Tracer Experimental Approaches

Three general isotopic approaches, pulse labeling, continuous labeling, and the 13C natural abundance have been used to assess carbon budgets and below-ground biomass. Techniques using 14C and pulse and continuous labeling techniques are beyond the scope of this chapter and can be found in Goh and Molloy (1979), Goy (1991), Paul et al. (1997), and Kuzyakov and Domanski (2000). The major advantage of isotopic approaches over non-isotopic approaches is that source tracking of individual pools can be conducted and the number of assumptions associated with the maintenance calculations can be reduced.

Of the three approaches, the 13C natural abundance approach is a technique that has been widely used to carbon turnover in production fields. The 13C isotopic approach is based on soil, C4, and C3 plants having different 813C signatures. The relative amount of below-ground biomass is calculated by multiplying the total amount of SOC at the end of the season times a weighting factor (Balesdent and Mariotti 1996). Kuzyakov and Domanski (2000) concluded that the 13C approach only provided a rough estimate of rhizodeposition because variability of 813C in soil and plants is between 1%e and 2%e. However, successful utilization of the 13C natural abundance by Kuzyakov and Cheng (2001) and Rochette and Flanagan (1997) dispute this claim. It is likely that the ability to use the 13C approach is site-specific. For example, if 2,000 kg C ha-1 with a 813C value of -11.2%c are added to a soil containing 80,000 kg of carbon with a 813C value of -17 %e, then the resulting 813C value of the soil will be -16.86%e. Some mass spectrometers and experimental techniques do not have the accuracy to measure this small difference (0.14%e). However, if the initial SOC level is 40,000 kg C ha-1, then the difference between treated and untreated soil will be much larger (0.28%e).

8.2.6.1 Root and Soil Respiration

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.

In whole system analysis, isotopic techniques are used to separate CO2 into CO2 derived from the plants or soil (Kuzyakov and Cheng 2001; Kuzyakov and Larionova 2005). When the natural abundance 13C approach is used, soil- and root-derived CO2 is trapped in a known amount in sodium hydroxide (NaOH). Total soil CO2 from areas containing plants and non-plant control areas are determined by titration where SrCl2 is used to precipitate the HCO3- and CO3-2 as SrCO3. The SrCO3 precipitate is then washed with deionized water, dried, mixed with V2O5 (catalyst) and analyzed for S13C (Kuzyakov and Cheng 2001). To separate CO2 into CO2 derived from the soil and plant, reference values for the plant (S13C value of the most recently expanded) leaf and soil (S13C value of the no-plant control area) are needed. Using this approach, plant-derived-respired CO2 (RPC) is calculated with the equation

where total CO2 Plant+soil is the total amount of CO2 trapped in the soil plus plant system, and f is the percentage of respired carbon from the plant. The fraction f is calculated with the equation f =

" plant ^soil CO2

where S13C

plant+soil CO2

is the S13C value of the CO2 containing both soil and plant carbon, S13C , , is the S13C value of the plant, and S13C

plant

.. CO2 is the S13C value of CO2

soil CO2 2

in an area not containing plants. Several recent reviews of rhizodeposition are available (Kuzyakov and Larionova 2005, 2006; Wichern et al. 2008). Rochette and Flanagan (1997) reported that the 13C natural abundance approach can be used to quantify rhizosphere respiration. Kuzyakov and Cheng (2001) reported that the 13C natural abundance and 14C pulse labeling under controlled laboratory conditions produced similar estimates of root-derived CO2 over a 7-day period. Based on their measurements, respired root-derived C was 17.3% and 20.6% of the total assimilated C for non-shaded and shaded plants, respectively. Kuzyakov and Larionova (2006) suggested that 40% of the rhizosphere CO2 efflux is due to root respiration and about 60% of this efflux is due to decomposing root exudates.

Several alternative approaches for separating CO2 into different components are available in Cheng et al. (2005) and Bostrom et al. (2007). These approaches often include collecting soil samples from fields and separating them into different depth increments and analyzing the resulting CO2 directly on a GC/MS (Bostrom et al. 2007) or using buried root chambers (Cheng et al. 2005).

The kinetics and timing of rhizodeposition are largely unknown. Melnitchouck et al. (2005) used a pyrolysis-field ionization mass spectrometer tool to show that day and night rhizodeposits of C, N, and S concentrations were 3-9.7 times larger than samples from non-cropped soil. They concluded that the diurnal dynamics in the molecular-chemical composition between day- and night-rhizodeposits resulted from the exudation of carbohydrates and amino acids during the photosynthetic period, the deposition of other root-derived compounds such as lipids, suberin, and fatty acids, and microbial metabolism of all available organic compounds in the rhizosphere.

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