Effect of continuous application of farmyard manure and inorganic fertilizer for 9 years on changes in phosphorus compounds in plow layer of an upland Andosol

The changes in total phosphorus (P) accumulation in the plow layer and in the contents of several P compounds in an upland Andosol with time and the effects of these changes on biomass production at the field level were investigated in relation to the continuous application of farmyard manure and compost (referred to as “farmyard manure”) and inorganic P for 9 y. 1) Inorganic P applied continuously without farmyard manure accumulated only in the plow layer of the upland Andosol, whereas the combined application of inorganic P and farmyard manure P led to the leaching below the plow layer, with a constant level (3.3–3.5 g kg^-1) of P accumulation in the plow layer within several decades. In order to reach a constant P accumulation level, higher application of P shortened the time required for the accumulation. 2) Bray No. 2-P value in the plow layer increased with time and tended to become maximum only under the application of both inorganic P and farmyard manure. 3) Aluminum (AI) bound-P (AI-P) fraction in the plow layer of the Andosol increased continuously and quadratically with the cultivation, along with an accelerated decrease in the amount of iron bound-P (Fe-P) and residual-P fractions (Res-P) during the 9-y period. 4) Continuous application of farmyard manure accelerated the decrease in the organic P (Org-P) fraction, presumably due to promoting microbial activity in the plow layer. 5) During the 9-y period, differences in crop growth with and without continuous application of P became attenuated, suggesting that the availability of accumulated P in the plow layer of the Andosol farm field before the experiment increased for the subsequent crop cultivation. Continuous increase in the AI-P fraction with time was closely correlated with the increase in P availability of accumulated P for crop growth.