Nitrogen Migration Research Articles

Transition metal load HZSM-5 catalyst assisted hydrothermal conversion of sewage sludge: Nitrogen transformation mechanism and denitrification effectiveness ofbio-oil

Sewage sludge is not only a kind of hazardous waste, but also biomass resources that rich in organic matter. Hydrothermal liquefaction (HTL) is an effective way of converting sewage sludge (SS) into fuels and chemicals. In this study, the HTL of SS was performed with the aid of transition metal supported HZSM-5 catalyst at 340 °C, conducted with methanol-water as the solvent. The existing forms of nitrogen in the full-component products were tested and analyzed, and the possible migration and transformation paths of nitrogen in the catalytic liquefaction process were proposed. The addition of catalyst has an effect on the physicochemical properties and yield of the liquefaction products. The highest bio-oil yield (28.55%, 26.16%) was achieved at 340 °C with the inclusion of 0.5%Co/HZSM-5 and 1%Ni/HZSM-5, which improved the bio-oil yield by 30% compared with the non-catalyzed group. Ni and Co modified HZSM-5 showed a similar trend of nitrogen transfer, the bio-oil obtained by using metal-loaded HZSM-5 catalysts has higher hydrogen content and lower nitrogen and oxygen content. Catalytic-liquefaction promoted the distribution of nitrogen to solid residues and aqueous. Furthermore, the addition of catalyst promoted the ring opening reaction of the polycyclic compounds and the conversion of the polycyclic nitrogen compounds to amines and amides. The content of monocyclic nitrogen compounds in bio-oil increased while polycyclic nitrogen compounds decreased.

Transformation and regulation of nitrogen and sulfur during pyrolysis of oily sludge with N/S model compounds

Pyrolysis of oily sludge (OS) is an alternative for energy recovery with less pollutants. However, the migration of nitrogen and sulfur during pyrolysis process remain challenges for environmental concern and subsequent utilization of the recovered fuel. In this work, stage pyrolysis experiments of polyacrylamide (PAM) and sodium dodecyl sulfonate (SDS) under OS surroundings were conducted in a thermogravimetry-mass spectrometry-infrared spectroscopy and a fixed reactor to investigate the transformation behaviors of N/S-containing pollutants. Ca(OH)2 and Fe2O3 were then used as inhibitors to regulate nitrogen and sulfur behaviors. Results showed that 3,3 dimethylpyrrolidine-2,5-dione (C6H9NO2), glutarimide (C5H7NO2) and NH3 were the primary N-containing pollutants in liquid and gaseous products, which were generated from PAM via deamination reactions at 200–400 °C. SO2 was the dominant S-containing pollutant, which was generated from SDS via desulfonation at temperature below 300 °C. The dosage of Ca(OH)2 catalyzed the deamination and cracking reactions, and thus led to more N/S-containing pollutants. Fe2O3 suppressed the N/S-containing pollutants significantly because it could increase the energy barrier of N/S-containing pollutants and fix the released NH3 and SO2 into solid residues. This work facilitated the understanding of the nitrogen and sulfur transformation behaviors and would provide a possible regulatory strategy for clean fuel recovery.

Hydrothermal liquefaction of municipal sludge: Coupling effects of temperature and time on nitrogen migration

Biocrude derived from hydrothermal liquefaction of municipal sludge has a high nitrogen content, which limits its direct use. This work explored coupling effects of reaction temperature and residence time on N transformation during hydrothermal liquefaction of municipal sludge. The results showed that 350 ℃ and 40 min were the optimal conditions, which led to the highest biocrude yield, HHV, energy recovery, and N removal efficiency after hydrothermal liquefaction of municipal sludge within 200–350 ℃ for 10–50 min. More than 60 wt% N in municipal sludge was distributed in the aqueous phase and NH4+ was the richest substance after hydrothermal liquefaction. Also, N transformation pathways were clarified by correlating the effects of reaction temperature and residence time on N distribution.

