Document: TRO-3-59-68

Using radiocarbon to estimate rates of nutrient turnover in a California grassland soil chronosequence.

BAISDEN, W.T.* ¹, R.AMUNDSON ¹, D.L.BRENNER ¹, J.HARDEN ² and A.C.COOK ³

University of California, Berkeley, CA 94720 USA ¹
U.S. Geological Survey, Menlo Park, CA 94025 USA ²
Lawrence Livermore National Laboratory, Livermore, CA USA ³

Abstract:
To predict NPP and C storage, terrestrial ecosystem biogeochemistry simulation models rely on assumptions about the mechanistic basis for carbon and nutrient mineralization within the soil organic matter (SOM) pool. Despite this, few (if any) empirical measurements of C and nutrient mineralization rates have been made which are suitable for calibrating an ecosystem model. We use radiocarbon measurements to empirically calibrate a 3-pool model of C and N dynamics for a chronosequence of California annual grassland sites ranging in age from <3000 to ~3,000,000 years, while other soil-forming factors remain constant. On these time scales, mineral weathering causes considerable changes in nutrient availability and soil texture, variables known to control SOM dynamics. We measured the ¹⁴C/¹²C ratio of soil organic matter (SOM) in 150 cm deep profiles from four terraces sampled circa 1949, 1978 and 1998-- a period spanning the pulse ¹⁴C enrichment of the atmosphere from H-bomb testing. An advective transport model is used to interpret SOM turnover and transport as a function of soil depth, so that SOM can be partitioned into 3 pools with distinct turnover rates and C:N ratios. We find that the C:N ratio of fast-cycling SOM increases with soil age from roughly 12-15 to 20-25, indicating decreasing litter quality. Similarly, the inferred residence time of slow cycling SOM increases from ~15 years to ~21 years. Overall, the model provides estimates of nutrient turnover which are consistent with those derived from biomass harvests. Thus, we believe that this approach-- combining %C, %N and ¹⁴C data with a mass balance model-- may provide valuable empirical constraints for ecosystem biogeochemistry modeling.

Keywords: SOM, Carbon, Nitrogen, Nutrients, Biogeochemistry, Radiocarbon, C-14, Soil Chronosequence

This abstract is being presented at: 9:15 AM in session:
Oral Session #25: Organic Matter Dynamics in Ecosystems.