Co-effect of phosphorus and cadmium interaction on fraction distribution and accumulation of phosphorus in mangrove-sediment system.

Mangrove [Kandelia obovata (S. L.)] seedlings were cultivated in rhizoboxes under different rates of phosphorus (P) and cadmium (Cd) level. The speciation distributions of P both in the rhizosphere and non-rhizosphere sediments were examined using sequential extraction procedures. P contents in different K. obovata (S.L.) tissues were also determined. Results showed that considerable differences existed in P speciation distribution between rhizosphere and bulk sediments. A higher proportion of iron-bound phosphate (Fe-P) was found in the rhizosphere sediments, while a relatively higher concentration of exchangeable phosphate (Ex-P) and Aluminum-bound phosphate was found in the bulk sediments. P accumulation in plant tissues was significantly positively correlated to Ex-P and Fe-P. Results indicated that root activities play an important role in the P cycling. And the coexistence of P and Cd induced higher P accumulation in mangrove plants. It is suggested that the root-induced chemical and biological changes in the rhizosphere environment play an important role in enhancing the bioavailability of soil P.

Effect of external phosphate addition on solid-phase iron distribution and iron accumulation in Mangrove Kandelia obovata (S. L.).

In this study, a pot experiment was conducted to evaluate the effect of phosphate (PO4 (3-)) addition on iron (Fe) cycling in mangrove ecosystem. Kandelia obovata (S. L.), one of the dominant mangrove species in the southeast of China, was cultivated in rhizoboxes under three different levels of P concentrations. Results showed the solid-phase Fe distribution and Fe(II)/Fe(III) values in both the root zone (rhizosphere) and bulk soil (non-rhizosphere) were comparable among all P levels (p > 0.05); P addition significantly decreased the pore water Fe content both in the rhizosphere and non-rhizosphere zone (p < 0.05); higher amount of reactive Fe was found in rhizosphere sediments, while in the non-rhizosphere sediments, higher concentration of crystalline Fe was determined; P significantly increased iron plaque formation and iron accumulation in K. obovata (S. L.) tissues (p < 0.05); P addition increased K. obovata (S. L.) biomass and chlorophyll content. It was suggested that P is implicated in the Fe cycling in mangrove plants; more reactive iron, higher abundance of root Fe-reducing bacteria (FeRB) and Fe-oxidizing bacteria (FeOB), and together with higher amount of K. obovata (S. L.) root organic acids exudation result in a rapid Fe cycling in rhizosphere, which contribute to comparable solid-phase iron distribution among different P levels.

Phosphorus and cadmium interactions in Kandelia obovata (S. L.) in relation to cadmium tolerance.

This study focused on the cadmium (Cd) tolerance of mangroves with application of phosphate (P) in order to explore whether exogenous P can alleviate Cd stress on these intertidal species. Kandelia obovata (S. L.) seedlings were cultivated in rhizoboxes under different levels of Cd and P concentrations. The speciation distributions of Cd in the rhizosphere and non-rhizosphere sediments were examined by sequential extraction procedures; organic acid in plant tissues and soil solution was measured by high-performance liquid chromatography; Cd and P accumulation in the plants was also determined. Results showed that considerable differences existed in Cd speciation distributions between rhizosphere and non-rhizosphere sediments. Root activity influenced the dynamics of Cd, P application increased the organic acid content in root tissues, P also increased Cd accumulation in roots whilst lowering Cd translocation from root to the above-ground tissues, and a significant positive correlation was found between Cd and P in roots (r = 0.905). It is postulated that Cd detoxification of K. obovata (S. L.) is associated with higher Cd immobilization in the presence of higher P and organic acid contents in root tissue.

Silicon alleviates cadmium toxicity in Avicennia marina (Forsk.) Vierh. seedlings in relation to root anatomy and radial oxygen loss.

The effects of Si on growth, the anatomy of the roots, radial oxygen loss (ROL) and Fe/Mn plaque on the root surface were investigated in Avicennia marina (Forsk.) Vierh. seedlings under Cd stress. Si prompted the growth of seedlings and reduced the Cd concentration in the root, stem and leaf of A. marina. Si prompted the development of the apoplastic barrier in the roots, which may be related to the reduction of Cd uptake. The higher amount of ROL and Mn plaque on the root surface due to Si were also related to the promotion of Cd tolerance in A. marina seedlings. Therefore, it is concluded that the alteration of the anatomy of the roots, the increase of ROL and Mn plaque of A. marina seedlings play an important role in alleviation of Cd toxicity due to Si.

Formation of iron plaque on mangrove Kandalar. Obovata (S.L.) root surfaces and its role in cadmium uptake and translocation.

In this study, a pot experiment was conducted to investigate the formation of iron plaque under Cd stress and its role in Cd uptake and translocation by mangrove Kandalar. Obovata (S.L.). Results showed: 1. the Fe in dithionite­citrate­bicarbonate (DCB) extracts increased with an increasing rate of Cd treatments. 2. the Cd in DCB extracts and in roots and above-ground tissues significantly increased with an increasing Cd application; 3. significant positive correlation between concentration of Cd and Fe in DCB extracts existed (p < 0.05); and that 4. the proportion of Cd in DCB extracts was significantly lower than that in roots or above-ground tissues (p < 0.001). In conclusion, formation of Fe plaque can precipitate Cd on root surfaces and impede its uptake and translocation in Kandalar. Obovata (S.L.). However, in comparison with root tissues, Fe plaque is of little significance. It is the mangrove root tissue that acts as the main buffer to Cd uptake and translocation.

Dependence of phenanthrene dissipation in mangrove sediment on the distance to root surface of Kandelia obovata L.

We explored the relationship between the distance from roots and rhizosphere-mediated phenanthrene (PHE) dissipation in planted mangrove sediment. A compartmentalized device (rhizobox) was used that separated root surface and four consecutive rhizosphere layers that extended 0-2, 2-4, 4-6, and >6 mm away from the root surface. Kandelia obovata L. Druce seedlings were grown for 4 months in sediment spiked with 10 mgkg(-1) PHE. PHE dissipation, microbial community structure and enzymatic activities at millimeter scale in the vicinity of plant roots were analyzed. Results indicated significant differences in PHE dissipation through the various layers in the planted rhizobox. PHE dissipation exhibited the most rapid loss in the 0-2 mm near-rhizosphere layer, the lowest in far-rhizosphere (>6 mm) layer. Microbial community structure as indicated by denaturing gradient gel electrophoresis (DGGE) profiles showed that special bacterial species and larger bacterial numbers were observed in near-rhizosphere layers. Depending on the distance to root surface, near-rhizosphere bacterial species and enzymatic activities were more abundant than root compartment and far-rhizosphere, resulting in rapid dissipation of PHE.