The Web of Life (78-3-1) Rhizosphere


The realm of soil under plant management is the rhizosphere. Plants can alter soil conditions in many ways. Plants make decisions as to what actions to take, depending upon their needs and perceived possibilities.

Dropping pH affords some increase in nutrient uptake, particularly phosphorus and nitrogen. This stratagem works by altering the ion balances in the rhizosphere.

To snag phosphorous, fava beans rapidly acidify their rhizosphere. They can drop pH 2 points in 6 hours.

Iron-deficient plants secrete siderophores: compounds which chelate iron and render it soluble. Microflora also have this knack. It may be that plant siderophores are meant mostly as an invitation to microbes, by providing an environment conducive to iron absorption, as microflora are much more efficient at providing soluble iron which plants can absorb.

To greatest effect, plants generally follow the adage of “a friend in need is a friend indeed,” by turning their rhizopheres into havens for the fungi and bacteria that can assist them in elemental nutrient uptake, by breaking minerals down into absorbable form. This often involves feeding the microflora: exuding sugary metabolites as solicitation and as payment for work well done.

Facing a deficiency of soil-based resources, a plant shifts its growth pattern, favoring root over shoot. Since nutrient uptake depends largely upon the geometry of the root system, the greatest probable return-on-investment (ROI) comes from maximizing root length. This favors fine roots, since they achieve the greatest root length for given weight and can be quickly extended.

The problem is more complex than simply spreading out. The root topology problem comes in architecting a root system that optimizes coverage without introducing competition between old roots and new and do so with minimal investment while maximizing ROI. Plants solve this 3-dimensional graphing conundrum with aplomb.

Certain branching patterns are more expensive to construct but are more efficient at exploring soil. A fractal herringbone pattern consists of axis roots with finer laterals running from the axis, in successive iterations.

Thicker axis roots come at a greater cost than finer ones. But the extensions from such fractal herringbone links are not proximate, and so do not compete for nutrient ions.

Roots scrounging for nutrients brings home the favorable economics of outsourcing. Symbiotic fungi (mycorrhiza) are much more efficient producers of inorganic supplies. For a given investment of resources, fungal filaments (hyphae) are at least 100 times more efficient than building roots; hence the widespread employment of certain fungi by plants.

Another issue is patchiness. Available sunlight is a gradient. Suboptimal return from leaves in the shade is overcome by shoot growth to take the leaves to a sunnier spot, or abandonment of the endeavor there, if the prospect for better light in the immediate vicinity is dim.

In contrast, soil often has a decided heterogeneity of barren areas and nutrient hotspots. When roots encounter a nutrient-rich patch, they intuitively proliferate.

Not all plants are so exacting in their extracting. The species that vigorously respond to nutrient-rich areas with precise root placement are small and slow-growing. Those that largely ignore fertile clumps and protrude roots more single-mindedly are large, fast growing, and highly competitive. Their strategy is to cover ground and crowd out any potential competitors.

The economics of root proliferation to exploit fertile spots is complex. To be advantageous, nutrient uptake from increased root density has to justify the cost of new roots.

If a rich patch taps out quickly, a small gain is had at the cost of maintaining roots there. Fine roots are easily forgone as a small investment.

The thicker the roots, the less likely that their construction cost will be recouped if potential return has been misjudged. Coarse-rooted plant species are less enthused about exploiting nutrient hotspots than species adept at fine rooting. Across global biomes, plants have consistently evolved thinner roots to improve their productivity.

Plants evolving thinner roots enabled them to markedly improve their efficiency of soil exploration per unit of carbon invested and to reduce their dependence on symbiotic mycorrhizal fungi. ~ Chinese botanist Zeqing Ma et al

Plants alter the spatial distribution of their roots based upon resource patchiness and competition from other plants; decisions based upon reward/risk analysis.

Roots are very adaptive at modifying growth throughout the root system to concentrate their efforts in the areas that are the most profitable. ~ English botanist Angela Hodge