Comparison of organic carbon properties in extracted soil solutions obtained underneath Cryptomeria japonica and Quercus acutissima and its implication on stream dissolved organic carbon

Dissolved organic carbon (DOC) in soils is released into streams, working as a main component of the carbon cycle. DOC in terrestrial and aquatic ecosystems participates in many biogeochemical reactions and processes such as heterotrophic respiration, sorption of metals, and transport of pollutants. In order to understand the connectivity of organic carbon among soil, soil water, and forest streams, we investigated the concentrations and dual carbon isotope ratios (delta C-13 and Delta C-14) of soil organic carbon (SOC) and water-extractable organic carbon (WEOC) obtained from soils beneath two tree species stands, and compared these with stream Delta C-14-DOC of the forest watershed. Soil samples were collected at different depths (0-10, 10-30, and 30-50 cm) beneath Japanese cedar (Cryptomeria japonica) and sawtooth oak (Quercus acutissima). Although the SOC concentration was not significantly different between the two tree species, the WEOC concentration ([WEOC]) of soil at 0-10 cm depth under Cryptomeria japonica was higher than that of Quercus acutissima, in general. The delta C-13-SOC and delta C-13-WEOC increased, while the Delta C-14-SOC and Delta C-14-WEOC decreased with increasing soil depth. The Delta C-14-WEOC was higher than the Delta C-14-SOC, indicating that WEOC could be primarily derived from the young, hydrophilic, and exchangeable fraction of SOC, rather than from SOC strongly bonded to mineral soils. However, the stream Delta C-14-DOC was lower than Delta C-14-WEOC in general, except during summer storms. The 14C-depleted DOC released from deep soils or groundwater might lower Delta C-14-DOC of a stream, suggesting that relatively old DOC could be released into streams during baseflow. This is contrary to the results of previous studies that have reported positive stream Delta C-14-DOC from temperate forests. The discrepancy warrants future research on forest stream Delta C-14-DOC across entire seasons, particularly under Asian monsoon climates.