MOVEMENT AND LOSS OF NITRATE FOLLOWING HEAVY APPLICATIONS OF SEWAGE SLUDGE TO A POORLY DRAINED SOIL

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Abstract
Three fields which received a heavy application of digested sewage sludge were studied over a 6-yr period to determine the accumulation, movement and losses of nitrate. The soils were fine textured and predominantly poorly drained. After an initial fallow year, the fields were cropped to cereals. Very high nitrate concentrations were found near the soil surface for several years. Downward movement of nitrate was very slow. There was a continuous loss of nitrate over the study period; this was attributed to denitrification. Laboratory experiments using 15N were conducted to determine the rate of denitrification and the soil conditions that were favorable to the reaction. The denitrification intensity of the soil profile decreased with depth, but was still appreciable at the greatest depth. When oxygen diffusion in the soil was slow and oxygen demand was high, denitrification occurred within a few centimetres of the soil surface or an air-filled pore. Poorly drained soils promote denitrification and offer some protection against the risk of nitrate pollution of groundwater.

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