ASSESSMENT OF MANURE-APPLICATION EFFECTS UPON THE RUNOFF WATER-QUALITY BY ALGAL ASSAYS AND CHEMICAL-ANALYSES

The effects of manure-application mode, rate to soil, and rainfall characteristics on the quality of agricultural runoff water have been assessed by means of the algal-growth-potential test (AGPT) and chemical analyses. This study used two modes of manure application (i.e. surface mode and incorporation mode), three manure-application rates (0, 150, 300 kg N ha-1), and two rainfall intensities and times (i. e. 11 mm h-1 for 142 min and 22 mm h-1 for 71 min). The effects of the dilution of runoff water on algal growth were also examined. The algal yields obtained with runoff from soil with the incorporated manure mode are similar to those from soil without manure application and are lower than those with the surface mode of manure application. A higher manure-application rate increases the load of nutrients in the runoff and subsequently the algal yield. The dilution of runoff water can stimulate or limit the algal growth, depending on the concentration of toxicants, N (nitrogen) and P (phosphorus) from runoff and in the aquatic diluting medium. A lower rainfall intensity plus a longer rainfall time increases algal productivity. This study showed that N is the limiting factor to algal growth at low dilution but that, at high dilution or with the incorporation mode of manure application, P becomes the limiting factor to algal growth.