Nitrogen and phosphorus losses in surface runoff due to rainfall in Manitoba

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Hargrave, Andrew Peter
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Four experimental field sites were monitored on the Gretna C, Leary SL, Ryerson SCL and Carroll CL for nitrogen and phosphorus losses in surface runoff in 1988 through 1990. At each site, 22.13 m x 4.6 m plots with a 9% slope withh five continuous crop-management systems - 1) alfalfa, 2) corn, 3) wheat (minimum till), 4) wheat (conventional till) and 5) fallow were randomly assigned one plot per site. The minimum till wheat was not included in the Gretna C and the Leary SL. Coshocton samplers were used to collect 1% of the runoff and sediment. Samples were filtered through borosilicate microfiber filters to separate the sediment fraction from the dissolved fraction. The dissolved fraction was stored at 4o C if necessary before analysis. Nitrite and nitrate nitrogen and total phosphorus were analyzed colorimetrically and ammonium was measured by electrode. Sediment was air dried for total Kjeldahl digestible nitrogen and total perchloric acid digestible phosphorus. Total nitrogen losses of up to 283 kg ha-1 were reported from the most severe storm on the Gretna C corn treatment. The alfalfa treatment were reduced to 1 kg ha-1 from the same storm and in most storms runoff was eliminated. Total nitrogen losses were more strongly influenced by a storm thirty minute intensity than by the rainfall amount, duration or erosivity index. Concentrations of total nitrogen in the sediment were higher in mid season than early or late season. Texture influenced both losses and concentration. The total nitrogen concentration was greatest in medium textured soils, but losses were greatest from fine textured soils. Coarse textured soils had both the lowest concentration and losses. Total nitrogen losses were highly correlated to soil loss. Total nitrogen concentration in runoff could be estimated using total rainfall, thirty minute intensity and soil loss. Total phosphorus losses of up to 99 kg ha-1 were reported from the most severe storm on the Gretna C (corn treatment). Losses from the alfalfa treatment were only 0.3 kg ha-1 from the same event. Total phosphorus losses were influenced by the thirty minute intensity. Total phosphorus concentration and losses were highest on the fine textured soils and lowest from the coarse textured soils. Total phosphorus losses were highly correlated to soil loss. Ammonium concentration in the dissolved fraction ranged from 0.03 to 3.69 ug g-1, which exceeds the 0.03 ug g-1 needed for accelerated algal growth. Concentrations of total phosphorus were as high as 2.15 ug g-1, much higher than the 0.01 ug g-1 needed to accelerate algal growth in standing waters. For the most part concentrations of nitrates were far below 8 ug g-1 which is generally regarded as the maximum concentration acceptable for drinking water. All concentrations were lower with larger storms.