Effects of maize straw-derived biochar on soil nutrient dynamics and nematode community structure in upland yellow soil

Biochar, a carbon-rich soil amendment, has considerable potential to enhance soil fertility, promote carbon sequestration, mitigate greenhouse gas emissions, and support sustainable agricultural development. However, in upland acidic yellow soil regions, the responses of soil nematode community structure, functional composition, and ecological indices to biochar application remain poorly understood, particularly at the field scale and in terms of dose–response relationships. Based on a three-year field experiment with five biochar application rates (0, 5, 10, 20, and 50 t·ha⁻¹), this study examined changes in soil physicochemical properties and enzyme activities, characterized shifts in nematode community composition, structure, and ecological indices, and further explored the dose-dependent effects of biochar on nematode communities and the underlying mechanisms mediated by soil properties. The results showed that biochar application significantly increased soil pH, electrical conductivity (EC), soil organic carbon (SOC), available nitrogen (AN), available phosphorus (AP), and urease activity (URE), with mean increases exceeding 10% across all variables. Soil enzyme activities and certain nutrient parameters exhibited nonlinear responses, with peak values observed at the 10 t·ha⁻¹ application rate. Biochar markedly increased the relative abundance of bacterivorous nematodes (by 19.8%–35.2%) while decreasing the abundance of omnivorous–predatory nematodes (by 18.2%–31.9%). Moreover, herbivorous nematodes tended to decrease under the 10 t·ha⁻¹ biochar treatment. Biochar application did not have a statistically significant effect on nematode diversity indices; however, both the Wasilewska index and the Basal index reached their highest values at the 10 t·ha⁻¹ treatment. Redundancy analysis and path analysis revealed that SOC, AN, AP, AK, and urease activity were key factors driving shifts in nematode trophic structure. Biochar indirectly regulated the nematode community by modifying these soil properties rather than exerting direct effects. In conclusion, biochar applied at 10 t·ha⁻¹ was most effective in enhancing upland yellow soil fertility and optimizing nematode community structure and function.