Potential DOC production from size-fractionated Arctic tundra soils

Soil organic carbon (SOC) accumulated in the Arctic regions has been subject to impacts of climate warming, but the interactions of SOC with aquatic environments and their biogeochemical consequences remain poorly understood. Arctic tundra soil samples were collected from the upper permafrost of arctic Alaska and size-fractionated into the > 2000 µm, 250–2000 µm, 53–250 µm and < 53 µm fractions by dry sieving. Production potential of dissolved organic carbon (DOC) from bulk and size-fractionated soils were evaluated under different extraction time, temperature, soil/water ratio, and preservation conditions. The soil DOC (< 0.7 µm) was further fractionated into high-molecular-weight (HMW, > 3 kDa) and low-molecular-weight (LMW, < 3 kDa) DOC fractions using ultrafiltration to examine the molecular weight distribution of soil DOC. Our results showed that, on average, the DOC yield was ~ 2.6% of total organic carbon (TOC) for moist acidic tundra (acidic) soils and ~ 1.8% of TOC for moist non-acidic tundra (non-acidic) soils. In general, soil extraction at higher temperature (22 °C vs. 2 °C) resulted in a 10–20% higher DOC. Degradation of soil DOC became noticeable after 24 h. Content of OC in different sized soil particles followed the order of 53–250 µm > 250–2000 µm > less than 53 µm. In both acidic and non-acidic soils, DOC yields increased with decreasing soil particle size, while TOC content increased with increasing particle size. Soil DOC released from bulk acidic soils contains 65% of HMW-DOC and 35% of LMW-DOC, but 44% and 56% respectively in non-acidic soil DOC. Acidic soils seemed to have higher potential of releasing DOC than non-acidic soils. On average, up to 2% of arctic SOC could be potentially released into aquatic systems during soil/water interactions. The fate of this newly released soil DOC and its biogeochemical impacts on aquatic systems needs further investigation.