Savanna-derived organic matter remaining in arable soils of the South African Highveld long-term mixed cropping:: Evidence from 13C and 15N natural abundance

Sustainable agriculture requires the formation of new humus from the crops. We utilized C-13 and N-15 signatures of soil organic matter to assess how rapidly wheat/maize cropping contributed to the humus formation in coarse-textured savanna soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) cropped for lengths of time varying from 0 to 98 years, after conversion from native grassland savanna (C4). We performed natural 13C and 15N abundance measurements on bulk and particle-size fractions. The bulk soil delta C-13 values steadily decreased from -14.6 in (C4 dominated) grassland to -16.5%o after 90 years of arable cropping. This delta C-13 shift was attributable to increasing replacement of savanna-derived C by wheat crop (C3) C which dominated over maize (C4) inputs. After calculating the annual C input from the crop yields and the output from literature data, by using a stepwise C replacement model, we were able to correct the soil delta C-13 data for the irregular maize inputs for a period of about one century. Within 90 years of cropping 41-89% of the remaining soil organic matter was crop-derived in the three studied agroecosystems. The surface soil C stocks after 90 years of the wheat/maize crop rotation could accurately be described with the Rothamsted Carbon Model, but modelled C inputs to the soil were very low. The coarse sand fraction reflected temporal fluctuations in C-13 of the last C-3 or C-4 cropping and the silt fraction evidenced selective erosion loss of old savanna-derived C. Bulk soil N-15 did not change with increasing cropping length. Decreasing delta N-15 values caused by fertilizer N inputs with prolonged arable cropping were only detected for the coarse sand fraction. This indicated that the present N fertilization was not retained in stable soil C pool. Clearly, conventional cropping practices on the South African highlands neither contribute to the preservation of old savanna C and N, nor the effective humus reformation by the crops. (c) 2005 Elsevier Ltd. All rights reserved.