Mangrove forests provide a suite of critical ecosystem services ranging from local to global scales.Soil conditions and associated micro-organisms play a fundamental role in maintaining these services, that include nutrient cycling, carbon sequestration and plant growth-promoting properties. Despite its importance, the microbial abundance and function of mangrove soil has received little effort in current research. On the ecotone between the land and the sea, mangrove soils are subject to high variability of geomorphological and ecological conditions that imply a strong distinction of ecological niches. These conditions can diversify the structure and function of the prevalent microbiome. To identify the structure of bacterial communities of mangrove soils, 16S rRNA gene sequencing techniques were applied on samples from arid Avicennia marina forests across different spatial scales: (i) at different depths of the soil profile(surface/subsurface); (ii) between two distinct zones within a given forest (seaward/landward); (iii) among forests with varying local hydrodynamic conditions (exposed/sheltered); and (iiii) among forests in different geographic regions (Saudi Arabia and Australia). This study found that the bacterial soil community varied more within each forest than between forests in different countries. Notably, differences between countries became more apparent at the finest taxonomic resolution (ASVs). Bacterial communities from the landward zone of the mangrove forest are more conserved across countries than those from the dynamic seaward zone. Theoretically assigned functions showed high levels of sulfate respiration and chemoheterotrophy as major metabolic pathways. Differences across local factors in the functional traits reflect the within forest variability. The distinct microbial assemblages from the landward zone were associated with high salinity and phosphorus, and nitrogen and larger grain size were associated with the seaward samples. Understanding patterns of microbial communities in mangrove soils is important to predict changes and mitigate anthropogenic pressures on the ecosystem.

The feasibility of predicting porewater salinity based on calibrated surface electromagnetic methods is discussed in a coastal wetland on the southern banks of Tampa Bay in West-Central Florida. This study utilizes a new method to float commercial land based electromagnetic (EM) instruments in shallow marine waters of less than 1.5 meters. The floating EM-31 (Geonics, Ltd.) effectively sensed the magnitude and lateral extent of high and low salinity porewaters within mangrove lined ditches and ponds. Resistivity and EM geophysical methods are merged with direct sampling data to calibrate layers in electromagnetic models to infer shallow (<30m) groundwater salinity patterns. Initial marine resistivity surveys are necessary to discriminate between equivalent EM model solutions for seafloor conductivities beneath shallow (0.1-1.5m) marine (~30 ppt) waters. Using formation factors computed from nearby resistivity surveys, porewater conductivity predictions based on surface EM-31 and EM-34 measurements are successful at distinguishing overall porewater salinity trends. At the Tampa Bay study site, the most distinctive terrain conductivity anomalies are associated with mangroves bordering marine waters. Highly elevated porewater conductivities are found within 5m of the mangrove trunks, falling sharply off within 10m, presumably due to saltwater exclusion by mangrove roots. Modeling indicates the shallow water EM-31 measurements probably lack the resolution necessary to image more subtle porewater conductivity variations, such as those expected in association with diffuse submarine groundwater discharge. However, the technique has potential application for locating high contrast zones of freshwater discharge and other salinity anomalies in shallow and nearshore areas not accessible to conventional marine resistivity or land-based arrays, and hence may be useful for interdisciplinary studies of coastal wetland ecosystems.