Land use and soil carbon accumulation patterns in South African savanna ecosystems

Soil organic matter (SOM) from representative South African savanna ecosystems (KwaZulu-Natal and Eastern Cape) was analysed to assess its potential for soil C sequestration and the impact of clearing and cultivation on humus characteristics. The different humic fractions and some structural features of the humic acid (HA) fraction were studied by visible and infrared (IR) derivative spectroscopies. Undisturbed soils showed low SOM accumulation (1-4g kg(-1)) and this potential accumulation seemed to depend on soil moisture and plant cover rather than on slope and vegetation type. In general, clearing and cultivation lead to decreased C levels accompanied by oxidative breakdown of the SOM. We found that the most diagnostic chemical descriptors for the contrasted situations under study were the amount of perylenequinonic pigments in HAs (not found in cultivated soils), the concentration of fulvic acids (FAs) extracted from soil with 2 M H3PO4 (significantly lower in soils with a pronounced slope) and the proportion of humic substances linked to clay and oxides which like the HA/FA ratio suggested an increased degree of association between SOM and mineral colloids in soils with low or no protection by forest canopy (open, dry savanna, cleared areas). Our results suggest a primary environmental control of the SOM turnover (affected by warm temperature, intense sunlight radiation and rain events) with an important secondary role of topographical and human disturbance factors. The interest of assessing SOM quality rather than its total concentration in soil is underlined.