Transformation and Migrant Mechanism of Sulfur and Nitrogen during Chemical Looping Combustion with CuFe2O4

Chemical looping combustion (CLC) is a key technology for capturing CO2. Different types of oxygen carrier (OC) particles are used in coal CLC. The migration and transformation behaviors of sulfur and nitrogen are basically the same when CaFe2O4 and Fe2O3/Al2O3 are used as OC. CLC can be divided into two reaction stages: coal pyrolysis and char gasification; SO2 and NO show bimodal release characteristics, both of which show a basic trend of rising first and then falling down. The contents of H2S and NO2 increased rapidly at the beginning of the reaction and then decreased slowly at the stage of char gasification. H2S is released rapidly during coal pyrolysis and discharged from the reactor with flue gas, and then part of H2S is converted to SO2 during the char gasification stage by OC particles. NO can be oxidized by OC particles and form NO2. The increase in the reaction temperature and oxygen-to-carbon ratio (O/C) contributes to the release of sulfur and nitrogen and higher reaction temperature and O/C can inhibit the formation of metal sulfide. O2 released by CuFe2O4 significantly increases the contents of SO2, H2S, NO and NO2 in flue gas. This work is helpful for improving control strategies for pollutants.

Thermal decomposition pathways of phenylalanine and glutamic acid and the interaction mechanism between the two amino acids and glucose

Understanding the migration and transformation of nitrogen during biomass pyrolysis and the influence of cellulose components on the nitrogen-containing components of biomass is important for the clean and efficient utilization of biomass. In this study, we performed pyrolysis-photoionization time-of-flight mass spectrometry (Py-PI-TOF-MS), pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS), and closed U-tube pyrolysis to analyze phenylalanine, glutamic acid, and their mixtures, the product composition of the primary and secondary reactions of amino acids, and the effect of glucose on the pyrolysis products of the two amino acids. The experimental results were combined with density functional theory calculations to reveal the pyrolysis path of the two amino acids and the mechanism of the effect of glucose on amino acid pyrolysis. The results revealed that phenylalanine undergoes deamination and decarboxylation reactions during pyrolysis. The hydrogen on the carboxyl group is transferred during this process to release CO2 and generate phenethylamine. Phenylalanine generates a large amount of N-phenethylpropane-2-imine through a secondary reaction. Glucose may play a role in the hydrogen supply and promote the deamination of phenethylamine to produce styrene. The primary reaction product of glutamic acid is a nitrogen-containing heterocyclic ring. The pyrolysis product, l-pyroglutamic acid, generates 2-pyrrolidinone through a decarboxylation reaction. Glucose can reduce the reaction energy barrier for the formation of 2-pyrrolidinone.

Nitrogen migration paths and source areas at different snowmelt periods in a seasonal freezing agricultural watershed

Study regionHeidingzi, a typical seasonal freezing agricultural watershed in northeastern China. Study focusThis study’s objectives were to: (1) confirm whether nitrogen source areas and migration paths have changed with the melting of snow and frozen soil; (2) find an effective way to recognize the change points and separate nitrogen export processes to different periods during snowmelt; and (3) identify the source areas, migration paths, and critical influencing factors of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) in river water during these snowmelt periods. New hydrological insights for the regionThe nitrogen export process during snowmelt can be divided into two stages according to the relationship between δ18O and nitrogen concentration. Nitrogen concentration had a linear relationship with δ18O during early snowmelt periods, while they varied within a small range during the late stages. Paddy field area proportion (PFP), Rural area proportion (RAP), Average Slope (AS), Slope area proportion < 2° (SAP < 2°), and SAP (6–15°) greatly affected water discharge, nitrogen concentration, and yield. Lower slope areas along the river expended water and nitrogen via soil saturation and depression detention. The NH4+-N and NO3−-N in these areas were flushed into rivers by water from steeper areas (slope > 6°). Abundant nitrogen reserves near the river compensated for the negative impact of water shortage on nitrogen yield during early snowmelt periods. Water and nitrogen in rural areas came mainly from snow-scattered areas during early snowmelt periods, and from frozen domestic sewage or refrozen snowmelt in drainage ditches during late snowmelt periods.

