Soil respiration, microbial biomass and nutrient availability in soil amended with high and low C/N residue – Influence of interval between residue additions

• High (H) and low (L) C/N residues were added as HL or LH with 10, 20 or 30 day interval.
• After 2nd addition available N and P did not differ between HL and LH with 10-day interval.
• Available N after 2nd addition was higher in HL than LH with 20 and 30 day interval.
• In LH, previous addition of L residue stimulated MBC after 2nd addition at all intervals.

It is well known that soil microbial biomass and activity and nutrient availability following plant residue addition are influenced by residue composition. In a previous study, Marschner et al. (2015) showed that in soil amended twice with residues of different C/N, the C/N of the previous residue addition influenced nutrient release after the second amendment which can be referred to as legacy effect. In that study, the interval between residue additions was 20 days. In this study, high C/N residue (mature wheat shoots, C/N 120, H) or low C/N residue (young kikuyu shoots, C/N 22, L) were added at 10 g kg−1 on day 0 to a silt loam followed by a second amendment (at 10 g kg−1) on days 10, 20 or 30 with low C/N residue following high C/N residue or vice versa (HL or LH). Respiration was measured between residue additions and over 30 days after the second addition of residues. Microbial biomass C, N and P and available N and P were measured every 10 days. Cumulative respiration over 30 days after the second residue addition was higher in HL than LH and higher when the second amendment occurred 10 days after the first compared to 30 days. There were no differences in microbial biomass C, N and P and available N and P between HL and LH when the second residue was added 10 days after the first, indicating a strong legacy effect. When the second residue was added 20 or 30 days after the first, available N concentrations were lower in LH than HL, but the MBC concentration was higher in LH than HL thus a legacy effect was present for MBC, but not for available N. It can be concluded that the legacy effect on N availability is short-lived (10 days), but the C/N ratio of a previously added low C/N residue can stimulate microbial growth after addition of high C/N residue even when added 30 days before.