LONG-TERM TRENDS IN TOTAL NITROGEN OF A VERTISOL SUBJECTED TO ZERO-TILLAGE, NITROGEN APPLICATION AND STUBBLE RETENTION

The effects of conservation practices, zero-tillage and stubble retention, on long-term trends in total N (0-0.1 m depth) of a Vertisol used mainly for wheat cropping were studied in a semi-arid subtropical environment (28-degrees 12' S. and 152-degrees 06' E.) in Queensland. Trends in total N content of a Vertisol (65% clay, pH 7.2) were discerned during a 22-year period of management practices including: zero-tillage (ZT) and conventional tillage (CT); stubble retention (SR) and stubble burning (SB); and fertilizer N application of nil (N1), 23 kg N ha-1 yr-1 (N2) and 69 kg N ha-1 yr-1 (N3). Soil total N (0-0.1 m) declined under all treatments at an overall rate of 25 +/- 2 kg N ha-1 yr-1 although after 22 years soil under ZT, SR and N3 treatments still contained higher soil total N than under CT, SB and Nl treatments. Apparent fertilizer N recovery in the soil-plant system was poor (34-64%) under CTSB, CTSR and ZTSB and ZTSR treatments, because N removed by the wheat crop was equivalent to less than 20% of fertilizer N in the first 12 years of management practices, due mainly to disease. Deep leaching losses of NO3-N was the likely factor for poor recovery of N. The ZTSR treatment showed better apparent N recovery than the CTSB treatment, most likely due to greater immobilization of fertilizer N, more N uptake in grain due to additional available soil water and hence less leaching losses of NO3-N. Under the current cultural practices, soil total N (0-0.1 m) may decline further to reach a steady state (about 1000 kg N ha-1). However, the apparent N recovery in the soil-plant system can be increased by disease control (for example, resistant cultivars and winter-summer crop rotations) and optimum utilisation of soil water (opportunity cropping) to minimize NO3-N leaching losses and to maximise production of crop biomass.