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

Abstract

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 kEC value of 0.45, only roughly 50% of the added labelled substrates were accountable and therefore we explored the full range of reported kEC 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.