Roots respiration

W. Cheng, D. Coleman, C. R. Carroll, and C. A. Hoffman, In situ measurement of root respiration and soluble C concentrations in the rhizosphere. Soil Biology and Biochemistry 25 1189 (1993). 

Several authors have applied in situ pulse labeling of plants (grasses and crops) with C-CO2 under field conditions with the objective of quantifying the gross annual fluxes of carbon (net assimilation, shoot and root turnover, and decomposition) in production grasslands and so assess the net input of carbon (total input minus root respiration minus microbial respiration on the basis of rhizodeposition and soil organic matter) and carbon fixation in soil under ambient climatic conditions in the field. 

We conclude that recent advances in the use of tracers, albeit C or C, offer a solution to some of the technological difficulties in quantifying and separating microbial from root respiration. Combinations of these tracers may even overcome the last of the problems in discriminating between all three sources of COt released from soil. 

Tolley MD, DeLaune RD, Patrick WH. The effect of sediment redox potential and soil acidity on nitrogen uptake, anaerobic root respiration, and growth of rice (Oryza saliva). Plant Soil. 1986 93 323-331. 

The partial pressure of C02 in the soil air controls the concentration of both dissolved C02 and undissociated carbonic acid. At 0.003 atm of C02 (g) as a reference level for soils, [H2C03°] is about 1.04 x 10 4 M (Lindsay, 1979). At a normal atmospheric level of 0.0003 atm C02 (g), [H2C03°] is approximately 1.04 x 10 5 M. In most soils, C02 (g) is higher than in the atmosphere. C02 is released from soil and plant root respiration. In flooded soils, C02 (g) partial pressure increases to 0.01-0.3 atm, about 1000-fold higher than normal upland soils due to strong microbiological activity (Lindsay, 1979). 

Rainwater recharges the top of the profile and reacted water drains from the bottom. We take discharge through the soil to be 4 m yr-1 and assume the dispersivity or (see Chapter 20) is 1 cm. The rainwater is dilute and in equilibrium with the CO2 fugacity of the atmosphere, 10-3 5. Within the soil, however, the soil gas is taken to contain additional CO2 as a result of the decay of organic matter, and root respiration. The pore fluid is assumed to maintain equilibrium with the soil gas and CO2 fugacity within the soil is held constant over the simulation, at 10-2. 

Two general approaches, component integration and whole system analysis have been used for to assess soil and root respiration (Anderson 1982 Hanson et al. 2000 Bostrom et al. 2007). In component integration the net respiration is determined by summing the respiration rates of the individual components (roots, plant residues, and soil). The disadvantage of this approach is the physical separation of these materials and that interactions between components cannot be evaluated. 

Kuzyakov Y, Larionova AA (2006) Contribution of rhizomicrobial and root respiration to C02 emission from soil (a review). Eurasian Soil Sci 39 753-764... 

In the model, the internal structure of the root is described as three concentric cylinders corresponding to the central stele, the cortex and the wall layers. Diffu-sivities and respiration rates differ in the different tissues. The model allows for the axial diffusion of O2 through the cortical gas spaces, radial diffusion into the root tissues, and simultaneous consumption in respiration and loss to the soil. A steady state is assumed, in which the flux of O2 across the root base equals the net consumption in root respiration and loss to the soil. This is realistic because root elongation is in general slow compared with gas transport. The basic equation is... 

The boundary conditions for solving Equation (6.1) are (a) at the root base, [02]g is the ambient value in the atmosphere and (b) at the root apex, [02]g is the minimum value required for root respiration 30ttmoldm (gas space)]. The equations are solved numerically. 

Figure 6.6 Effect of cortical porosity of primary root and fraction of root covered with laterals on (a) maximum primary root length, (b) absorbing root surface per unit root mass, and (c) absorbing root surface per primary root as a function of net O2 consumption, and (d) O2 consumed in root respiration and loss to the soil. Numbers on curves are porosities other parameters have standard values (Kirk, 2003). Reproduced by permission of Blackwell Publishing...

Because very large concentrations of dissolved CO2 develop in submerged soil, in spite of root respiration the CO2 pressure outside the root may be greater than that inside it, resulting in a flow of CO2 from the soil to the atmosphere through the aerenchyma. Net removal of CO2 by the root decreases the concentration of the acid H2CO3 near the root, and this may offset the acidity produced in oxidation and excess cation uptake. 

Bowden, R. D., K. J. Nadelhoffer, R. D. Boone, J. M. Melillo, and J. B. Garrison. 1993. Contributions of aboveground litter, belowground litter, and root respiration to total soil respiration in a temperate mixed hardwood forest. Canadian Journal of Forest Research 23 1402-1407. 

Despite its importance in ecosystem C fluxes, soil respiration has limitations as a constraint on SOM turnover, for two main reasons. First, it is difficult to partition soil respiration into its two sources (1) decomposition of SOM by microbes (heterotrophic respiration) and (2) respiration from live plant roots (autotrophic respiration) (Kuzyakov, 2006). As a result, an increase in soil respiration may indicate not only an increase in SOM decomposition but also an increase in root respiration. Second, it is likely that in most soils only a small fraction of total SOM contributes to heterotrophic respiration. As a result, respiration measurements provide information about the dynamic fraction of SOM (particularly when combined with 14C measurements of respiration) but do not provide information about the large, stable pools unless they are destabilized and contribute to respiration (detectable with 14C02 respiration measurements). Attributing the sources of respiration from different SOM reservoirs, which may respond differently to climatic variables, is not... 

Kuzyakov, Y., and Siniakina, S. V. (2001). Siphon method of separating root-derived organic compounds from root respiration in non-sterile soil. J. Plant Nutr. Soil Sci. 164, 511-517. 

Lee M. Nakane K. Nakatsubo T. and Koizumi H. (2005). Long-term carbon exchanges at a Takayama, Japan forest The importance of root respiration in annual soil carbon fluxes in a cool-temperate deciduous forest. Agricultural and Forest Meteorology, 134(1 -4), 95-101. 

Now we must attempt to determine the source of the rest of the HCO3 in river waters. The HCO3 is derived from two major sources atmospheric CO2 that has interacted directly with silicate minerals or has been incorporated into plants by photosynthesis, later to be released by plant decay or root respiration and CO2 produced by oxidation of fossil carbon already present in rocks (Garrels and Mackenzie, 1971a, 1972 Holland, 1978). 

Rain and surface water dissolve small amounts of atmospheric C02. Significantly more C02 is added to water percolating through the soil layer, as soil air contains about 100 times more C02 as compared to free air. Soil C02 is produced by biological action such as root respiration and decay of plant material. This C02 was tagged by the atmospheric 14C concentrations, that is, about 100 pmc in pre nuclear bomb times (pre-1952) and up to 200 pmc in post-bomb years. 

HSgberg, P., Nordgren, A. Agren, G. I. (2002). Carbon allocation between tree root growth and root respiration in a boreal pine forest. Oecologia, 132, 579-81. 

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