Soil δ15N and nutrients under exotic tree plantations in the southwestern Ethiopian highlands

This study was undertaken to evaluate the effect of tree species on soil properties. Soil δ15N, 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 0–20 cm depth. Assuming a linear rate of loss, total N declined by 90.6 kg ha−1 year−1 or by 38% (6.8 Mg ha−1) of the level in native forest during the 75 years of cultivation. The 15N 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 0–50 cm depth increased by 5.7 Mg ha−1 under Cupressus lusitanica and 2.0 Mg ha−1 under Pinus patula. The decrease in δ15N 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 Ca2+ increased significantly at 0–5 cm. Exchangeable K+ increased significantly within 0–30 cm 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 δ15N, TN, exchangeable Ca2+ and K+ to native forest. The residual effect of 15N-depleted C4 pasture grasses might explain the lack of difference in soil δ15N profile below 10 cm 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.