WT. One event, i.e., FAM13, showed an increase in yield as much as 27.9% and higher plant biomass as much as 27.4% compared to WT under the low nitrogen condition. The lead events also showed higher absorption NUE, agronomical NUE, and grain NUE as compared to WT under the low nitrogen condition.

Conclusions

The results of this study showed that root-specific expression of cucumber alanine aminotransferase2 gene improved nitrogen use efficiency in transgenic rice, which indicate the conservation of function of this gene in monocot and dicot.

Appropriate Ammonium-Nitrate Ratio Improves Nutrient Accumulation and Fruit Quality in Pepper (Capsicum annuum L.)

Ammonium (NH4+) and nitrate (NO3−) are the two forms of inorganic nitrogen essential for physiological and biochemical processes in higher plants, but little is known about how the NH4+:NO3− ratio may affect nitrogen metabolism. This study determined the effect of NH4+:NO3− ratios on plant growth, accumulation, and distribution of nutrient elements, fruit quality, enzyme activity, and relative expression of genes involved in nitrogen (N) metabolism in pepper (Capsicum annuum L.). In a pod experiment, the NH4+:NO3− ratios of 0:100, 12.5:87.5, 25:75, 37.5:62.5, and 50:50 were arranged in a complete randomized design with three replicates. The application of NH4+:NO3− at 25:75 resulted in highest dry matter and N, phosphorus (P), and potassium (K) accumulation. Pepper treated with 25:75 ratio increased root length, surface areas, and root volume and tips. The contents of vitamin C, soluble sugar, soluble protein, total phenols, flavonoids, and capsaicinoids in the fruits were significantly higher with the NH4+:NO3− ratio of 25:75 compared with 0:100 treatment, while lowering nitrate content was found in NH4+:NO3− ratios of 25:75, 37.5:62.5, and 50:50 treatments. Activity of glutamine synthetase (GS), glutamate synthases (GOGAT) enzyme and the levels of relative expression of genes coding these enzymes were superior when the NH4+:NO3− ratio of 25:75 were applied. Therefore, an appropriate ratio of NH4+:NO3− (25:75) in nitrogen application can stimulate root development, promote enzyme activities, and enhance the productivity and fruit quality in pepper.

Different impacts of external ammonium and nitrate addition on plant growth in terrestrial ecosystems: A meta-analysis

Terrestrial plant growth is strongly limited by the availability of nitrogen (N). Atmospheric deposition of N has been rapidly increasing since the industrial revolution, associated with fast compositional shifts between ammonium- (NH₄⁺) and nitrate-N (NO₃^-) globally. However, whether and how such composition changes of deposition will affect the response of terrestrial plant growth to N deposition remains unclear. To fill the gaps, this study quantified the different responses of terrestrial plants to external NH₄⁺-N and NO₃^--N additions. A meta-analysis was applied to compare the growth responses of 367 plant species to different forms of N addition from 210 N-fertilization experiments. In general, a greater response of plant growth to NH₄⁺- N (+6.3% per g N) than NO₃^--N (+1.0% per g N) addition was detected across all species. The larger response of plant growth to NH₄⁺-N than NO₃^--N addition was found in trees and forbs but not in shrubs and grasses. The NH₄⁺-N and NO₃^--N additions had contrasting effects on biomass allocation. For example, the NO₃^--N addition increased biomass allocation to above-ground tissues, whereas the NH₄⁺-N addition enhanced below- but not above-ground growth. These results generally reveal a higher response of plant growth to NH₄⁺- N than NO₃^--N addition in terrestrial ecosystems. The findings suggest that future predictions on the vegetation response to atmospheric N enrichment could benefit from a better understanding of plant strategies for acquiring different forms of N.

Hydroponic Solutions for Soilless Production Systems: Issues and Opportunities in a Smart Agriculture Perspective

