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Soil, land care and environmental research

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Change in soil organic carbon and nitrogen stocks eight years after conversion of sub-humid grassland to Pinus and Eucalyptus forestry

Mirriam Lebenya , Cornelius van Huyssteen , Christiaan du Preez


Scientific studies report a decrease, an increase or negligible changes in soil organic C stocks upon afforestation. These studies further neglect the potential role of total N, tree species, and soil drainage class on changes in soil organic C stocks. This paper therefore aimed to quantify the change in soil organic C and total N stocks in the Weatherley catchment eight years after conversion of grassland to forestry. Twenty-seven soil profile sites in the Weatherley catchment, situated in the north-eastern corner of the Eastern Cape Province of South Africa, were sampled to determine the soil organic C and total N concentrations for the estimation of stocks. These sites represented different vegetation (P. elliottii, P. patula, E. nitens, and grass) and soil drainage class (poorly drained, moderately drained, and freely drained soils) areas. Eighteen of the 27 sites studied had decreases, while nine sites had increases in organic C stocks in the 0-300 mm soil layer after eight years of afforestation. Total N decreased in 18 sites and increased at nine sites. Eight years of afforestation with P. elliottii and E. nitens significantly decreased stocks of soil organic C (from 47.6 to 38.8 Mg/ha) and, total N (from 3.22 to 2.87 Mg/ha), while P. patula only slightly increased stocks of soil organic C (from 43.8 to 48.6 Mg/ha) and total N (from 2.80 to 3.68 Mg/ha). Both soil organic C and total N stocks decreased in all three soil drainage classes upon afforestation. It is proposed that these findings be corroborated after another eight to ten years of afforestation.

SR17171  Accepted 29 November 2017

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