INTRODUCTION
This study explores how differing irrigation regimes and organic amendments shape selenium (Se) behaviour in naturally Se‑rich paddy soils. By comparing continuous flooding (CF) with alternating wet‑dry (AWD) cycles and evaluating cotton‑straw biochar (BC) versus sheep manure (SM) at two dosage levels, the work seeks to clarify why Se sometimes remains locked in soil and how it can be mobilised for healthier rice production.
WATER‑MANAGEMENT STRATEGIES
Switching from CF to AWD proved pivotal: periodic drainage not only elevated root‑surface iron‑plaque formation but also boosted rhizospheric affinity for Se. AWD further hastened the shift from weakly organic‑bound forms toward soluble and exchangeable fractions, creating a more plant‑available Se pool without relying solely on chemical inputs.
ORGANIC AMENDMENTS AND RATES
Amendment chemistry mattered. A modest 10 g kg⁻¹ SM dose maximised Se bioavailability—especially under AWD—while BC repeatedly suppressed it. Manure’s nutrient cocktail and labile carbon likely spurred reductive processes that free Se, whereas BC’s high sorption capacity and pH buffering may have locked Se into less accessible complexes.
SELENIUM BIOAVAILABILITY DYNAMICS
Increases in Fe(II) and dissolved organic carbon (DOC) under SM applications promoted the dissolution of Se‑bearing compounds. AWD strengthened these effects by enhancing redox fluctuations that dissolve iron plaques, releasing adsorbed Se into soil solution where roots can take it up, thereby tying water management tightly to Se fate.
MICROBIAL COMMUNITY SHIFTS
Manure and flooding patterns reshaped bacterial assemblages: sulfur‑oxidising Thiobacillus and Se‑reducing Pseudarthrobacter showed strong positive correlations with bioavailable Se. These taxa likely mediate key redox transformations, underscoring that microbial‑driven chemistry—rather than simple geochemistry—governs Se mobilisation in paddy ecosystems.
IMPLICATIONS FOR AGRONOMIC PRACTICE
Pairing AWD irrigation with low‑rate sheep‑manure return offers a practical, eco‑friendly route to enrich rice Se content while minimising external fertiliser inputs. Understanding the synergistic roles of water regime, organic carbon supply, and microbially mediated redox cycling can guide precise nutrient‑fortification strategies for Se‑enriched rice without compromising soil health.
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