Soilless cultivation represent a valid opportunity for the agricultural production sector, especially in areas characterized by severe soil degradation and limited water availability. Furthermore, this agronomic practice embodies a favorable response toward an environment-friendly agriculture and a promising tool in the vision of a general challenge in terms of food security. This review aims therefore at unraveling limitations and opportunities of hydroponic solutions used in soilless cropping systems focusing on the plant mineral nutrition process. In particular, this review provides information (1) on the processes and mechanisms occurring in the hydroponic solutions that ensure an adequate nutrient concentration and thus an optimal nutrient acquisition without leading to nutritional disorders influencing ultimately also crop quality (e.g., solubilization/precipitation of nutrients/elements in the hydroponic solution, substrate specificity in the nutrient uptake process, nutrient competition/antagonism and interactions among nutrients); (2) on new emerging technologies that might improve the management of soilless cropping systems such as the use of nanoparticles and beneficial microorganism like plant growth-promoting rhizobacteria (PGPRs); (3) on tools (multi-element sensors and interpretation algorithms based on machine learning logics to analyze such data) that might be exploited in a smart agriculture approach to monitor the availability of nutrients/elements in the hydroponic solution and to modify its composition in realtime. These aspects are discussed considering what has been recently demonstrated at the scientific level and applied in the industrial context.

Tomato plants rather than fertilizers drive microbial community structure in horticultural growing media

Synthetic fertilizer production is associated with a high environmental footprint, as compounds typically dissolve rapidly leaching emissions to the atmosphere or surface waters. We tested two recovered nutrients with slower release patterns, as promising alternatives for synthetic fertilizers: struvite and a commercially available organic fertilizer. Using these fertilizers as nitrogen source, we conducted a rhizotron experiment to test their effect on plant performance and nutrient recovery in juvenile tomato plants. Plant performance was significantly improved when organic fertilizer was provided, promoting higher shoot biomass. Since the microbial community influences plant nitrogen availability, we characterized the root-associated microbial community structure and functionality. Analyses revealed distinct root microbial community structure when different fertilizers were supplied. However, plant presence significantly increased the similarity of the microbial community over time, regardless of fertilization. Additionally, the presence of the plant significantly reduced the potential ammonia oxidation rates, implying a possible role of the rhizosheath microbiome or nitrification inhibition by the plant. Our results indicate that nitrifying community members are impacted by the type of fertilizer used, while tomato plants influenced the potential ammonia-oxidizing activity of nitrogen-related rhizospheric microbial communities. These novel insights on interactions between recovered fertilizers, plant and associated microbes can contribute to develop sustainable crop production systems.

Resource heterogeneity structures aquatic bacterial communities

Microorganisms are strongly influenced by the bottom-up effects of resource supply. While many species respond to fluctuations in the concentration of resources, microbial diversity may also be affected by the heterogeneity of the resource pool, which often reflects a mixture of distinct molecules. To test this hypothesis, we examined resource-diversity relationships for bacterioplankton in a set of north temperate lakes that varied in their concentration and composition of dissolved organic matter (DOM), which is an important resource for heterotrophic bacteria. Using 16S rRNA transcript sequencing and ecosystem metabolomics, we documented strong relationships between bacterial alpha-diversity (richness and evenness) and the bulk concentration and the number of molecules in the DOM pool. Similarly, bacterial community beta-diversity was related to both DOM concentration and composition. However, in some lakes the relative abundance of resource generalists, which was inversely related to the DOM concentration, may have reduced the effect of DOM heterogeneity on community composition. Together, our results demonstrate the potential metabolic interactions between bacteria and organic matter and suggest that changes in organic matter composition may alter the structure and function of bacterial communities.

Importance of matching soil N transformations, crop N form preference, and climate to enhance crop yield and reducing N loss

The preferential uptake of nitrogen (N) by plant species is generally considered to be tightly associated with soil N transformation characteristics and climatic conditions. In the present study, the yield, N recovery, and N balance of wheat (an NO₃^--preferring crop) and rice (an NH₄⁺-preferring crop) planted in two sites with significantly different soil N transformations and precipitation were compared to test the connections among soil N transformations, species-specific N preferences, crop yield, N use efficiency (NUE), N loss, and climate. Nitrogen recoveries of ¹⁵N applied as either an NH₄⁺ or NO₃^- based fertilizer in crops and soil were determined, and N losses were calculated using the ¹⁵N balance. The results indicated that the match (or mismatch) of crop N preference, soil N transformation characteristics, and climate conditions could significantly affect the crop yield, recovery, and loss of applied ¹⁵N. A crop preferring NH₄⁺ (e.g., rice) planted in soil with low nitrification and low pH tended to have a higher yield and NUE, and a lower N loss compared with plants growing in soil characterized by a higher nitrification rates. In contrast, NO₃^--preferring crops (e.g. wheat) planted in soil with higher nitrification rates and lower precipitation achieved a higher yield and lower N loss than those planted in acid soil with a lower nitrification rate and higher precipitation. Replacement of ammonium N fertilizer with nitrate N fertilizer improved the NUE for NO₃^--preferring crops (wheat) planted in arid and semi-arid regions, but not in humid regions because of the mobility of NO₃^-. Thus, a good match of crop N preference, soil N transformation characteristics, and climate conditions is critical to enhance crop yield and reduce N loss. This intimate connection needs to be taken into consideration in terms of the choice of crop species and N fertilizer type to optimize N fertilization management.

