EFFECT OF CROPPING, SUMMERFALLOW AND FERTILIZER NITROGEN ON NITRATE-NITROGEN LOST BY LEACHING ON A BROWN CHERNOZEMIC LOAM

Abstract

In 1982, six crop rotation treatments that were initiated in 1967 on a Orthic Brown Chernozemic loam were sampled for soil NO₃-N and moisture to a depth of 240 cm. Soil samples were taken on 18 May and 10 June from all treatments, on 2 Sept. on fallow treatments only, and on 14 Oct. from cropped treatments. Precipitation during the sampling period was about 23% above the long-term average. It was estimated that at least 123 kg NO₃-N∙ha⁻¹ were leached from the top 240 cm of fallow soils. Leaching appeared to result from a portion of the precipitation moving through macro soil pores. There was evidence that water and NO₃-N might also move upwards from below the 240-cm depth. Of the six rotations examined, the 2-yr and 3-yr spring wheat (Triticum aestivum L.) rotations lost the most NO₃-N. The presence of fall rye (Secale cereale L.) in a fallow-rye-wheat rotation was very effective in reducing NO₃-N losses. Spring wheat, when grown continuously, was also very effective in reducing NO₃-N losses but even here there was some evidence of leaching beyond the root zone. Application of fertilizer N and P at amounts based on soil test recommendations reduced NO₃-N leached. It was estimated from long-term precipitation data, that over the past 100 yr about 20% of the soil organic N that was present at the time of breaking the land has been lost from the soil via leaching. It was concluded that leaching losses of N from the soils on the Canadian prairies had been greatly underestimated and were partly responsible for losses attributed to the more visible wind erosion. Key words: Nitrate movement, crop rotations, fertilizer and leaching, summerfallow and leaching, bimodal leaching