Experimental and Simulation Research on the Process of Nitrogen Migration and Transformation in the Fluctuation Zone of Groundwater Level

The fluctuation of groundwater causes a change in the groundwater environment and then affects the migration and transformation of pollutants. To study the influence of water level fluctuations on nitrogen migration and transformation, physical experiments on the nitrogen migration and transformation process in the groundwater level fluctuation zone were carried out. A numerical model of nitrogen migration in the Vadose zone and the saturated zone was constructed by using the software HydrUS-1D. The correlation coefficient and the root mean square error of the model show that the model fits well. The numerical model is used to predict nitrogen migration and transformation in different water level fluctuation scenarios. The results show that, compared with the fluctuating physical experiment scenario, when the fluctuation range of the water level increases by 5 cm, the fluctuation range of the nitrogen concentration in the coarse sand, medium sand and fine sand media increases by 37.52%, 31.40% and 21.14%, respectively. Additionally, when the fluctuation range of the water level decreases by 5 cm, the fluctuation range of the nitrogen concentration in the coarse sand, medium sand and fine sand media decreases by 36.74%, 14.70% and 9.39%, respectively. The fluctuation of nitrogen concentration varies most significantly with the amplitude of water level fluctuations in coarse sand; the change in water level has the most significant impact on the flux of nitrate nitrogen and has little effect on the change in nitrite nitrogen and ammonium nitrogen, and the difference in fine sand is the most obvious, followed by medium sand, and the difference in coarse sand is not great.

Nitrogen biogeochemical reactions during bank filtration constrained by hydrogeochemical and isotopic evidence: A case study in a riverbank filtration site along the Second Songhua River, NE China

River eutrophication and nitrogen pollution poses a potential threat to the groundwater quality in riverbank filtration systems. The river filtration process is often accompanied by complex redox reactions. Therefore, identification of nitrogen biogeochemical processes is essential to scientifically characterize the cause of NO3− attenuation and NH4+ accumulation in groundwater, optimize the design of groundwater pumping wells, and design protection strategies in sudden environmental pollution accidents. Hydrogeochemical and stable isotope tracing techniques were employed in this study at a typical riverbank filtration site located in Songyuan, NE China, to explore the main geochemical reactions controlling the migration and transformation of nitrogen along the groundwater flow path during riverbank filtration. The results indicate that mixing, adsorption, organic nitrogen mineralization, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) represent the major geochemical reactions. Denitrification primarily occurs within 10–20 m from the riverbed surface along the filtration path and is the primary reaction accounting for NO3− attenuation, whereas DNRA typically occurs between 1.5 and 6 m from the riverbed surface along the filtration path and is more active in the wet season, characterized by high temperatures (>15–17 °C), low dissolved oxygen (DO, < 2–4 mg/L), and high carbon load i.e., organic carbon (OC):NO3− > 10–15. This work confirms that DNRA is an important nitrogen geochemical reaction during riverbank filtration, and emphasizes its role in NH4+ enrichment.

Ammonia (NH3)/nitrogen (N2) torrefaction pretreatment of waste biomass for the production of renewable nitrogen-containing chemicals via catalytic ammonization pyrolysis: Evolution of fuel-N under a N2/NH3-rich atmosphere

