Respiration in a sand amended with clay - Effect of residue type and rate

Clay may bind organic matter and thus affect decomposition rates. To investigate if the effect of clay depends on residue type and rate, two experiments were conducted in a sand without clay or with 300 g clay kg−1. The clay was isolated from a Vertosol subsoil. Clay and finely ground residues were mixed into the sand and a microbial inoculum derived from the Vertosol subsoil was added. In the first experiment, three residues were added at a rate of 20 g kg−1: pea residues (low C/N, high concentration of water-soluble C), wheat residues (high C/N, medium concentration of water-soluble C) and sawdust (high C/N, low concentration of water-soluble C). Cumulative respiration per g C added (with residues and clay) after 34 days was highest with pea residues and lowest with sawdust. Clay addition increased cumulative respiration with wheat residues and sawdust but decreased it with pea residues. In the second experiment, wheat residues were added to the sand with or without clay at 0-20 g residues kg−1. Cumulative respiration per g C added after 32 days was reduced by clay up to residue addition rates of 2.5 g kg−1 but not at higher residue rates.