Mechanistic insights into sulphur-enriched biochar: Enhancing soil enzymatic activities, nutrient cycling, and early maize growth

Modern agriculture faces the dual challenge of enhancing crop productivity while minimizing environmental impacts, particularly due to inefficient phosphorus (P) fertilizer use. Sulphur-enriched biochar (SEB) may help address this by increasing P availability through soil acidification and microbial interactions. However, its combined effects on soil biogeochemistry, microbial function, and plant physiology remain poorly understood, reducing potential adoption by farmers. This study investigated how SEB application affects P fertilization regimes and influences soil nutrient availability, microbial biomass and enzyme activities, and early plant growth. We conducted a controlled 120-day experiment testing two P levels (full dose; P100 and reduced dose; P75), with soils amended with pristine biochar (PB) and two SEB formulations: low-sulphur biochar (SEB1) and high-sulphur biochar (SEB2), each at 1% and 2% (w/w). Maize plants were harvested after 30 days, allowing us to test longer-term impacts on soil alone. Our results showed that SEB2 significantly increased availability of sulphur (S), potassium (K), and P compared to the control (CK). SEB1 enhanced microbial biomass (C, N, and P) and stimulated selected enzyme activities related to nutrient release: alkaline phosphatase (ALP, 42.9%), N -acetylglucosaminidase (NAG, 92.4%), and leucine aminopeptidase (LAP, 58.4%) on day-120. These improvements promoted root growth (e.g., root surface area increased by 25%), photosynthetic efficiency (chlorophyll content +26.9%; quantum yield of photosystem II (PSII) + 45.4%), and plant biomass (dry weight + 60.9%), with more pronounced effects under reduced P fertilization. Structural equation modeling (SEM) revealed that integrated effect of P levels, biochar types and biochar levels significantly influenced nutrient availability (AN, AK, and AS), which led to improved P availability. Our findings demonstrate that SEB sustains early plant growth under reduced P inputs by mobilizing inherent soil nutrients and stimulating microbial nutrient cycling.