Relationship between vertical distributions of Bray II P content and organic P content in the Ap horizon of paddy rice fields

We examined the vertical distributions of total, Bray II and organic phosphorus (P) in the Ap horizons of five paddy rice fields including no tillage treatment. Soil samples were collected from the Ap horizons as nine thin layers of 0-1, 1-2, 2-3, 3-4, 4-6, 6-8, 8-10, 10-15 and 15-20 cm from the soil surface after harvest. In three lowland paddy soils of no-tillage Hachirogata (HA-NT), conventional-tillage Hachirogata (HA-T) and conventional-tillage Furukawa (FU-T), the total P (TP) content was relatively low. In these soils, the modified Bray II P (soil : solution = 1:20; shaking time 60s) content decreased and the organic P (OP) content increased at a depth of 0-1 cm compared with the underlying layers, possibly because of high microbial activities as suggested from the increased total C and N contents in this layer. In contrast, the decrease in Bray II P content and the increase in the OP content of the surface layer were not evident in two Andosols of the conventional-tillage Utsunomiya (UM-T) and conventional-tillage Kawatabi (KA-T). Even TP content fluctuated in the thin surface layers of 0-3 cm depending on the sampling site. Accordingly, percentages of Bray II P and OP to TP were calculated to compare the vertical distribution curves between Bray II P and OP. The percentage of Bray II P to TP of the 0-1 cm layer was lower and the percentage of OP to TP in the layer was higher than the underlying layers for HA-T, HA-NT and FU-T. The decrease in Bray II P was nearly compatible with the increase in OP for the surface of the 0-1 cm layers of HA-NT, HA-T and FU-T. This result suggests that a part of Bray II P was converted to OP in the thin surface layer of the Ap horizons in these paddy soils. This change in chemical form of P was more pronounced in the HA-NT by accumulating the effect of the microbial activities over a longer period of time because of no tillage compared with HA-T and FU-T. These findings add a new dimension to P behavior in submerged rice soils under field conditions, particularly near the soil surface. Moreover, we must be careful in soil sampling for P availability evaluation, and soil samples should represent the vertical distribution of P in the whole Ap horizon of a paddy field. Changes in OP with depth in UM-T and KA-T may have been masked by the high OP content as well as by the high TP content in these Andosols.