Enhancement of microbial assimilation of inorganic nitrogen (N) by straw addition is believed to be an effective pathway to improve farmland N cycling. However, the effectiveness of differently pretreated straws on soil N2O emissions and soil N-acquiring enzyme activities remains unclear. In this study, a pot experiment with four treatments (I, no addition, CK; II, respective addition of maize straw, S; III, composted maize straw under no fungi inoculation, SC; and IV, composted maize straw under fungi inoculation, SCPA) at the same quantity of carbon (C) input was conducted under the same amount of inorganic N fertilization. Results showed that the seasonal cumulative N2O emissions following the SCPA treatment were the lowest at 4.03 kg N ha-1, representing a significant reduction of 19 % compared with the CK treatment. The S and SC treatments had no significant effects on N2O emissions. The decrease of soil N2O emissions following the SCPA treatment was mainly attributed to the increase of microbial N assimilation and the increased abundance of functional genes related to N2O reductase. The SCPA treatment significantly decreased soil alkaline phosphatase (ALP) activity and increased leucine aminopeptidase (LAP) activity at the basal fertilization, while increased soil ALP and LAP activity, decreased soil N-Acetyl-β-D-Glucosidase (NAG) activity at harvest. Compared with the CK treatment, the S, SC, and SCPA treatment significantly increased soil β-glucosidase (β-GC) activity at harvest. The decrease in the (NAG+LAP)/ALP ratio following the SCPA treatment indicated that the composted maize straw under fungi inoculation alleviated microbial N limitation at harvest. Moreover, PICRUSt analysis also suggested that the SCPA treatment increased the abundance of bacterial genes associated with N assimilation and N2O reduction, whereas the S and SC treatment did not significantly affect the abundance of N2O reduction genes compared with the CK treatment. Our results suggest that the composted maize straw under fungi inoculation would reduce the risk of N2O emissions and effectively mitigate the microbial N limitation in dryland wheat system.

CONCLUSIONS: Exogenous application of 24-epibrassinolide can alleviate oxidative damage, improve photosynthetic capacity, and regulate carbon and nitrogen assimilation, thus improving the tolerance of grapevine (Vitis vinifera L.) to drought stress. Brassinosteroids (BRs) are a group of plant steroid hormones in plants and are involved in regulating plant tolerance to drought stress. This study aimed to investigate the regulation effects of BRs on the carbon and nitrogen metabolism in grapevine under drought stress. The results indicated that drought stress led to the accumulation of superoxide radicals and hydrogen peroxide and an increase in lipid peroxidation. A reduction in oxidative damage was observed in EBR-pretreated plants, which was probably due to the improved antioxidant concentration. Moreover, exogenous EBR improved the photosynthetic capacity and sucrose phosphate synthase activity, and decreased the sucrose synthase, acid invertase, and neutral invertase, resulting in improved sucrose (190%) and starch (17%) concentrations. Furthermore, EBR pretreatment strengthened nitrate reduction and ammonium assimilation. A 57% increase in nitrate reductase activity and a 13% increase in glutamine synthetase activity were observed in EBR pretreated grapevines. Meanwhile, EBR pretreated plants accumulated a greater amount of proline, which contributed to osmotic adjustment and ROS scavenging. In summary, exogenous EBR enhanced drought tolerance in grapevines by alleviating oxidative damage and regulating carbon and nitrogen metabolism.

A novel A. pittii J08 with heterotrophic nitrification and aerobic denitrification (HN-AD) isolated from pond sediments could rapidly degrade inorganic nitrogen (N) and total nitrogen (TN-N) with ammonium (NH4+-N) preference. N degradation rate of NH4+-N, nitrite (NO2--N) and nitrate (NO3--N) were 3.9 mgL-1h-1, 3.0 mgL-1h-1 and 2.7 mgL-1h-1, respectively. In addition, strain J08 could effectively utilize most of detected low-molecular-weight carbon (LMWC) sources to degrade inorganic N with a wide adaptability to various culture conditions. Whole genome sequencing (WGS) analysis revealed that assembled genome of stain J08 possessed the crucial genes involved in dissimilatory/assimilatory NO3--N reduction and NH4+-N assimilation. These results indicated that strain J08 could be applied to wastewater treatment in aquaculture.

Plants adapt to changing environmental conditions by adjusting their growth physiology. Nitrate (NO3-) and ammonium (NH4+) are the major inorganic nitrogen forms for plant uptake. However, high NH4+ inhibits plant growth, and roots undergo striking changes, such as inhibition of cell expansion and division, leading to reduced root elongation. In this work, we show that high NH4+ modulates nitrogen metabolism and root developmental physiology by inhibiting iron (Fe)-dependent Jasmonate (JA) signaling and response in Arabidopsis (Arabidopsis thaliana). Transcriptomic data suggested that NH4+ availability regulates Fe and JA-responsive genes. High NH4+ levels led to enhanced root Fe accumulation, which impaired nitrogen balance and growth by suppressing JA biosynthesis and signaling response. Integrating pharmacological, physiological, and genetic experiments revealed the involvement of NH4+ and Fe-derived responses in regulating root growth and nitrogen metabolism through modulation of the JA pathway during NH4+ stress. The JA signaling transcription factor MYC2 directly bound the promoter of the NITRATE TRANSPORTER 1.1 (NRT1.1) and repressed it to optimize the NH4+/Fe-JA balance for plant adaptation during NH4+ stress. Our findings illustrate the intricate balance between nutrient and hormone-derived signaling pathways that appear essential for optimizing plant growth by adjusting physiological and metabolic responses during NH4+/Fe stress.

