Seasonal regulation of selenium biofortification in tea through Se-rich organic fertilizer application

Selenium (Se) deficiency affects approximately one billion people globally, and Se biofortification of tea represents a promising strategy to enhance dietary Se intake while improving tea quality; however, the seasonal dynamics and dose-dependent effects of Se-enriched organic fertilizers in perennial tea systems remain poorly understood. This study evaluated the impacts of three Se-enriched organic fertilizer application rates on soil properties, Se accumulation, enzymatic activities, and tea quality across spring, summer, and autumn harvests in a subtropical tea plantation in Guangxi, China. Soil available Se followed a consistent dose hierarchy (T3 > T2 > T1 > CK) and declined progressively across seasons, yet T3 demonstrated superior retention, declining only 29.1% from spring to autumn versus 53.3% for CK, sustaining autumn soil Se at 0.0559 mg kg^-1 (+175.4% over CK). Leaf Se content increased 2.04-3.54-fold over CK across seasons, with T2 achieving the greatest autumn enhancement (0.6660 mg kg^-1). Soil enzyme responses were highly season-specific: summer urease activity under T2 collapsed by 55.3% relative to CK before reversing to a +174.7% peak in autumn, while summer invertase was dose-dependently suppressed (up to -45.8% under T3) and acid phosphatase was uniformly elevated (up to +34.1%). In tea leaves, high-dose Se (T2/T3) elevated spring GSH-Px activity ~94% above CK, while autumn CAT activity increased 7.3-fold under T2. Se treatments protected summer chlorophyll from stress-induced degradation, raising total chlorophyll by up to 18.8%. Regarding tea quality, T3 most effectively improved summer tea by reducing the polyphenol-to-amino acid (P/A) ratio by 22.6% and increasing free amino acids by 20.9%, while T2 was optimal for autumn tea, maximizing water extractive content at 37.53% (+12.6%) and free amino acids at 3.03% (+9.8%). These findings establish season-specific optimal application rates for simultaneous dietary Se enrichment and tea quality improvement, providing an evidence base for commercial Se biofortification recommendations.