SOME EFFECTS OF PARTIAL STERILIZATION ON MINERAL NITROGEN IN A LIGHT SOIL

Field plots on loamy sand with and without the soil fumigant ‘D-D’ (dichloropropane-dichloropropene mixture) were sampled at 0–12, 12–24, and 24–64 cm depths during May 1966–8. All plots received 125 kg/ha nitrogen fertilizer early in March each year and were cropped with sugar beet. Fumigation increased the total amount of mineral (NH₄⁺+ NO₃^?) nitrogen to a depth of 64 cm by 67 kg/ha on average, largely by increasing NH₄⁺-nitrogen. In unfumigated plots some mineral nitrogen leached below 12 cm every year but there was at least 125 kg/ha in the surface 64 cm in 1966 and 1968; during the wet spring of 1967 much leached below 64 cm. Sequential sampling in 1968 confirmed that some of the fertilizer nitrogen moved down the profile during spring. Plots partially sterilized either one or two years previously and not again, contained the same quantity of mineral nitrogen as plots that had never been fumigated. Fumigation increased the mineral nitrogen in plots by a similar amount whether or not they had been fumigated previously. Thus, on this soil, partial sterilization with ‘D-D’ seemed to have little residual effect on mineral nitrogen.