Occurrence of seasonal water replacement in mangrove soil and the trunk growth response of Avicennia alba related to salinity changes in a tropical monsoon climate

Abstract

Salinity is a crucial factor regulating mangrove growth. We evaluated seasonal variations in soil water salinity and the water replacement process in a tropical monsoon mangrove forest of eastern Thailand during 2015–2018. Trunk growth of Avicennia alba was monitored monthly using dendrometer bands and was analyzed in relation to water replacement. Soil water salinity showed remarkable seasonal variation that was influenced by the infiltration of inundated water from the river, with a salinity level similar to that of seawater during the middle of the dry season and to that of fresh water during the middle of the rainy season. Patterns of soil water salinity shifted seasonally in both horizontal and vertical distributions, highlighting soil water replacement between the two seasons. In the middle of the rainy season, soil water salinity across the horizontal distribution was nearly fresh at most sampling points on the river side but gradually increased landward along a 120‐m transect. The vertical distribution of soil water salinity showed relatively low salinity at the surface horizon, which gradually increased downward to a 100‐cm depth. In the dry season, the soil salinity gradient in both distributions was opposite to that in the rainy season. This seasonal change in soil water salinity was significantly related to the trunk growth of A. alba, which increased greatly during the rainy season. Seasonal water replacement causes fluctuations in soil salinity and probably nutrient availability. The combination of low soil water salinity and large nutrient influx might enhance trunk growth during the rainy season.

Daily inundation induced seasonal variation in the vertical distribution of soil water salinity in an estuarine mangrove forest under a tropical monsoon climate

“…Mangroves utilize both freshwater and seawater for growth under natural conditions (Santini, Reef, Lockington, & Lovelock, 2015; Sternberg & Swart, 1987). Trunk growth of mangrove trees was accelerated in the rainy season when freshwater was abundant (Komiyama et al, 2019; Robert et al, 2014; Santini et al, 2015). In catchment basins, the largest source of freshwater is rainfall.…”

“…In such estuarine areas under monsoon climates, mangrove soils often show different levels of salinity between the dry and rainy seasons (for Florida, see Chen & Twilley, 1999; for French Guiana, see Marchand, Baltzer, Lallier–Vergés, & Albéric, 2004; for Bangladesh, see Chowdhury, De Ridder, & Beeckman, 2016). The relatively low salinity during the rainy season is due to freshwater discharge from rivers and rainfall (for Kenya, see Robert, Schmitz, Kirauni, & Koedam, 2009; for Thailand, see Komiyama et al, 2019). At this time, the frequency of inundation is a significant factor controlling soil salinity (for Australia, see Boto & Wellington, 1984; for Florida, see Chen & Twilley, 1999; for Mozambique, see Hogaune, Hill, Simpson, & Bowers, 1999; for Brazil, see da Cruz et al, 2013).…”

Organic Carbon Stabilization Mechanisms in Mangrove Soils: A Review

“…The salinity-induced immobilization of mangrove OM almost reached its upper limit at salinity of around one-tenth of that of seawater [9]. On the other hand, high salinity can be a stress to mangroves and decrease its productivity [52]. Saline soils do not necessarily have favorable conditions for mangroves, although they may have suitable conditions to store OM from a geochemical viewpoint.…”

“…Desorbed OM is susceptible to microbial degradation and eventually mineralized [38]. As mangrove soils experience daily and seasonal fluctuations in salinity [52], sorption-desorption properties of OM may be critical for its stability and warrant in-situ investigation.…”

“…Hence, the role that salinity plays on C stabilization may be more important and spatiotemporally dynamic in vegetated coastal ecosystems including mangroves than in marine sediments. Although information on the spatiotemporal variation in salinity in mangrove soils is scarce, Komiyama et al recently observed that soil water salinity (up to 1 m depth) showed remarkable seasonal variation with salinity similar to that of seawater during the dry season and to that of freshwater during the rainy season in Trat mangrove forest, Thailand [52]. This fluctuation in salinity should affect soil C stabilization, for instance through dissolution and loss of OM under low salinity [79].…”