Mangroves are unique evergreen vegetation found along the coastlines of 124 tropical and subtropical countries. These species possess specialized adaptations to cope with challenging environmental conditions, including salinity, anaerobic soils from tidal inundation, unstable muddy substrates, nutrient-poor soils, and high levels of temperature, wind, and solar radiation. However, the degree of adaptation varies among species. Mangrove ecosystems provide a wide range of ecosystem services (provisioning, regulating, cultural, and supporting) at local, regional, and global scales ( Tomlinson, 1986 ; Mahmood et al., 2008 ; Ribeiro et al., 2019 ). Bangladesh, as a coastal nation, contains approximately 4% of the world’s total mangrove forest area, including the Sundarbans, the largest contiguous natural mangrove forest in the world. The Sundarbans is rich in floral and faunal diversity. This forest has three distinct salinity zones [e.g. Oligohaline/Less-saline zones (<2 dS m⁻¹) (0.5 - 5 ppt), Mesohaline/Moderately saltwater (2 - 4 dS m⁻¹) (5 - 18 ppt), Polyhaline/Saltwater (>4 dS m⁻¹) (18 - 30 ppt)] ( Siddiqi, 2001 ; Ahmed et al., 2011 ; Mahmood 2015 ). Heritiera fomes, Excoecaria agallocha, Sonneratia apetala, Bruguiera sexangula, Avicennia officinalis and Ceriops decandra are common species. Heritiera fomes and E. agallocha constitute the major vegetation types of the Sundarbans ( Mahmood, 2015 ). Heritiera fomes is believed to be a climax species of this forest. The species composition and their dominance in different zones of the Sundarbans found to influenced by the salinity ( Ahmed et al., 2011 ). The species dominance, abundance, frequency, density, canopy cover, volume, and biomass of the major species of the Sundarbans have changed since 1960 ( Siddiqi, 2001 ). Many natural and anthropogenic issues are believed to be responsible for the vegetation changes ( Mahmood et al., 2021 ). Increased soil and water salinity of the Sundarbans habitat are major concerns for the changes in species composition ( Ahmed et al., 2022 ), survival, and growth ( Mahmood et al., 2014a ; Nasrin et al., 2016 ; Chowdhury et al., 2019 ). The water and soil salinity of the Sundarbans found to vary with seasons and is believed to increase with time due to the decreasing flow of freshwater from upstream and climate change-induced salinity ( Siddiqi, 2001 ). This increased salinity poses stresses to the mangrove species by limiting the uptake of nutrients ( Alongi, 2009 ; Reef et al., 2010 ; Nasrin et al., 2016 ; Siddique et al., 2017 ).

Mangroves adopt various mechanisms to conserve and reuse nutrients for survival and growth in high-saline environments ( Tomlinson, 1986 ; Mahmood et al., 2014a ; Nasrin et al., 2019 ). Nutrient levels in soil and plant parts of a forest ecosystem fluctuate with species, plant growth form, species-specific development capacity, geomorphological settings, and climate variabilities ( Mahmood, 2004 ; Vergutz et al., 2012 ; Limon et al., 2025 ). Mangrove plants usually shed their older leaves and bark to eliminate excessive accumulated salt from their tissues ( Tomlinson , 1986 ; Acosta-Motos et al., 2017 ). Mangrove plants have a high rate of litterfall and are mostly washed by the tidewater ( Mahmood et al., 2005 ), which may result in poor nutrient status in mangrove soil ( Mahmood, 2004 ). The nutrient resorption mechanism in mangroves may be an important adaptation tool to cope with saline and nutrient-poor soil ( Saenger, 2002 ; Hu et al., 2023 ), and the nutrient resorption mechanism and efficiency of mangroves vary with species and sites ( Alongi, 2009 ; Alam et al., 2019 ; Nasrin et al., 2019 ). It is reported that this adaptation (nutrient resorption efficiency) is not only related to the salinity but also to the nutrient level in the soil and element mobility ( Feller et al., 2003 ; Alongi, 2009 ; Blanco et al., 2009 ; Acosta-Motos et al., 2017 ). The variation in nutrient resorption efficiency will help to understand the nutrient conservation strategy and adaptation at the species level in a stress condition. H. fomes is a less salt-tolerant species than the E. agallocha in the Sundarbans ( Siddiqi, 2001 ). It is reported that the vegetation coverage by H. fomes in the Sundarbans has decreased over time and more salt-tolerant species like E. agallocha and C. decandra are taking over ( Chowdhury et al., 2019 ; Ahmed et al., 2022 ). Moreover, a declining growth rate was observed for H. fomes in the Sundarbans ( Siddiqi, 2001 ).

