Even though sensitivity analysis of carbon budget maintenance equations shows that below-ground biomass estimates influence SOC maintenance rate calculations, most experiments do a poor job at estimating below-ground biomass. To overcome this problem simplifying assumptions are accepted. In most situations, the impact of these assumptions on the recommendations is not tested. The minimum data required to estimate SOC maintenance requirements are SOCinitial, SOCfinal, and the amount of NHC returned to the soil during the study period. This minimum data set is not available for most studies.

Many experiments are conducted on a specific soil and do not consider how landscape position impacts carbon turnover. The few studies that have been conducted show that landscape position has a large impact on carbon turnover (Fig. 8.7). Campbell et al. (2005) reported that in Colorado, soil organic C gains increased with cropping intensity and tended to be the highest in the lowest evaporation sites and least in the toe slope area. Footslope areas generally have higher turnover rates than summit shoulder areas (Campbell et al. 2005; Clay et al. 2005; Soon and Malhi 2005). Soon and Malhi (2005) reported that the timing of the mineralization may also be impacted by landscape position. In the upper landscape positions, N mineralization was suppressed. Landscape differences can result from two interrelated factors, higher soil water contents, and amount of SOC in footslope than summit/ shoulder areas (Clay et al. 2001).

Carbon turnover in production fields can be determined, using non-isotopic techniques, by combining historical soil samples, current soil samples, and whole field yield monitor data. Sensitivity analysis of such data shows that the amount of above-ground biomass that could be harvested decreases with root to shoot ratio (Table 8.1). For example, if root biomass is ignored, analysis suggests that only 20-30% of the above-ground biomass can be harvested, whereas if the root to shoot ratio is 1.0, then between 40% and 70% of the residue could be harvested.

The impact of the root to shoot ratios on calculated maintenance requirements is important because root to shoot ratios are highly variable and almost always underestimate below-ground biomass. Amos and Walters (2006) reported that the net below-ground C deposition in corn at physiological maturity was 29% ± 13% of the shoot biomass (leaves, stems, and husks) in 41 studies. The use of these values is further complicated by the use of different definitions for root to shoot ratio. Converting Amos and Walters (2006) units to units used by Johnson et al. (2006) would reduce the reported values from 0.29 to 0.15 (harvest index 50%).

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