Decoupling of dissolved organic matter patterns between stream and riparian groundwater in a headwater forested catchment

Abstract

Abstract. Streams are important sources of carbon to the atmosphere, though knowing whether they merely outgas terrestrially derived carbon dioxide or mineralize terrestrial inputs of dissolved organic matter (DOM) is still a big challenge in ecology. The objective of this study was to investigate the influence of riparian groundwater (GW) and in-stream processes on the temporal pattern of stream DOM concentrations and quality in a forested headwater stream, and whether this influence differed between the leaf litter fall (LLF) period and the remaining part of the year (non-LLF). The spectroscopic indexes (fluorescence index, biological index, humification index, and parallel factor analysis components) indicated that DOM had an eminently protein-like character and was most likely originated from microbial sources and recent biological activity in both stream water and riparian GW. However, paired samples of stream water and riparian GW showed that dissolved organic carbon (DOC) and nitrogen (DON) concentrations as well as the spectroscopic character of DOM differed between the two compartments throughout the year. A simple mass balance approach indicated that in-stream processes along the reach contributed to reducing DOC and DON fluxes by 50 and 30 %, respectively. Further, in-stream DOC and DON uptakes were unrelated to each other, suggesting that these two compounds underwent different biogeochemical pathways. During the LLF period, stream DOC and DOC : DON ratios were higher than during the non-LLF period, and spectroscopic indexes suggested a major influence of terrestrial vegetation on stream DOM. Our study highlights that stream DOM is not merely a reflection of riparian GW entering the stream and that headwater streams have the capacity to internally produce, transform, and consume DOM.

Carbon limitation leads to thermodynamic regulation of aerobic metabolism

“…In contrast, nutrient limitation regulates respiration when bioavailable carbon is in excess, and in this scenario, organic N may be a key constraint on respiration. This is consistent with previous reports of a strong role from organic N cycling in hyporheic zones and freshwater systems 10,12,43,44 . While our work represents a single system, our conceptual model is meant to provide proof-of-concept for more spatially extensive studies that will allow broader transferability.…”

High-frequency measurements of dissolved organic carbon quantity and quality in a headwater catchment

“…However, they used grab samples from different hydroclimatic situations and streams, thus potentially masking the eventscale dynamics of DOC mobilization as revealed in the current study. But also the magnitude of instream processing and biodegradation could further influence DOC composition and hence SUVA 254 and S 275-295 measurements in stream water (Bernal et al, 2018;Hansen et al, 2016). Precipitation events can get buffered and retarded in the soils (low Q hf ) (state warm & dry, Fig.…”

High-frequency measurements explain quantity and quality of dissolved organic carbon mobilization in a headwater catchment

“…This buildup of exportable DOC pools in turn is strongly related to the hydroclimatic conditions like temperature and soil moisture content prior to an event (Birkel et al, 2017;Broder et al, 2017;Christ and David, 1996;Garcia-Pausas et al, 2008;Preston et al, 2011), which to some degree also define the potential for hydrological connectivity and transport during the event (Birkel et al, 2017;Köhler et al, 2009;Shang et al, 2018). On a seasonal scale (roughly 1-3 months) hydroclimatic variables control intra-annual variability of DOC concentration and quality (Ågren et al, 2007;Hope et al, 1994;Köhler et al, 2009) and are hence considered important drivers of seasonal DOC export dynamics (Ågren et al, 2007;Birkel et al, 2014;Köhler et al, 2009;Seibert et al, 2009). In summary, DOC concentration and quality are jointly controlled by the hydrologic conditions during events (defining the timing and magnitude of DOC export) and the antecedent hydroclimatic conditions (defining size and quality of exportable DOC pools in the soil), resulting in a highly dynamic system with processes interacting at timescales ranging from the event scale of hours to days to timescales of seasons.…”