Aquaculture wastewater may weaken mangroves’ iron-based carbon storage and shift nitrogen cycling toward recycling

Mangrove soils store “blue carbon” and help clean coastal waters by processing excess nitrogen. Some carbon is preserved when organic matter binds to iron minerals, forming a protective shield against microbial decomposition. However, wastewater from nearby aquaculture ponds may disrupt both functions.

A study published in Forest Ecosystems examined mangroves in the Maowei Sea reserve in China’s Beibu Gulf. Researchers from East China Normal University compared three sites representing different levels of aquaculture influence: a restored reference site where nearby pond discharge had stopped about nine years earlier, and two sites beside active pond systems. They measured soil and porewater chemistry and examined microbial genes involved in carbon, nitrogen and iron cycling.

At the most heavily affected site, porewater DOC was about 99% lower than at the restored site. Although aquaculture wastewater brings nutrients and organic matter, nutrient enrichment appeared to stimulate microbial consumption of available carbon.

This enhanced carbon consumption was associated with, and likely contributed to, more reducing soil conditions, weakening the coupling between iron minerals and organic carbon. Around mangrove roots, the relatively persistent pool of organic carbon associated with crystalline iron minerals was 67% to 76% smaller at the disturbed sites. Carbon associated with a more reactive form of iron increased at the most affected site, suggesting a shift from long-term mineral protection to a less stable carbon cycle.

Aquaculture influence also changed how microbes processed nitrogen. Some microbes remove nitrate by converting it into gases through denitrification. Others use a pathway called DNRA (dissimilatory nitrate reduction to ammonium), which converts nitrate into ammonium and keeps nitrogen within the ecosystem. The microbial evidence suggested that both pathways became active together under aquaculture stress. Greater ammonium retention could make the mangrove a stronger nitrogen recycler, rather than a system that permanently removes excess nitrogen from coastal waters.

The researchers also detected a marked transition in nitrogen cycling around a dissolved organic carbon concentration of 32 mg·L^–1. While this value may serve as a potential site-specific indicator of ecosystem change, further studies are needed before it can be applied as a general management threshold.

The study shows how aquaculture discharge can affect two connected mangrove services: durable carbon storage and coastal nitrogen regulation. Reducing nutrient inputs and tracking carbon, iron and microbial activity could help guide efforts to protect or restore these functions.

DOI Link:

https://doi.org/10.1016/j.fecs.2026.100488