Significant Bomb  14 C Enrichment in Deep Soil: A Previously Unrecognized Decadal C Pool? W. Troy Baisden and Roger L Parfitt Landcare Research, Palmerston North, New Zealand Funded by NZ Foundation for Research Science and Technology Ministry of Agriculture and Forestry US-NZ Climate Change Partnership through NZ Govt

Depth (cm) ∆ 14 C (‰, pre-bomb atm.= 0) % Modern Carbon (1950 atmosphere) California Grassland Soil

Baisden et al 2002, Global Biogeochemical Cycles

Koputaroa, New Zealand Goh et al, 1976 Syers et al, 1970

Judgeford, New Zealand O’Brien and Stout, 1978

Understanding Carbon Storage and Turnover The balance between inputs and outputs Soil Organic Matter Inputs Outputs

The Model DOM v DOM k DOM Concentration

Model Formulation for DRDSC (Decadally Reactive Deep Soil C) Mass (g/m 2 ) Downward Transport (cm/yr) First Order Loss Coefficient (yr -1 ) E-folding Depth (cm) No interactions with other 3 pools Is DRDSC from roots or dissolved organic matter (DOM)? –Progressive enrichment in most profiles –Below rooting zone

Data Assimilation via a Genetic Algorithm Evolves a chromosome of parameter values toward the best value for the model fit (based on an ‘objective function’) Inefficient but searches parameter space on both sides of the bomb-C spike Monte-Carlo style variation in  14 C and %C data Parameter uncertainty estimated from Monte-Carlo –Greater of: Standard deviation of best fits with data PDFs Difference between best fit with and without PDFs included Emphasizing fitting DRDSC pool sizes

Koputaroa Soil, New Zealand  14 C (per mil) Cumlative Soil Mass (g cm -2 )

DRDSC as a Fraction of Total SOC 10-20% of Deep Soil C has Decadal Dynamics Possibly increasing with Depth? Better understanding of DOC needed! 10-20% of Deep Soil C has Decadal Dynamics Possibly increasing with Depth? Better understanding of DOC needed! Koputaroa