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

Abstract

This study was undertaken to evaluate the effect of tree species on soil properties. Soil δ 15N, 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 δ 15N 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 2+ 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 δ 15N, TN, exchangeable Ca 2+ and K + to native forest. The residual effect of 15N-depleted C 4 pasture grasses might explain the lack of difference in soil δ 15N 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.