Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland .V. Rate of loss of total nitrogen from the soil profile and changes in carbon : nitrogen ratios

The kinetics of total N loss from the top (0-0.1 m) and the subsoil (up to 1.2 m depth) of six southern Queensland soils after different periods (0-70 years) of cultivation and cereal cropping, were studied. The equation: Nt = Ne + (No - N,)exp(- kt), where No, Ne and N, are total N concentrations initially, at equilibrium and at time t, respectively, and k is the rate of loss of total N from soil, described total N loss from only three soils. For the 0-0.1 m depth, the kw values (based on weight of total Nholume of soil) were 0.061, 0.115 and 0.275 year-1, respectively for Waco (black earth; initially grassland), Langlands-Logie (grey, brown and red clays; brigalow) and Cecilvale soil (grey, brown and red clays; poplar box). The kw values decreased to less than half at 0-0.6 m depths of those at 0-0.1 m depth. In the other three soils, Billa Billa (grey, brown and red clays; belah), Thallon (grey, brown and red clays; coolibah) and Riverview (red earth; silver-leaved ironbark), total N declined linearly over the 20-25 years of cultivation period studied. Average annual rates of N loss from the profiles of the six soils, respectively, were 31.3, 67.1, 34.5, 50.8, 35.8 and 32.0 kg N ha-1 year-1 , from Waco, Langlands-Logie, Cecilvale, Billa Billa, Thallon and Riverview soils. Except for Langlands-Logie, these losses could be accounted for by crop N removal. In the Langlands-Logie soil, besides N removal by crop (51 kg N ha-1 year-1, 1982-1984 period), leaching of N below the root-zone appeared to be the likely factor for N loss. C:N ratios generally increased with depth in the five clay soils but decreased with depth in the red earth (Riverview). Cultivation had no significant effect on the C:N ratios of Cecilvale, Thallon and Riverview soils, but it caused a decrease in Langlands-Logie soil (up to 0.6 m depth) and an increase in Waco soil (up to 1.2 m depth). It was inferred, therefore, that in the latter, remaining soil N was likely to mineralise more slowly with increasing period of cultivation, resulting in a fertility loss which may be greater than that shown by the decrease in total N