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

Abstract

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.