Effect of cadmium stress on inorganic and organic components in xylem sap of high cadmium accumulating rice line (Oryza sativa L.)

Physiological properties involved in cadmium (Cd) transport were investigated in the high Cd accumulating rice line (Lu527-8) in comparison with the normal rice line (Lu527-4) through a soil culture experiment. The results showed that Cd contents in xylem saps of Lu527-8 were 1.68-2.55 times higher than those of Lu527-4 under Cd stress. A high-positive correlation between Cd contents in xylem saps and Cd contents in shoots was observed. Lu527-8 owned a more rapid and effective transport of Cd to above-ground part. By analyzing the relationship between inorganic anions, organic components and Cd contents in xylem saps, the lower HPO₄^2- and oxalate contents were considered to be related to the higher Cd transport in xylem sap of Lu527-8. As for citrate, tartaric and histidine content, significant increases were observed with the increasing Cd contents in xylem saps of two rice lines, and their contents of Lu527-8 were significantly higher than those of Lu527-4. Citrate, tartaric and histidine could take part in root-to-shoot Cd transport in xylem.

Human Urine as a Fertilizer in the Cultivation of Snap Beans ( Phaseolus vulgaris) and Turnips ( Brassica rapa)

The main reason for implementing human urine diversion is to produce a local and renewable source of fertilizer for agriculture. Accordingly, the goal of this research was to compare human urine fertilizer and synthetic fertilizer in the cultivation of snap beans and turnips by evaluating the yield, plant tissue chemical composition, nutrient uptake efficiency, soil nutrient content, and leachate nutrient content between plots. Four fertilizer treatments were evaluated: (1) synthetic fertilizer, (2) urine supplemented with synthetic fertilizer, (3) urine only, and (4) a no-fertilizer control, referred to as treatments 1, 2, 3, and 4, respectively. Plants fertilized by treatments 1 and 2 produced the highest yield for fall turnips and spring snap beans. The turnip yield for the urine-only treatment was significantly higher than the no-fertilizer control. Overall, the results showed that supplemented urine fertilizer can be used as an alternative to synthetic fertilizer with comparable yields, and urine-only fertilizer can significantly increase yields over the no-fertilizer control. The results also suggest that nutrients in urine are available in a form favorable for plant uptake.

Organic and inorganic nitrogen uptake by 21 dominant tree species in temperate and tropical forests

Evidence shows that many tree species can take up organic nitrogen (N) in the form of free amino acids from soils, but few studies have been conducted to compare organic and inorganic N uptake patterns in temperate and tropical tree species in relation to mycorrhizal status and successional state. We labeled intact tree roots by brief 15N exposures using field hydroponic experiments in a temperate forest and a tropical forest in China. A total of 21 dominant tree species were investigated, 8 in the temperate forest and 13 in the tropical forest. All investigated tree species showed highest uptake rates for NH4+ (ammonium), followed by glycine and NO3- (nitrate). Uptake of NH4+ by temperate trees averaged 12.8 μg N g-1 dry weight (d.w.) root h-1, while those by tropical trees averaged 6.8 μg N g-1 d.w. root h-1. Glycine uptake rates averaged 3.1 μg N g-1 d.w. root h-1 for temperate trees and 2.4 μg N g-1 d.w. root h-1 for tropical trees. NO3- uptake was the lowest (averaging 0.8 μg N g-1 d.w. root h-1 for temperate trees and 1.2 μg N g-1 d.w. root h-1 for tropical trees). Uptake of NH4+ accounted for 76% of the total uptake of all three N forms in the temperate forest and 64% in the tropical forest. Temperate tree species had similar glycine uptake rates as tropical trees, with the contribution being slightly lower (20% in the temperate forest and 23% in the tropical forest). All tree species investigated in the temperate forest were ectomycorrhizal and all species but one in the tropical forest were arbuscular mycorrhizal (AM). Ectomycorrhizal trees showed significantly higher NH4+ and lower NO3- uptake rates than AM trees. Mycorrhizal colonization rates significantly affected uptake rates and contributions of NO3- or NH4+, but depended on forest types. We conclude that tree species in both temperate and tropical forests preferred to take up NH4+, with organic N as the second most important N source. These findings suggest that temperate and tropical forests demonstrate similar N uptake patterns although they differ in physiology of trees and soil biogeochemical processes.

