The subtropical highlands of Mexico have been under intensive cultivation for centuries with maize being the dominant crop. Heavy tillage before seeding, monocropping and crop residue removal is common farmers' practice. However, these cropping systems are not sustainable, since they result in serious water and wind erosion, soil fertility decline and low yields (<3 t ha^-1). Since the beginning of the 1990's, several long-term field trials have been established under rain-fed conditions at CIMMYT's experimental station “El Batan” near Mexico City (2240 m asl) to evaluate alternative management practices. Results indicate that small-scale maize and wheat farmers may expect considerable yield improvements through zero tillage and residue retention. An important explanation for these yield improvements is the increased moisture availability and spatial homogeneity due to less runoff and evaporation. These benefits can be ascribed to the presence of the residue cover on the soil surface. However, enhanced earthworm activity, soil porosity and stable aggregation, resulting from residue inputs and absence of mechanical soil disturbance, are also expected to contribute.

To test these hypotheses earthworm abundance and stable macro and microaggregation were measured in a long term trial comparing the following treatments: Conventional vs. zero till under maize monocropping, with and without residue retention (CT+R/MM, CT-R/MM, ZT+R/MM and ZT-R/MM) and zero tillage with residue retention under rotation of maize and wheat (ZT+R/MW and ZT+R/WM). In addition, the qualitative and quantitative effect of earthworms on soil porosity and macro and microaggregation is studied in thin sections using micromorphological techniques. Total earthworm numbers were significantly higher under zero-till with residue retention, especially when maize and wheat are rotated (on average 173 earthworms m-2). Under zero till without residues retention or conventional till, either with- or without residues, only 49 earthworms m-2 were found (averaged among the three treatments). Their effects on aggregation and soil structure morphology are now being studied. The results will be presented on the poster.