Quantity, 14C age and lability of desorbed soil organic carbon in fresh water and seawater

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.