Rhizosphere properties in salt marshes are shaped by both soil genetic horizons and halophyte species

Salt marshes are harsh ecosystems due to seawater intrusion that causes waterlogging and salt accumulation in soil. Halophytes are the primary species able to resist these stress conditions by impacting physicochemical and biological properties of the rhizosphere. This study investigated how properties of soil genetic horizons affect soil enzyme activities under two halophytes (Arthrocaulon macrostachyum and Juncus maritimus) and examined changes in bulk and rhizosphere soil properties under salt and hypoxia stress. Results indicated that organic matter was the primary driver of biological activity in the superficial horizon (Az), particularly under J. maritimus. This is supported by the elevated enzyme activity recorded under this halophyte, which corresponded with higher levels of total organic carbon and its fractions content. In contrast, A. macrostachyum mitigated salinity stress by reducing salt accumulation in the Az horizon compared to J. maritimus, showing a threefold decrease in the content of cations and anions. The anoxic conditions caused by the rising water table reduced biochemical processes in the bulk soil of Bgz horizon under both plant species compared to the rhizosphere. In response, plants enhanced potential enzyme activity in the rhizosphere (on average, three times higher than bulk soil), likely due to plant-soil gas exchange mechanisms. In contrast, the decrease in root abundance and coarser soil texture of the 2Bgz horizon regulated the chemical and biochemical behaviour of the horizon itself. Overall, halophyte adaptability to harsh conditions such as those in salt marshes is influenced by numerous factors, including a significant association with soil genetic horizon properties and plant species. Therefore, assessing soil by genetic horizons rather than fixed depths facilitates a more accurate understanding of tolerance mechanisms under field conditions.