The impact of nutrient increase on nutrient resorption from senescent leaves is an important topic, which is not completely understood. Mangrove species are characterized by nutrient conservation mechanisms as a consequence of adaptation to nutrient-poor environments. The understanding of the response of the traits related to nutrient conservation strategies to nutrient availability is limited. This study aims to address whether nutrient resorption of mangrove species is responsive to long-term nutrient enrichment.

Methods

A field-based study was carried out in a Kandelia obovata dominated mangrove stand where a nutrient gradient was generated by the long-term discharge of aquaculture wastewater. Seasonal variations of nutrient resorption and nutrient availability were assessed.

Important findings

The data from summer and winter showed the same results: 1) resorption proficiency or the nutrient concentrations (N and P) in senescent leaves did not varied with nutrient increases except the plot that the N availability was extremely high; 2) the resorption efficiency of both N and P was positively correlated with nutrient availability, and N resorption efficiency was also positively correlated to leaf N: P ratio; 3) N resorption was complete while the resorption of P was incomplete. These results indicate that resorption proficiency of K. obovata is less sensitive than resorption efficiency to nutrient availability, and the positive response of N resorption efficiency is related to the imbalance of nutrient availability, while the positive response of P resorption efficiency is unrelated to plant nutrient status. We suggest that caution should be taken in using the resorption efficiency of mangrove species to predict plant nutrient status.

We selected a mangrove stand (24°26' N; 117°54' E) between two creeks, where Kandelia obovata (a common mangrove species in China) is the predominant species. These creeks have been received the effluents from aquaculture ponds for nearly ten years. We collected the leaves and sediment cores (top 10 cm) in January (winter) and in August (summer) across six plots between two creeks (Creek I to the north side and Creek II to the south, the distance between these two creeks was ~500 m; Fig S1). For each plot, it is around 9 m² (3×3 m) and the distance between plots was ~ 80 m. Ten to 20 leaves (mature or senescent) were sampled from each of the three trees in each plot. The leaves collected from the same tree were mixed representing for one sample, thereby three samples were obtained for each plot. The senescent leaves were obtained by shaking the branches lightly and selected the healthy yellow leaves just dropped off (Lin & Wang, 2001). The sediment samples collected at four directions for each tree in each plot and mixed into one sample, then three sediment samples obtained in each plot.