The effect of the addition of 13C labelled artificial root exudates on carbon cycling in intact peat bog mesocosms

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.

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^-1 yr^-1). 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 ¹³C following Basiliko et al. (2012) at 15 cm depth. We monitored carbon dioxide (CO₂), and methane (CH₄) and ¹³CO₂ and ¹³CH₄ 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 ¹³C content was determined and DOC samples were analysed for C and ¹³C content.

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 CO₂. No priming effect was detected; the spiked cores did not release more CO₂ and CH₄ than the control cores. ¹³C concentration in peat at 5 and 25 cm depth showed no increased ¹³C concentration.

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 CH₄ release rates reported by Drollinger et al. (2019b).

 

 

References:

 

Basiliko, N., Stewart, H., Roulet, N.T., Moore, T.R. (2012): Do Root Exudates Enhance Peat Decomposition? Geomicrobiology Journal 29: 374-378.

 

Crow SE, Wieder RK. 2005. Sources of CO2 emission from a northern peatland:

root respiration, exudation, and decomposition. Ecology 86:1825–1834.

 

Drollinger, S., Kuzyakov, Y., Glatzel, S. (2019a): Effects of peat decomposition on d13C and d15N depth profiles of Alpine bogs. Catena 187: 1-10.

 

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.

 

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.