The composition of soil nitrogen pool is an important indicator reflecting the sustainable productivity of soil. By quantifying the response relationship between changes in soil nitrogen pool components and the application rate of organic and chemical fertilizers, the cultivation characteristics of different nitrogen components in soil by organic and chemical fertilizers were clarified, so as to provide a theoretical basis for soil fertilization and optimizing nitrogen management in farmland soil.

This study used a long-term location monitoring experiment with different dosages of organic fertilizer and chemical fertilizer (starting in 2006) as a platform. Both organic fertilizer and chemical fertilizer were set at 10 nitrogen fertilizer levels (calculated as N, with crop nitrogen application rates of 0, 60, 120, 180, 240, 300, 360, 420, 500, and 600 kg·hm^-2 per season). The planting system was a double cropping system of winter wheat and summer maize per year. Plant samples were taken during the maturity period of maize in the summer of 2024 to determine nitrogen content, biomass, and maize grain yield. After the summer maize harvest, soil samples were collected in 0-20 cm depth to determine the total nitrogen and inorganic nitrogen content of the soil. The Bremner method was used to determine the organic nitrogen component content of the soil.

(1) The total nitrogen content of the soil in both fertilization treatments showed a linear increase with the increase of nitrogen application rate. Compared with the no fertilization treatment, the increase in soil total nitrogen content in organic fertilizer and chemical fertilizer treatments was 19.1%-273.5% and 8.8%-25.0%, respectively. For soil inorganic nitrogen, with the increase of nitrogen application rate, the soil nitrate nitrogen content significantly increased under both fertilization treatments. Under the same nitrogen content, the nitrate nitrogen content in soil treated with organic fertilizer was significantly higher than that of chemical fertilizer, ranging from 1.44 to 5.17 times, but the difference in ammonium nitrogen was not significant. (2) For soil organic nitrogen components, the content of total acid hydrolyzable nitrogen (THN) and non-acid hydrolyzable nitrogen (NHN) in organic fertilizer treatment increased with the increase of nitrogen application rate. The THN content of fertilizer treatment also increased with the increase of nitrogen application rate, but there was no significant difference between the treatments. In the components of acid hydrolyzed nitrogen, the content of acid hydrolyzed ammonium nitrogen (AN), amino acid nitrogen (AAN), and amino sugar nitrogen (ASN) increased with the increase of nitrogen application rate in both organic and chemical fertilizer treatments. However, the trend of changes in the content of acid hydrolyzed unknown nitrogen (HUN) was opposite, with organic fertilizer treatment increasing with increasing nitrogen application, while chemical fertilizer treatment showed a decreasing trend. The linear fitting results indicated that there was no significant correlation between NHN and nitrogen application rate in fertilizer treatment. Under both fertilization treatments, the content of various organic nitrogen components showed a significant linear positive correlation with nitrogen application rate. Among them, the HUN of fertilizer treatment showed a significant linear negative correlation with nitrogen application rate. Long term application of organic and chemical fertilizers has changed the proportion of various acid hydrolyzed nitrogen components to total soil nitrogen. The distribution ratio of chemical fertilizer treatment was AN (28.3%-32.5%)>AAN (25.1%-29.5%)>HUN (5.9%-10.0%)>ASN (2.6%-3.5%); the allocation ratio for organic fertilizer treatment was AAN (23.7%-31.4%)>AN (19.6%-27.6%)>HUN (7.9%-12.6%)>ASN (2.3%-3.2%). (3) Correlation analysis showed that AN, AAN, and ASN were significantly positively correlated with nitrogen uptake in the aboveground parts of summer maize (P<0.01).

Under the winter wheat-summer maize rotation system in the North China Plain, the long-term application of chemical fertilizer and long-term application of organic fertilizer could significantly increase the soil nitrogen pool capacity. In acid hydrolyzed organic nitrogen, compared with the application of chemical fertilizers, organic fertilizer treatment significantly increased the content of AN, AAN, ASN, and HUN, thereby improving soil nitrogen supply capacity. AN, AAN, and ASN exhibited a highly significant positive correlation with the nitrogen absorption in the aboveground parts of summer maize, making them potential effective nitrogen sources for the absorption and utilization by summer maize.

Sugars regulate the nitrogen transformation process in soil. However, the effects of different polymerization degrees of dextran on nitrogen conversion were not clear. This study focused on investigating the transformation characteristics of dextran-modified urea with varying degrees of polymerization in calcareous fluvo-aquic soil, analyzing the relationship between the polymerization degree of dextran and nitrogen efficiency, and revealing its functioning mechanism. It aimed to provide a solid theoretical basis for the application of different polymerization degrees of dextran in urea.

