When Roots Follow the Path of Least Resistance
AUGUST 18, 2009
Roots growing in hostile soils use previous root channels as preferential pathways when they cannot penetrate through the subsoil. Root proliferation in the subsoil facilitates access to water, since subsoil has a higher water-holding capacity. This is critical for crops in low-rainfall areas. But do the roots follow the old channels only because they offer the path of least resistance or are they chemically superior to the surrounding soil? It is well known that roots alter the chemical composition of the rhizosphere by taking up particular ions and releasing exudates, but do these features persist in a soil after the roots decay? Using the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne National Laboratory, researchers mapped the heterogeneous chemistry of plant micropores in situ. They found that, while the chemical composition is different, chemical speciation does not vary with distance from the pore. This information can be used to better manage soils and crops in hostile environments, such as low-rainfall areas.
Previously performed bulk soil analyses revealed the chemistry of a soil on a macro scale, but not on a micro-, spatially resolved scale. “We assumed that destructive studies done before—where they scraped out several millimeters of soil and studied some chemical properties—might have missed some information because the studies ‘diluted out’ the micrometer-scale differences in nutrient concentrations and their chemistry,” said Ganga Hettiarachchi of Kansas State University, who authored the 2009 Soil Science of America Journal paper along with her student, Laurence Jassogne of the University of Western Australia, and David Chittleborough and Ann McNeill of the University of Adelaide.
Understanding the chemistry of the soil around pores and channels at the microscale requires non-invasive synchrotron-based x-ray techniques. The researchers used the x-ray fluorescence spectroscopy (XRF) microprobe at X-ray Operations and Research/PNC 20-ID beamline at the APS to map the distribution of nutrients Ca, Mn, Fe, Cu, and Zn at the micrometer scale in and around the soil pores. Using these XRF maps, the scientists located element hotspots immediately adjacent to the micropore and further into the soil matrix for µ-x-ray absorption spectroscopy (XAS) analysis. They analyzed the speciation of Mn, Fe, Cu, or Zn in the hotspots using µ-XAS (µ-x-ray absorption near edge structure and µ-extended x-ray absorption fine structure) spectroscopy.
The scientists chose duplex soils (Fig. 1), which make up about 20% of the Australian continent. Duplex soils exhibit an abrupt transition from horizon A (the top layer of a soil profile) (or E, the leached lower regions of the topmost soil layer) to horizon B immediately below horizon A, including as much as a 10-fold increase in clay content in a distance of <2 cm. The clays inhibit the growth and proliferation of plant roots and restrict water uptake. In duplex soils, 80% of roots are located in the macropore sheath, the soil surrounding a root channel, crack, or other structural void.
Nutrient enrichment/depletion around the pores was still evident: Ca was concentrated in the pores; Mn, Zn, and Cu accumulated around the micropores, and Fe was similar throughout (Fig. 2). However, XAS spectra indicated that the chemical form of the elements was no different in, near, or away from the micropore. So while Zn, Mn, and Cu are concentrated within the vicinity of the micropore, the chemical form of the elements was not different from the rest of the soil.
“This understanding can be used to manage soils in hostile environments; in other words, we could use physical and chemical amelioration of hostile subsoil as a management strategy,” said Hettiarachchi. For example, vegetation could be planted to increase the numbers of biopores in the subsoil, which would improve the structure and permeability of the subsoil, and also increase the number of preferential pathways for exploitation by roots of subsequent vegetation. — Dana Desonie