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In Rwanda, mulch is applied in coffee fields to control soil erosion. The objective of this paper is to quantify the effects of different types of mulch on soil properties and soil erodibility in coffee farming systems in three different agro-ecological zones of the highlands of Rwanda. The treatments consisted of five mulching systems (T1: Cympobogon spp.; T2: Panicum spp.; T3: Cympobogon spp. and Panicum spp.; T4: Eucalyptus spp. and Cympobogon spp.; T5: mixed residues) and control (un-mulched coffee - T6). The mulching systems were selected in coffee plantations to which the same mulch material had been applied annually at an average rate of 15–21 t mulch ha− 1 for at least three consecutive years before we started field experiments in 2007. We tested the effects from 2007 to 2008 on two soil types. A factorial two-way ANOVA was used to assess the effects of the application of different mulch types on the soil organic carbon, bulk density, wet aggregate stability, and soil erodibility at each site. The application of mulch significantly reduced the bulk density and soil erodibility (P < 0.001) and significantly increased the soil organic carbon and wet aggregate stability (P < 0.001), mainly at Kibirizi and Karongi. In addition, the effects of the interactions between the type of mulch and type of soil were site specific (P < 0.001). Low bulk densities and soil erodibility as well as high soil organic carbon and wet aggregate stability values were observed mainly in response to the application of a mixture of mulches, particularly T4 and T5 at rates of above 17 t ha− 1. The maximum erodibility values coincided with the lowest values of soil organic carbon and wet aggregate stability and the highest bulk density. In addition, soil erodibility was also affected by soil texture; high sand and very fine sand fractions increased the vulnerability of soils to erosion. Mulching demonstrated positive effects on soil properties and soil erodibility, but the effects were site specific, depending on the agro-ecological conditions of the study sites and the types of mulch used.

The contribution of local knowledge to ecological sciences has not been fully exploited: there is still a gap between the recognition of farmers' knowledge as valid and an effective use of such knowledge in activities aimed at sustainable development. This study explores the use of farmers' indicators of erosion for developing a rapid tool for water erosion assessment at field level in the West Usambara Mountains (Tanzania). Two extensive field surveys were conducted in the research area concurrently. One survey consisted of applying an established erosion assessment method, the Assessment of Current Erosion Damage (ACED). According to the erosion features observed, fields were classified into five erosion classes, from very slightly eroded to very severely eroded. The second survey consisted of recording the type and number of indicators of erosion listed by farmers and present in the fields. The number of farmers' indicators per field increased with erosion intensity, from less than four in slightly eroded fields to more than eight in severely eroded fields. All farmers' indicators were positively correlated to the ACED erosion assessment classes. However, two groups of farmers' indicators could be distinguished in terms of erosion assessment: strong indicators, which were observed in more than 70% of cases in severely eroded fields, and weak indicators, which were observed more frequently in slightly and moderately eroded fields. Weak indicators appeared to be indicative of other land degradation phenomena, such as chemical fertility decline. Strong indicators and number of indicators were used to create a field erosion assessment tool in the form of a classification tree. The tree was built using one half of the field survey data and validated using the other half. The tree consisted of a hierarchical sequence of questions. Presence of rills and number of farmers' indicators were the most important factors of the tree. The validation yielded a highly significant Spearman's rho correlation coefficient (0.81). The contingency table showed that more than 80% of very severely eroded fields were correctly classified, whereas most misclassification occurred among slightly and moderately eroded fields. Farmers include land degradation phenomena and soil fertility decline in their definition of soil erosion. Soil and water conservation planning should address this broader farmers' perception by including, e.g., soil fertility improvements beside soil conservation. The distinction between strong and weak indicators of erosion is important in recommending the right intervention in the right spot, e.g., by counteracting soil erosion where strong indicators are present and by improving chemical fertility where weak indicators are present. The classification tree is of empirical nature and may need adaptation before being applied to other areas. The proposed methodology can be easily replicated and showed a high potential to provide extensionists with a field tool for erosion assessment. The classification tree was a successful example of integrating different types of knowledge for enhancing the co-operation between stakeholders involved in the erosion-control activities