Denitrification risks in relation to fertilizer nitrogen losses from vertisols and phaoezems

Denitrification is of particular concern to agronomists with respect to fertilizer nitrogen (N) recovery by crops grown on soils of poor drainage. The objectives of this study were to determine the effects of (i) soil moisture content, (ii) available carbon, (iii) nitrogen rate, and (iv) nitrogen source on denitrification in a Vertisol (heavy clay) and a Phaoezem (loam). Three experiments were undertaken to examine the influences of soil moisture content and available carbon (Experiment 1), the effects of nitrogen rate (Experiment 2), and the effects of nitrogen source (Experiment 3) on denitrification. The critical moisture level for denitrification to commence in the studied soils was 60% WHC. Available C was an important limiting factor for denitrification, more so in Vertisol than in Phaoezem. The highest denitrification N losses were obtained at 140% WHC with additional available C, and were 29 and 55 mg N kg− 1 soil for Vertisol and Phaoezem, respectively. Denitrification rates at 100 and 400 mg NO3-N kg− 1 soil for the Vertisols were 206 and 99 mg N kg− 1 soil, respectively. For Phaoezem the rates were 931 and 729 mg N kg− 1 soil. The differences in denitrification losses between the N sources were attributed to the NO3-N contents of the applied N-forms. The initial lag phase as well as the level and length of soil wetness were important in determining duration of denitrification. Under field conditions the soil's hydrological properties and the precipitation pattern which determine the magnitude and duration of soil wetness are the major determinants of denitrification risk. It is concluded that Vertisols have higher denitrification risk than Phaoezems, although the latter have higher C and NO3-N contents.

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