SOIL AND FERTILIZER-N TRANSFORMATIONS UNDER SIMULATED ZERO TILL: EFFECT OF TEMPERATURE REGIMES

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Abstract
Cool soil temperature regimes with initial soil temperatures of 5 °C rising to 20 °C at the heading stage reduced the rate of growth of barley by approximately one-third compared to 15–25 °C but did not change the barley yield or the fate of the applied fertilizer N in the soil biomass, roots, or tops of the plant or that lost by denitrification. The primary isotope data, % Ndff or ’A’ values remained relatively constant irrespective of whether the straw was placed on the surface or mixed throughout the soil. In contrast, the nitrogen balance data verified that fertilizer N loss, presumably due to denitrification, was as high as 35% in certain treatments, and further that up to 40% of the added fertilizer N was immobilized where the straw was uniformly mixed in the soil. The nitrogen balance data were used to correct the original rate of fertilizer N application. When this was done, A values calculated on the basis of the revised rates of application showed that the amount of soil N which was denitrified or immobilized was approximately double that of the applied fertilizer N. Thus, it is possible where a N balance is included in an investigation to quantitatively assess the effect of management practices on available soil N. It is further concluded that differential immobilization or denitrification of the 15N fertilizer standard may invalidate yield-dependent isotope-derived data, such as dinitrogen fixation unless nitrogen balance data are available to permit the appropriate corrections to be made. Key words: Zero till, N-cycle, temperature, crop residues, barley