Determination of the fate of regularly applied ¹³C-labeled-artificial-exudates C in two agricultural soils

Exudates are part of the total rhizodeposition released by plant roots to soil and are considered as a substantial input of soil organic matter. Exact quantitative data concerning the contribution of exudates to soil C pools are still missing. This study was conducted to reveal effects of ¹³C-labeled exudate (artificial mixture) which was regularly applied to upper soil material from two agricultural soils. The contribution of exudate C to water-extractable organic C (WEOC), microbial biomass C (MBC), and CO₂-C evolution was investigated during a 74 d incubation. The WEOC, MBC, and CO₂-C concentrations and the respective δ¹³C values were determined regularly. In both soils, significant incorporation of artificial-exudate-derived C was observed in the WEOC and MBC pool and in CO₂-C. Up to approx. 50% of the exudate-C amounts added were recovered in the order WEOC << MBC < CO₂-C in both soils at the end of the incubation. Newly built microbial biomass consisted mainly of exudates, which substituted soil-derived C. Correspondingly, the CO₂-C evolved from exudate-treated soils relative to the controls was dominated by exudate C, showing a preferential mineralization of this substrate. Our results suggest that the remaining 50% of the exudate C added became stabilized in non-water-extractable organic fractions. This assumption was supported by the determination of the total organic C in the soils on the second-last sampling towards the end of the incubation. In the exudate-treated soils, significantly more soil-derived C compared to the controls was found in the WEOC on almost all samplings and in the MBC on the first sampling. This material might have derived from exchange processes between the added exudate and the soil matrix. This study showed that easily available substrates can be stabilized in soil at least in the short term.