Residue addition combined with rewetting of dry soil - Effect of timing of residue addition on soil respiration, microbial biomass, nutrient availability and legacy effect

Harvest residues may be incorporated into soil just after a rainfall event in the field. The aim of this experiment was to determine the effect of residue addition upon rewetting on microbial activity, biomass and nutrient availability. A loamy soil was exposed to two wet-dry cycles with 5 days moist and 5 days dry each. Residues with high C/N ratio (H) or low C/N ratio (L) were added in eight residue treatments at different rates (10 or 20 g kg(-1) soil) and timing (day 0 or day 10, before rewetting): 20 g kg-1 soil of H, L or a 50:50 mix of H and L was added on day 0 or day 10; 10 g kg-1 of H or L was added on day 0 followed by 10 g kg-1 of L or H on day 10 giving treatments HL and LH. Microbial biomass C, N and P and available N and P were measured on days 1, 5, 10, 11, 15 and 20. In the first 10 days, microbial biomass, N and P availability were higher with L than with H and higher at higher residue addition rate. Respiration rates decreased during the dry period and were very low prior to rewetting on day 10. Available N and P concentrations on day 11 were similar as on day 1 suggesting that if residues are added upon rewetting, nutrient release is not greater than if residues are added to moist soil. However, in the treatment where L had been added only on day 0, rewetting of dry soil induced N release from partially decomposed L residue left in the soil from the first period. When H was added to moist soil (day 0), MBN on day 1 and 5 was higher than in unamended soil. But when H was added on day 10, MBN increased only on day 11 indicating that with H microbial utilisation of residue N may be restricted if addition of residue is combined with rewetting. Microbial biomass C and MBN changed little during the first dry period, but decreased during the second dry period. This suggests that prior exposure to drying and rewetting reduced the ability of microbes to survive in dry soil even in treatments where residues were added upon rewetting. From day 11 to 20, MBN, available N and P were lower in LH than in HL, indicating that the second residue had a strong effect and thus the legacy effect was weaker than if the soil was moist throughout the experiment. We conclude that addition of residue upon rewetting enhanced microbial C and N uptake and reduces the legacy effect of the previously added residue.