Stocks and soil-plant transfer of macro-nutrients and trace metals in temperate New Zealand estuarine mangroves

Aims Rapid expansion of temperate mangroves in New Zealand over the last decades has prompted an increase in resource consents for removal. However, little is known about the capacity of temperate mangroves to store elements, including pollutants. The main objectives of this study were to assess the stocks and soil-plant transfer of macro-nutrients (C, N, P, K, Mg, Ca, S) and trace metals (Fe, Mn, Al, Ni, Cu, Zn, Co, Cr) within temperate mangrove soils. Methods Soil samples and root, leaf and wood tissues were analysed for their total and available elemental concentrations over a wide range of environmental conditions along the head-mouth textural, salinity and nutrient gradients of the estuary. Physico-chemical characteristics of the soil of the different sites studied were also determined. Results Mangrove soils with lower Eh and currents upstream of the estuary trapped larger amount of macro-nutrients and heavy metals in the soils than downstream. This results in higher translocation of macro elements from the soil toward the above-ground biomass and higher translocation of heavy metals from the soil toward the litterfall, likely as a mechanism to avoid long term metal toxicity in the root system. Conclusions This multi-elemental study provides a comprehensive understanding of soil- plant transfers in temperate mangroves and can be used to better evaluate the ecological services of these ecosystems.