Oxygen transport in soil and the vertical distribution of roots
F. J. Cook A B D , J. H. Knight A and F. M. Kelliher CA CSIRO, Land and Water, 120 Meiers Road, Indooroopilly, Qld 4068, Australia.
B The University of Queensland, St Lucia, Qld 4067, Australia.
C Manaaki Whenua Landcare Research, PO Box 69, Lincoln, New Zealand.
D Corresponding author. Email: freeman.cook@csiro.au
Australian Journal of Soil Research 45(2) 101-110 https://doi.org/10.1071/SR06137
Submitted: 6 October 2006 Accepted: 7 March 2007 Published: 28 March 2007
Abstract
An analytical solution for steady-state oxygen transport in soils including 2 sink terms, viz roots and microbes with the corresponding vertical distribution scaling lengths forming a ratio p, showed p governed the critical air-filled porosity, θc, needed by most plants. For low temperature and p, θc was <0.1 but at higher temperatures and p = 1, θc was >0.15 m3/m3. When root length density at the surface was 104 m/m3 and p > 3, θc was 0.25 m3/m3, more than half the pore space. Few combinations of soil and climate regularly meet this condition. However, for sandy soils and seasonally warm, arid regions, the theory is consistent with observation, in that plants may have some deep roots. Critical θc values are used to formulate theoretical solutions in a forward mode, so different levels of oxygen uptake by roots may be compared to microbial activity. The proportion of respiration by plant roots increases rapidly with p up to p ≈2.
Synthesis of vertical root biomass density, L [= L0 exp(–z/Zr), z is the depth positive down (m)] (m/m3), data using an exponential function to represent the distribution suggested that, on average, 70 ± 10% of fine roots in 10 terrestrial biomes were located in the upper 0.1 m of soil. Integrated over the root-zone, LT is given by the product of the function’s 2 parameters, the surface value of L, L0 (m/m3), and length scale, Zr (m). As postulated, negative correlations were obtained between L0 and Zr. For a maize (Zea mays L.) crop, significantly different distributions were measured during relatively dry and wet seasons and predicted by our model. For woody and herbaceous plants, Zr (the value determines the rate of decrease in L with depth) averaged 0.3 and 0.2 m, respectively, while the corresponding averages for Rm0 [= L0.ρr, ρr is root density (kg/m)] were 2.7 and 1.1 kg/m3.
Additional keywords: oxygen, roots, root respiration, soil respiration, soil aeration.
Acknowledgments
The authors are grateful for encouragement from colleagues to continue this work, much of it done privately. F. M. Kelliher was funded by the New Zealand Foundation for Research, Science and Technology (contract C09X0212). The authors would like to thank Dr G. Clarke Topp for his help in obtaining the Dwyer et al. datasets and for B. A. Ma for retrieving and supplying us with these data.
Chabot R,
Boufarfa S,
Zimmer D,
Chaumont C, Duprez C
(2002) Sugarcane transpiration with shallow water-table: sap flow measurements and modelling. Agricultural Water Management 54, 17–36.
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[K]
f is the soil porosity [L3/L3]
g is a constant defined in Eqn 4
I0 is a modified Bessel function of first kind and zero order
K0 is a modified Bessel function of second kind and zero order
L root length density as a function of depth [L/L3]
L0 is root length density at z = 0 [L3/L3]
LT total root length in the soil profile per unit area [L/L2]
M(z,T) is the microbial respiration as related to depth and temperature [M/L3.T]
M0(T) is the microbial respiration at the soil surface (z = 0) as related to temperature [M/L3.T]
M* is the reference microbial respiration rate defined in Eqn 7 [M/L3.T]
p = Zr/Zm, is the ratio of the root and microbial length scales. This determines the difference in the rate of decrease with depth of the root length density and microbial respiration rate
Qp is the proportion of the total sink due to the root sink and defined in Eqn 9
Q is the total soil oxygen sink [M/L3.T]
R(z) is the respiration rate of roots in unit volume of soil [M/L3.T]
R* is the normalised root sink
Rm is root mass density [M/L3]
Rm0 root mass density at the surface [M/L3]
RmT is the total root mass in the soil profile [M/L2]
RT is the total respiration rate per unit area of soil due to roots [M/L2.T]
T is temperature [K]
T0 is a base temperature (227.1 K) in Eqn 7 [K]
W is the radius of an a of saturated soil around the root plus the root [L]
X = 2Zrg1/2 exp(−z/Zr) is a modified space coordinate [L]
X0 is X at z = 0
z is depth positive downward direction [L]
Zr is the length scale for root length, it determines the rate at which the root length density decreases with depth [L]
Zm is the length scale for microbial respiration rate, it determines the rate at which the microbial respiration rate decreases with depth [L]
Zr is the length scale for the root length density; it determines the rate at which the root length density decreases with depth
Y is a dummy variable
α is the Bunsen coefficient [L3/L3]
β is a parameter in the power function relationship between root length density and depth [L/L3]
λ = M0(T)/Da is a parameter in Eqn 3 [M/L2]
θ is the volumetric water content [L3/L3]
θa is the air-filled porosity of the soil [L3/L3]
θc is the critical air-filled porosity of the soil [L3/L3]
ρr is the root density [M/L3]
