Crop yields in sandy soils can be increased by addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C/N ratios. A loamy sandy soil(7% clay) was amended with a clay-rich subsoil(73% clay) at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control(sandy soil alone) with no clay addition. The sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg~(-1): mature wheat(Triticum aestivum L.) straw with a C/N ratio of 68, mature faba bean(Vicia faba L.) straw with a C/N ratio of 39, or their mixtures with different proportions(0%–100%, weight percentage) of each straw. Soil respiration was measured over days 0–45 and microbial biomass C(MBC), available N, and p H on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C/N ratio of the residues or their mixtures, but C use efficiency(cumulative respiration per unit of MBC on day 15) was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of wheat and faba bean in the mixture or more faba bean. Soil N availability and soil p H were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be concluded that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, whereas available N and p H were influenced by clay addition to the sandy soil studied. Crop yields in sandy soils can be increased by addition addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C / N ratios. A loamy sandy soil (7% clay) was amended with a clay-rich subsoil (73% clay) at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control (sandy soil alone) with no clay addition. sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg -1: mature wheat (Triticum aestivum L.) straw with a C / N ratio of 68, mature faba bean (Vicia faba L.) straw with a C / N ratio of 39, or the mixtures with different proportions (0% -100%, weight percentage) of each straw. Soil respiration was measured over days 0-45 and microbial biomass C (MBC), available N, and p H on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C / N ratio of the residues or their mixtures, but C use efficiency (cumulative respir ation per unit of MBC on day 15) was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of Soil N availability and soil p H were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, while available N and p H were influenced by clay addition to the sandy soil studied.