Impact of Pore-Size-Class on Carbon Turnover in Peat Soils

The loss of carbon from peatlands occurs through gaseous emissions and a substantial fraction from aquatic fluxes, specifically dissolved organic carbon (DOC), during mineralisation and degradation processes. Our study hypothesises that DOC production is dependent on pore size, with higher concentrations occurring in finer pores. To investigate this, pore water was extracted at specific pressure heads (-60 and -600 hPa), representing macro- and mid-size pore domains, from degraded peat samples. Soil organic matter content was measured at 34 wt% in the topsoil and 57 wt% in the subsoil. Notably, the more degraded topsoil exhibited significantly higher average DOC concentrations than the subsoil, with levels 1.5 times greater at -60 hPa and 2.4 times higher at -600 hPa. These trends indicate that degraded peat soils are prone to release greater amounts of DOC. Additionally, DOC concentrations in topsoil samples were consistently higher at -600 hPa compared to -60 hPa.
The negative correlation between soil organic matter (SOM) and DOC at -600 hPa (r = - 0.53; p < 0.0001) aligns with degradation-driven reductions in SOM and porosity. Degraded topsoil exhibited high DOC variability for SOM < 40 wt%, stabilising below 50 mg/L for SOM ≥ 40 wt%. Through a graphical illustration, we infer that the elevated DOC export is likely due to the higher surface-to-volume ratio observed in mid-sized pores (-60 to -600 hPa), further enhanced by the dual-porosity structure of the degraded topsoil. This structural variation contributes to differences in carbon turnover rates. Additionally, microbial communities and their abundance differ across pore size classes, causing pore size-dependent reactions that influence DOC export.