The immersion test was conducted in an agricultural drainage canal (at point 1 in Fig. 4) where reddish-brown sediment accumulated, and sheathed bacteria (Leptothrix spp.) were found to be abundant (Fig. 6). The test was performed during the irrigation period for paddy fields (from May to September 2009) and the non-irrigation period (from October 2009 to April 2010), because the canal is mainly fed by drainage water from paddy fields via surface outlets and underdrains, and the water quality is affected by the paddy field irrigation.

In this test, the woody carrier was placed in a container of non-woven bag and lowered to the bottom of the canal. The carrier in the container was removed from the water after immersion for 4 weeks. The Fe collected on the immersed carrier was analyzed by the 1,10- phenanthroline method (Stucki & Anderson, 1981), and the P adsorbed on the Fe oxides was analyzed by the Bray-2 method (Byrnside & Sturgis, 1958). The Bray-2 P is a portion of the soil P and is one of the indexes of available P for plant uptake. In this study, the adsorbed P is expressed as g/kg instead of the conventional expression of Bray-2 P (mg P2O5/100 g dry material). In addition, the water samples were collected at weekly intervals and the water quality of D-Fe and PO4-P was analyzed by the above-mentioned methods.

1.2 Elemental analysis

Elemental analysis of the immersed carrier was carried out by X-ray fluorescence spectrometry system (Shimadzu, EDX-720) at a voltage of 50 kV and a current of 1 mA.