Effect of different nitrogen forms on the toxicity of Zn in wheat seedling root: a modeling analysis

Heavy metal stress in culture media is always rhizotoxic. Our study aims to investigate the role of negative potential (ψ ₀) at root cell membrane surface (CMs) on modeling Zn²⁺ toxicity to wheat seedling roots and to examine the effects of different nitrogen forms (NH₄⁺ and NO₃^-) on ψ ₀ and Zn rhizotoxicity. Solution culture experiments were conducted to measure the root elongation and Zn accumulation under Zn²⁺ exposure. The role of two nitrogen forms in affecting Zn²⁺ toxicity was compared, giving particular consideration to ψ ₀ and Zn²⁺ activities at CMs ({Zn²⁺}₀). Results showed that NH₄⁺ alleviates Zn²⁺ rhizotoxicity and NO₃^- increases Zn²⁺ rhizotoxicity. In modeling the rhizotoxicity, root length correlated better with {Zn²⁺}₀ than {Zn²⁺}_b, and the predictive accuracy (r ²) of NH₄⁺ treatment increased from 0.748 to 0.917 when incorporation of {Zn²⁺}₀ and {Ca²⁺}₀ into analysis. Oppositely, ψ ₀ played a limited role in modeling Zn²⁺ rhizotoxicity and bioavailability in NO₃^- treated medium (r ² = 0.609). Moreover, higher concentration of Zn in roots was found in NO₃^- treatment, compared with the NH₄⁺ treatment. ψ ₀ rather than the rhizotoxicity data correlated better with Zn accumulation especially in the NO₃^- treatment (r ² > 0.7), which meant the electrical driving force at CMs playing a dominant role in modeling the metal accumulation. In conclusion, the alleviatory role of NH₄⁺ on Zn toxicity and uptake was well explained and modeled by electrostatic effects at CMs. Though our data do not explore mechanisms for the NO₃^--Zn²⁺ interactions, we propose that ψ ₀ worked better in affecting the driving force for root Zn uptake, than influencing metal bioavailability at CMs.

Inorganic nitrogen form: a major player in wheat and Arabidopsis responses to elevated CO2

Critical for predicting the future of primary productivity is a better understanding of plant responses to rising atmospheric carbon dioxide (CO2) concentration. This review considers recent results on the role of the inorganic nitrogen (N) forms nitrate (NO3-) and ammonium (NH4+) in determining the responses of wheat and Arabidopsis to elevated atmospheric CO2 concentration. Here, we identify four key issues: (i) the possibility that different plant species respond similarly to elevated CO2 if one accounts for the N form that they are using; (ii) the major influence that plant-soil N interactions have on plant responses to elevated CO2; (iii) the observation that elevated CO2 may favor the uptake of one N form over others; and (iv) the finding that plants receiving NH4+ nutrition respond more positively to elevated CO2 than those receiving NO3- nutrition because elevated CO2 inhibits the assimilation of NO3- in shoots of C3 plants. We conclude that the form and amount of N available to plants from the rhizosphere and plant preferences for the different N forms are essential for predicting plant responses to elevated CO2.

Nutrient constraints on terrestrial carbon fixation: The role of nitrogen

Carbon dioxide (CO₂) concentrations in the earth's atmosphere are projected to rise from current levels near 400ppm to over 700ppm by the end of the 21st century. Projections over this time frame must take into account the increases in total net primary production (NPP) expected from terrestrial plants, which result from elevated CO₂ (eCO₂) and have the potential to mitigate the impact of anthropogenic CO₂ emissions. However, a growing body of evidence indicates that limitations in soil nutrients, particularly nitrogen (N), the soil nutrient most limiting to plant growth, may greatly constrain future carbon fixation. Here, we review recent studies about the relationships between soil N supply, plant N nutrition, and carbon fixation in higher plants under eCO₂, highlighting key discoveries made in the field, particularly from free-air CO₂ enrichment (FACE) technology, and relate these findings to physiological and ecological mechanisms.

