Soil respiration, microbial biomass and nutrient availability in soil after repeated addition of low and high C/N plant residues

It is well known that nutrient availability and microbial biomass and activity in the soil following plant residue addition are influenced by residue composition. But, less is known about the effect of repeated addition of residue with different decomposability on these soil properties and how they are affected by the order in which the residues are added. In this study, low C/N residue (young kikuyu shoots) and high C/N residue (mature wheat shoots) were added four times (on days 0, 10, 20 and 30) at a rate of 7 g C kg(-1) in a different order to a silt loam. Respiration was measured over 40 days and microbial biomass C (MBC), N (MBN) and P (MBP) and available N and P were measured on days 0, 10, 20, 30 and 40. Cumulative respiration in the 10 days following low C/N residue addition was higher than with high C/N residue addition. It decreased with increasing proportion of high C/N residue addition compared to four times addition of low C/N residue. Treatments with three or four additions of high C/N residue had lower MBC, MBN and MBP, and available N and P concentrations compared to treatments with three or four times addition of low C/N residue, irrespective of the order in which the residues were added. It can be concluded that with repeated residue addition, microbial biomass and N and P availability at the end of a given 10-day period are mainly influenced by the proportion of low or high C/N residue added previously, whereas the C/N ratio of residue added at the beginning of the period has little effect. Further nutrient availability was not affected by the order in which low or high C/N residue was added.