Potential Errors When Estimating Soil Organic Carbon Maintenance Requirements.

A change in cropping philosophy from food and fiber to food, fiber, and energy production has the potential to increase agricultural intensification and reduce soil quality. To maintain the long term soil sustainability, research has been initiated at numerous locations. Isotopic and non-isotopic approaches can be used for measuring soil organic matter turnover in production fields. The non-isotopic approach is based on the equation, [NHCa /SOCtime0] = ksoc/kNHC+[(dsoc/dt)(1/(kNHC*SOCtime0)) where, NHCa is the nonharvested biomass returned to soil, SOCtime0 is the soil organic matter (SOC) at the start of the experiment, ksoc is the mineralization rate of the relic soil organic C, kNHC is the mineralization rate of non-harvested carbon, and dSOC/dt is the change in SOC with time. This equation makes the assumption that above and below ground biomass equally impact SOC and that the amount of below ground biomass is known. Calculations show that maintenance requirement is influenced by the estimated amount of below ground biomass. To overcome this problem, actual measurements of below ground biomass are needed. If the 13C isotopic approach is used to develop C budgets, serious errors occur if the 13C fractionation during SOC turn over is not considered. Not considering this fractionation can result in underestimating the SOC half life by 50%, overestimating the C4 plant contribution and underestimating the C3 plant contribution to SOC. To overcome this problem, experiments should include a no-plant control where 13C isotopic discrimination during relic SOC mineralization is measured. A failure to account for either problem can result in maintenance recommendations that are incorrect.