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.

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.