Publication

Soil organic carbon (OC) resulting from terrestrial photosynthesis is one of the largest reactive pools of sequestered atmospheric C-2. As an intermediate storage mechanism for organic material through time, the vertical distribution of carbon in a soil profile generally shows an age continuum with depth. Aged soil carbon is often the result of sorptive preservation, a physical mechanism that protects organic matter (OM) from microbial decay. Sorptive preservation is, however, a reversible process. Upon desorption, previously protected OM could be available for decomposition. Here we present simple fresh and salt water desorption experiments designed to determine the amount, C-14 age and lability of "reversible" OM. Results suggest that 1.1-3.9% and 1.5-6.7% of OC, and 0.7-3% and 0.6-5% of total soil nitrogen can be desorbed using fresh and salt water, respectively. Radiocarbon data suggest this desorbed carbon is Delta C-14-enriched compared to the bulk soil, with Delta C-14 values close to modern atmospheric Delta(CO2)-C-14 at the soil surface, and aging with depth. Of this extractable aged carbon, upwards of 23-56% is labile across all treatments and with depth. Additional carbon is extracted with salt water compared to fresh water and this surplus is non-labile and younger than the labile component. Radiocarbon dating shows that the C-14 age of soil carbon increases with depth and that there is a pool of Delta C-14-depleted OC that is readily remineralized upon entering an aquatic/marine environment. These results indicate that eroded soil particles can release OC and organic N when entering rivers and oceans and that up to 50% of this carbon is available for aquatic metabolism. (C) 2007 Elsevier Ltd. All rights reserved.