Sensitivity Analysis of the Nonisotopic Approach Root to Shoot Ratio

Sensitivity analysis of the non-isotopic calculation approaches indicates that the rate constants and maintenance calculations are sensitive to NHC and soil depth (Clay et al. 2005, 2008; Fig. 8.4). The NHC value is the sum of above- and below-C pools. Above-ground biomass is easily measured. However, obtaining "good" measures of below-ground biomass is very difficult (Kuzyakov and Domanski 2000; Amos and Walters 2006). In the past, nearly all efforts have underestimated this value because destructive soil sampling techniques (soil sampling, rinsing, and weighing) do not measure small roots and root exudates. Molina et al. (2001) predicted that 24% of the plant's net fixed carbon was released from the corn plants during the growing


• Footslope o Lower backslope T Backslope v Upper backslope ■ Shoulder - y=34.6+39.4x;r=0.92

Fig. 8.4 Relationship between root to shoot ratio and the amount of above-ground biomass that can be harvested and still maintain the SOC level at the current level season. Kuzyakov (2001) reported that about 1/3 of the below-ground carbon was either respired or exudated. Root exudation may actually reduce the mineralization of other carbon sources (Torbert et al. 2000). Efforts to measure root respiration and the impact of root exudates on soil respiration have relied on the measurement of CO2 released in areas with and without plants. Kuzyakov and Domanski (2000) suggest that approximately half of the below-ground C is incorporated into root tissue, one third is respired by roots and rhizosphere microorganisms, with the remaining one sixth of the carbon incorporated into the soil and microorganisms.

Below-ground biomass is typically estimated from the root to shoot ratio (Johnson et al. 2006; Bolinder et al. 2007). Extreme care must be used when using published root to shoot ratios because different scientists define root to shoot ratios differently. For example, Johnson et al. (2006) defined root to shoot ratios for corn (Zea mays) as the ratio between root biomass and total above-ground biomass (grain, stover, and cob), whereas Amos and Walters (2006) defined this value as the ratio between root biomass and corn stover. In addition, a standardized root to shoot ratio has not been used in maintenance calculations. For example, Barber (1978) used a value of 0.17 for corn, Huggins et al. (1998) used a value of 0.53, and Larson et al. (1972) did not consider roots.

Sensitivity analysis showed that the amount of corn stover that could be harvested increased with root to shoot ratio (Fig. 8.4). If roots were not considered in the NHC value, then the estimated amount of above-ground biomass that could be safely harvested was about 35%, whereas if the root to shoot ratio was 1.00 then 70% of the above-ground biomass could be harvested. These findings are attributed to a relative increase in importance of the below-ground biomass. Based on these calculations, underestimating the root to shoot ratio will result in underestimating corn stover removal rates, which, while having a positive influence on future organic matter content of the soil, may cost producers valuable income if above-ground biomass is sold as a commodity.

In addition to highly variable root to shoot ratios (0.01-1.22) the use of these values is complicated by: (1) ratios that are hybrid, variety, and species-specific; and (2) below-ground allocations that are impacted by stress (Herbert et al. 2001; Bradford et al. 2005; Amos and Walters 2006; Johnson et al. 2006). For example, Johnson et al. (2006) used root to shoot ratios of 0.82, 0.55, and 0.62 for wheat (Triticum aestivum), corn, and soybean (Glycine max), respectively; whereas Amos and Walters (2006) reported that root to shoot ratios increased with N and P deficiencies and decreased with increasing water stress, population, shade, and soil compaction.

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