Cumulative effects of cropping systems on the structure of the tilled layer in northern France

A field experiment was initiated in 1989 in northern France to evaluate the cumulative effects of cropping systems on the structure of the tilled layer of a loamy soil. Three cropping systems involving different crop rotations and cultivations (early or late sowing, early or late harvesting) were compared. Them soil structure was evaluated after each winter wheat sowing by a morphological analysis of the ploughed layer. We determined the proportion of highly compacted zones, i.e. the zones with a massive structure and no visible macropores in the soil profile. These zones, defined as Delta zones, result from severe anthropogenic compaction. The creation of Delta zones depended largely on the soil moisture at the time of field operations and the characteristics of the machinery used. Maximum compaction occurred during harvesting in wet conditions because of high axle loads. In contrast, little compaction was produced by seedbed preparation, which involved lower axle loads and wide lyres. Consequently, changes in soil structure depended to a large extent on the cropping system. However, the proportion of Delta areas was not stable, but fluctuated greatly from one year to the next. Delta zones could quickly disappear from the ploughed layer. We, therefore, detected no irreversible effects on the structure of the ploughed layer, even for the cropping system that produced the highest annual risk of soil compaction. The combined effects of tillage and climate led to fewer compacted zones in the surface layer. Because this layer was mixed with deeper layers during soil inversion at the next ploughing, this contributed to fewer Delta zones in the whole layer. The loss of Delta zones over the whole cultivated layer could not be explained by this effect alone and the reduction in soil compaction was probably due to the combined effects of loosening by mouldboard ploughing and climatic and soil fauna activities in the ploughed layer below the seedbed. The initial soil structure had a major effect on seedbed fragmentation. When preparing seedbeds in the autumn, the proportion of remaining clods depended greatly on the initial state of the ploughed layer even when using a power rotary harrow. In spring, the number of remaining clods was still dependent on the initial compaction, but they were fewer and no differences were observed between rotary and combination harrows. (C) 2002 Elsevier Science B.V. All rights reserved.