To investigate the nitrogen migration and transformation mechanisms that produce high value-added N-containing chemicals, the NH3/N2 torrefaction pretreatments of waste biomass for the production of renewable N-containing compounds were explored, and the effects of torrefaction temperature (200, 225, 250, 275, 300 °C) on fuel properties, structure, crystallinity, surface functional groups and thermal kinetics were evaluated by element analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier infrared spectroscopy (FTIR) and thermogravimetric analysis (TG-DTG). The effects of subsequent catalytic ammonization pyrolysis upgrading of the torrefied biomass with Co/HZSM-5/HZSM-5 catalysts under an NH3/N2 atmosphere on product yield and N-containing compound selectivity were also evaluated with GC/MS and UV-fluorescence spectrometry. Furthermore, nitrogen fixation and migration mechanisms were also proposed in depth. The results showed that NH3 torrefaction was more effective in enhancing N fixation and retarding the release of N-containing volatiles in torrefied biochar (0.26%–9.43%) and reducing the oxygen content (43.18%–21.81%) via Maillard reactions compared with N2 torrefaction. With increasing torrefaction severity, the N-containing compound content, higher heating values (HHV), deoxygenation ratio (DEO) and nitrogen-doping ratio (NDR) increased, which were much higher than those of conventional N2 torrefaction. Higher temperatures promoted the evolution of graphitic-N and oxidized-N and reduced pyrrolic-N and pyridinic-N group formation in torrefied biochar. The maximum content of N-containing compounds of 88.71% was obtained at a torrefaction temperature of 275 °C under NH3 pyrolysis coupled with the Co/HZSM-5 catalyst, and was achieved via substrate interaction with the -Co-O-Si- catalyst sites and Maillard reactions. Furthermore, the N2 atmosphere combined with the HZSM-5 catalyst significantly promoted the conversion of HC compounds (∼56.97% at 225 °C during N2 torrefaction), while the Co/HZSM-5 catalyst combined with the NH3 atmosphere significantly improved N-containing compound formation. NH3 pyrolysis facilitated the formation of pyrroles and pyrazines while reducing the formation of amides by deamination and dehydrogenation reactions. Cellulose was conducive to the transformation of pyrrole in NH3 pyrolysis, while lignin was conducive to the formation of aniline. These results could offer strategies to regulate nitrogen functional groups to enhance N-containing chemicals.

Analysis of urea nitrogen leaching under high-intensity rainfall using HYDRUS-1D

The leaching of nitrogen in farmland could lead to groundwater nitrogen pollution, while precipitation is the driving factor. Nevertheless, the influences of extreme precipitation on soil organic nitrogen leaching, such as urea, is not clear. In this study, we used soil column experimental data to construct a nitrogen migration and transformation model. The constructed model was applied to unsaturated zones in farmlands in the Dagu River Basin of China. Three rainfall scenarios, namely rainstorms, heavy rainstorms, and extreme rainstorms, were simulated to analyze the variation in urea nitrogen leaching to groundwater. As the rainfall intensity increased, the quantity of urea nitrogen leaching increased from 0.04% to 18.09%. The cumulative urea nitrogen leaching flux is related to the initial soil moisture content. The time interval between fertilization inversely influenced the urea nitrogen leaching flux. Applying urea-based fertilizers shortly before high-intensity rainfall significantly increases the soil-crop system urea leaching loss and decreases crop nitrogen utilization. These findings imply that urea leaching under extreme rainstorms should not be ignored, and based on weather forecasting, fertilizer application in farmland should be avoided before high-intensity rainfall.

Nitrogen migration in products during the microwave-assisted hydrothermal carbonization of spirulina platensis

Nitrogen has a vital influence on the properties of the microwave-assisted hydrothermal carbonization (MHTC) products of Spirulina platensis (SP). The effects of hydrothermal temperature (140–220 °C) and time (1–4 h) on the product distribution and nitrogen migration of SP in MHTC were studied. Increasing temperature led to an increase in the carbon content, and a decrease in the nitrogen content in hydrochar. Protein-N was the major nitrogen-containing species in hydrochar. The total nitrogen in liquid phase increased significantly with increasing temperature. Carbon dots were found to be one of the valuable products in the liquid phase. Higher temperatures improved the amine-N level and reduced the quaternary-N content in carbon dots. A close correspondence was found between the N-containing species and the luminescence centers of carbon dots. A possible nitrogen migration mechanism was proposed to provide guidance for the potential application of the products.

