Seasonal trends in the content of mineral nitrogen in solodized solonetz soil

Abstract

Fluctuations of mineral nitrogen were studied for three years in two solodized solonetz soils at Narrabri, New South Wales, under a wheat summer-fallow management system. Changes in mineral nitrogen were related to plant uptake, soil environment characteristics (water, temperature and oxygen), and climate. NO3-N was the major form of soil mineral nitrogen, generally exceeding NH4-N with NO2-N occurring only occasionally (<1 ¦g N g-1). NO3-N accumulated during fallow to 67-111 kg N ha-1 in the profile just after sowing. Crop growth depleted NO3-N to 0-11 kg N ha-1 at flowering. Several significant falls in soil NO3-N (11-27 kg N ha-1) could not be accounted for by plant uptake, and were apparently caused by denitrification. Decreases due to leaching (including lateral leaching) were not detected. Redistribution of NO3-N down the profile was observed, the efficiency of leaching increasing with increased intensity of rainfall. NH4-N generally amounted to less than 25 kg N ha-1. Fluctuations in NH4-N which occurred within the profile, could not be related to observed factors and are unexplained. The effect of a drought year, 1965, was to inhibit mineral nitrogen accumulation, lessen the depth of leaching of NO3-N and depress plant uptake of nitrogen.