An experiment was carried Out to investigate the decomposition and microbial use of maize leaf Straw incubated in packed soil columns at different depths. The straw was incorporated into the top layer at 0-5 cm depth and into the bottom layer at 15-20 cm depth of a sandy or a loamy soil. Microbial biomass C was significantly increased after adding straw to the bottom layer of both soils. After adding straw to the top layer, this increase was significantly lower in the sandy soil and significantly higher in the loamy soil. Maize straw application significantly increased the ergosterol-to-microbial biomass C ratio in both soils from 0.26\% to a mean content of 0.72\% after adding straw to the top layer and to a mean content of 1.11\% after adding straw to the bottom layer. The calculation of the maize-derived CO(2) production revealed that the mineralization rates of maize C were always higher in the sandy soil, with a mean of 20\%, than in the loamy soil, with a mean of 14\%. The application of maize straw always significantly increased the soil organic matter-derived CO(2) production. This increase was stronger in the loamy soil than in the sandy soil and stronger after application of the maize straw to the top layer than to the bottom layer. On average, 100\% of the maize straw C was recovered in the different fractions analysed. In the layers with maize leaf straw application, 28\% of the maize C was recovered as particulate organic matter (POM) > 2 mm and 32\% as POM 0.4-2.0 MM, Without a significant difference between the two soils and the depth of application. In the layers with maize leaf straw application, 19\% of the maize C was recovered as microbial residue C and 3.1\% as microbial biomass C. In the three layers without straw, the microbial biomass incorporated a further 2.4\% of the maize C in the sandy soil, but only 0.9\% in the loamy soil. Considerable amounts of substrate C were transferred within the microbial biomass over a decimetre distance. The finer pore space of the loamy soil seems to obstruct the transfer of maize-derived C. This was especially true if the maize leaf straw was added to the bottom layer. (C) 2009 Elsevier Masson SAS. All rights reserved.