Growing media constituents determine the microbial nitrogen conversions in organic growing media for horticulture

Vegetables and fruits are an important part of a healthy food diet, however, the eco‐sustainability of the production of these can still be significantly improved. European farmers and consumers spend an estimated €15.5 billion per year on inorganic fertilizers and the production of N‐fertilizers results in a high carbon footprint. We investigated if fertilizer type and medium constituents determine microbial nitrogen conversions in organic growing media and can be used as a next step towards a more sustainable horticulture. We demonstrated that growing media constituents showed differences in urea hydrolysis, ammonia and nitrite oxidation and in carbon dioxide respiration rate. Interestingly, mixing of the growing media constituents resulted in a stimulation of the function of the microorganisms. The use of organic fertilizer resulted in an increase in amoA gene copy number by factor 100 compared to inorganic fertilizers. Our results support our hypothesis that the activity of the functional microbial community with respect to nitrogen turnover in an organic growing medium can be improved by selecting and mixing the appropriate growing media components with each other. These findings contribute to the understanding of the functional microbial community in growing media and its potential role towards a more responsible horticulture.

Temperature determines size and direction of effects of elevated CO2 and nitrogen form on yield quantity and quality of Chinese cabbage

Rising atmospheric CO2 concentrations (e[CO2 ]) are presumed to have a significant impact on plant growth and yield and also on mineral nutrient composition, and therefore, on nutritional quality of crops and vegetables. To assess the relevance of these effects in future agroecosystems it is important to understand how e[CO2 ] interacts with other environmental factors. In the present study, we examined the interactive effects of e[CO2 ] with temperature and the form in which nitrogen is supplied (nitrate or ammonium nitrate) on growth, amino acid content and mineral nutrient composition of Chinese cabbage (Brassica pekinensis Rupr.), a crop characterised by its high nutritional value and increasing relevance for human nutrition in many developing countries. Higher temperature, ammonium nitrate and e[CO2 ] had a positive impact on net photosynthesis and growth. A stimulating effect of e[CO2 ] on growth was only observed if the temperature was high (21/18 °C, day/night), and an interaction of e[CO2 ] with N form was only observed if the temperature was ambient (15/12 °C, day/night). Mineral nutrient composition was affected in a complex manner by all three factors and their interaction. These results demonstrate how much the effect of e[CO2 ] on mineral quality of crops depends on other environmental factors. Changes in temperature, adapting N fertilisation and the oxidation state of N have the potential to counteract the mineral depletion caused by e[CO2 ].

Biochar-induced concomitant decrease in ammonia volatilization and increase in nitrogen use efficiency by wheat

Ammonia (NH3) volatilization is a major nitrogen (N) loss from the soil, especially under tropical conditions, NH3 volatilization results in low N use efficiency by crops. Incubation experiments were conducted using five soils (pH 5.5-9.0), three N sources such as, urea, di-ammonium phosphate (DAP), and poultry manure (PM) and two biochars such as, poultry litter biochar (PL-BC) and macadamia nut shell biochar (MS-BC). Ammonia volatilization was higher at soil with higher pH (pH exceeding 8) due to the increased hydroxyl ions. Among the N sources, urea recorded the highest NH3 volatilization (151.6 mg kg(-1)soil) followed by PM (124.2 mg kg(-1)soil) and DAP (99 mg kg(-1)soil). Ammonia volatilization was reduced by approximately 70% with PL-BC and MS-BC. The decreased NH3 volatilization with biochars is attributed to multiple mechanisms such as NH3 adsorption/immobilization, and nitrification. Moreover, biochar increased wheat dry weight and N uptake as high as by 24.24% and 76.11%, respectively. This study unravels the immense potential of biochar in decreasing N volatilization from soils and simultaneously improving use efficiency by wheat.

Hypoxia Affects Nitrogen Uptake and Distribution in Young Poplar (Populus × canescens) Trees