Four types of modified urea were prepared by incorporating 1% glucose (monosaccharide, GU), maltose (2-poly, MU), oligomeric dextran (≈5-poly, OU), and polydextrose (≈20-poly, PU) into molten urea. Six treatments were designed: no fertilization (CK), conventional urea (U), and four dextran-modified urea treatments. A soil incubation experiment was conducted to evaluate nitrogen transformation, and ¹³C nuclear magnetic resonance (¹³C-NMR) spectroscopy was used to characterize the chemical structures, to uncover the urea conversion mechanism affected by dextran polymerization degree and structural changes.

(1) Dextran-modified urea contained a Schiff base structure. (2) Compared with U, dextran-modified urea slowed urea hydrolysis and increased soil amide N content by 15.3%-37.1%, with the highest value in OU. (3) After the application of nitrogen fertilizer, the urease activity of the U treatment peaked at 1 d, whereas the urease activity of the dextran-modified urea treatment peaked at 2 d of incubation. (4) Ammonia volatilization accumulation of dextran-modified urea was reduced by 2.2%-12.9% compared with U. With the increase of the degree of polymerization of dextran, the accumulation of ammonia volatilization showed a gradually increasing trend. (5) At the end of the incubation, NO₃^--N content of dextran-modified urea was increased by 14.1%-30.5%. As the degree of polymerization of dextran increased, the nitrate nitrogen content first increased and then decreased. When the degree of polymerization was 3.35, the nitrate nitrogen content was the highest.

Dextran-modified urea effectively delayed urea hydrolysis, reduced ammonia volatilization, and enhanced soil NO₃^--N content. GU had the best impact on reducing ammonia volatilization, while dextran with a medium polymerization degree (3.35) had the best effect on improving the NO₃^--N content in the soil.

The conversion characteristics of different carboxyl-containing organic acids modified urea combined with carboxyl groups and other active functional groups in calcareous fluvo-aquic soil were investigated, so as to provide a theoretical basis for the development of high-efficiency nitrogen fertilizers.

The citric acid (carboxyl group + hydroxyl group), humic acid (carboxyl group + phenolic hydroxyl group/carbonyl group/aldehyde group, etc.), polyglutamic acid (carboxyl group + amino group) and polyacrylic acid (carboxyl group) were added into the molten urea at 0.5% addition amount, to prepare the test fertilizers, exactly, citric acid urea (CAU), humic acid urea (HAU), polyglutamic acid urea (PGAU) and polyacrylic acid urea (PAAU). The treatments of without urea (CK), common urea (U), CAU, HAU, PGAU and PAAU were set up to study the effects of different carboxyl-containing organic acids modified urea on the soil amide nitrogen, NH₄⁺-N, NO₃^--N and soil urease activity by using soil incubation method. The influence mechanism of different carboxy-containing organic acids modified urea in soil conversion was revealed with the results from Fourier transform infrared spectroscopy (FTIR) of U and different carboxy-containing organic acids modified urea.

(1) Compared with U, the four kinds of carboxyl-containing organic acids modified urea delayed the urea hydrolysis in the soil from 6 h to 2 d. HAU and PGAU had a better performance than other treatments, and their residual amount of soil urea nitrogen increased by 22.3% and 23.7% than that with U, respectively. (2) Compared with the appearing time of NH₄⁺-N content peak treat with U (2 d), HAU application delayed to the appearing time of NH₄⁺-N content peak to the third day. In 6 h-2 d, the average value of NH₄⁺-N content with HAU treatment decreased by 16.9% than that with U, while increased by 3.2% in 3-14 d. (3) Compared with U, four kinds of carboxyl-containing organic acids modified urea significantly increased soil NO₃^--N content in the later period of incubation, and the HAU treatment showed the highest value with an average increase of 17.4 mg·kg^-1 than the U treatment. (4) Compared with U, four kinds of carboxyl-containing organic acids modified urea inhibited soil urease activity in 1-2 d, among which HAU had the strongest inhibitory effect, and the urease activity was reduced by 30.9% compared with U. However, HAU enhanced the soil urease activity in 2-14 d.

Carboxyl-containing organic acids modified urea could delay the hydrolysis and transformation of urea in the soil by inhibiting urease activity in the early stage of incubation delaying the transformation of NH₄⁺-N to NO₃^--N in the middle stage of incubation, and increase the NO₃^--N content of cultivated soil in the late stage of incubation to reduce nitrogen loss. The above results were mainly attributed to the reaction of carboxyl groups and other active functional groups with urea. The reaction degree with urea was the deepest when the carboxyl group and a variety of active functional groups (phenolic hydroxyl group/aldehyde group/carbonyl group) existed at the same time, which attributed the slow release of urea to the best.

Promoting the dissolution of phosphate fertilizer and reducing its fixation are important ways to ensure the phosphate supply and to improve the utilization rate of phosphate fertilizer. In this study, the test products of phosphate fertilizer by adding hydroxyethylidene diphosphonic acid (HEDP) and sodium gluconate (SG) into diammonium phosphate were prepared to investigate its dissolution, anti-fixation and fertisphere transformation characteristics, so as to provide a theoretical and technical basis for the efficient utilization of phosphate fertilizer.

