Long-term effects of combined application of chemical fertilizers and organic materials on maize yield stability and microbial functional potential and soil microbial community structure in alkaline soils

Aims Sustainable agriculture in China’s Northeast Black Soil Region faces a critical challenge: naturally alkaline soils with elevated pH restrict nutrient availability, impair microbial functions, and cause yield fluctuations under long-term chemical fertilization. Methods A five-year field experiment (2021–2025) in Anda, Heilongjiang compared conventional chemical fertilization alone (CK) with chemical fertilizer plus humic acid (HA) or chemical fertilizer plus composted chicken manure (JF), applied at recommended rates. The organic materials were additional to a common mineral fertilizer dose, so the treatments were not nutrient-balanced. Soil and sequencing samples were subsamples within single plots rather than independent replicates; results are therefore reported descriptively. Results Over the five years, the average yields under HA and JF treatments over five years were 12.47 t/hm² and 12.50 t/hm², respectively, representing increases of 45.90% and 46.27% relative to CK. Because the treatments were not nutrient-balanced and were not independently replicated, these differences are reported without inferential testing. For descriptive purposes, HA showed the steepest yield increase over time, whereas JF showed the lowest coefficient of variation. Both HA and JF showed slightly but consistently lower soil pH than CK, whereas bulk density, alkaline nitrogen, available phosphorus, available potassium, and organic carbon fractions were similar across treatments. Microbial community alpha diversity indices were similar across treatments; community composition differed only descriptively. HA was descriptively associated with higher relative abundance of Luteitalea and Nocardioides, and JF with Bradyrhizobium and Lysobacter. Conclusions The combined application of humic acid and chicken manure benefit to enhances the multi-year average yield of corn. We note that organic-amended treatments received additional N, P, and K inputs from organic sources; therefore yield differences cannot be definitively attributed to organic-material intrinsic properties, pH shifts, or microbial community changes alone. Soil pH was identified as an important correlate of microbial community differentiation across treatments.