The effects of natural hydrological fluctuations on the nature and bioavailability of soil phosphorus (P) in relation to iron (Fe) and aluminum (Al) chemistry and root mass were studied along a flooded tropical forest gradient in Mapire river, Venezuela. Soil samples were collected following a complete natural hydroperiod: end of the dry season (May 2004), end of the rainy season (November 2004) and end of the subsequent dry season (May 2005), and from three zones subjected to different flooding intensities: MAX, MED and MIN zones inundated for 8, 5 and 2 months per year respectively. The results showed that flood induced the increase of resin-Pi in the MAX zone, but not in the MED and MIN zones. Flood in the soil of the MAX zone also induced the increase of the NaOH-Pi fraction, which removes inorganic P sorbed onto secondary Fe and Al minerals. Changes in this redox-sensitive P form can be considered indirect evidence that P in the MAX zone can be released from the dissolution of iron oxyhydroxide. This field study also showed that along the flooded forest gradient, fine root mass declined during the flood event. However, such decline was more pronounced in the MIN zone than in the MAX zone. In this zone fine root mass was higher than in the other zones.
