Mulching is known to influence soil nitrogen (N) cycling via microbial activity; however, the response of N-cycling functional genes and microbial communities to novel soil film mulching remains unclear. This study aimed to characterize these responses under different application rates of a synthetic soil film-forming material. A spring wheat field experiment was conducted with four application rates of carboxymethyl cellulose ammonium (CMC-NH4). Soil properties were measured, and metagenomic sequencing was used to analyze N-cycling functional genes and associated microbial communities. Soil film mulching significantly increased soil temperature, moisture, and most soil physicochemical properties, with stronger effects at higher CMC-NH4 rates. Compared to the control, mulching enhanced the abundance of genes involved in dissimilatory nitrite reduction (nirK) and nitrite ammonification (nirB, nirD), while reducing genes for ammonia oxidation (amoC) and nitrogen fixation (nifH). Proteobacteria and Actinobacteria were the dominant phyla associated with these functional shifts. Shifts in microbial community composition and N-cycling gene abundance were strongly associated with changes in multiple soil properties, particularly soil temperature, moisture, and the availability of potassium and nitrogen. Soil film mulching modifies the soil hydrothermal regime and nutrient availability, which collectively regulate microbial N-cycling functions by shifting functional gene abundance and community structure. These findings suggest that CMC-NH4 soil film amendment can be a viable practice for modulating soil N-cycling processes to improve nutrient management in dryland wheat systems.