Assessment of magnetite as a sediment tracer in the study of ephemeral gully erosion: Application and distribution in the soil

There are few works where magnetic tracers are used for the assessment of water erosion, focusing on the traceability of sediments over the soil surface and in small areas (i.e., for laminar erosion survey). The main limitation of magnetic tracers is associated with its preparation and application, as the tracer is typically mixed with a large amount of soil and then spread manually over experimental sites susceptible to erosion. In gully erosion studies, the experimental sites (field areas where a gully is expected to develop) can be quite large. Consequently, the high volume/weight of the material (soil + tracer) to be prepared, transported, and applied renders this technique unfeasible. Moreover, eroded channels can reach a depth of several centimeters, requiring the tracer to be (re)located in the soil profile ideally at this same depth. This work presents a novel technique for the application and distribution of magnetite in the soil profile to track sediments in gully erosion assessments. Experiments were carried out in microplots, where a mixture of magnetite and water was applied over the soil surface using a backpack sprayer. Redistribution of the tracer in the soil profile was carried out with different conventional tillage implements. The control plots were subjected to the same experiments, but using a soil-tracer mixture (conventional technique). The application technique proposed herein enables a correct application of magnetite over large areas, being a promising alternative to the conventional technique. Hoeing tillage implements, such as the rotavator, ensure an approximately homogeneous redistribution of tracer in the soil profile when good soil moisture content is present.