Phosphorus transformation in a soybean-cropping system in Andosol: effects of winter cover cropping and compost application

Understanding phosphorus (P) transformation is necessary to develop sustainable P management practices on Andosol with large P-fixing capacity. This study was conducted during 2005–2007 in northeastern Japan to determine how the amounts of inorganic P (P_i) and organic P (P_o) fractions change in a Silandic Andosol under soybean production Glycine max (L.)]. Two treatments were examined: application of composted cattle manure (0 (P0), 61 (P1), and 122 or 183 (P2) kg P ha⁻¹ year⁻¹) and winter cover cropping (no cover crop, rapeseed Brassica napus], and cereal rye Secale cereale L.]). Compost was applied before soybean seeding; cover crops were seeded after soybean harvest without further fertilization. Soil P was extracted sequentially with anion exchange resin (P_i), 0.5M NaHCO₃ (P_i, P_o), 0.1M NaOH (P_i, P_o), and 0.5M H₂SO₄ (P_i). Soybean removed 42.3 and 48.5 kg P ha⁻¹ (only 23 and 10% of the added P), respectively, in P1 and P2. In the P2 soil, 64% of excess P was distributed into P_i fractions, mainly resin-P_i and NaOH–P_i (29 and 19%, respectively). In P0, despite no P addition, soybean removed 41.5 kg P ha⁻¹ concurrently with a decrease in NaOH–P_i, suggesting its potential contribution to soybean P uptake. Neither of cover crops had significant effect on soil P fractions during the 3 year period.