基于pH分级法的腐植酸结构分析及其对尿素转化的影响机制

Humic acid is playing an important role in chemical fertilizer modification and efficiency-enhancement. Humic acid combined with urea is applied comprehensively, which can inhibit soil urease activity, reduce ammonia volatilization losses and improve nitrogen use efficiency. However, the efficiency-enhanced effect and technological upgrading of humic acid urea are limited because that very few studies on the reaction features of humic acid structure and urea chemical bond and the relationship between the humic acid structure characteristics, urea transformation rule and soil urease activity were conducted. 11 small fractions could be obtained from humic acid by “pH fractionating method” from 5 main production places and the reaction features between humic acid structure and urea chemical bond, the relationship between humic acid structure characteristics, urea transformation, ammonia volatilization and soil urease activity would be studied and ascertained. And the humic acid’s effect mechanism on urea transformation could be clarified. Therefore, the study could guide the activation, extraction, isolation and modification of humic acid and provide basis for the utilization of humic acid in the field of urea effect-enhanced to develop the effect of humic acid in urea N fertilizer efficiency-increasing and quality-improving, promote the high efficiency utilization of humic acid resource and urea N fertilizer industry technology upgrading.

腐植酸在尿素改性增效方面发挥了重要作用，腐植酸与尿素结合后，具有抑制土壤脲酶活性，降低氨挥发损失的作用，可提高氮肥利用率，在生产中应用广泛。但目前腐植酸结构与功能的关系尚不明确，限制了腐植酸在尿素氮肥增效中的科学应用。本研究利用“pH分级法”将5个主要产地的腐植酸均细分为11个级分，获得不同结构的腐植酸试验样品并研究其结构特征；研究不同pH范围腐植酸尿素结构变化与尿素转化之间的关系，明确有效基团及其结构，结合腐植酸与尿素化学键的反应特点，探明腐植酸结构对尿素转化的影响机制，指导腐植酸的活化、提取、分离和改性等，为腐植酸在尿素改性增效领域的应用提供理论依据，促进不同区域腐植酸资源的高效利用和尿素产业的技术升级。

本研究以风化煤为原料，通过pH分级方法制备不同结构性的腐植酸级分，研究腐植酸及其与尿素结合产物的结构特征，分析腐植酸尿素在土壤中转化的影响，结合腐植酸和腐植酸尿素的结构特征，探究其调控机理。主要研究结果如下：.（1）不同pH级分腐植酸的分子量。腐植酸的分子量并非随pH升高而增加，而是呈现先升高后降低趋势。各级分腐植酸分子量范围为1.322×10-5g/mol~2.029×10-5g/mol，以pH 6~7 时获得的腐植酸组分分子量最大。pH＞8或pH3~5时，腐植酸分子量均较中性时降低20%左右，pH＜3时，腐植酸分子量降低幅度达30%。.（2）不同pH级分腐植酸的官能团特征。低pH溶液（pH3~6）中浸提出的腐植酸级分具有更多的羧基官能团，随着浸提溶液pH的增加，羧基官能团减少；HA7~10级分腐植酸具有更高的芳香基团。以pH6~7为界，pH＜6，则腐植酸羧基含量更高；pH＞7，则芳香基团含量更高，这也说明了利用pH分级的方法可得到结构不同的腐植酸。.（3）不同pH级分腐植酸与尿素反应特点。pH分级所获得腐植酸级分制得的尿素-腐植酸反应产物以UHA6~7的总氮和肥料氮的含量最高，是UHA3~4和UHA9~10的1.61倍和2.96倍。因此，腐植酸羧基碳含量越高，其与尿素的结合量越大；此外，也有一定的酚羟基碳参与了腐植酸与尿素结合的反应。.（4）腐植酸结构与对尿素转化的调控机制。腐植酸对尿素的作用效果均随着原料腐植酸浸提溶液pH的增加而逐渐增强。pH 9~10级分腐植酸尿素对氨挥发抑制和脲酶后期稳定效果最好， pH 6~7和pH 9~10级分腐植酸尿素延缓了铵态氮向硝态氮的转化速度，这可能是由于这两个级分腐植酸芳香度高于pH 3~4级分，对脲酶抑制作用更强。另一方面，这两个级分的芳香碳非质子化程度较高，结构稳定，与尿素的结合作用更为牢固。