The role of abiotic processes on dissolved organic matter (DOM) production is often underappreciated. However, abiotic processes appear to be especially important in subsoils where, with increasing depth, microbial activity declines and soil organic matter (SOM) becomes a progressively more important contributor to DOM. Within three soil depths (20, 40, and 60 cm) in a temperate forest, soil temperature was positively associated with dissolved organic carbon (DOC) concentration (R2 = 0.23–0.77) and the DOM humification index (R2 = 0.35–0.72) for soil solutions in slow and fast flowpaths. With increasing soil temperature from 5 to 24 °C, average DOC concentrations increased by 86% at 20 cm, 12% at 40 cm and 12% at 60 cm soil depths. Our data suggest that DOM supply, especially in subsoils, is temperature dependent. We attribute this to the influence of temperature on DOM replenishment through direct processes such as SOM dissolution, diffusion and exchange reactions as well as indirect processes such as rhizodeposition and exoenzyme activity. In contrast, negative relationships (R2 = 0.71–0.88) between temperature and nitrate concentrations in subsoil suggested that the temperature-dependent supply of DOM drives microbial processes such as dissimilatory and assimilatory nitrate consumption.
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