Effects of magnetite- and attapulgite-modified biochar with organic fertiliser: mineral-driven microbial mechanisms regulating the nitrogen cycle

The mismatch between nutrient release from organic fertilisers and crop nitrogen demand often results in low nitrogen use efficiency and substantial nitrogen losses. To address this issue, a pot experiment was conducted to assess the effects of co-applying magnetite or attapulgite modified biochar with organic fertiliser on soil nitrogen transformation, microbial community structure, and rapeseed growth. The results showed that both mineral-modified biochar composites effectively improved soil nitrogen dynamics. The 5% magnetite modified biochar treatment performed best, increasing rapeseed yield to 1200 kg·ha−1, along with superior plant height, root length, and chlorophyll content. Soil analysis revealed that magnetite enhanced the retention of ammonium and nitrate, whereas attapulgite promoted adsorption and delayed nitrogen release through its porous structure. Microbial analysis showed that magnetite increased the abundance of Bacillota and upregulated ammonia-oxidising bacterial (AOB) genes, thereby enhancing nitrification. In contrast, attapulgite modified biochar regulated the abundance of Pseudomonadota and upregulated the nosZ gene, thereby optimising denitrification and mitigating nitrogen losses. This study provides theoretical and practical insights into the use of mineral modified biochar composites as nitrogen enhancers for sustainable agriculture.