Legacy effect of previous residue addition—influence of length of the moist period between residue additions on soil respiration, microbial biomass and nutrient availability

In previous studies, we showed that microbial activity and nutrient availability after the second residue addition are influenced by the C/N ratio of the previously added residue, referred to as legacy effect. The aim of this experiment was to assess the influence of soil water content after the first residue addition on the legacy effect. A silt loam was amended with high C/N residue (mature wheat shoots, C/N 120, H) or low C/N residue (young kikuyu shoots, C/N 22, L) at 10 g kg −1 twice, on day 0 and on day 29 with low C/N residue following high C/N residue or vice versa (HL or LH). After the first residue addition, the soil was maintained either constantly moist for 4 weeks or dried immediately after residue addition or after 1, 2 or 3 weeks. All dried treatments were rewetted on day 28 and the second residue added on day 29 after which all soils were maintained moist until end of the experiment (day 57). Respiration was measured over 57 days and microbial biomass C (MBC), N (MBN) and P (MBP) and available N and P were determined on days 0, 29 (before second residue addition), 43 and 57. Compared to constantly moist soil, only soil drying immediately after the first residue addition reduced cumulative respiration and increased MBC on day 29, and with low C/N residue, increased available N concentration. After the second residue addition, the previous moisture treatment influenced the legacy effect in HL, but not LH. Compared to the treatment that was maintained moist after the first residue addition, cumulative respiration and MBC and available N concentration were higher in the treatment that was dried immediately after the first amendment. This can be explained by the greater amount of the previously added H residue left in the soil when the L residue was added compared to the treatment that had been moist throughout the experiment. It can be concluded that soil drying influences the legacy effect only when it occurs shortly after addition of the first residue, which is when decomposition rates are high.