Green leaf nutrients, not soil nutrients, mediate trace element resorption in mangrove plants across wet and dry seasons

Leaf nutrient resorption represents a vital nutrient conservation strategy for plants. While trace element resorption patterns have been extensively studied in upland terrestrial plants, they remain poorly characterized in mangrove ecosystems. This study investigated the nutrient resorption efficiency (NuRE) of seven trace elements-iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), sodium (Na) and aluminum (Al)-in mangroves, comparing them with upland terrestrial plants and evaluating their ecological implications under seasonally dry and wet conditions. Field sampling was conducted in Dongzhaigang National Nature Reserve, China, across dry and wet seasons, and green and senesced leaves from 10 mangrove species were analyzed. Our findings revealed distinct resorption strategies between mangroves and upland terrestrial plants. Compared with upland terrestrial species, mangroves presented net accumulation (negative NuRE) of Na (-29.06 ± 6.87%), Mn (-72.71 ± 11.79%), B (-77.36 ± 14.49%), Fe (-123.63 ± 17.98%) and Al (-164.91 ± 33.21%), demonstrating significantly lower NuRE values for these elements. In contrast, mangroves presented a greater NuRE for Cu (57.80 ± 3.50%) than their upland terrestrial counterparts did, whereas Zn resorption (17.39 ± 4.00%) did not differ significantly between the two systems. Our analysis revealed that Na resorption patterns exhibited strong seasonal variations across ecological gradients. During dry seasons, Na accumulation (more negative NaRE) was significantly greater in low intertidal zones, tree species and isobilateral leaves (characterized by symmetrical mesophyll organization). In contrast, wet seasons completely reversed these patterns, favoring accumulation in high intertidal zones, shrubs and bifacial leaves (with dorsiventral mesophyll organization). Green-leaf nutrient concentrations emerged as the primary driver of NuRE, outweighing soil nutrient availability across dry and wet seasons. These findings highlight mangroves' unique nutrient conservation strategies and underscore the importance of foliar nutrient status in predicting ecosystem resilience under seasonal hydroclimatic variations.