Isotopic studies of the nitrogen balance in a cracking clay. III. Nitrogen recovery in plant and soil in relation to the depth of fertilizer addition and rainfall

Abstract

15N-labelled nitrate nitrogen was added at either 15 cm or 45 cm depth to 10-cm diameter undisturbed cores of black earth soil. Cores were either fallowed or planted to wheat and received either 444 mm or 255 mm of simulated and natural rain during a 17-week growing season under field conditions. Between 92 and 98 % of added 15N could be accounted for in planted or fallow cores receiving the lower rainfall, while those receiving the higher rainfall contained 75-94 %. The loss was larger where fertilizer was added at 15 cm than at 45 cm depth. Fallow cores lost more 15N than planted cores. These losses are ascribed mainly to denitrification and are discussed in relation to the results of earlier work with this particular soil-plant system. Plants yielded more, and recovered more of the fertilizer nitrogen, in those cores which received the higher rainfall. Plants recovered more applied nitrogen from 45 cm depth than from 15 cm depth. At harvest, over 40% of the applied nitrogen remained as nitrate in the planted cores which received the lower rainfall. In these cores, only 38-47 % of the fertilizer was recovered in plant tops. The significance of these results for fertilizer efficiency is discussed.