Studying the priming effect by adding 13 C-labelled glucose to peat samples in two forestry-drained peatland soils with different nutrient status

Land-use changes, such as forestry-drainage, modify the characteristics of peatland soil, which further affect the peatland carbon (C) balance. It has been suggested that peat soil nutrient status, which is mainly a result of the original peatland type, has further an impact on the processes affecting the C balance after drainage. The respiration and priming effect (PE) of peat soils collected from two forestry-drained boreal peatlands with different nutrient status were examined in this two-week laboratory experiment. Half of the samples were labelled with ¹³ C-glucose to study the effect of fresh C addition on the old soil C decomposition. The ¹³ CO₂ -samples were analyzed with IRMS (Isotope Ratio Mass Spectrometer). A two-pool mixing model was applied to separate the soil- and sugar-derived respirations and determining the PE. The nutrient-rich peat soil respired generally more than the nutrient-poor peat. A negative PE was observed in both peat soils, suggesting that the addition of fresh C did not increase the SOM decomposition, but on contrary decreased it. The negative PE was significantly more pronounced in nutrient-poor peat soil than in the nutrient-rich peat treatments, thus higher nutrient availability seemed to suppress the negative PE. These results imply that the microbes prefer utilizing fresh C instead of old C in short-term and that the peat decomposition might be suppressed in presence of fresh C inputs from vegetation at forestry-drained peatlands. These effects are even stronger in peat soils with less nutrients available.

Refining the role of phenology in regulating gross ecosystem productivity across European peatlands

The role of plant phenology as a regulator for gross ecosystem productivity (GEP) in peatlands is empirically not well constrained. This is because proxies to track vegetation development with daily coverage at the ecosystem scale have only recently become available and the lack of such data has hampered the disentangling of biotic and abiotic effects. This study aimed at unraveling the mechanisms that regulate the seasonal variation in GEP across a network of eight European peatlands. Therefore, we described phenology with canopy greenness derived from digital repeat photography and disentangled the effects of radiation, temperature and phenology on GEP with commonality analysis and structural equation modeling. The resulting relational network could not only delineate direct effects but also accounted for possible effect combinations such as interdependencies (mediation) and interactions (moderation). We found that peatland GEP was controlled by the same mechanisms across all sites: phenology constituted a key predictor for the seasonal variation in GEP and further acted as a distinct mediator for temperature and radiation effects on GEP. In particular, the effect of air temperature on GEP was fully mediated through phenology, implying that direct temperature effects representing the thermoregulation of photosynthesis were negligible. The tight coupling between temperature, phenology and GEP applied especially to high latitude and high altitude peatlands and during phenological transition phases. Our study highlights the importance of phenological effects when evaluating the future response of peatland GEP to climate change. Climate change will affect peatland GEP especially through changing temperature patterns during plant phenologically sensitive phases in high latitude and high altitude regions.