Small scale variability of vertical water and dissolved organic matter fluxes in sandy Cambisol subsoils as revealed by segmented suction plates
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Dissolved organic matter (DOM) is considered as a major carbon source in subsoils. As soil water fluxes are highly variable at small scale, and transport versus sorptive retention of DOM is related to water flux and associated contact time with minerals, knowledge of the small scale spatial variability of the dissolved organic carbon (DOC) concentrations and fluxes into the subsoil is decisive for a solid estimation of organic carbon (OC) translocation into the subsoil. Here, we made advantage of novel segmented suction plates (4 × 4 segments, each 36 cm2) to analyze the small scale spatial and temporal variability of DOC transport at 10, 50 and 150 cm depth of three subsoil observatories (approximately 50 m apart) in a sandy Dystric Cambisol under beech in the Grinderwald, 40 km northwest from Hannover, Germany. Water fluxes, DOC concentrations and fluxes as well as the specific UV absorbance (SUVA) at 280 nm were determined in weekly samples from August 2014 to November 2015 for each individual segment. The DOC fluxes decreased with depth (19.6 g C m−2 year−1, 10 cm; 1.2 g C m−2 year−1, 150 cm) and were strongly related to the water fluxes. The SUVA at 280 nm also decreased with depth (0.03 L mg C−1 cm−1, 10 cm; 0.01 L mg C−1 cm−1, 150 cm), indicating a selective retention of aromatic moieties, that was eased with increasing water flux at least in the subsoil. The proportion of temporal fluctuations and small scale variability on the total variance of each parameter where determined by the calculation of intra class correlations. The seasonal heterogeneity and the small scale spatial heterogeneity were identified to be of major importance. The importance of the small scale spatial heterogeneity strongly increased with depth, pointing towards the stability of flow paths and suggesting that at a given substrate hydrological processes rather than physicochemical processes are decisive for the sorptive retention of DOM and the variability of OC accumulation in the subsoil. Our results clearly show the demand of small scale sampling for the identification of processes regarding carbon cycling in the subsoil.
KeywordsDOC flux SUVA Segmented suction plates Small scale variability Beech forest
Funding of the research was provided by the Deutsche Forschungsgemeinschaft DFG within the research unit FOR 1806 “The Forgotten Part of Carbon Cycling: Organic Matter Storage and Turnover in Subsoils (SUBSOM)”. We would like to thank Dr. Stefan Wessel-Bothe of ecoTech Umwelt-Meßsysteme GmbH for help with establishing the soil observatories and Heike Steffen, Anne Kathrin Herwig and numerous student helpers for support in sample processing.
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