Global-scale climate-related variations in the carbon isotope composition of soil organic carbon in coarse-textured soils M. Bird, Y. Zhou, L. Vellen, J. Cowley, J.O. Carter and G.D. Farquhar

Global-scale climate-related variations in the carbon isotope composition of soil organic carbon in coarse-textured soils

M. Bird, Y. Zhou, L. Vellen, J. Cowley, J.O. Carter and G.D. Farquhar

Soil organic carbon (SOC) is the major terrestrial reservoir of carbon and contributes substantially to the flux of CO2 between the terrestrial biosphere and the atmosphere. Carbon isotopes in the SOC pool are effected by several processes, including isotopic fractionation effects that accompany microbial degradation of SOC. Furthermore, the SOC pool is not in isotopic equilibrium with the modern atmosphere, but is subject to a 'lag' between photosynthetic uptake of CO2 respired from SOC and microbial respiration that is a function of decomposition rate.

 

Owing to the mixed C3/C4 biomes in the tropics and sub-tropics, the delta13C value of SOC is affected by a highly heterogeneous distribution of C3 and C4 vegetation. Hence, this study utilizes stratified sampling to provide weighted delta13C values for SOC. About 8,000 cores from coarse-textured soils were collected since 1997 from 41 regions of widely different climate, mostly in Australia but also in Botswana (4), Siberia (3), Canada (5), the Czech Republic (1) and Sarawak (1). Mean annual temperature for these regions range from -14 to +28°C and precipitation from 120 to 3,300 mm. Samples were collected at each locality from beneath trees and away from trees. Multiple cores were collected in sealed bags from 0-5 cm depth and 0-30 cm. A visual estimate of crown cover was made at each location. Five localities over several kilometres comprises a transect, and five transects represent a region (200 cores in total).

 

Soil bulk densities ranged from 0.19 to 1.73 g/cc for the 0-5 cm interval and 0.58 to 1.54 g/cc for the 0-30 cm interval. Weighted carbon inventories over the full 0-30 cm interval ranged from 36 mg/sq cm in hot, dry regions to a maximum of 960 mg/sq cm in wet, cool temperate locations. In pure C3 biomes, delta13C values for SOC in the 0-5 cm interval ranged from -29.4permil in wet tropical forests to -26.2permil in arctic tundra regions, and the values from beneath trees and grass agreed to ± 0.07permil. Figure 6 shows the variation with temperature and rainfall. The pattern is different where C4 plants are a significant fraction of the total biomass. Here, theweighted delta13C values increases rapidly in hot, drier environments characterized by tropical and sub-tropical savannas and desert, owing to increased proportions of C4 plants in the total biomass. To a lesser extent, the increase also reflects the tendency for isotope fractionation during C3 photosynthesis to decrease under water-limited conditions, with a resultant increase in the delta13C value of C3-derived carbon.

Finally, the weighted delta13C value of the 5-30 cm interval is enriched by 0.9 ± 0.6 permil compared to the 0-5 cm interval. This may result from several factors: (i) SOC age generally increases with depth, and older SOC will have been formed in equilibrium with the higher delta13C atmosphere that existed prior to industrialization, (ii) the effect of microbial degradation is to increase the delta13C value of the remaining SOC, and (iii) the relative contribution of carbon from woody roots increases with depth.

Figure : (a) Relationship between the weighted delta13C value of SOC from exclusively C3 biomes and mean annual temperature (MAT), and (b) for mean annual precipitation (MAP)