Rhizodeposit Carbon Gradients: Potentials and Limitations of Destructive Rhizosphere Sampling on a Millimeter-Scale

Abstract

Background

Despite the availability of modern techniques for high-resolution non-destructive rhizosphere analysis, destructive examinations yielding a certain minimum soil amount are often required to provide detailed insights into organic matter composition.

Methods

We compared an established approach for destructive rhizosphere sampling via root brushing to a new millimeter-scale gradient sampling approach, expecting that the latter allows to characterize spatial patterns of rhizodeposit-carbon (C) distribution and relate them to root traits and soil texture. A tool to sample soil in 2 mm steps around a root was developed. Maize with and without root hairs was grown under field conditions until the end of tassel emergence, either in loam or in sand, and labeled with ¹³CO₂ one day before harvest.

Results

Both approaches showed an enrichment of C and ¹³C in sandy and partially in loamy rhizosphere, but no δ¹³C gradient could be statistically demonstrated due to high variability. The major uncertainty of both approaches was the potential masking of bulk soil organic C concentration and isotopic composition by non-target roots. The new gradient sampling approach offers uniform, pre-defined, and thus neutral conditions with respect to sampling distance independent of root and soil properties; yields at least 100–200 mg of soil on a millimeter-scale from one individual root segment; and can be applied in natural settings without root growth artifacts. The presented techniques integrated signals from fine roots and root hairs.

Conclusions

For root systems with longer unbranched segments, the new approach has potential for tracing ¹³C released by roots and for analyzing plant and microbial remains at the millimeter-scale.