Nitrogen (N) is of great significance to the absorption, distribution and detoxification of cadmium (Cd). Ectomycorrhizal fungi (EMF) are able to affect the key processes of plant N uptake to resist Cd stress, while the mechanism is still unclear. Therefore, we explored potential strategies of Cenococcum geophilum (C. geophilum) symbiosis to alleviate Cd stress in Pinus massoniana (P. massoniana) from the perspective of plant N metabolism and soil N transformation. The results showed that inoculation of C. geophilum significantly increased the activities of NR, NiR and GS in the shoots and roots of P. massoniana, thereby promoting the assimilation of NO3- and NH4+ into amino acids. Moreover, C. geophilum promoted soil urease and protease activities, but decreased soil NH4+ content, indicating that C. geophilum might increase plant uptake of soil inorganic N. qRT-PCR results showed that C3 symbiosis significantly up-regulated the expression of genes encoding functions involved in NH4+ uptake (AMT3;1), NO3- uptake (NRT2.1, NRT2.4, NRT2.9), as well as Cd resistance (ABCC1 and ABCC2), meanwhile down-regulated the expression of NRT7.3, Cd transporter genes (HMA2 and NRAMP3) in the roots of P. massoniana seedlings. These results demonstrated that C. geophilum was able to alleviate Cd stress by increasing the absorption and assimilation of inorganic N in plants and inhibiting the transport of Cd from roots to shoots, which provided new insights into how EMF improved host resistance to abiotic stress.

Salt stress is one of the major adverse factors affecting plant growth and crop production. Rapeseed is an important oil crop, providing high-quality edible oil for human consumption. This experiment was conducted to investigate the effects of salt stress on the phenotypic traits and physiological processes of rapeseed. The soil salinity was manipulated by setting three different levels: 0 g NaCl kg-1 soil (referred to as S0), 1.5 g NaCl kg-1 soil (referred to as S1), and 3.0 g NaCl kg-1 soil (referred to as S2). In general, the results indicated that the plant height, leaf area, and root neck diameter decreased with an increase in soil salinity. In addition, the biomass of various organs at all growth stages decreased as soil salinity increased from S0 to S2. The increasing soil salinity improved the distribution of biomass in the root and leaf at the seedling and flowering stages, indicating that rapeseed plants subjected to salt stress during the vegetative stage are capable of adapting their growth pattern to sustain their capacity for nutrient and water uptake, as well as leaf photosynthesis. However, as the soil salinity increased, there was a decrease in the distribution of biomass in the pod and seed at the maturity stage, while an increase was observed in the root and stem, suggesting that salt stress inhibited carbohydrate transport into reproductive organs. Moreover, the C and N accumulation at the flowering and maturity stages exhibited a reduction in direct correlation with the increase in soil salinity. High soil salinity resulted in a reduction in the C/N, indicating that salt stress exerted a greater adverse effect on C assimilation compared to N assimilation, leading to an increase in seed protein content and a decrease in oil content. Furthermore, as soil salinity increased from S0 to S2, the activity of superoxide dismutase (SOD) and catalase (CAT) and the content of soluble protein and sugar increased by 58.39%, 33.38%, 15.57%, and 13.88% at the seedling stage, and 38.69%, 22.85%, 12.04%, and 8.26% at the flowering stage, respectively. In summary, this study revealed that salt stress inhibited C and N assimilation, leading to a suppressed phenotype and biomass accumulation. The imbalanced C and N assimilation under salt stress contributed to the alterations in the seed oil and protein content. Rapeseed had a certain degree of salt tolerance by improving antioxidants and osmolytes.

CONCLUSIONS: The genetic diversity in tetraploid wheat provides a genetic pool for improving wheat productivity and environmental resilience. The tetraploid wheat had strong N uptake, translocation, and assimilation capacity under N deficit stress, thus alleviating growth inhibition and plant N loss to maintain healthy development and adapt to environments with low N inputs. Tetraploid wheat with a rich genetic variability provides an indispensable genetic pool for improving wheat yield. Mining the physiological mechanisms of tetraploid wheat in response to nitrogen (N) deficit stress is important for low-N-tolerant wheat breeding. In this study, we selected emmer wheat (Kronos, tetraploid), Yangmai 25 (YM25, hexaploid), and Chinese spring (CS, hexaploid) as materials. We investigated the differences in the response of root morphology, leaf and root N accumulation, N uptake, translocation, and assimilation-related enzymes and gene expression in wheat seedlings of different ploidy under N deficit stress through hydroponic experiments. The tetraploid wheat (Kronos) had stronger adaptability to N deficit stress than the hexaploid wheats (YM25, CS). Kronos had better root growth under low N stress, expanding the N uptake area and enhancing N uptake to maintain higher NO3- and soluble protein contents. Kronos exhibited high TaNRT1.1, TaNRT2.1, and TaNRT2.2 expression in roots, which promoted NO3- uptake, and high TaNRT1.5 and TaNRT1.8 expression in roots and leaves enhanced NO3- translocation to the aboveground. NR and GS activity in roots and leaves of Kronos was higher by increasing the expression of TANIA2, TAGS1, and TAGS2, which enhanced the reduction and assimilation of NO3- as well as the re-assimilation of photorespiratory-released NH4+. Overall, Kronos had strong N uptake, translocation, and assimilation capacity under N deficit stress, alleviating growth inhibition and plant N loss and thus maintaining a healthy development. This study reveals the physiological mechanisms of tetraploid wheat that improve nitrogen uptake and assimilation adaptation under low N stress, which will provide indispensable germplasm resources for elite low-N-tolerant wheat improvement and breeding.

