Continental-scale Organic Carbon and Carbon Isotope Inventory of Australian Soils.

Continental-scale Organic Carbon and Carbon Isotope Inventory of Australian Soils.

J.G. Wynn, M.I. Bird, L. Vellen, J. Cowley, D. Barrett1 and J. Carter2
1 CSIRO, Plant Industry

Program B2(c) of the CRC for Greenhouse Accounting (Wynn & Bird, RSES, EE) has completed a continental-scale inventory of soil organic carbon (SOC) storage in well-drained sandy soils of the Australian continent. The SOC reservoir is known to vary spatially with primary variables of climate, texture, drainage, and vegetation conditions, which have all been addressed by this study to date. Early phases of this project developed a sampling and carbon analysis program for the determination of SOC and 13-Carbon pools on a continental scale, which is based on a stratified sampling methodology and simplified analytical techniques. The current analytical phase of this project has completed work on soils of coarse texture (sandy) in well-drained conditions, and established relationships of two depth-specific SOC pools to climatic conditions, while also taking into account the local variability with respect to the distribution of trees and grass (Figure 26). Forty three sites have been sampled across a wide range of climate regimes and analysed for percent organic carbon and _13C, among other key soil physical and chemical properties. Subsequent analysis has produced best-fit empirical relationships of the 0-5 cm and 0-30 cm pools of SOC and

13-Carbon isotopic ratios to mean annual temperature and mean annual precipitation, such that greater than 70 percent of the variance measured can be attributed to climatic constraints.

Although simple regression analyses shows good correlation with the observed data, these simple relationships do not allow one to explore more complex issues such as the effect grazing pressure, wind transport, or other factors affecting soil carbon input or output. In order to do so, we are employing conceptual models, such as the Century 4 model being used by J. Carter (QDNR), in conjunction with the model's design team (Dr. Bill Pardon), or the VAST model being developed by D. Barrett (CSIRO). The data set provided by this study is of great utility in both model validation and parameterization of these carbon cycle models.

Current research of this project is under way to assess the variability of SOC and 13-Carbon with soil textural conditions (particle size distribution), by analysis along soil texture gradients within specific climatic regions. The next analytical phase (Feb.-Jun. 2003) will continue with 14-Carbon analysis of SOC in order to establish turnover time of the SOC pool.

Figure 26: Measured and modelled stable carbon isotopic composition (vertical axis, PDB scale) of the soil organic carbon pool at 0-5 cm depth across the Australian sites measured. MAP = mean annual precipitation (mm); MAT = mean annual temperature (°C). Similar relationships can be defined for total SOC, and the 0-30 cm depth interval. Simple equations can be fit to the observations by least-squared regression, with R2 values > 0.7.