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A Simple Effects Analysis of the Effects of Phosphogypsum Application on Phosphorus Transport to Runoff Water and Eroded Soil Sediments

Soil and Irrigation Res. Inst., P/Bag X-79, Pretoria 0001, South Africa;

B. E. Eisenberg

Dep. of Agric. Dev., P/Bag X640, Pretoria 0001, South Africa.

^* Corresponding author.

ABSTRACT

Phosphorus contamination of surface waters as a result of land application of phosphogypsum (PG—a soil amendment containing orthophosphate-P) has not yet been investigated. A laboratory rainfall simulator study was conducted to assess the initial and residual effects of PG application on P transport to runoff water and eroded soil sediments. The experiment was carried out using three soils of various dispersivity and four rates of PG application (0, 2, 5, and 10 Mg ha^–1, in three replicates). Simulated rainfall was applied during three consecutive wet-dry cycles consisting of a 1-h rainfall event followed by a 4- to 6-d drying period. A split-plot design was used, consisting of soils and rates of PG application as the main plot treatments, and the storm as the split-plot treatment. A simple effects analysis was used to interpret the interactive effects between storms, PG rate, and soils. The beneficial effects of PG in reducing the amounts of runoff and sediment yield depended on the soil dispersivity. In a highly dispersive soil the positive effect was apparent at the lower rates of application (<5 Mg ha^–1) and gradually leveled off at the higher application rates. Conversely, on a soil with low dispersivity the beneficial effect of PG was obtained only at the higher rates of application (>5 Mg ha^–1). Differences in response to the PG application were explained in terms of the complementary effects of PG on chemical dispersion and mulching. The effects of PG decreased in subsequent storms indicating a progressive removal of PG from the soil surface by runoff and downward percolation. The orthophosphate-P concentrations in the runoff were generally much greater than 0.025 mg L^–1, which is considered as the threshold concentration limiting algae proliferation in surface waters. Phosphorus removal by runoff and erosion gradually decreased in subsequent storms. The data indicated that P experts were determined by the combined effect of PG rate on the amounts of labile P available for transport, and its effects on the amounts of runoff and sediment yield. The results of the study suggesthat application of high PG rates may resalt in P being exported to the waterways adjacent to the site of application and hence it may contribute to the enhanced eutrophication of the waterways.

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