Nanoscale agrochemicals have been widely used in sustainable agriculture and may potentially affect the nitrogen fixation process in legume crops. The present study investigated the size-effects of copper oxide nanoparticles (CuO NPs) on nitrogen assimilation in soybean (G. max (L.) Merrill) plants, which were treated with different sizes (20 and 50 nm) of CuO NPs at low use doses (1 and 10 mg/kg) for 21 days under greenhouse conditions. The results showed that 50 nm CuO NPs significantly increased the fresh biomass more than 20 nm CuO NPs achieved at 10 mg/kg. The activities of N assimilation-associated enzymes and the contents of nitrogenous compounds, including nitrates, proteins, and amino acids, in soybean tissues were greatly increased across all the CuO NP treatments. The use doses of two sizes of CuO NPs had no impact on the Cu contents in shoots and roots but indeed increased the Cu contents in soils in a dose-dependent fashion. Overall, our findings demonstrated that both 20 and 50 nm CuO NPs could positively alter soybean growth and boost N assimilation, furthering our understanding that the application of nanoscale micro-nutrient-related agrochemicals at an optimal size and dose will greatly contribute to increasing the yield and quality of crops.

Nitrogen (N) is the nutrient most applied in agriculture as fertilizer (as nitrate, Nit; ammonium, A; and/or urea, U, forms) and its availability strongly constrains the crop growth and yield. To investigate the early response (24 h) of N-deficient tomato plants to these three N forms, a physiological and molecular study was performed. In comparison to N-deficient plants, significant changes in the transcriptional, metabolomic and ionomic profiles were observed. As a probable consequence of N mobility in plants, a wide metabolic modulation occurred in old leaves rather than in young leaves. The metabolic profile of U and A-treated plants was more similar than Nit-treated plant profile, which in turn presented the lowest metabolic modulation with respect to N-deficient condition. Urea and A forms induced some changes at the biosynthesis of secondary metabolites, amino acids and phytohormones. Interestingly, a specific up-regulation by U and down-regulation by A of carbon synthesis occurred in roots. Along with the gene expression, data suggest that the specific N form influences the activation of metabolic pathways for its assimilation (cytosolic GS/AS and/or plastidial GS/GOGAT cycle). Urea induced an up-concentration of Cu and Mn in leaves and Zn in whole plant. This study highlights a metabolic reprogramming depending on the N form applied, and it also provide evidence of a direct relationship between urea nutrition and Zn concentration. The understanding of the metabolic pathways activated by the different N forms represents a milestone in improving the efficiency of urea fertilization in crops.

The algal-bacterial wastewater treatment process has been proven to be highly efficient in removing nutrients and recovering nitrogen (N). However, the recovery of the valuable N-rich biopolymer, cyanophycin, remains limited. This research explored the synthesis mechanism and recovery potential of cyanophycin within two algal-bacterial symbiotic reactors. The findings reveal that the synergy between algae and bacteria enhances the removal of N and phosphorus. The crude contents of cyanophycin in the algal-bacterial consortia reached 115 and 124 mg/g of mixed liquor suspended solids (MLSS), respectively, showing an increase of 11.7 %-20.4 % (p < 0.001) compared with conventional activated sludge. Among the 170 metagenome-assembled genomes (MAGs) analyzed, 50 were capable of synthesizing cyanophycin, indicating that cyanophycin producers are common in algal-bacterial systems. The compositions of cyanophycin producers in the two algal-bacterial reactors were affected by different lighting initiation time. The study identified two intracellular synthesis pathways for cyanophycin. Approximately 36 MAGs can synthesize cyanophycin de novo using ammonium and glucose, while the remaining 14 MAGs require exogenous arginine for production. Notably, several MAGs with high abundance are capable of assimilating both nitrate and ammonium into cyanophycin, demonstrating a robust N utilization capability. This research also marks the first identification of potential horizontal gene transfer of the cyanophycin synthase encoding gene (cphA) within the wastewater microbial community. This suggests that the spread of cphA could expand the population of cyanophycin producers. The study offers new insights into recycling the high-value N-rich biopolymer cyanophycin, contributing to the advancement of wastewater resource utilization.