Soil erosion and erosion thresholds in an agroforestry system of coffee (Coffea arabica) and mixed shade trees (Inga spp and Musa spp) in Northern Nicaragua

Abstract

Soil erosion by water is one of the main environmental problems in tropical mountain regions of Central America. Agroforestry systems have been shown to reduce erosion through their canopy cover and their contribution to the litter layer. This paper analyzes the influence of the cover and height of coffee strata and mixed shade trees, the ground cover (weeds and litter), the slope gradient and various soil features and characteristics relevant to erosion in six farms that apply the agroforestry system of coffee growing (Coffea arabica) with mixed shade trees of Musa spp and Inga spp. We determine the corresponding erosion thresholds, i.e. the value (erodibility factor) beyond which the reduction in erosion becomes statistically significant. A purpose-designed verification method was applied to determine whether the thresholds corresponded to the effective control of erosion. The study area is located in the hills around El Cuá, in northern Nicaragua, an area with a humid tropical climate and a mean annual rainfall of 2770mm. An analytical method, based on the use of visual indicators, was used to analyze soil erosion by water. This approach enabled us to replicate the method in a sufficiently large number of cases for the results obtained to be considered reliable and representative of the study area. These results indicate that in the agroforestry system examined, for the cultivation of coffee and mixed shade trees (Musa spp and Inga spp), an average of 10.4% of the area is affected by erosion. The erodibility factors identified were litter layer and slope gradient. The first of these factors accounted for 66% of the variability of erosion, compared with 4% in the second case. The erosion threshold value was between 60 and 65% of the litter layer; beyond this level, the reduction in erosion was statistically significant. With this threshold, the soil was relatively little affected by erosion, around 13% of the cultivated area.