Fine-earth translocation by tillage in stony soils in the Guadalentin, south-east Spain: an investigation using caesium-134

Tillage erosion is increasingly recognised as an important soil erosion pro cess on agricultural land. In view of its potential significance, there is a clear need to broaden the experimental database far the magnitude of till age erosion to include a range of tillage implements and agricultural envir onments. The study discussed in this paper sought to address the need for s uch data by examining tillage erosion by a duckfoot chisel plough in stony soils on steep slopes in a semi-arid environment. Results of the investigat ion of coarse fraction (rock fragment) translocation by tillage in this env ironment have been presented elsewhere and the paper focuses on tillage tra nslocation and erosion of the fine earth. Tillage translocation was measure d at 10 sites, representing both upslope and downslope tillage by a duckfoo t chisel plough on five different slopes, with tangents ranging from 0.02 t o 0.41. A fine-earth tracer, comprising fine earth labelled with Cs-134, wa s introduced into the plough layer before tillage. After a single pass of t he plough, incremental samples of plough soil were excavated and sieved to separate the fine earth from the rock fragments,Translocation of the fine-e arth tracer was established by analysing the 134Cs content of the samples o f fine earth. These data were used to establish translocation distances for each combination of slope and tillage direction. Translocation distances o f the fine earth were not significantly different from translocation distan ces of the coarse fraction. For all sites, except uphill on the 0.41 slope, translocation distances were found to be linearly related to slope tangent . The soil flux due to tillage for each site was calculated using the trans location distance and the mass per unit area of the plough layer. For slope s with tangents <0.25, the relationship between soil flux and tangent was l inear and the soil flux coefficient derived was 520-660 kg m(-1) per pass. This is much larger than the coefficients found in other studies and this h igh magnitude is attributed to the non-cohesive nature and high rock fragme nt content of the soil in this investigation. A second contrast with previo us studies was found in non-linearity in the relationship between soil flux and tangent when steeper slopes were included. This was a product of varia tion in plough depth between the steepest slopes and the remainder of the s tudy area. On the basis of the study it is suggested that an improved under standing of tillage erosion may be obtained by considering the dual process es of tillage detachment (mass per unit area of soil subject to tillage) an d tillage displacement (equivalent to translocation distance per pass) in a ssessing, comparing and modelling tillage translocation. An improved model is proposed that recognises the complexity of soil redistribution by tillag e, provides a framework for process-based investigation of the controls on tillage fluxes, and allows identification of potential self-limiting condit ions for tillage erosion. (C) 1999 Elsevier Science B.V. All rights reserve d.