Components of the nitrogen cycle measured for cropped and grassland soil-plant systems

Summary Inputs and outputs to the N balance of a clay soil catchment (Evesham and Kingston series) under grassland and cereals at Wytham near Oxford were measured over 2 years. Soil mineral N (NH₄+NO₃) was measured to 1 m depth at intervals of 2 to 8 weeks. The frequency distribution of these values was approximately log-normal and the geometric mean was used as an estimate of central tendency. Overall, soil mineral N tended to decrease during the study period, but marked fluctuations were observed in autumn (October–November) and early spring (February–March) in the grassland due to mineralization of soil organic N, and in the arable soil in April–May following the application of N fertilizer to the spring barley and winter wheat.N lost by leaching, including a little surface runoff, was calculated from the NO₃ concentration of the catchment drainage and the volume of drainage. The estimate of N leached using concentrations unweighted for flow rate was only 14 per cent less than that based on flow-weighted concentrations. The differences in the uptake of N by cereals and grass between fields were explicable partly in terms of soil type and partly in terms of the timing and amounts of fertilizer added. The results are discussed in the context of steady-state equilibrium of N in the soil-plant system. However, an N balance could not be struck because N input due to mineralization, and N outputs due to gaseous losses and immobilization of N in the soil and root biomasses, were not measured and could not be accurately estimated.