Bioretention system mediated by different dry–wet alterations on nitrogen removal: Performance, fate, and microbial community

In laboratory experiments, the nitrogen migration and transformation in the stormwater bioretention system under different dry–wet alterations were studied. The removal efficiency showed that nitrogen could be removed efficiently in bioretention system under all dry–wet alterations, and the shorter antecedent dry days (ADDs) (1–5 days) were beneficial to the removal of nitrogen before plants decay, compared to the longer ADDs (7–22 days). Using a new method combined with Hydrus-1D model, water transport was simulated and nitrogen migration in bioretention system was quantified, indicating that NH4+-N was mainly removed in the planting layer, and the removal of NO3−-N was occurred in the submerged layer. Fate experiment showed the main fate of the nitrogen was microorganisms (1–5 ADDs) and soil immobilization (7–22 ADDs). Microbial analysis showed that shorter ADDs (1–5 days) were suitable for Firmicutes growth, while Proteobacteria and Actinobacteria accounted for greater abundance under longer ADDs (7–22 days). Canonical correlation analysis (CCA) revealed the relationships between microbial community and environmental factors. Soil moisture content, soil organic matter (SOM), TN (water), root length, and NO3−-N (water) were significantly correlated with bacterial community. This work may give new insights into nitrogen migration and transformation, and can provide a reference for the further mechanism study and construction of stormwater bioretention systems.

Evolution of carbon and nitrogen during chemical looping gasification of rapeseed cake with Ca-Fe oxygen carrier

• Higher OC/RC ratio is beneficial to the transformation of carbon. • NH 3 and HCN are the major N species in CLG of rapeseed cake. • Fe and Ca in CaFe 2 O 5 can increase the yields of NH 3 and HCN. • The reaction pathways of nitrogen in CLG are summarized. • CLG is promising process for the sustainable production of NH 3 . Rapeseed cake (RC) is an attractive fuel for its high caloricity, but the high-nitrogen content seriously hinders its utilization. Meanwhile, chemical looping gasification (CLG) is a promising technology for the production of N 2 free high-quality syngas. Inspired by using RC as a source of C for syngas production, it also can be a possible source of N and H for the sustainable production of NH 3 . Hence, recycling N and H from RC via CLG was proposed for NH 3 production. This paper evaluates the effects of oxygen carrier (CaFe 2 O 5 (CaFe) and 60% Fe 2 O 3 /Al 2 O 3 (FeAl)) on kinetic parameters, as well as the evolutions of carbon and nitrogen during CLG of rapeseed cake using TG-FTIR. Results show that the gasification behavior is remarkably influenced by oxygen carrier at both devolatilization and char reaction stages. CaFe shows a remarkably higher reduction activity compared to FeAl, especially at char reaction stage. The superior activity of CaFe can be also reflected in the lower activation energy compared to FeAl. As regards to nitrogen emission, NH 3 and HCN are the dominant nitrogen species, whereas HNCO is minor. Meanwhile, the migration of nitrogen includes catalytic pyrolysis and oxidation by lattice oxygen (O*). It is feasible to control the migration path of nitrogen by adjusting the operating conditions. Thus, CLG is an attractive pathway for efficiently utilizing rapeseed cake, which can not only produce tunable syngas, but also possibly make the most use of nitrogen as valuable chemicals.

Simultaneous recovery of nutrients and power generation from source-separated urine based on bioelectrical coupling with the hydrophobic gas permeable tube system

Source-separated urine has been regarded as a precious treasure on account of its rich nitrogen content and is suitable for fertilizer production. In this study, a novel bioelectrical coupling with hydrophobic gas permeable tube system (BGTS) was developed to treat urine, for removing organic matter, and recover nitrogen as value-added products in the form of nitrogen fertilizer. In the presence of the electric field, the hydrolysis process of urea in the anode chamber was accelerated, and the NH4+ driven by electric field force and concentration difference reached the cathode through the cation exchange membrane. The cathode made use of oxygen and electrons to produce alkali in situ to promote the conversion of NH4+ to NH3, which was straightforwardly absorbed in hydrophobic gas permeable tube circulating sulfuric acid solution, so as to promote the rapid migration of nitrogen and build an efficient dynamic recovery of nitrogen. After a 48-h cycle, the BGTS achieved a 95.28 ± 0.60% COD removal ratio, 91.60 ± 0.29% nitrogen recovery efficiency, and 3.48 kg m−3 ammonium sulfate fertilizer. Economic analysis indicated a profit of 5.75 $ associated with the utilization of the BGTS system for nitrogen fertilizer recovery from source separation in urine. Consequently, this study manifested that the BGTS system can recover nitrogen from human urine in a high-recovery and cost-effective way, and is of great significance in the sustainable recovery of nitrogen resources.

