Land use change affecting soil humic substances in three semi-arid agro-ecosystems in South Africa

Abstract

Productivity of cropland soils, especially in semi-arid to arid zones, is declining due to increasing losses of stable organic matter. Therefore, it was found important to evaluate alternative management systems that can control, and if possible combat, these escalating losses of recalcitrant fractions of soil organic matter (SOM). The aim of this study was to quantify the influence of land use changes on humic substances of the semi-arid Plinthustalfs. Topsoil samples from distinctive agro-ecosystems at Harrismith, Tweespruit and Kroonstad were collected in the virgin, cultivated and restored Plinthustalfs and analysed for C and N in crude humic substances (Cs and Ns), extractable humic substances (Ce and Ne), humic acids (Ch and Nh) and fulvic acids (Cf and Nf). The results indicated that cultivation depleted the former virgin soils of crude humic substances, extractable humic substances, humic acids and fulvic acids across the three agro-ecosystems, with the exception of fulvic acids, which were higher in the cultivated soils than in the virgin soils of Kroonstad. Restored soils had substantial amounts of these humic fractions, except fulvic acids that behaved differently in the Kroonstad agro-ecosystem. Some correlations were observed between climate variables and humic fractions. In general, mean annual rainfall (MAR) and aridity index (AI) contributed to the loss of crude humic substances, extractable humic substances, humic acids and fulvic acids, while mean annual temperature (MAT) did not. These three climate variables did not influence the buildup of humic fractions. No trends were observed regarding the C/N ratios of the examined humic fractions; however, most were found to be closer to a range of 10–12, suggesting that SOM was approaching equilibrium state. Although it is known that humic substances are recalcitrant in nature, our results revealed that management practices that prohibit soil disturbance are needed to complement the refractory nature of humic substances against biological oxidation, and hence reduce C and N losses into the atmosphere.