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.