Characterization of nitrogen and sulfur migration during pressurized coal pyrolysis and oxy-fuel combustion

In the present work, pressurized coal pyrolysis and oxy-fuel combustion experiments were carried out in a pressurized drop tube furnace to investigate the migration and transformation characteristics of nitrogen and sulfur. The pyrolysis experiments showed that high pyrolysis pressure inhibited the release of nitrogen and sulfur. The content of oxidized nitrogen (N-X), pyrite, sulfide, sulfoxide and sulfate in char decreased, while the content of pyridine nitrogen (N-6), pyrrole nitrogen (N-5), quaternary nitrogen (N-Q), thiophene and sulfone in char increased with the increase of pyrolysis pressure. The elevated pyrolysis temperature was conducive to the decomposition of nitrogen and sulfur-containing functional groups, and it also promoted the conversion of nitrogen and sulfur to functional groups with high stability such as N-Q and thiophene. The release of volatiles and the decomposition of nitrogen and sulfur-containing functional groups in coal were promoted by CO2 gasification. The increase of the total CO2 gasification pressure led to more nitrogen and sulfur remaining in char. During pressurized oxy-fuel combustion in O2/CO2 atmosphere, the emissions of NO, NO2 and SO2 were lower than those in air combustion. The emissions of NO, NO2 and SO2 gradually decreased with the increase of pressure at air and oxy-fuel atmospheres. The increase of temperature increased the emissions of NO and SO2 at different pressures, but the NO2 emission decreased at elevated temperature. In addition, the emissions of NO and NO2 increased, but the emission of SO2 decreased with the increase of oxygen concentration.

Effects of Biochar Addition on Soil Nitrogen Mineralization and Leaching Characteristics in Riparian Zone of Taihu Lake

Nitrogen mineralization in riparian soil changes the migration and utilization efficiency of nitrogen, which is closely related to the control of water eutrophication. The differences in soil properties caused by land use alter nitrogen retention and transport capacity. Therefore, the soil of three land use types (woodland, grassland, and cultivated land) in the western riparian zone of Taihu Lake were selected for research on the dynamic changes in nitrogen mineralization amount using an incubation experiment and a leaching characteristics by soil column leaching experiment, and their environmental effects were also studied under different biochar addition conditions (0%, 1%, and 5%). The results showed that, in general, the addition of biochar inhibited nitrogen mineralization in forest land and grassland soil, whereas the effect of biochar on nitrogen mineralization in cultivated land was promoted in low concentrations but inhibited in high concentrations. Leaching experiments showed that the biochar addition reduced the loss of soil mineral nitrogen, and the reduction rate in ammonia nitrogen was 23.28%-39.79%, whereas there was little difference between the three land use types. The nitrate decreased by 17.20%-44.49%, and the reduction rate of cultivated land was smaller than that of forest land and grassland. In conclusion, the input of biochar into grassland and cultivated land can better maintain soil fertility and reduce soil nitrogen loss in riparian soil.

