Phosphorus fractions and sorption characteristics in a subtropical paddy soil as influenced by fertilizer sources

Continuous excessive application of phosphorus (P) fertilizer and/or manure can lead to P accumulation in soils and progressive saturation of soil P sorption capacities, thus resulting in increased P loss from soil to aquatic ecosystems. Yet the responses of P fractions and sorption characteristics to P fertilization in paddy soil, a special type of arable soil with long periods of waterlogged conditions, are not fully understood. The aim of this study was to determine the P fractions and its sorption indices in the plow layer of a typical subtropical paddy soil under different fertilization regimes. We selected 15 sites that varied in three fertilization regimes (i.e., chemical fertilizer [CF, n = 6], rice straw [RS, n = 3], and swine manure [SM, n = 6]) with equivalent P input (59 kg P ha− 1 yr− 1) from a long-term field experiment station (initiated in 1981). A sequential fractionation scheme identified that NaOH-extractable inorganic P (NaOH-Pi) was the primary P form in the paddy soil followed by residual P (Pres). The paddy soil that received long-term SM application presented significantly (p < 0.05) higher contents of KCl-extractable P (PKCl), NaOH-Pi, and total P than the soil that received only CF. In addition, SM significantly increased the proportion of NaOH-Pi, but reduced that of Pres (p < 0.05). In contrast, paddy soil treated with RS incorporation showed a similar allocation of P fractions as in CF treatment. Compared with CF, SM and RS increased the Langmuir P sorption maximum (Smax) of the paddy soil by 30 and 10%, respectively (p < 0.05). Organic amendments (i.e., SM and RS) significantly increased the contents of amorphous Fe and Al (i.e., oxalate-extractable Fe and Al, p < 0.05), and this appeared to contribute mostly to the increased P sorption capacity and variations in P fractions in the paddy soils. Thus, soil P buildup and related environmental assessments need to consider the increment of amorphous Fe and Al in paddy soil due to organic amendments.