Mangrove soils have a deficient nutrient concentration (Lovelock et al. 2005), and the concentration varies among mangroves of the world and the dominant types (Feller et al. 2003; Hossain and Nuruddin, 2016). The nutrient availability in mangroves is controlled by different factors such as salinity, soil type, tidal inundation, elevation in the tidal frame, etc. (Reef et al. 2010). In comprising of other studies (e.g. 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). In monsoon time, nutrients become more available in the soil because of the influence of the nutrient input and output sources (Alongi et al. 1992; Mahmood 2004) and lower soil salinity due to higher flow of freshwater flow (Mahmood 2004). Therefore, plants do not need more nutrients to reabsorb with more nutrients in the soil (Feller et al. 1999). After the onset of the monsoon period, the leaching and decomposition afterward help to build up the soil nutrient reserves (Mahmood et al. 2014b). Soil nutrient status during the growth phases supports our findings, as nutrient concentrations remain high during the post-monsoon season, indicating that nutrients have been taken up by the plants, resulting in nutrient deficiency. Drought and increased evaporation (Naskar and Bakshi 1987) combined with reduces flow of freshwater (Siddiqi 2001) influence the saline intrusion (Basar 2012). This situation results in increased soil salinity during the pre-monsoon season, which reduces the availability of nutrients (Saenger 2002; Singh et al. 2005; Alam et al. 2018).
The efficiency of nutrient uptake and utilization varies between species and stages of life form due to nutrient availability, leaf longevity, leaf type, and the nature of nutrient pools (Lovelock and Feller 2003; Mahmood 2004). This could be the reason for observing variation in leaf nutrient composition and nutrient resorption efficiency among the studied species and seasons of the present study. Generally, plants resorb about 50% of N and 52% of P (Aerts and Chapin 2000). Our current study found resorption efficiency of N (76.94–14.15% and 63.33–31.76%) and P (44.44–34.33% and 53.33–16.24%) for H. fomes and E. agallocha, respectively. McKee and Faulkner (2000) claimed that mangroves resorb between 20% and 60% of N and 30–70% of P, that depends on the nutrients availability in soil. Wang et al. (2003) also found comparatively higher resorption efficiency (72.22 and 57.53%, respectively, for N and P) for Kandellia candel. It indicates that many N and P were retranslocated out of the senescing leaves for reuse, though mangrove plants exchange leaves frequently to eliminate excess salt accumulation in their bodies.
In the case of P resorption efficiency, the studied species were comparatively lower efficiency than the study of Aerts and Chapin (2000). Comparatively higher P concentration in the soil of the study area with similar concentration throughout the seasons may influence the lower resorption efficiency for H. fomes and E. agallocha. The resorption efficiency in K may be overestimated, as it is a readily leachable element. Alam et al. (2018) and Noor et al. (2015) observed a higher resorption efficiency of K for Avicennia officinalis and Rhizophora spp. Higher K resorption efficiency was observed during the monsoon than in other seasons. The readily leaching nature of K from the leaf litter (Mahmood et al. 2014b) may influence the higher resorption during the monsoon season. Resorption has been considered the most important nutrient conservation strategy for plants in nutrient-poor or stressed sites or environments (Hörtensteiner and Feller 2002). However, environmental factors have resulted in similar adaptations among mangrove species in various habitats, such as water relations (Macinnis-Ng et al. 2004) and architecture, which may explain why the species have varying levels of nutrient resorption efficiency. Additionally, the capacity for salt tolerance varies among the studied species. The different nutrient resorption efficiency between species may be due to species convergence, which is unique to each species.