Dual effects of tidal stress on carbon turnover by biochar incorporation in estuarine wetlands: Obscuring the promotion of plant CO2 fixation while magnifying carbon stabilization of sediments.

Estuarine wetlands, crucial in mitigating global warming, have suffered significant degradation, necessitating innovative restoration techniques to enhance their carbon sink capacity. This study investigated the dual effects of biochar incorporation in estuarine wetlands with tidal stress, utilizing a field experiment in the Chongming Dongtan wetland and a meta-analysis for cross-study comparison. Results indicated that while biochar incorporation slightly obscured the promotion of plant CO2 fixation (with a 1.24% decrease), it still augmented carbon sink capacity by magnifying the inhibition of organic carbon (OC) mineralization, evidenced by a 39.74% reduction in sediment respiration (SR) rate compared to control. Conversely, straw incorporation facilitated plant CO2 fixation (with a 25.07% increase) but also raised SR by 53.33% compared to biochar, offering a less pronounced carbon sink enhancement. Unlike terrestrial ecosystems, biochar under tidal influences did not protect soil NH4+-N from loss nor reduce bulk density, which were proved to be primary drivers of plant growth and SR by redundancy analysis. The inputs of recalcitrant carbon substrates, along with nitrogen and mass transfer limitations in sediments caused by biochar under tidal conditions, significantly inhibited microbial-driven OC mineralization, thereby compensating for the weaker plant CO2 fixation and ultimately enhancing carbon sink. The OC mineralization suppressed by biochar with tidal stress was attributed to anaerobic microbial taxa enrichment, particularly Chloroflexota, indicating a distinct microbial response. Our findings emphasized the necessity for habitat-specific biochar strategies, highlighting its potential to boost the carbon sink capacity of estuarine wetlands through pathways and patterns distinct from those in agricultural fields.