Characteristics and formation of nitrogen-containing products from the pyrolysis of maple wood and maize straw

Biomass is widely used in residential and industrial sectors as a renewable and carbon-neutral energy source, however, NOx emissions caused by the heat treatment of biomass are of concern. The nitrogen element in biomass plays an important role in N-containing products generated from pyrolysis, while the migration behaviors of fuel-N are still unknown. Slow pyrolysis experiments of maple wood and maize straw were conducted at 350–650 °C in this study. The TG, XPS and GC/MS technologies were performed to investigate the nitrogen migration pathways during prolysis. Results demonstrated that the fuel-N in the two species mainly exists as protein-N, amide-N and inorganic-N functional groups. Char-N occupied the most of total fuel-N from 350 °C to 450 °C, while the yields of tar-N and NOx precursors-N (NH3-N and HCN-N) increased gradually with the rising temperatures. NH3 was found to be the main NOx precursor for the both species, accounting for 15.01–20.19 wt% and 3.18–5.91 wt% of fuel-N in maple wood and maize straw, respectively. More than 85% of NH3-N was produced between 350 °C and 450 °C, attributing to the conversion of fuel-N and tar-N in the devolatilization stage. Differently, around 50% of HCN-N yields were produced by the secondary decomposition of tar-N and char-N at higher temperatures (≥550 °C). Particularly, char-N showed a greater thermal stability in maize straw, while a much higher conversion ratio of NOx precursors-N was observed from maple wood pyrolysis than that of maize straw. These results could provide some guidances on the control of NOx emissions from biomass energy utilization.

Biogenic Migration of Nitrogen and Phosphorus in Saline Drying Lakes in Crimea

Biogenic Migration of Nitrogen and Phosphorus in Saline Drying Lakes in Crimea

Biochar impacts on the soil environment of soybean root systems

Biochar has been widely studied as a soil amendment, but little is known about the “biochar-freeze-thaw soil-crop root system” interface in seasonally frozen soil areas. In the second year after the application of biochar, we conducted research on the morphological characteristic indicators of the soybean root system and the nutrient migration of the soil in the root zone under different biochar application periods (spring and autumn mixed, autumn, and spring biochar application) and different biochar application rates (3 kg·m−2, 6 kg·m−2, 9 kg·m−2, and 12 kg·m−2). The effects of different biochar treatments on the growth and development of soybean roots were examined. The soil organic carbon, ammonium nitrogen and nitrate nitrogen contents of the soil were measured at different locations in the root zone, and the migration processes of these nutrients in the soil were explored. The conclusions drawn from the experiments are as follows. (i) The biochar application rate and application method together determine the root morphological characteristic indicators of soybean plants. During long freeze-thaw periods, the freeze-thaw cycles change the internal environment of the biochar-freeze-thaw soil complex. (ii) Biochar tends to move towards the root system, which can increase soil organic carbon content, but the effect of biochar on root characteristics is not caused by the change in soil organic carbon content. (iii) Biochar promotes nitrogen cycling in the soil and the migration of soil nitrogen to the root sheath, increasing the number of nitrogen compounds that can be directly absorbed and utilized by crops. (iv) From a comparison of the effects of various biochar treatments on crop roots and farmland soils, we suggest that the 9 kg·m−2 biochar application rate under spring and autumn mixed biochar application is the optimal treatment.

Water–fertilizer coupling effect on the growth traits of winter wheat under conditions of light and small sprinklers

Light and small sprinklers were used to irrigate winter wheat fields. The distribution and migration of water and nitrogen in soil during winter wheat growth period were analyzed. Moreover, the growth traits of winter wheat were monitored. It is demonstrated that water–fertilizer coupling effect had a significant impact on the migration of water and nitrogen in soil and the growth traits of winter wheat under conditions of light and small sprinklers. Under the same condition of irrigation quota, although the content of moisture in soil was slightly reduced with the increase of the amount of fertilizer, the impact of the amount of fertilizer on the content of moisture in soil is much more insignificant than that of irrigation quota. The content of nitrate nitrogen (NO3--N) in soil was increased with an increasing amount of fertilizer, especially when the value of irrigation quota was low. On the other hand, under the same condition of amount of fertilizer, the content of nitrate nitrogen (NO3--N) in soil was decreased with an increasing value of irrigation quota, especially when the value of irrigation quota was high. Hence, an ultra-high value of irrigation quota will aggravate the leach of nitrate nitrogen in soil. The water–fertilizer coupling effect and yield of winter wheat could be optimized by improving irrigation scheme, optimizing irrigation quota, and optimizing the amount of fertilizer.