Formation and remobilisation of soil microbial residue. Effect of clay content and repeated additions of cellulose and sucrose

In two studies, we assessed the mass balance of added ¹⁴C-labelled sucrose and ¹⁵NH₄¹⁵NO₃ by measuring ¹⁴CO₂, ¹⁴C and ¹⁵N in soil microbial biomass (SMB) and ¹⁴C and ¹⁵N in soil solution. Specifically, we assessed the potential of recently added ¹⁴C to be re-mobilised by cryptic growth using subsequent additions of sucrose and cellulose and the effect of physical protection on the stabilisation of the labelled substrate. We used both a constructed soil with low soil organic matter content and varied the clay content as well as a natural soil. We observed a substantial initial as well as a later stage transfer of ¹⁴C into unidentifiable form, hypothesised to be microbial residues. When using a standard k _EC value of 0.45, only roughly 50% of the added labelled substrates were accountable and therefore we explored the full range of reported k _EC values to assess the mass balance. Subsequent application of unlabelled sucrose and cellulose did not substantially increase turnover ¹⁴C and ¹⁵N. Contrary to our expectation, there was no effect of clay content on the amount of unidentified ¹⁴C and ¹⁵N. The unidentified ¹⁴C and ¹⁵N is ascribed to formation of soil microbial residue. The low recovery of added isotope suggests that our mechanistic models are missing a large and important pool in order to realistically simulate organic matter turnover in soil.