Previous residue addition rate and C/N ratio influence nutrient availability and respiration rate after the second residue addition

Abstract In previous studies we showed the existence of a legacy effect, that is, the C/N ratio of the previously added residue influences soil respiration and nutrient availability after the second residue addition. We hypothesised that the legacy effect is due to microbes decomposing both the remaining previously added residue and the second residue. Thus, the extent of the legacy effect would depend on the amount of previously added residue left in the soil when the second residue is added. In the present study low C/N (L, young faba bean shoots, C/N 19) or high C/N residue (H mature wheat straw, C/N 73) was added at 2.5, 5 or 10 g kg − 1 on day 0, the control was left unamended. The second residue which had the same or a different C/N ratio than the first, was added on day 14 at 10 g kg − 1 . Thus, there were 14 treatments, unamended soil in the first period, then H or L residue (Control-H, Control-L). In the other treatments, 2.5, 5 or 10 g kg − 1 of either H or L were followed by 10 g kg − 1 H or L residue, giving the treatments HH, HL, LL and LH. Soil respiration was measured continuously, available N and P and microbial biomass C, N and P (MBC, MBN, MBP) were measured on days 0, 14 (before second residue addition) and 28 (end of experiment). At the end of the first period, cumulative respiration, available N, P and MBC, MBN and MBP were higher with L compared to H. In both H and L, compared to the lowest addition rate, cumulative respiration was about two and four fold higher with 5 and 10 g kg − 1 . Microbial biomass C, N and P on day 14 increased with addition rate with a greater increase with L compared to H. Available N on day 14 increased with addition rate in L, but decreased with H. There was a clear legacy effect after the second residue addition because for example, MBC on day 28 was similar in HL and LH whereas available N was lower in HL and Control-L than LL. The extent of the legacy effect was influenced by the initial addition rate. For example, available N on day 28 in HL decreased with rate of H in the first period, but increased with rate of L. It was about 40% higher in LH10 than in LH2.5. It can be concluded that the legacy effect depends on the amount of previously added residue left in the soil when the second residue is added.