Soil carbon and nitrogen stocks under chronosequence of farm and traditional agroforestry land uses in Gambo District, Southern Ethiopia

Abstract

Conversion of forests to farm lands without trees and farm lands with scattered trees (traditional agroforestry systems) may lead to decline of soil organic carbon (SOC) and N stocks provided that they have similar original status. This study was conducted on soils with the age chronosequences of 12, 20, 30, 40, 50 years of farm (F), traditional agroforestry (AF) and the adjacent natural forest (NF) lands. We studied the changes in the concentration and stocks of SOC, total N and their distribution in the soil profile of an Andic Paleustalfs in Gambo District, Southern Ethiopia. Soil samples were collected at 10, 20, 40, 60, 100 cm depth interval from pits of 1 m depth in all land use types and they were analyzed for their SOC and N stock. The results showed that the greater proportion of SOC and N was concentrated in 0–20 cm depth and that their concentration in AF and F land uses was significantly lower than that under the NF. Soils in traditional agroforestry land use showed a trend of higher SOC stocks in all chronosequences than those under the corresponding farm lands. The SOC stock under the chronosequence of 12–50 years of AF and F land uses varied from 28.2 to 98.9 Mg ha⁻¹ or 12 to 43 % of the stock under the NF. The SOC was less by 6.2 Mg ha⁻¹ year⁻¹ for AF₁₂ and 0.9 Mg ha⁻¹ year⁻¹ for AF₅₀ compared with NF. The corresponding values for farm lands were 6.6 and 1.3 Mg ha⁻¹ year⁻¹. The N values of all land uses were also less than that of the NF. The SOC and N stocks tended to be less in farm lands than in the traditional agroforestry. However, the SOC stocks were not significantly higher with AF compared with F suggesting that the parkland systems as practiced is not sufficient to overcome other effects of cultivation.