The present study with young poplar trees aimed at characterizing the effect of O2 shortage in the soil on net uptake of NO3- and NH4+ and the spatial distribution of the N taken up. Moreover, we assessed biomass increment as well as N status of the trees affected by O2 deficiency. For this purpose, an experiment was conducted in which hydroponically grown young poplar trees were exposed to hypoxic and normoxic (control) conditions for 14 days. 15N-labelled NO3- and NH4+ were used to elucidate N uptake and distribution of currently absorbed N and N allocation rates in the plants. Whereas shoot biomass was not affected by soil O2 deficiency, it significantly reduced root biomass and, consequently, the root-to-shoot ratio. Uptake of NO3- but not of NH4+ by the roots of the trees was severely impaired by hypoxia. As a consequence of reduced N uptake, the N content of all poplar tissues was significantly diminished. Under normoxic control conditions, the spatial distribution of currently absorbed N and N allocation rates differed depending on the N source. Whereas NO3- derived N was mainly transported to the younger parts of the shoot, particularly to the developing and young mature leaves, N derived from NH4+ was preferentially allocated to older parts of the shoot, mainly to wood and bark. Soil O2 deficiency enhanced this differential allocation pattern. From these results we assume that NO3- was assimilated in developing tissues and preferentially used to maintain growth and ensure plant survival under hypoxia, whereas NH4+ based N was used for biosynthesis of storage proteins in bark and wood of the trees. Still, further studies are needed to understand the mechanistic basis as well as the eco-physiological advantages of such differential allocation patterns.

Suppression of soil nitrification by plants

Nitrification, the biological oxidation of ammonium to nitrate, weakens the soil's ability to retain N and facilitates N-losses from production agriculture through nitrate-leaching and denitrification. This process has a profound influence on what form of mineral-N is absorbed, used by plants, and retained in the soil, or lost to the environment, which in turn affects N-cycling, N-use efficiency (NUE) and ecosystem health and services. As reactive-N is often the most limiting in natural ecosystems, plants have acquired a range of mechanisms that suppress soil-nitrifier activity to limit N-losses via N-leaching and denitrification. Plants' ability to produce and release nitrification inhibitors from roots and suppress soil-nitrifier activity is termed 'biological nitrification inhibition' (BNI). With recent developments in methodology for in-situ measurement of nitrification inhibition, it is now possible to characterize BNI function in plants. This review assesses the current status of our understanding of the production and release of biological nitrification inhibitors (BNIs) and their potential in improving NUE in agriculture. A suite of genetic, soil and environmental factors regulate BNI activity in plants. BNI-function can be genetically exploited to improve the BNI-capacity of major food- and feed-crops to develop next-generation production systems with reduced nitrification and N2O emission rates to benefit both agriculture and the environment. The feasibility of such an approach is discussed based on the progresses made.

Ammonium tolerance and toxicity of Actinoscirpus grossus--a candidate species for use in tropical constructed wetland systems

Actinoscirpus grossus, a native species in tropical wetlands of South-East Asia, North Australia and the Pacific islands, has been reported to perform well in experimental scale constructed wetland (CW) systems. However, little is known about how high NH4(+) concentrations prevailing in wastewater affect growth and performance of this species. We examined growth, morphological and physiological responses of A. grossus to NH4(+) concentrations of 0.5, 2.5, 5, 10 and 15mM under hydroponic growth conditions. The relative growth rates (RGR) of the plants were highest at 2.5mM NH4(+) but significantly reduced at 10 and 15mM NH4(+). The roots of the plants were stunted and produced subepidermal lignified-cell layers at exposure to 10 and 15mM NH4(+). The photosynthetic rates did not differ between treatments (average An=21.3±0.4µmolCO2m(-2)s(-1)) but the photosynthetic nitrogen and carbon use efficiency (PNUE and PCUE) were significantly depressed at 10 and 15mM NH4(+) treatments. The concentration of NH4(+) in the roots, but not in the leaves, reflected the NH4(+) concentration in the growth medium suggesting that the species is unable to regulate the NH4(+) uptake. The high root respiration rates in concert with high tissue NH4(+) and declined C/N ratio at 10 and 15mM NH4(+) suggest that the NH4(+) assimilation occurs primarily in the roots and the plant has inadequate C-skeletons for NH4(+) assimilation and exudation at high NH4(+) concentration in the external solution. The concentrations of mineral cations were generally reduced and the root membrane permeability increased at high external NH4(+) concentrations. Our study shows that A. grossus tolerates NH4(+) concentrations up to 5mM which is characteristic of most types of wastewater. Hence, A. grossus is a good native candidate species for use in CW systems in tropical and subtropical climates in South-East Asia, North Australia and the Pacific islands.

Ecological significance and complexity of N-source preference in plants

BACKGROUND

Plants can utilize two major forms of inorganic N: NO3(-) (nitrate) and NH4(+) (ammonium). In some cases, the preference of one form over another (denoted as β) can appear to be quite pronounced for a plant species, and can be an important determinant and predictor of its distribution and interactions with other species. In many other cases, however, assignment of preference is not so straightforward and must take into account a wide array of complex physiological and environmental features, which interact in ways that are still not well understood.

