Bed substrate influences leaf litter decomposition rates and leached dissolved organic matter quality in subsurface flow constructed wetlands

Providing endogenous labile carbon (C) is crucial when designing constructed wetlands (CWs) to treat C-limited but nitrate rich-wastewater. In subsurface flow CWs, the main sources of C are the bed substrate and vegetation. When senescent plant leaves fall on the CW bed surfaces, they release C and nutrients during decomposition, fuelling microbial reactions and enhancing CW performance. However, the type of bed substrate not only influences plant and microbial growth but may also affect the chemical composition of leaves and their role as a C-source. To study this, we designed an experiment to analyze the effect of different bed substrates (gravel only, gravel + soil and gravel + biochar) on i) leaf litter decomposition rates and ii) the chemical quality and potential microbial uptake of leaf leachates. Our goal was to advance in the selection of a CW substrate that promotes leaf litter decomposition as a suitable and effective source of labile C for microorganisms. We observed varying decomposition rates among substrates, which appeared to result from differences in environmental conditions at the habitat scale rather than differences in leaf chemical composition. Photodegradation mechanisms dominated in gravel beds, driving decomposition at rates similar to those observed with soil addition, where microbial activity played a major role. In contrast, the addition of biochar inhibited decomposition. C leached from plants growing in soil and biochar substrates exhibited the highest microbial uptake, likely due to the presence of essential nutrients. This study supports that adding natural soil to gravel is the most favourable option for promoting labile C supply via plant decomposition, thereby enhancing microbial activity. Furthermore, our results suggest that management strategies allowing leaf litter to remain on CW bed surfaces would provide a valuable supply of DOC, helping to mitigate labile C limitation in the treatment of irrigated agricultural drainage water.