Losses of carbon and nitrogen with prolonged arable cropping from sandy soils of the South African Highveld

A knowledge of the kinetics of organic matter transformations in arable soi ls is important for managing them sustainably. Our aim in this study was to elucidate the effects of cropping period on pools of C and N in coarse-tex tured savanna soils of the South African Highveld. Composite samples were t aken from the top 20 cm of soils (Plinthustalfs) that have been cropped for lengths of time varying from 0 to 98 years in each of three different agro -ecosystems in the Free State Province of South Africa. Thereafter, soil or ganic C and N concentrations were determined in the bulk soil (< 2 mm) as w ell as in the clay (< 2 mum), silt (2-20 mum), fine sand (20-250 mum), and coarse sand (250-2000 mum) separates. Long-term cultivation of native grass land reduced soil C and N concentrations by 65 and 55%, respectively. Losse s of soil organic matter occurred from all particle-size separates, althoug h rate loss constants increased as particle size increased. The concentrati ons of organic C reached equilibrium after 34 years for the bulk soil and a fter 55 years for clay-size separates. Nevertheless, organic matter attache d to silt continued to be lost as the cropping continued, probably due to w ind erosion. Changes in soil properties thereby continued even after almost 100 years of cultivation.