SCOPE

This Viewpoint presents a discussion of the key, and often co-occurring, factors that join to produce the complex phenotypic composite referred to by the deceptively simple term 'N-source preference'.

CONCLUSIONS

N-source preference is much more complex a biological phenomenon than is often assumed, and general models predicting how it will influence ecological processes will need to be much more sophisticated than those that have been so far developed.

A paradigm shift towards low-nitrifying production systems: the role of biological nitrification inhibition (BNI)

BACKGROUND

Agriculture is the single largest geo-engineering initiative that humans have initiated on planet Earth, largely through the introduction of unprecedented amounts of reactive nitrogen (N) into ecosystems. A major portion of this reactive N applied as fertilizer leaks into the environment in massive amounts, with cascading negative effects on ecosystem health and function. Natural ecosystems utilize many of the multiple pathways in the N cycle to regulate N flow. In contrast, the massive amounts of N currently applied to agricultural systems cycle primarily through the nitrification pathway, a single inefficient route that channels much of this reactive N into the environment. This is largely due to the rapid nitrifying soil environment of present-day agricultural systems.

SCOPE

In this Viewpoint paper, the importance of regulating nitrification as a strategy to minimize N leakage and to improve N-use efficiency (NUE) in agricultural systems is highlighted. The ability to suppress soil nitrification by the release of nitrification inhibitors from plant roots is termed 'biological nitrification inhibition' (BNI), an active plant-mediated natural function that can limit the amount of N cycling via the nitrification pathway. The development of a bioassay using luminescent Nitrosomonas to quantify nitrification inhibitory activity from roots has facilitated the characterization of BNI function. Release of BNIs from roots is a tightly regulated physiological process, with extensive genetic variability found in selected crops and pasture grasses. Here, the current status of understanding of the BNI function is reviewed using Brachiaria forage grasses, wheat and sorghum to illustrate how BNI function can be utilized for achieving low-nitrifying agricultural systems. A fundamental shift towards ammonium (NH4(+))-dominated agricultural systems could be achieved by using crops and pastures with high BNI capacities. When viewed from an agricultural and environmental perspective, the BNI function in plants could potentially have a large influence on biogeochemical cycling and closure of the N loop in crop-livestock systems.

Is the C:N ratio a reliable indicator of C allocation to primary and defence-related metabolisms in tomato?

Plant growth and defence are both fuelled by compounds synthesized from a common pool of carbon and nitrogen, implying the existence of a competition for carbon and nitrogen allocation to both metabolisms. The ratio of carbon to nitrogen (C:N) of an organ is often regarded as a convenient indicator of growth and quality. The purpose of this work was to assess whether or not it is possible to extend its use to characterize the trade-off between growth and defence processes. Therefore, we calculated C:N ratios in the pool of resources and in the total plant, and correlated them to the concentrations of diverse compounds of the primary and secondary metabolisms in young tomatoes. Plants were grown hydroponically at N availabilities either limiting (0.1 mM) or not (7 mM) for growth in two glasshouses maintained either under ambient or enriched (700 vpm) air CO(2). These conditions yielded a large array of C:N in fully developed leaves, developing leaves, stem and roots, sampled 27, 35 and 47 days after sowing. Growth parameters and tissue concentrations of primary metabolites (carbohydrates, starch), defence-related compounds (polyphenols, glycoalkaloids), lignin, nitrate, ammonium, C and N were analyzed. Net CO(2) exchange rate was also measured at the last sampling date. Low N limited plant growth more than photosynthesis. The C:N in the resource pool was far higher than the total C:N. Starch was the most responsive compound, attaining high concentration under high C:N, whereas lignin remained stable. Chlorogenic acid, rutin, kaempferol-rutinoside and tomatine concentrations correlated positively to C:N. The same patterns were observed for most organs and molecules, except soluble carbohydrates in fully developed leaves whose concentration was not influenced. Among the organs, developing leaves showed the highest concentrations of secondary compounds and were the most responsive to C:N variations. Neither the biochemical nature of the compounds (C-based or N- containing metabolites) nor the calculation mode of C:N, influenced the patterns observed. Within the range of N availabilities considered (up to N limitation but not deficiency), the C:N can be considered as a good indicator of the secondary compounds concentrations in organs, especially for those involved in the chemical defence.