Transformation of a Podocarpus falcatus dominated natural forest into a monoculture Eucalyptus globulus plantation at Munesa, Ethiopia: soil organic C, N and S dynamics in primary particle and aggregate-size fractions

摘要

Changes in land use and management can affect soil structure, soil organic carbon (SOC) and other nutrients reserve (such as N, P, S). We analyzed organic carbon (OC), total nitrogen (N), and total sulfur (S) in particle-size, aggregate-size and size/density fractions of soil organic matter (SOM) in order to identify the SOM pools most affected by the conversion of a Podocarpus falcatus dominated mixed natural forest into a monoculture Eucalyptus globulus plantation 21 years ago on a reddish brown Nitisol at Munesa, Ethiopia. Bulk soil OC, N, and S concentrations and stocks in soil to 20 cm depth were not significantly changed after the conversion of the natural forest into Eucalyptus plantation, but C/N ratio narrowed significantly. Soil organic C, N and S concentrations, and C/N and C/S ratios in sand and silt separates from the plantation samples were significantly reduced, while clay N and S concentrations had slightly increased. The losses of SOC, N and S in the sand fraction were more pronounced than that in the silt. Aggregate stability and total SOC, N and S concentrations of the aggregates were not significantly different in samples from the Eucalyptus plantation and the natural forest. In the plantation samples, both the free light fraction (LF) and the intra-particulate organic matter (iPOM) C, N and S concentrations associated with the macro-aggregates were significantly reduced. Differences in the total amount of the free LF (on the basis of water-stable aggregates proportion) between the two forest types were not apparent, suggesting that SOM quality is more prone to changes in land use and soil management strategies than the total amount of SOM. The loss of iPOM was higher than that of free LF probably due to gaseous losses of organic matter (OM) inside the aggregates caused by high fire temperatures during clearing and site preparation. In both forest types, the LF OM comprised the highest percentage of whole soil OM and the loss of particulate organic matter (POM) accounted for much of the losses of OM. Overall, the results showed that analysis of OC, N and S concentrations in soil particle and aggregate-sizes, and size/density fraction of SOM allowed sensitive detection of changes in SOM dynamics and soil fertility resulting from changes in land use. (c) 2004 Elsevier B.V. All rights reserved.