Soil Organic Matter Mean Residence Time Measurement: Characterization of Analyitical and Numerical Approaches for the Bombspike Model Resolution

Anthropogenic actions, fossil fuel use and land use change, are drastically altering the global carbon cycle. CO2 in the atmosphere, the most sensitive compartment of the entire cycle, has never been exceeded in the last 650 ka. Terrestrial ecosystems preserve in the soil organic matter(SOM) 1600 PgC, an amount of C twice with respect to the atmosphere. The amounts of carbon in SOM, its exchange fluxes with the atmosphere, the observed sequestering times(ranging from years to centuries)and the possibility of regulation of C in future land management, has led the scientific community to look at a more precise soil C cycle. Recent studies pointed out how, because of the multi compartment nature of soils, SOM cannot be considered as a homogeneous reservoir. The coexistence of several phases in the soil compartment is evidenced by the presence of different SOM carbon pools (fractions) each one characterized by a homogeneous C mean residence time. Considering SOM as a composite compartment drastically increases difficulties in the methodological approach to the study of these pools resulting in complex feedback responses to changing climate conditions. Atmospheric tests of nuclear weapons (banned in 1963 after the test ban treaty) enriched atmospheric 14CO2 doubling its background levels. The signal (bombspike) decreases over time, with an exponential trend (annual rate of 0.4%), because of the net uptake of CO2 from oceans and vegetation; and the dilution caused by fossil CO2 releases. Observed bombspike decrease rate allows dating of atmospheric CO2 with a precision of ± 1 year. Once globally distributed in the atmosphere the bombcarbon signal becomes a SOM marker allowing a sensitive mean residence time measurement of C by means of dynamic models simulating its fate in the soil (bombspike models). Bombspike models rely on the SOM box model, in which first derivative of soil C content over time is expressed by the net balance between net inputs of C and outputs mainly due to decomposition. Decomposition flux is related to the carbon content by means of the decomposition constant (k) implying a first degree kinetic relationship among outputs and C content. SOM C over time can be analytically expressed only under some particular edge conditions (i.e. constant input scenario). Combining the bomb spike properties (i.e. each annual 14C input) and the box model it is possible to model SOM 14C content as the sum of a contribution of each annual input and hence express SOM 14C/12C ratio. Annual radiocarbon input over time are not constant (because of the bomb spike) hence the constant input scenario is not verifiable for 14C and model predicted SOM 12C are not be free of bias.The main goal of this contribution is to quantify the biases introduced during the analytical resolution of the bombspike model by comparing it with the alternative numerical approach. The same results will be shown for the resolution of more complex model simulating the relationships occurring among fractions as described by different soil fractionation protocols (i.e. cascade three boxes model).