13C signature of CO 2 evolved from incubated maize residues and maize-derived sheep faeces

Abstract

Analyses of the spatial and temporal variations in the natural abundance of 13C are frequently employed to study transformations of plant residues and soil organic matter turnover on sites where long continued vegetation with the C 3-type photosynthesis pathway has been replaced with a C 4-type vegetation (or vice versa). One controversial issue associated with such analyses is the significance of isotopic fractionation during the microbial turnovers of C in complex substrates. To evaluate this issue, C 3-soil and quartz sand were amended with maize residues and with faeces from sheep feed exclusively on maize silage. The samples were incubated at 15 °C for 117 days (maize residues) or 224 days (sheep faeces). CO 2 evolved during incubation was trapped in NaOH and analysed for C isotopic contents. At the end of incubation, 63 and 50% of the maize C was evolved as CO 2 in the soil and sand, respectively, while 32% of the faeces C incubated with soil and with sand was recovered as CO 2. Maize and faeces showed a similar decomposition pattern but maize decomposed twice as fast as faeces. The δ 13C of faeces was 0.3‰ lower than that of the maize residue ( δ 13C −13.4‰), while the δ 13C of the C 3-soil used for incubation was −31.6‰. The δ 13C value of the CO 2 recovered from unamended C 3-soil was similar or slightly lower (up to −1.5‰) than that of the C 3-soil itself except for an initial flush of 13C enriched CO 2. The δ 13C values of the CO 2 from sand-based incubations typically ranged −15‰ to −17‰, i.e. around −3‰ lower than the δ 13C measured for maize and faeces. Our study clearly demonstrates that the decomposition of complex substrates is associated with isotopic fractionation, causing evolved CO 2 to be depleted in 13C relative to substrates. Consequently the microbial products retained in the soil must be enriched in 13C.