Optical properties and 14C ages of stream DOM from agricultural and forest watersheds during storms

Forest and agricultural land use affects the concentration and composition of dissolved organic carbon (DOC) in streams and rivers. To elucidate the impacts of forest and agricultural land use on stream DOC during storm events, we investigated DOC concentration ([DOC]), optical properties of dissolved organic matter (DOM), and Delta C-14-DOC in both forest- and agriculture-dominated headwater streams in South Korea in the summer of 2012. One forested and five agricultural streams were investigated. During storms, the peak [DOC] of forest stream increased to 5.8 mg L-1, approximately two times larger than that of the most agricultural stream (3.2 mg L-1), demonstrating the weaker storm responses of the [DOC] of agricultural streams to hydrological change. Five PARAFAC components were identified, including three terrestrial humic-like substances (C1, C2, C3), one microbial humic substance (C4), and one microbial protein-like substances (C5). The mean (C4+C5)/(C1+C2+C3) of all storm events at the most agricultural stream was 1.5 times larger than that of the most forested stream, suggesting that more protein-like DOM is exported from agricultural watersheds. Whereas a forest stream was primarily composed of terrestrially derived and C-14-enriched modern DOC, the C-14-age of the most agricultural stream was up to similar to 1000 years old. The results suggest that agricultural practices could decrease the old organic carbon pools from soils. However, how quickly the aged DOC can be degraded to CO2 in streams is unknown, warranting future investigation on lability of the aged DOC and their effects on CO2 evasion from rivers and estuaries downstream. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study found that forest and agricultural land use impact the concentration and composition of dissolved organic carbon (DOC) in stream during storm events; agricultural streams have lower peak DOC values than forest streams, indicating weaker response to hydrological changes; agricultural watersheds export more protein-like DOM compared to forested streams.