Surface water phosphorus dynamics in rice fields receiving fertiliser and manure phosphorus

A long-term randomised block field experiment was established in 1997 to study the dynamics of total P and dissolved P in the surface waters of rice fields receiving two application rates of fertiliser P and one rate of combined fertiliser and manure P. Preliminary results from the first two crops show that concentrations of both total P and dissolved P in the surface waters increased significantly following P application, especially during the first 2 weeks after application. P concentrations subsequently declined sharply within about 10 days, then declined steadily and remained almost constant from about 1 month after application. The initial increase in P concentration of surface waters was higher with increasing rate of fertiliser P, and the P concentration at the highest fertiliser rate peaked within about 1 week of application. The elevated P concentrations following fertiliser P application declined more rapidly than those following the combined application of fertiliser and manure P. When fertiliser and manure P were applied together, about 7 days later the surface water P concentrations were significantly higher than when the same rate of P (or double) was applied as fertiliser only. Disturbance of the surface soil by hand harrowing further increased the P concentrations in surface waters, with a subsequent decline to a steady value after about 1 week. Application of P fertiliser to the high P status soil in this experiment gave no crop yield response and may have increased the risk of pollution of adjacent surface waters through drainage from heavy rainfall events during the rice growing season. Therefore, fertiliser P should not be applied to such soils. If, however, fertiliser or manure P is applied, the application should be made during the dry winter to reduce P losses. Manure should be applied with particular care because of the higher risk of P losses to surface water arising from the relatively long period of high P concentrations in surface waters and the potential for greater release of P to field surface waters from the soil. Hand harrowing should also be avoided during wet weather to protect water quality.