The effect of natural infrastructure on water erosion mitigation in the Andes

The Andes Mountains stretch over about 8900 km and cross tropical, subtropical, temperate and arid latitudes. Very few, if any, of the diverse physiographic, climatic and biogeographic regions in the Andes have been preserved from human impact. Land use and management have significantly altered the magnitude and frequency of erosion events: deforestation and agricultural practices (such as soil tillage and cattle grazing) have modified erosion rates, river sediment loads and landslide occurrences.

There is an urgent need to identify which soil conservation and management practices are most effective to combat soil erosion and to mitigate the on-site and off-site effects in the Andean region. Three large groups of water-related interventions can be identified: interventions based on land use and protective land cover including (1) restoration and protection of native ecosystems, such as montane forests or grasslands and (2) forestation with native or exotic species and (3) soil and water conservation measures including crop management, conservation tillage and slow-forming terraces and the implementation of linear elements such as vegetation strips and check dams. To expand the knowledge base on natural infrastructure for erosion mitigation in the Andes, it is necessary to move beyond case-by-case empirical studies to comprehensive assessments.

This study reviews the state of evidence on the effectiveness of interventions to mitigate soil erosion by water and is based on Andean case studies. Based on a systematic review of peer-reviewed and grey literature involving more than 120 local case-studies from the Andes, this study addressed the following research questions: (1) Which erosion indicators allow us to assess the effectiveness of natural infrastructure? (2) What is the overall impact of working with natural infrastructure on on-site and off-site erosion mitigation? (3) Which locations and types of studies are needed to fill critical gaps in knowledge and research?

From the suite of physical, chemical and biological indicators commonly used in soil erosion research, two indicators were particularly relevant: soil organic carbon of topsoil and soil loss rates at plot scale. The protection and conservation of natural vegetation has the strongest effect on soil quality, with 3.01 ± 0.893 times higher soil organic carbon content in the topsoil compared to control sites. Soil quality improvements are significant but lower for forestation and soil and water conservation measures. Soil and water conservation measures reduce soil erosion to 62.1 ± 9.2 %, even though erosion mitigation is highest when natural vegetation is maintained.

The systematic review of the existing literature allowed us to identify critical gaps in knowledge and research. There is a need for future empirical work on soil quality, erosion and sediment yield before/after interventions in data-scarce regions, such as high elevations, regions with either low or high relief, and low to very low or very high precipitation. Besides, most erosion assessments are based on short-term measurements that tend to miss the impact of rare high-magnitude events. Further research is needed to evaluate whether the reported effectiveness holds during extreme events related to, for example, El Niño–Southern Oscillation.