Version définitive du manuscrit publié dans / Final version of the manuscript published in : European Journal of Soil Science, 2007, vol.58, no.6, 1273-1284 Manuscrit d’auteur / Author manuscript Manuscrit d’auteur / Author manuscript Manuscrit d’auteur / Author manuscript 1 Detecting zones of abrupt change in soil data, with an applica- tion to an agricultural field E. Gabriel a , D. Allard b , B. Mary c & M. Gu´ erif d a Department of Mathematics and Statistics, Lancaster University, Lancaster LA1 4YF, UK, b Unit´ e Biostatistique et Processus Spatiaux, INRA, Agroparc, 84914 Avignon, France, c Unit´ e d’Agronomie, INRA, Rue Fernand Christ, 02007 Laon, France, and d Unit´ e Climat Sol et Environnement, INRA, Agroparc, 84914 Avignon, France Summary We propose a new method for estimating and testing the zones where a variable has discontinuities or sharp changes in the mean. Such zones are called Zones of Abrupt Change (ZACs). Our method is based on the statistical properties of the estimated gradient of the variable. The local gradient is first interpolated by kriging. Then we test whether the estimated local gradient exceeds some critical threshold computed under the null hypothesis of a constant mean. The locations where the local test is rejected define the potential ZACs, which are then tested globally. Using this method, we analyse soil data from an agricultural field. The analysis of the main soil components of the ploughed layer (clay, silt and sand particles and calcium carbonate content) reveals the structural variations in the field, linked to boundaries between soil types. Its application to non-permanent variables (soil water and mineral nitrogen content of the [0, 120cm] soil profile taken at several dates) shows that water content has the same ZACs for all dates, whereas mineral nitrogen has none. Introduction a Correspondence: E. Gabriel. E-mail: e.gabriel@lancaster.ac.uk