Determination of the fate of 13C labelled maize and wheat exudates in an agricultural soil during a short-term incubation

M. Marx, F. Buegger, A. Gattinger, Á Zsolnay, J. C. Munch

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A broader knowledge of the contribution of carbon (C) released by plant roots (exudates) to soil is a prerequisite for optimizing the management of organic matter in arable soils. This is the first study to show the contribution of constantly applied 13C-labelled maize and wheat exudates to water extractable organic carbon (WEOC), microbial biomass-C (MB-C), and CO 2-C evolution during a 25-day incubation of agricultural soil material. The CO2-C evolution and respective δ13C values were measured daily. The WEOC and MB-C contents were determined weekly and a newly developed method for determining δ13C values in soil extracts was applied. Around 36% of exudate-C of both plants was recovered after the incubation, in the order WEOC < MB-C < CO2-C for maize and MB-C < WEOC < CO2-C for wheat. Around 64% of added exudate-C was not retrieved with the methods used here. Our results suggest that great amounts of exudates became stabilized in non-water extractable organic fractions. The amounts of MB-C stayed relatively constant over time despite a continuous exudate-C supply, which is the prerequisite for a growing microbial population. A lack of mineral nutrients might have limited microbial growth. The CO2-C mineralization rate declined during the incubation and this was probably caused by a shift in the microbial community structure. Consequently, incoming WEOC was left in the soil solution leading to rising WEOC amounts over time. In the exudate-treated soil additional amounts of soil-derived WEOC (up to 110 μg g-1) and MB-C (up to 60 μg g-1) relative to the control were determined. We suggest therefore that positive priming effects (i.e. accelerated turnover of soil organic matter due to the addition of organic substrates) can be explained by exchange processes between charged, soluble C-components and the soil matrix. As a result of this exchange, soil-derived WEOC becomes available for mineralization.

Original languageEnglish
Pages (from-to)1175-1185
Number of pages11
JournalEuropean Journal of Soil Science
Issue number5
StatePublished - Oct 2007
Externally publishedYes


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