Impact of Exotic Tree Plantations on Carbon and Nutrient Dynamics in Abandoned Farmland Soils of Southwestern Ethiopia
Year: 2006
Type: dissertation
Abstract: The impact of exotic tree plantations in Ethiopia on soil organic carbon (SOC), soil nutrients and decomposition processes was analysed and factors contributing to differences in SOC sequestration identified using the CO2Fix model. Forest clearing followed by continuous cultivation for 75 years reduced total SOC by 43% (75.4 Mg ha-1), and forest-derived SOC by 73% (128.4 Mg ha-1) within 0-50 cm depth. Total N decreased by 38% (6.8 Mg ha-1) and base cations and available P were significantly decreased within 0-20 cm depth, while δ15N increased within 0-10 cm depth. Farmland afforestation led to net SOC accretion of 69.6 Mg ha-1 (63.3 Mg ha-1 tree-derived) under Cupressus lusitanica and 29.3 Mg ha-1 (24.2 Mg ha-1 tree-derived) under Pinus patula stands after 20 years. Afforestation with Eucalyptus grandis on an area with pasture plus cropping restored SOC stocks to pre-deforestation levels. Total N increased by 5.7 Mg ha-1 under C. lusitanica and 2.0 Mg ha-1 under P. patula (0-50 cm soil depth). After afforestation, δ15N decreased more in surface layers with C. lusitanica than with P. patula. Afforestation had little impact on available P and exchangeable Mg++. Exchangeable K+ increased significantly (0-30 cm depth) under C. lusitanica and P. patula, while Ca++ increased significantly (0-5 cm depth) only under C. lusitanica. The δ15N pattern in E. grandis soil reflected residual effects of C4 organic matter from pasture. Leaf litter decomposed in the order C. lusitanica> native forest> E. grandis~ P. patula, whereas fine roots decomposed in order P. patula> native forest> C. lusitanica~ E. grandis. SOC sequestration under the exotic trees might be explained by the higher leaf litter decomposition transferring fresh C to mineral soil and lower fine root decomposition, which occurs directly in mineral soil. The CO2Fix model confirmed that total litter input and proportion of fine woody litter explained differences in SOC. Increasing degree of litter decomposition decreased extractives concentration, holocellulose concentration and O-alkyl C spectral regions, and increased spectral regions of alkyl, methoxyl, aromatic and phenolic C and proximate lignin concentration. Fast-growing trees can thus influence SOC and nutrient status within 20 years through their litter input and litter mineralisation rate.
Author: Bekele Lemma
Institution: Swedish University of Agricultural Sciences
Language: English
Subfield: Soil Science
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2148632353