We hypothesized that the recent increasing abundance of E. agallocha in the Sundarbans may be linked to its superior nutrient resorption efficiency and greater salt tolerance than H. fomes. These traits will allow E. agallocha to thrive in increasingly saline and nutrient-poor conditions, enabling it to outcompete H. fomes, which exhibits lower resorption efficiency and salt tolerance. The physiological advantage of E. agallocha will contribute to shifts in species dominance over time, particularly in response to rising salinity levels and changing nutrient dynamics in the Sundarbans ecosystem. Therefore, the present study aimed to evaluate i) the seasonal variation in nutrients (N, P, and K) concentration in soil, mature and senescent leaves of H. fomes and E. agallocha; ii) nutrients (N, P, and K) resorption efficiency for H. fomes and E. agallocha; iii) the relationship among the soil parameters (N, P, K, and EC) and nutrient resorption efficiency of H. fomes and E. agallocha.

Nutrients are limiting in mangrove soil ( Lovelock et al., 2005 ), and the concentration varies among mangroves of the world and the dominant types ( Feller et al., 2003 ). Compared to the studies of Mahmood (2004) and Hossain and Nuruddin (2016) , the present study site contained relatively lower N and higher P in soil. The variation in soil nutrients among the mangroves and seasons is highly related to the natural process of weathering, environmental conditions, and nutrient flux in the mangroves ( Feller et al., 2003 ; Mahmood, 2004 ; Reef et al., 2010 ). In the monsoon, nutrients become more available in the soil because of the influence of nutrient input and output sources ( Alongi et al., 1992 ; Mahmood, 2004 ). Conversely, higher litter export during the monsoon may reduce the organic matter and nutrient levels in mangroves ( Mahmood et al., 2005 ; Mahmood & Hoque, 2008 ). These consequences may control the fluctuation of soil nutrient levels in the mangroves. Our study found no significant variation in available soil nitrogen, phosphorus, and potassium concentrations across different seasons. This suggests that seasonal changes have minimal influence on soil nutrient availability in the studied mangrove environment. Such stability in soil nutrient levels could be attributed to the buffering capacity of mangrove soils, which are often rich in organic matter and maintain relatively consistent biogeochemical conditions despite external environmental fluctuations. Similar observations were reported by Alongi et al. (2000) , who noted that nutrient concentrations in mangrove soils tend to remain stable due to slow nutrient turnover and strong retention capacity. Additionally, Krishna and Mohan (2017) highlighted that mangrove ecosystems, particularly in intertidal zones, experience limited seasonal shifts in soil nutrient levels due to reduced leaching and continuous input from tidal exchanges and litterfall decomposition.

Nutrient concentration in leaves of mangroves varied with species, age of leaves, sites, seasons, and salinity ( Mahmood, 2004 ; Alongi, 2009 ). High saline conditions limit the availability of nutrients to mangroves ( Saenger, 2002 ; Singh et al., 2005 ; Alam et al., 2018 ). The present study showed that nutrient composition in green mature and senescent leaves of H. fomes responds to seasons or salinity. Mahmood et al. (2014a) observed a similar trend of decreasing nitrogen, phosphorus, and potassium concentrations in H. fomes seedlings as salinity increased. On the other hand, E. agallocha showed less seasonal variation in nitrogen and phosphorus levels in both mature green and senescent leaves. Nutrient composition in green leaves was found to vary with the species even if they are growing at the same site. The ability of mangroves to take up nutrients in response to salinity varies greatly between species (e.g., Reef et al., 2010 ; Mahmood et al., 2014a ; Alam et al., 2019 ; Nasrin et al., 2019, 2021 ).

McKee and Faulkner (2000) claimed that mangroves resorb between 20 and 60% of N and 30 and 70% of Wang et al. (2003) found comparatively higher resorption efficiency of N (72%) and P (58%) for Kandellia candel. The range of the resorption efficiency of N, P, and K in the present study was 14 to 76%, 26 to 48%, and 28 to 68%, respectively. Mangrove species exhibit varying ranges of nutrient resorption efficiency depending on their physiological and ecological adaptations, which are very much species- and site-specific ( Feller et al., 1999, 2003 ; Reef et al., 2010 ). In the case of P resorption efficiency, the studied species showed lower efficiency than the study of McKee and Faulkner (2000) and Wang et al. (2003) . Comparatively higher and similar available concentrations of P in soil throughout the seasons may influence the lower resorption efficiency for H. fomes and E. agallocha. This statement is supported by the findings of Feller et al. (2003) . They observed higher P resorption efficiency for mangroves growing in highly saline conditions and soil with poor P content. The readily leaching nature of K from the leaf ( Mahmood et al., 2014b ) may influence the higher resorption during the monsoon. Alam et al. (2018) and Noor et al. (2015) observed a higher resorption efficiency of K for Avicennia officinalis and Rhizophora spp. during the monsoon. Higher nutrient resorption efficiency reflects the adaptive strategies of mangroves to maintain productivity and support ecosystem resilience in nutrient-limiting and saline conditions ( Reef et al., 2010 ).

