Warming accelerates belowground litter turnover in salt marshes – insights from a Tea Bag Index assay

Salt marshes play an important role in the global carbon (C) cycle due to the large amount of C stored in their soils. Soil C input in these coastal wetland ecosystems is strongly controlled by the production and initial decomposition rates of plant belowground biomass and litter. This study used a field warming experiment to investigate the response of belowground litter breakdown to rising temperature (+1.5°C and +3.0°C) across whole-soil profiles (0-60 cm soil depth) and the entire flooding 15 gradient ranging from pioneer zone via low marsh to high marsh. We used standardized plant materials, following the Tea Bag Index approach, to assess the initial decomposition rate of ( k ) and the stabilization factor ( S ) of labile organic matter (OM) inputs to the soil system. While k describes the initial pace at which labile (= hydrolyzable) OM decomposes, S describes the part of the labile fraction that does not decompose during deployment in the soil system and stabilizes due to biochemical transformation. We show that warming strongly increased k consistently throughout the entire soil profile and across the entire 20 flooding gradient, suggesting that warming effects on the initial decomposition rate of labile plant materials are independent of the soil aeration (i.e. redox) status. By contrast, negative effects on litter stabilization were less consistent. Specifically, warming effects on S were restricted to the aerated topsoil in the frequently flooded pioneer zone, while the soil depth to which stabilization responded increased across the marsh elevation gradient via low to high marsh. These findings suggest that reducing soil conditions can suppress the response of belowground litter stabilization to rising temperature. In conclusion, our 25 study demonstrates marked differences in the response of initial decomposition rate vs. stabilization of labile plant litter to rising temperature in salt marshes. We argue that these differences are strongly mediated by the soil redox status along flooding and soil-depth gradients. Warming effects on litter breakdown in coastal wetlands may be strongly controlled by hydrology and resulting soil redox gradients. For instance, warming studies conducted in boreal peatlands demonstrated a dramatic reduction of warming effects on soil decomposition processes in waterlogged and thus strongly reducing subsoils compared to less reducing topsoils (Hopple al., 2020; Wilson al., 2016). The authors suggest that the absence of oxygen can inhibit warming effects on soil microbial 60 activity. In order to understand warming effects on litter breakdown in coastal wetland ecosystems, it is therefore necessary to study temperature effects across gradients in both flooding frequency (i.e. along the marsh elevation gradient) and in relation to soil depth. The present study uses a unique field experiment to assess the effects of rising temperatures on litter breakdown in relation to 65 soil depth and across the marsh elevation gradient ranging from pioneer zone via low marsh to high marsh. The MERIT (Marsh Ecosystem Response to Increased Temperature) experiment operates in a NW European salt marsh and induces active temperature manipulation to 1 m soil depth. We investigated litter breakdown over two consecutive growing seasons, year 1 and year 2 of the experiment, to gain insight into potential differences between short- and mid-term warming effects. We hypothesize (1) that warming will increase litter initial decomposition rate, k , and decrease the ability of litter stabilized, S , in 70