Processes in Roots and the Rhizosphere - The Biogeochemistry of Submerged Soils - page 170

species Phragmites australis , and found that for the appropriate combination of
root types, properties and dimensions, and a large but realistic soil O 2 demand,
the ratio of O 2 consumption in root respiration to that in loss to the soil was
13:1 for adventitious roots but 0.15:1, i.e. reversed, for laterals. Evidence for
preferential loss of O 2 from laterals in rice includes measurements of Fe oxide
coatings on roots placed in deoxygenated agar containing Fe(II) (Trolldenier,
1988); changes in redox potential as roots grew across rows of Pt electrodes in
anaerobic soil (Flessa and Fischer, 1993); and the abundance of methane oxidiz-
ing bacteria, which are obligate aerobes, along rice lateral roots in anaerobic soil
(Gilbert et al ., 1998).
Although O 2 leakage compromises the root's internal aeration, some leakage is
desirable for a number of purposes. These include oxidation of toxic products of
anaerobic metabolism in submerged soil such as ferrous iron (van Raalte, 1944;
Bouldin, 1966; van Mensvoort et al ., 1985); nitrification of ammonium to nitrate,
there being benefits in mixed nitrate-ammonium nutrition (Kronzucker et al .,
1999, 2000); and mobilization of sparingly soluble nutrients such as P (Saleque
and Kirk, 1995) and Zn (Kirk and Bajita, 1995) as a result of acidification due
to iron oxidation and cation-anion intake imbalance.

6.2.1 MODEL OF ROOT AERATION VERSUS NUTRIENT ABSORPTION

Kirk (2003) has developed a simple model to compare root requirements for
aeration with those for efficient nutrient acquisition in rice. The main features
of the rice root system are summarized in Figure 6.4. The model considers roots
in the anoxic soil beneath the floodwater—soil interface, receiving their oxygen
solely from the aerial parts of the plant.

Structure of the Root System

The distribution of primary roots beneath a hill of plants is approximately hemi-
spherical with the individual roots randomly distributed with respect to the
vertical and horizontal directions. Thus if there are N primary roots per hill,
the length of primary roots per unit soil volume, L VP , at any distance r from the
centre of the hill is

d N/ d r
d V/ d r =

N
2 πr 2

L VP (r) = ( 6 . 2 )

About each primary root there is a cylinder of laterals, increasing in density
with distance from the root base (Figure 6.5). The laterals may develop up to
sixth-order branches. A simple equation to describe this is:

r 2
(k + r) 2

L VL (r) = L VLmax ( 6 . 3 )