Phosphorus fractions in soils of the mangrove, restinga and Atlantic forest ecosystems from Cardoso Island, Brazil
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The availability of phosphorus (P) for the growth of plants often depends on the solubilisation of inorganic phosphates (P fractions) in the soil. The aim of this study was to investigate soil P fractions in terms of availability and soil characteristics of mangrove soil compared with the Atlantic forest and restinga soils that occur at Cardoso Island, Brazil. Soil samples were collected during summer at depths of 0-2, 2-5 and 5-10cm. Except for Ca-P, the quantity of P fractions in the soils decreased in the order: Atlantic forest>mangrove>restinga. Principal components analysis showed that treatments were clustered into two groups based on ecosystem proprieties; P in mangrove soil was associated with pH, moisture and salinity, and P in Atlantic forest soil with soil organic matter (SOM). The P in restinga soil was not associated with any variable. Among fractions, iron (Fe)-P was dominant, followed by calcium (Ca)-P and aluminium (Al)-P. The most recalcitrant P fractions, reductant Fe-P, occluded (o) Fe-P and oAl-P were detected in minor amounts. Soluble (sol)-P was detected only in the Atlantic forest soil. Soil P fractions varied little with depth in the mangrove and restinga soils. The P fractions from Atlantic forest soil were found at depths of 0-2cm (Fe-P, Ca-P) and 2-5cm (sol-P, Al-P, Fe-P, oAl-P and oFe-P). Positive correlation was obtained for all P fractions with moisture (except sol-P and Fe-P) and SOM (except Ca-P) contents and negative correlation with soil pH (except Ca-P) and salinity (except Fe-P and Al-P). Quantities and qualities of the P fractions varied among the studied ecosystems, influenced by the parent material and physico-chemical characteristics of the soil and vegetation.