South Africa - Inkaba.org

Quantification of the soil-plant carbon stock in the Weatherley
catchment eight years after conversion from grassland to
forestry
R.M. Lebenya 1 , C.W. van Huyssteen 2 , C.C. du Preez 3
1. Department of Soil, Crop and Climate Sciences, University of the Free State, P.O. Box 339, Bloemfontein
9300, Republic of South Africa, r.lebenya@yahoo.com.
2. Department of Soil, Crop and Climate Sciences, University of the Free State, P.O. Box 339, Bloemfontein
9300, Republic of South Africa, vhuystc@ufs.ac.za
3. Department of Soil, Crop and Climate Sciences, University of the Free State, P.O. Box 339, Bloemfontein
9300, Republic of South Africa, dpreezcc@ufs.ac.za
ABSTRACT
The Weatherley catchment was dominated by grassland before being converted to commercial forestry in 2002.
Three tree species were planted: Pinus patula, Pinus elliotii, and Eucalyptus nitens. A baseline study was
conducted before planting to quantify the soil organic matter content in the different soils and to characterize the
grassland vegetation and its biomass yield. Twenty five sample sites were selected and two control sites outside the
afforested area. Four replicates were taken at each site. Organic carbon and total nitrogen were determined on all
soil samples. The bulk densities of these soil layers were also quantified for every site. A linear decrease from
0.015-0.020 Mg m -3 to 0.005 Mg m -3 organic carbon was observed from the surface to a depth of 650 mm followed
by a uniform decrease to 0.003 Mg m -3 at 1150 mm. The total organic carbon content to a depth of 1200 mm for
the different soil groups and tree species areas were: 111, 85, 97, and 88 Mg ha -1 for soil groups A, B, C, and H
respectively; and 90, 93, 74, and 75 Mg ha -1 for the P. Patula, P. Elliottii, E. Nitens areas, and control sites
respectively. Carbon sequestration efficiency by the grassland in the catchment was estimated to be 2.1 kg C ha -1
yr -1 mm -1 rain. The present study is conducted eight years after afforestation of the catchment. The first objective is
to quantify whether the soil-plant carbon stock in the Weatherley catchment have changed significantly after eight
years of afforestation. The second objective is to establish whether changes in soil-carbon stock in the Weatherley
catchment are related to either tree species or soil types. It is hypothesised that the soil-plant carbon stock will
increase on account of afforestation of grassland.
KEYWORDS: forestry, grassland, nitrogen, organic carbon, vegetation
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