Significant Bomb 14C 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 TechnologyMinistry of Agriculture and Forestry

US-NZ Climate Change Partnership through NZ Govt

-400 -200 0 200

0

50

100

150

Dep

th (

cm)

∆14C (‰, pre-bomb atm.= 0)

% Modern Carbon (1950 atmosphere)

80 100 12060

1952

1997

California Grassland Soil

Baisden et al 2002, Global Biogeochemical Cycles

Koputaroa, New ZealandGoh et al, 1976Syers et al, 1970

Judgeford, New ZealandO’Brien and Stout, 1978

Understanding Carbon Storage and Turnover•The balance between inputs and outputs

Soil Organic Matter

Inputs Outputs

The Model

DOM

vDOM

kDOM

Concentration

Model Formulation for DRDSC(Decadally Reactive Deep Soil C)

• Mass (g/m2)

• 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 14C 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

14C (per mil)

Cu

mla

tive

So

il M

ass

(g c

m-2)

Soil Organic Carbon (SOC) Pools Estimated by the ModelSite Max.

DepthIncluded

inAnalysis

(m)

Total SOC

(MgC/ha)

SOCwith turnover <100 yr

(MgC/ha)

DRDSC

(MgC/ha)

Koputaroa 0.8 148±4 81±4 10±3Judgeford 1.0 192±10 131±7 10.3±0.7*

TurlockLake

1.1 33±2 13±1 4.7±3.3

Riverbank 3.6 39±3 12±2 9.0±1.3*Model appears to underestimate DRDSC pool size.

Model-Estimated Soil Organic Carbon (SOC) PoolsSite Max.

DepthIncluded

inAnalysis

(m)

Ratio ofDRDSC toTotal SOC

Ratio ofDRDSC toSOC withturnover<100 yr

Koputaroa 0.8 0.07±0.02 0.13±0.04Judgeford 1.0 0.054±0.005* 0.078±0.008*

TurlockLake

1.1 0.14±0.10 0.37±0.27

Riverbank 3.6 0.23±0.05 0.74±0.05*Model appears to underestimate DRDSC pool size.

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