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

Mesele Negash,Janne Kaseva,Helena Kahiluoto

doi:10.1007/s10113-022-01905-3

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.

Highlights

In Ethiopia, cash cropping of khat has been traditionally integrated within agroforestry systems but is increasingly conducted by monocropping
No evident difference existed in the concentrations of soil organic carbon (SOC) and N in the deeper (20–40 cm) soil layer between the native forest and agroforestry systems (p > 0.05); both were higher than the concentrations of SOC and N in the same deeper soil layer of the khat plots
Our findings indicate that the SOC and N losses due to the conversion of native forests to perennial monocropping were substantially greater than due to conversion to agroforestry, and the annual rates of loss due to the conversion to khat monocropping were 3.0 and 3.4 times higher, respectively, than those due to the conversion to multistrata agroforestry

Summary

Introduction

Soil represents the largest terrestrial carbon (C) pool and can preserve captured C longer than aboveground biomass (Lehmann 2006). 2008; Mellisse et al, 2018), is expanding at the expense of native forest and agroforestry reducing C and N stocks in Southeast Asia, Latin America, and East Africa. In Ethiopia, cash cropping of khat has been traditionally integrated within agroforestry systems but is increasingly conducted by monocropping. Khat monocropping is replacing forest and coffee farms in Eastern, South Central, and South Western Ethiopia (Lemessa 2001; Dessie and Kinlund 2008; Dube et al 2014; Woldu et al 2015). This development may create a serious threat to SOC and N stocks

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