Globally, peatlands have been recognized as important carbon sinks while only covering approximately 3% of the earth’s land surface. Root exudates are known key drivers of C cycling in soils and rhizosphere priming effects have been studied extensively in terrestrial ecosystems. Their role for decomposition of peat still remains unclear, as little research about their fate and potential priming effects in peat exists. In this study, we aimed to evaluate pathways of root exudates and their short-term priming effects by daily determination of stable carbon isotope fluxes of CO2 and CH4. As the drainage of peatlands strongly alters processes of decomposition, we included measurements after drainage as well. Results revealed the immediate respiration of root exudates in peat, mainly as CO2, while CH4 release was associated with a lag time of several days. However, the largest proportion of added root exudates remained in the solid and liquid phase of peat. In conclusion, our findings suggest that no priming occurred as added substrates remained immobile in peat.

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<p>Peat accumulation is the result of a small imbalance between the formation and decomposition of plant litter. Changing environmental conditions alter the vegetation cover in peatlands and therefore litter quality inputs. Litter mixing effects, describing variable interactions between different litter types and decomposition rates, have been studied, but observations and directions of non-additive effects are not consistent. To better understand litter mixture effects of an ombrotrophic bog, where the encroachment of vascular plants has been observed, we incubated pure litter (<em>Sphagnum</em> (S), <em>Betula</em> (B), <em>Calluna</em> (C)) and three resultant mixtures (SB, SC, BC) over 70 days.</p> <p>We hypothesized that decomposition pattern of pure substrates differs from mixtures. Also, substrate specific decomposition patterns develop at the beginning of the experiment, which should harmonize with increasing time. Mixtures containing S litter have lower decomposition rates than their pure constituents, while mixtures without S (<em>i.e. </em>BC) show higher decomposition rates.</p> <p>For our incubation study, we collected three litter types (<em>Calluna vulgaris</em> (L.) Hull., <em>Sphagnum</em> <em>capillifolium </em>(Ehrh.) Hedw., <em>Betula pubescens</em> Ehrh.) from an ombrotrophic bog (Pürgschachen Moor, Austria). Oven-dried (60 °C) and sieved (< 2 mm) litter was used for litter bags containing 1 g of pure litter (S, B, C) or mixtures (SB, SC, BC). Bags were inoculated with bog water for 24 h and incubated in 50 mL conical tubes containing 4.5 mL of saturated K<sub>2</sub>SO<sub>4</sub> (glass marbles were used to avoid contact) to ensure constant relative humidity. For every sampling day (0, 2, 14, 28, 70) four replicates of each substrate were prepared. Three bags per day were used for measurements of CO<sub>2</sub> production rates, water extractable organic carbon (WEOC) and nitrogen (TN-L), mass loss and total carbon analysis. We measured the specific ultraviolet absorbance at 254 nm (SUVA<sub>254</sub>) to monitor aromaticity of organic compounds in WEOC. In addition, one litter bag was used for the analysis of C-, N-, P-degrading enzymes using a fluorometric microplate assay. Cube root transformed data was used for k-means clustering to detect litter specific decomposition pattern over time.</p> <p>As hypothesized, results show that S litter has a constant, low decomposition pattern over the whole experimental time. Other substates share a similar (low decomposition) pattern on day 0 and day 2 (high decomposition). After 14 days, pure substrates develop a specific pattern, while all mixtures share a common pattern. S containing mixtures (SB, SC) behave similar over time but remarkably different than related pure components only on day 28. Our results indicate that, especially in the beginning, patterns of decomposition are mainly time depend, possibly covering litter specific decomposition patterns. In conclusion, whole decomposition patterns showed no clear litter mixing effects, although some measured variabales indicate shifts with increasing time.</p>

<p>Root exudates are a key driver of carbon cycling in peatlands. They have been found to influence substrate quality in and methane release from peat (Ström et al., 2003), peat decomposition (Crow & Wieder, 2005) and to cause priming effects (Basiliko et al., 2012). However, investigating the fate of added root exudates in peatlands is very challenging, as it requires the consideration of the gaseous, liquid, and soil phase, a traceable substrate, and as little disturbance as possible.</p><p>We sampled 6 undisturbed peat cores from Pürgschachen Moor, Austria in September 2019. Following transport of the cores to the laboratory in Vienna, we stored the mesocosms in daylight with intact vegetation at 22°C and created ports for pore water sampling in 5, 15, and 25 cm depth. The water table was set to 3 cm below surface by daily addition of artificial Pürgschachen rainfall (20 kg N ha<sup>-1</sup> yr<sup>-1</sup>). After 1 week of incubation for establishment of a baseline, three cores were spiked with 140 mg artificial root exudates consisting of 99% glucose-, acetic acid- and amino acid <sup>13</sup>C following Basiliko et al. (2012) at 15 cm depth. We monitored carbon dioxide (CO<sub>2</sub>), and methane (CH<sub>4</sub>) and <sup>13</sup>CO<sub>2</sub> and <sup>13</sup>CH<sub>4</sub> efflux from the cores daily and sampled dissolved organic carbon (DOC) weekly from the ports. Three weeks after spiking, all cores were drained, drainage water collected, and peat at 5, 15, and 25 cm depth sampled. Upon drying at 60°C, peat C and <sup>13</sup>C content was determined and DOC samples were analysed for C and <sup>13</sup>C content.</p><p>Results show that ca. 20% of spiked substrates were incorporated into peat, but this effect was restricted to 15 cm peat depth and ca. 30% were respired as CO<sub>2</sub>. No priming effect was detected; the spiked cores did not release more CO<sub>2</sub> and CH<sub>4</sub> than the control cores. <sup>13</sup>C concentration in peat at 5 and 25 cm depth showed no increased <sup>13</sup>C concentration.</p><p>These results indicate a low mobility of DOC and a limited effect of root exudate derived substrate in peat bogs with a low water table oscillation, explaining remarkably constant CH<sub>4</sub> release rates reported by Drollinger et al. (2019b).</p><p> </p><p> </p><p>References:</p><p> </p><p>Basiliko, N., Stewart, H., Roulet, N.T., Moore, T.R. (2012): Do Root Exudates Enhance Peat Decomposition? Geomicrobiology Journal 29: 374-378.</p><p> </p><p>Crow SE, Wieder RK. 2005. Sources of CO2 emission from a northern peatland:</p><p>root respiration, exudation, and decomposition. Ecology 86:1825–1834.</p><p> </p><p>Drollinger, S., Kuzyakov, Y., Glatzel, S. (2019a): Effects of peat decomposition on d13C and d15N depth profiles of Alpine bogs. Catena 187: 1-10.</p><p> </p><p>Drollinger, S., Maier, A. Glatzel, S. (2019b): Interannual and seasonal variability in carbon dioxide and methane fluxes of a pine peat bog in the Eastern Alps, Austria. Agricultural and Forest Meteorology 275: 69-78.</p><p> </p><p>Ström, L. Ekberg, A., Mastepanov, M., Christensen, T.R. (2003): The effect of vascular plants on carbon turnover and methane emissions from a tundra wetland. Global Change Biology 9: 1185-1192.</p><p> </p>