HEDP and SG were physically mixed with powdered diammonium phosphate at the addition ratio of 0, 0.5%, 1%, 3% and 5%, respectively, and the extrusion granulation was used to obtain the nine granular diammonium phosphate test products, specifically, common diammonium phosphate (P), HEDP_0.5%+P, HEDP_1%+P, HEDP_3%+P, HEDP_5%+P, SG_0.5%+P, SG_1%+P, SG_3%+P, and SG_5%+P. The dissolution rate and fixation rate of water-soluble phosphorus in different diammonium phosphate particles were analyzed and compared by using water dissolution rate method and CaCl₂ precipitation method, respectively. A soil cultivation was conducted to investigate the transformation characteristics of diammonium phosphate in the fertisphere.

(1) Adding HEDP and SG increased the dissolution rate of diammonium phosphate on average by 27.7% and 20.0%, respectively. When HEDP and SG were added at 5% and 0.5%, the dissolution rates were 5.1%·min^-1 and 4.8%·min^-1, respectively, higher than other additive rate, and also higher than P by 39.2% and 32.6%, respectively. (2) When HEDP and SG were added, the fixation rates of water-soluble phosphorus in diammonium phosphate decreased by 10.3% and 6.6%, respectively. When the additive rate of chelating agent was at 5% and 3%, the fixation rate of water-soluble phosphorus was lower than P by 58.1% and 61.3% for HEDP, 17.0% and 11.8% for SG, respectively. (3) The content of available phosphorus in the fertisphere soil treated with HEDP+P and SG+P significantly increased by 39.5% and 21.2%, respectively, and HEDP+P had a better performance. When the additive rate of HEDP and SG was at 3%, the available phosphorus content in fertisphere soil was 2.9 g·kg^-1 and 2.5 g·kg^-1, respectively, higher than that of P by 53.3% and 31.9%, respectively. (4) The Ca₂-P contents in the fertisphere soil treated with HEDP+P and SG+P were higher than that of P by 38.2% and 43.0%, respectively, and both of HEDP and SG had a better performance when their additive amount at 3%. The Ca₈-P content in the 5 mm fertisphere soil treated with HEDP+P and SG+P was lower than that of P by 33.6% and 14.5%, respectively. When the additive amount of HEDP and SG was at 0.5% and 3%, the Ca₈-P content of fertisphere was 0.9 g·kg^-1 and 1.5 g·kg^-1, respectively, lower than that of P by 53.4% and 25.8%, respectively. (5) The contents of available phosphorus and Ca₂-P in the fertisphere soil were positively correlated with the dissolution rate of phosphorus fertilizer and negatively correlated with the fixation rate of water-soluble phosphorus.

The addition of HEDP and SG could effectively enhance the dissolution of diammonium phosphate and reduce phosphorus fixation. HEDP had a better performance, and especially when its addition amount was at 3%, HEDP could more improve the available phosphorus content and decrease the transformation from Ca₂-P to Ca₈-P in the fertisphere soil.

Saccharides could regulate nitrogen transformation and promote crop growth. However, the effect and mechanism of carbohydrate reaction with urea with different degrees of polymerization on urea nitrogen utilization and crop growth are still unclear. In this paper, the dextran modified urea were prepared by incorporated of glucose (dextran) with different polymerization degrees into ¹⁵N labelled urea. The relationship between structure of dextran modified urea and wheat yield and nitrogen utilization was analyzed, in order to provide scientific basis for the application of dextran in improving nitrogen utilization efficiency.

In the current study, the dextran modified ¹⁵N labelled-urea (glucose, maltose, oligomeric maltose, and polydextrose modified urea are represented by GU, MU, OU, and PU, respectively) and ordinary ¹⁵N labelled-urea (U) was prepared, and the effects of dextran modified urea on winter wheat (Jimai 22) growth and the fate of fertilizer nitrogen were studied by using soil column cultivation experiments. The relationship between the dextran polymerization degree and the winter wheat yield or fertilizer nitrogen fate was explored by structure investigation of the dextran modified urea using Fourier Transform Infrared (FTIR) and ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy.

