Nitrate and chloride distribution and balance under continuous potato cropping

Abstract

A study was conducted between 1977 and 1982 to determine N losses from sandy loam soil under continuous potato cropping in Southern Ontario, Canada. A monthly climatic water budget was used to evaluate the timing and amount of soil drainage in relation to changes in the distribution of NO 3 −-N and Cl − in the soil profile. Significant temporal variations in NO 3 −-N and Cl − concentration were associated with percolation. The presence of concentration peaks below 183 cm, adjacent to a shallow water-table aquifer underlying the field, indicated definite leaching patterns. Ground water in the aquifer and in springs located down-gradient from the field had NO 3 −-N concentrations > 10 mg l −1. Little leaching occurred during the summer, but considerable amounts of NO 3 −-N and Cl − were lost during major episodes of soil drainage in autumn before freeze-up and in the early spring thaw period. The similar pattern of NO 3 −-N and Cl − concentration in the 0–3-m soil profile suggested that denitrification was not an important N-loss mechanism. This conclusion was supported by laboratory incubations of soil samples which showed that 8–20% of the initial NO 3 −-N was denitrified over a 14-day period in samples from 0–15-cm depth, whereas soils from greater depths exhibited very low denitrification potentials. Mass balance data for 1979–1982 indicated that annual N losses from the 0–183-cm soil profile ranged between 78 and 220 kg ha −1 under potato crops which received annual fertilizer applications of 160–210 kg N ha −1. Above-normal precipitation in late summer and autumn may have increased the rates of NO 3 −-N leaching into the shallow aquifer during the study period.