Perennial monocropping of khat decreased soil carbon and nitrogen relative to multistrata agroforestry and natural forest in southeastern Ethiopia

Abstract

Monocropping of perennial cash crops providing livelihood for smallholders is replacing native forest throughout the tropics, but there is no direct empirical evidence on the impact on soil organic carbon (SOC) and nitrogen (N) relative to multistrata-agroforestry-based cash cropping. In particular, the impact of the conversion of forests and multistrata-agroforestry-based cash cropping to a rapidly expanding perennial monocropping of khat (Catha edulis Forskal) is not known. We investigated the potential of cash cropping integrated in multistrata agroforestry to alleviate SOC and N loss from converted native forest, relative to cash monocropping. We assessed empirically SOC and N stocks in the 40-cm-deep soil surface layer of three matched adjacent plots of native forest, multistrata agroforestry, and perennial cash monocropping, within nine replicate groups of the three land uses. The fixed mass method was applied. The estimated rates of the annual SOC and N losses were 3.0 and 3.4 times greater, respectively, in areas converted to khat monocropping than in agroforestry systems producing both coffee (Coffea arabica) and khat. Additionally, the carbon and N contents in leaf litter and fine roots were greater in agroforestry than in khat. The results indicated that multistrata-agroforestry-based cash cropping maintains most of the SOC and N stocks of converted native forests lost in conversion to cash monocropping khat than in agroforestry-based cash cropping. This warrants economic incentives to prevent the loss of the current stocks, while enabling cash crop income by smallholders. Reducing forest SOC and N stock decline in agroforestry through system management deserves attention as well.