(1) Compared with U, the dextran modified urea showed a weaker vibration intensity of primary amide at 3 343 cm^-1 and 1 601 cm^-1 in FTIR spectra, and a new chemical shift peak was detected at 158-171 ppm in ¹³C NMR spectra, which was a sign that the aldehyde group of dextran and the amine group of urea reacted and that a C=N structure attributed to Schiff base was formed. (2) The wheat yield treated with GU, MU, OU and PU was higher than that of U by 1.9%, 9.2%, 10.3% and 12.3%, respectively. The yield increase was mainly attributed to the improvement of the ears number and grains per ear. (3) The total nitrogen uptake and fertilizer nitrogen uptake of grain of the dextran modified urea treatments were also higher than that under the U treatment by 8.7%-20.0% and 6.1%-13.9%, respectively. MU, OU and PU treatments had a higher nitrogen uptake than GU. (4) Compared with U, the ¹⁵N utilization rate and the fertilizer residue rate of dextran modified urea was enhanced by 2.0-6.1 and 1.3-4.9 percentage points, respectively, while the nitrogen loss rate of dextran modified urea was reduced by 6.9-7.4 percentage points. The ¹⁵N utilization rate of MU, OU and PU treatments was significantly higher than that of U. (5) Correlation analysis showed that the yield of wheat was significantly positively correlated with the polymerization degree of dextran, while the content of Schiff base was significantly negatively correlated. Among them, the relationship between Schiff base content and wheat yield and ¹⁵N utilization rate could be significantly fitted by using one quadratic equation. When the polymerization degree of dextran was 5-8, wheat yield and fertilizer nitrogen utilization rate were the highest.

Compared with common urea, the dextran modified urea could increase wheat yield, promote nitrogen absorption and utilization, increase fertilizer nitrogen residue and reduce urea loss. Within a certain range, the wheat yield and nitrogen absorption gradually increased, while the residual fertilizer nitrogen in soil decreased along with the increase of the polymerization degree of dextran. When the polymerization degree of dextran was 5-8, the corresponding modified urea would have the best performance.

The aim of this study was to investigate the effects of dextran modified phosphate fertilizer prepared by the reaction between dextran and phosphate fertilizer with different polymerization degrees on the growth and soil phosphorus effectiveness of wheat, so as to provide the scientific support and theoretical basis for the application of dextran in phosphate fertilizer.

By using the reaction method, glucose (monomer), maltose (2-polymer), oligomaltose (≈5-polymer) and polydextrose (≈20-polymer) were added to a mixture of phosphoric acid and potassium hydroxide at 1% addition to prepare glucose-modified phosphate fertilizer (GP), maltose-modified phosphate fertilizer (MP), oligomaltose-modified phosphate fertilizer (OP) and polydextrose-modified phosphate fertilizer (PP), and the normal phosphate fertilizer (P) was prepared without the addition of dextran. The structural characteristics of the reaction between dextran and phosphate fertilizer were investigated by Fourier infrared transform spectroscopy (FTIR) and ³¹P nuclear magnetic resonance spectroscopy (³¹P NMR). Five treatments, including P, GP, MP, OP, and PP, were set up according to the principle of equal phosphorus amount, and the control (CK) was applied with only nitrogen and potassium fertilizers. The effect of different polymeric dextran modified phosphate fertilizers on wheat yield and fertilizer utilization was investigated by soil column cultivation.

(1) Compared with P, the FTIR spectra of dextran modified phosphate fertilizer showed a new vibration peak at 975 cm^-1, and the ³¹P NMR spectra showed a new displacement peak at 3.09-4.51 ppm, which might be due to the reaction between the hydroxyl group of dextran and phosphoric acid to form orthophosphate monoester. (2) Wheat yields were increased by 5.1%, 9.3%, 11.2% and 1.4% for the treatments with different polymerization degrees of dextran modified phosphate fertilizers (GP, MP, OP and PP) compared with P, respectively, mainly through the number of spikes, followed by the number of grains. (3) Compared with P, the total phosphorus uptake of wheat was significantly higher by 8.2%-21.4% under different polymerization degrees of dextran modified phosphate fertilizer treatments, among which, OP treatment was significantly higher than the other treatments. (4) The apparent phosphate fertilizer utilization rate of dextran modified phosphate fertilizer treatment was increased by 4.4-11.5 percentage points compared with P. The phosphate fertilizer bias productivity and phosphorus fertilizer agronomic efficiency were increased by 1.4%-11.2% and 1.6%-13.1%, respectively. The phosphate fertilizer utilization rate of both MP and OP treatments were significantly higher than P. (5) Compared with P, the soil fast-acting phosphorus content of dextran modified phosphate fertilizer treatment was significantly higher 10.2%-29.9%, and the OP treatment was significantly higher than the other dextran modified phosphorus fertilizer treatments.

Compared with common phosphate fertilizer, all dextran modified phosphate fertilizers with different polymerization degrees could improve wheat yield, promote the uptake and utilization of phosphorus in wheat, increase soil fast-acting phosphorus content, and reduce phosphorus fertilizer fixation. With the increase of dextran polymerization degree, wheat yield and apparent phosphorus fertilizer utilization increased first and then decreased. The best effect of dextran polymerization on the modification and efficiency of phosphate fertilizer was achieved when the polymerization degree of dextran was 4-6.