Aquaculture stress destabilizes iron–organic carbon coupling and alters nitrogen pathways in mangrove soils

Mangrove carbon sequestration and nitrogen removal processes are threatened by coastal aquaculture, but the underlying biogeochemical mechanisms remain poorly understood. Using geochemical profiling, metagenomics, and structural equation modeling, we examined aquaculture effects on nitrogen cycling, porewater chemistry, and iron-bound organic carbon (Fe–OC) coupling in China's Beibu Gulf mangroves. Aquaculture caused ~99% loss in dissolved organic carbon (DOC), creating a “eutrophication paradox” where nutrient inputs triggered microbial respiration that consumed DOC faster than it could be replenished. This DOC depletion acted as the primary cascade trigger, showing power-law correlations with NH₄⁺/NO₃⁻ ratios (R² = 0.680) and revealing a critical threshold at ~32 mg·L⁻¹. Below this point, redox potential (Eh) collapsed (dropped 77.1%), destabilizing crystalline Fe–OC pools and promoting dynamic microbially-mediated complexation. Stable crystalline iron-bound organic carbon (OC) declined 76% while labile organically complexed iron-bound OC increased, representing a shift from long-term geochemical carbon stabilization to short-term biological carbon stabilization. Notably, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) exhibited seemingly synergistic rather than competitive interactions (Spearman's ρ = 0.503, p = 0.036), with DNRA positively influencing nitrogen removal efficiency (β = 0.50, p = 0.035). This cooperation transforms mangroves from nitrogen “sinks” to “internal nitrogen recyclers”, retaining bioavailable NH₄⁺ rather than permanently removing it. Enhanced carbon–nitrogen–phosphorus (C–N–P) cycle coupling (r > 0.95) at disturbed sites suggests ecological instability under aquaculture stress. Our results demonstrate that aquaculture fundamentally alters mangrove biogeochemistry through DOC-mediated cascades, providing functional gene markers and quantitative thresholds for ecosystem management while highlighting the critical importance of maintaining DOC dynamics in mangrove conservation strategies.