Non-isotopic and 13C Isotopic Approaches to Calculate Soil Organic Carbon Maintenance Requirement

To meet long-term food, energy, and fiber security requirements, production systems must be sustainable. A critical component in sustainable agricultural systems is the maintenance of soil organic carbon (SOC). SOC maintenance requires, over time, the amount of carbon added to soil to be equal to the amount of relic carbon mineralized. Obtaining the information required for maintenance calculations is difficult and, therefore, many projects assume that: (1) above- and below-ground biomass have similar impacts on SOC turnover; (2) root to shoot ratios can be used to estimate below-ground biomass; and (3) 13C enrichment during SOC mineralization is insignificant. This chapter reviews non-isotopic and 13C isotopic approaches used to develop carbon budgets, and it investigates the implications of simplifying assumptions on SOC turnover calculations. Sensitivity analysis of carbon-budget equations showed that: (1) if the root to shoot ratio is underestimated, then the above-ground biomass needed for maintenance is overestimated, whereas the reverse is true if the root to shoot ratio is overestimated; and (2) in systems where C4 residue is applied to soil derived from C4 and C3 plants, the calculated half-life was higher when 13C fractionation during relic carbon mineralization was considered. For C3 plants, the reverse was true. For 13C natural abundance calculation these errors can be minimized by testing the assumptions that 13C isotopic discrimination during mineralization of relic carbon and fresh biomass does not occur. Correction values for 13C isotopic discrimination of relic carbon can be calculated from data collected from control areas where plant growth is prevented. Temporal changes in relic SOC 13C isotopic discrimination can be measured in this zone. A mathematical approach for using 13C isotopic discrimination during relic carbon mineralization in SOC budget calculations is provided