Objectives This study explored the effect of humic acid urea (HAU) application on the decomposition process of maize straw and identified the key differential microbial groups involved in the decomposition of straw, aiming to provide nutrient management method for the efficient utilization of straw resources in North China Plain.

Methods The experiment was carried out in Modern Agricultural Research and Development Base of Henan Province, where a long-term experiment of new fertilizers was conducted since 2016. In October 2022 after winter wheat sowing, maize straw bags were buried into the urea and humic acid urea treatment plots, respectively, and straw bags were collected at different burying days during the wheat season (200 days in total) to determine the residual straw weight, the organic carbon and nitrogen content, enzyme activity, bacterial and fungal diversity.

Results Compared with urea treatment, HAU accelerated the decomposition process of maize straw, increased the decomposition rate, nitrogen and carbon release rate of straw by 6.98%, 32.29%, and 6.56%, respectively; enhanced the release rate of cellulose, hemicellulose and lignin by 12.27%, 9.35% and 14.27%, respectively. The activities of β-glucosidase, cellullobiose hydrolase, leucine aminopeptidase and laccase were also improved. The dominant bacteria involved in straw decomposition were Proteobacteria, Bacteroidota, Actinobacteriota and Firmicutes. And the dominant fungi were Ascomycota, Mucoromycota and Basidiomycota. HUA application enriched bacterial groups included Agromyces, Microterricola, Paenarthrobacter, Pseudomonas, Chryseobacterium, Paenarthrobacter, Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium, and Mesorhizobium, etc. And the enriched fungal groups included Actinomucor, Hannaella, Trichocladium, Aspergillus, etc. Straw N content, lignin/N ratio and β-glucosidase activity were the most important factors affecting the community structure of straw bacteria and fungi.

Conclusions The long-term application of humic acid urea proves to be more advantageous than the use of common urea in fostering more diverse bacteria and fungi species associated with straw decomposition across various stages. This includes the proliferation of distinct nitrogen-fixing bacteria such as Paenarthrobacter, Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium, among others, as well as enhancing the activities of enzymes responsible for carbon and nitrogen transformation. Consequently, this accelerates the decomposition process of maize straw. Therefore, the application of humic acid urea emerges as an effective strategy for achieving efficient utilization of straw nutrient resources in the North China Plain.