Soil aggregate and microbial biomass in a permanent bed wheat–maize planting system after 12 years

In northwest Mexico wheat is planted as a winter crop in rotation with maize in summer. Permanent beds as a form of conservation tillage is an alternative planting system for grain production in this area. The planting system on permanent beds reduces the operational costs up to 40% and enhances physical and biological soil quality parameters. Research plots were established in the 1992 crop season to study the effect of conventional tilled-bed (CTB) with all plant residues incorporated versus permanent beds (PB) with four plant residue alternatives (all burned, removed, as stubble, and partially removed, maize stover removed and wheat straw retained). The objective of this work was to compare, after 10 and 12 years of plot establishment, the effect of CTB with PB management on biological and physical soil attributes during the wheat phase of the rotation. Results indicate that degree of soil aggregation, estimated by means of fractal dimension (D) from dry sieving, was comparable for CTB-all residues incorporated and PB-all residues retained as stubble. But the soil dispersion index, as shown by the numerical difference of D from wet and dry sieving (ΔD), was less for PB-all residues retained as stubble, indicating that soil aggregates are greater in size and more stable for this treatment. By contrast, soil aggregates from dry and wet sieving for PB-all residues burned were highly fractionated. The largest degree of dispersion between the wet and dry aggregate size distribution in all PB treatments was for the 1–2 mm soil fraction, while for the CTB-all residues incorporated, a constant and high degree of dispersion occurred to each of the soil fractions from 0.053 to 8 mm. The amount of C and N from microbial biomass for PB treatments was bigger, as the amount of crop residues left on the soil surface increased. Kernel weight from PB treatments was negatively related to the N mineralized from microbial biomass (r = −0.57 in 2002 and r = −0.39 in 2004).