Low Denitrification, Anammox, and Dissimilatory Nitrate Reduction to Ammonium Rates in a Low Disturbance Mangrove‐Saltmarsh System Using an in Situ Intact Chamber Method

Abstract

Mangroves and saltmarshes can remove anthropogenic nitrogen (N) moving from the land to the ocean, but many previous N cycling measurements used destructive sampling procedures that excluded important habitats such as crab burrows, pneumatophores, and saltmarsh vegetation. In this study, a new intact chamber method using ¹⁵ NO ₃ ⁻ labeling was developed to quantify in situ denitrification, anammox, and DNRA rates in different habitats across a low disturbance mangrove‐saltmarsh system. No anammox was detected, likely due to the low nutrient concentrations. Denitrification rates were 0.4–36 times higher than DNRA rates. The highest denitrification rates were measured in crab burrows (36.4 μmol N m ⁻²  h ⁻¹ ) and saltmarsh (26.6 μmol N m ⁻²  h ⁻¹ ), while DNRA rates were highest in saltmarsh (7.4 μmol N m ⁻²  h ⁻¹ ) and pneumatophores (2.1 μmol N m ⁻²  h ⁻¹ ). Rates were scaled using a detailed habitat map, giving a total of 17.1 kg N ha ⁻¹  yr ⁻¹ being removed via denitrification in the mangrove system, 17.0 in the saltmarsh system, 17.5 in the open water system, and 17.1 in the mangrove‐saltmarsh‐open water system. Furthermore, 2.5 kg N ha ⁻¹  yr ⁻¹ was recycled by DNRA in the mangrove system, 2.9 in the saltmarsh system, 1.9 in the open water system, and 2.5 in the saltmarsh‐mangrove‐open water system. The low denitrification and DNRA rates compared to previous studies likely reflect minimal anthropogenic disturbance and associated low nitrogen availability at the study site, but this needs to be confirmed with replicated measurements using the new intact chamber method at a high nutrient concentration mangrove‐saltmarsh site.