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

A long-term randomised block field experiment was established in 1997 to st udy the dynamics of total P and dissolved P in the surface waters of rice f ields receiving two application rates of fertiliser P and one rate of combi ned fertiliser and manure P. Preliminary results from the first two crops s how that concentrations of both total P and dissolved P in the surface wate rs increased significantly following P application, especially during the f irst 2 weeks after application. P concentrations subsequently declined shar ply within about 10 days, then declined steadily and remained almost consta nt from about 1 month after application. The initial increase in P concentr ation of surface waters was higher with increasing rate of fertiliser P, an d the P concentration at the highest fertiliser rate peaked within about 1 week of application. The elevated P concentrations following fertiliser P a pplication declined more rapidly than those following the combined applicat ion of fertiliser and manure P. When fertiliser and manure P were applied t ogether, about 7 days later the surface water P concentrations were signifi cantly higher than when the same rate of P (or double) was applied as ferti liser only. Disturbance of the surface soil by hand harrowing further incre ased 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 in creased the risk of pollution of adjacent surface waters through drainage f rom heavy rainfall events during the rice growing season. Therefore, fertil iser P should not be applied to such soils. If, however, fertiliser or manu re P is applied, the application should be made during the dry winter to re duce 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 gre ater release of P to field surface waters from the soil. Hand harrowing sho uld also be avoided during wet weather to protect water quality. (C) 2000 E lsevier Science Ltd. All rights reserved.