Determining deep root activity in arable fields by the core-labelling technique (CLT)

Introduction

Deep roots have the potential to exploit plant resources otherwise inaccessible. The aim of this study was to develop a method, the Core-Labelling Technique (CLT), to measure root activity down to 2.5 m of soil depth under field conditions.

Materials and Methods

Two stainless steel ingrowth-cores, 0.1 m in diameter and 0.55 m in length, were filled with soil labelled with nutrient tracers, i.e., LiCO₃, CsCO₃, Na₂SeO₄, RbCO₃ and ¹⁵NH₄Cl. The labelled cores were placed into an access-tube having openings at 1.0 m and 2.5 m of soil depth. They were kept under a lucerne (Medicago sativa) crop for 60 days, after which, the root length density (RLD) and concentration of tracers in shoot biomass (including a control) were measured.

Results

RLD of lucerne measured at 1.0 m and 2.5 m of soil depth were 0.022 and 0.007 cm cm^-3, respectively. Effects of core-labelling on shoot samples were significant for ¹⁵N, Li, Cs and Se across the soil depth but not for Rb. On average 6 times higher concentration of ¹⁵N (‰) was found at the labelled spot compared to the control. Li and Cs (mg kg^-1) at the labelled area showed 2 and 3 times higher concentration than the control, respectively. Regardless of the treatment, core-labelling at 1 m of soil depth resulted in higher concentration of ¹⁵N and Cs in comparison to 2.5 m of soil depth.

Discussion

To our knowledge, this report illustrates the deepest application of an ingrowth-core method as well as its first adoption of the tracer technique. Our results suggest that ¹⁵N, Li, Cs and Se can be feasible tracers for root activity detection in arable subsoil.

Conclusions

The CLT can be used as an effective tool for determination of root activity in arable subsoil. We suggest that root research should progress beyond the generic depth-scale in order to discover the relevant yet hidden function of deep roots in crops in the field.