Effects of 34-year-old Pinus taeda and Eucalyptus grandis plantations on soil carbon and nutrient status in former miombo forest soils

摘要

There is a strong need in Mozambique to counteract decades of deforestation and forest degradation by planting new forests. Plantations of Pinus/Eucalyptus species and maintenance of mature miombo forests are activities supported by the REDD+ mechanism (Reducing Emissions from Deforestation and forest Degradation) in climate negotiations. This study examined the effects of first-rotation Pinus taeda L. (Loblolly pine) and Eucalyptus grandis Hill ex Maiden plantations (ca. 34 years old) on soil carbon status compared with adjacent dry miombo forest. At three study sites located in the Western Highlands of Manica Province, Mozambique, study plots with Pinus taeda, Eucalyptus grandis and mixed-deciduous miombo species were delineated. The selection criteria were (i) forest stand of first-rotation plantation of Pinus/Eucalyptus, located adjacent to miombo forest, (ii) plantations established on soils similar to miombo forest soils, and (iii) former land use similar to that at current miombo sites. Stocks of soil organic carbon (SOC), total nitrogen (N) and extractable phosphorus (P) were quantified. Soil pH (H2O), cation exchange capacity (CEC) and base saturation (BS) were measured in soil extracted with ammonium acetate. Plantations of P. taeda and E. grandis increased total SOC stocks (0-50 cm) and N stocks in the top 10 cm. Assuming steady state in the miombo stands, the estimated net stock change in soil carbon was 1.41 Mg ha-1 yr-1 in P. taeda and 1.53 Mg ha-1 yr-1 in E. grandis stands. Estimated N accumulation rate was 32 kg ha-1 yr-1 in P. taeda and E. grandis stands. Pinus taeda had no significant effect on extractable P, soil pH and BS, but had significantly higher CEC compared with miombo forest soil. Eucalyptus grandis decreased P stocks, but increased soil pH and BS. Overall, P. taeda and E. grandis plantation had a large impact on SOC in dry miombo forest and also affected soil acidity and soil nutrient status, except for total soil N stocks. These effects of tree plantation on soil reflected differences in management practices between miombo forest and plantations, with the latter being subjected to better protection against fires and illegal cutting.