Granular Mg-enriched biochar-based compound fertilizer: Enhanced slow-release performance and effects on maize growth under reduced application rates

Mg-enriched biochar serves as an ideal carrier for biochar-based fertilizers due to its excellent slow-release nutrient properties and soil remediation potential. Nevertheless, the influence of granulation techniques on its properties and the effects of biochar-based fertilizer application rates on crop yields remain inadequately investigated. This study aimed to develop a granular biochar-based organic-inorganic compound fertilizer (GBCF) using Mg-enriched biochar derived from wheat straw as the carrier and evaluate its efficacy in enhancing maize growth, yield, and nutrient use efficiency (NUE) under reduced nitrogen rates. The results demonstrated that GBCF produced via a two-step granulation process exhibited significantly improved nutrient retention compared to both conventional granular chemical fertilizer (GCF) and one-step granulated biochar fertilizer (GBCF-M). Specifically, the 48-hour nutrient release rates for GBCF were 50.08 % (N) and 28.44 % (P), substantially lower than those of GCF (82.43 % N, 78.08 % P) and GBCF-M (60.63 % N, 42.86 % P). Remarkably, GBCF maintained maize yield equivalent to that of GCF even with a 20 % reduction in nitrogen application, while simultaneously enhancing root volume by 9.8 % and improving NUE by 23.11 %. These improvements were mainly attributed to the sustained nutrient supply, promoted soil enzyme activities, and optimized functional fungal communities resulting from GBCF application. GCF provided a short-term nutrient pulse, yielding higher Shannon diversity than GBCF, while GBCF released nutrients steadily, promoting dominance of adaptive fungal communities; Neocosmospora and key OTUs were enriched under GBCF with N reduction. Co-occurrence network analysis showed that GBCF with N reduction resulted in more stable and cooperative fungal communities than GCF. The findings provide theoretical and practical support for the commercial adoption of biochar-based fertilizers in sustainable agricultural systems. • Two-step granulation enhanced N/P slow-release of biochar-based fertilizer (GBCF). • GBCF with 20 % less N than chemical fertilizer (GCF) kept equivalent maize growth. • GBCF (-20 % N) markedly enhanced maize N uptake & yielded a 23.11 % higher NUE than GCF. • GBCF enhanced soil enzyme activities and nutrient availability. • GBCF increased fungal diversity and enhanced community stability.