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

Abstract

Scientific studies report decreases, increases, or negligible changes in soil organic carbon (C) stocks upon afforestation; however, these studies neglect the potential role of total nitrogen (N), tree species, and soil drainage class on these changes. 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 this 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 (Pinus elliottii, P. patula, Eucalyptus nitens, and grass) and soil drainage class (poorly, moderately, and freely drained soils) areas. Eighteen of the 27 sites studied had decreases, and 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), whereas P. patula only slightly increased the corresponding stocks from 43.8 to 48.6 Mg C/ha and from 2.80 to 3.68 Mg N/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 8–10 years of afforestation.