Soil delta super(1) super(5)N and nutrients under exotic tree plantations in the southwestern Ethiopian highlands

摘要

This study was undertaken to evaluate the effect of tree species on soil properties. Soil delta super(1) super(5)N, total N, C:N ratio, base cations and available P were analyzed under farmland, native forest and tree plantations established on abandoned farmland at Belete forest. Changes associated with land use were evaluated using a comparative approach. Forest clearing followed by continuous cultivation of annual crops resulted in a significant decline in total N, base cations and available P within 0-10 or 0-20cm depth. Assuming a linear rate of loss, total N declined by 90.6kgha super(-) super(1)year super(-) super(1) or by 38% (6.8Mgha super(-) super(1)) of the level in native forest during the 75 years of cultivation. The super(1) super(5)N enrichment of soil and litter N in the farmland may indicate losses of N through leaching and harvest. The degree of change in soil properties under exotic trees after 20 years of afforestation of former farmland depended on tree species. Total N within 0-50cm depth increased by 5.7Mgha super(-) super(1) under Cupressus lusitanica and 2.0Mgha super(-) super(1) under Pinus patula. The decrease in delta super(1) super(5)N value with afforestation was greater for C. lusitanica than for P. patula, indicating that N cycling under C. lusitanica progressed more towards 'native forest like' conditions. Under C. lusitanica, exchangeable Ca super(2) super(+) increased significantly at 0-5cm. Exchangeable K super(+) increased significantly within 0-30cm depth under both C. lusitanica and P. patula. The increased C:N ratio under these tree species was attributed to recent organic matter (OM) addition. The soil under Eucalyptus grandis, established on former mixed land use (pasture plus farmland), had nearly similar delta super(1) super(5)N, TN, exchangeable Ca super(2) super(+) and K super(+) to native forest. The residual effect of super(1) super(5)N-depleted C sub(4) pasture grasses might explain the lack of difference in soil delta super(1) super(5)N profile below 10cm depth between E. grandis and native forest. Greater OM inputs by E. grandis probably account for the greater C:N ratio relative to native forest soil.