The seasonal fluctuation of available concentrations of nutrients and salinity levels strongly controls the nutrient resorption efficiency of mangrove plants. Even the same mangrove species in different geomorphological settings usually show different ranges of nutrient resorption efficiency ( McKee & Faulkner, 2000 ; Lovelock & Feller, 2003 ; Reef et al., 2010 ). The present study demonstrated that H. fomes prioritizes nitrogen and phosphorus resorption in relation to salinity. Nitrogen resorption efficiency increases as mangroves prioritize nutrient conservation in saline conditions ( Nasrin et al., 2019 ). Soil salinity plays a critical role in nutrient resorption strategies ( Alongi, 2009 ), nutrient mobility, and ionic balance ( Krauss et al., 2008 ). In the case of E. agallocha, the negative correlation between soil salinity and P resorption efficiency explains the luxury consumption hypothesis-plants retain less phosphorus when it is abundant ( Aerts, 1996 ). Here, the negative correlation between soil salinity (measured as EC) and phosphorus resorption efficiency may reflect a context of sufficient phosphorus availability, where the species exhibits lower resorption in line with the luxury consumption hypothesis, where plants resorb less of a nutrient when it is sufficiently available in the environment, regardless of salinity stress. While the positive correlation between salinity and P resorption efficiency of H. fomes indicates that phosphorus resorption efficiency can be species-specific, which demands further study. The ionic competition between potassium and sodium can result in a negative relation for K resorption efficiency and salinity ( Reef et al., 2010 ). The weak correlations between soil nutrients and nutrient resorption efficiency suggest that soil nutrient levels do not strongly affect resorption in H. fomes and E. agallocha. This matches our finding that soil N, P, and K did not change much across seasons. Salinity is known to affect nutrient uptake in mangroves ( Alongi, 2009 ; Reef et al., 2010 ). However, the salinity at our study site was relatively low and showed little change (0.96 - 5.03 mS/cm), and this range falls within the lower salinity level of the less saline zone of the Sundarbans ( Siddiqi, 2001 ). The low variation in salinity and relatively stable nutrient concentration likely control resorption efficiency through species-specific traits. These may include nutrient storage, selective ion use, and internal recycling ( Krauss et al., 2008 ). The species-specific differences in nutrient resorption strategies, particularly the superior efficiency of E. agallocha, may lead to long-term shifts in mangrove species composition under increasing salinity conditions in the Sundarbans. These findings underscore the need to integrate ecological understanding into mangrove management, particularly in the context of natural forest resilience and biodiversity conservation. The management actions need to focus on monitoring species dynamics in the Sundarbans and evaluating the salinity thresholds for vulnerable species like H. fomes.

Heritiera fomes and Excoecaria agallocha are two key species of the Sundarbans, exhibiting distinct nutrient conservation strategies. This study reveals significant differences in nutrient resorption efficiency for nitrogen, phosphorus, and potassium across species, seasons, and varying soil salinity. Excoecaria agallocha consistently showed higher nitrogen resorption efficiency during the monsoon season and higher potassium resorption efficiency in the post-monsoon season, showing its strong ability to adapt to changing conditions. For E. agallocha, the negative link between soil salinity and phosphorus resorption efficiency supports the luxury consumption idea—the plant holds on to less phosphorus when there is already some available in the soil. These findings suggest that E. agallocha is robust in salinity tolerance, which may facilitate its continued expansion and potentially reshape species dynamics within the Sundarbans ecosystem. Such a shift may alter forest structure, reduce canopy complexity, and negatively impact existing biodiversity. However, this study was limited to two species within a single salinity zone of the Sundarbans. Broader spatial coverage and inclusion of additional species and related parameters are recommended for future research.

The authors acknowledge the Nutrient Dynamics Laboratory, Khulna University, and the Bangladesh Forest Department for their laboratory and logistical support.