Temperature-dependent Degradation of Soil Organic Matter in an Incubation Experiment

Climate change induced warming of soils will have a strong impact on the carbon cycle, especially the decomposition of soil organic matter (SOM) is likely to increase with rising temperature. Alpine regions are especially prone to those changes with earlier and higher expected temperature increase compared to the global average. Carbon cycling in these regions has been also increasingly influenced by land-use changes, such as afforestation, the abandonment of alpine pastures and the resulting bush encroachment, as well as an increasing elevation of the tree line. However, it is still largely unknown how these changes affect the degradation of different compound classes of soil organic matter. A one-year laboratory incubation experiment was carried out to investigate the degradation of SOM at a molecular level.

Two soils with different land-use histories including a soil from an afforested subalpine forest site and a nearby pasture soil from the same site located near Jaun (Canton Fribourg, Switzerland) were incubated under controlled conditions. The incubation was carried out under three different temperatures, the current average growing season temperature (12.5 °C) as a control, as well as two increased temperature treatments of +4 °C (16.5 °C) and +8 °C (20.5 °C) that represent the range of temperature increase expected for Alpine regions under a high emission scenario. To trace the decomposition of organic matter input, ¹³C-labelled plant litter was added to a subset of the incubated samples. The incubation ran for a period of one year with six different sampling times (14, 28, 56, 168, 360 days).

In samples without labelled litter, the bulk carbon (C) concentration decreased for pasture and forest soils from initial C concentrations of 45.5 and 43.3 mg/g, respectively, by 3.3 % and 5.6 % on average. This is also reflected in lignin concentrations, with a decrease of 13.8 % for pasture and 20.2 % for forest soils.

With litter addition, the degradation was higher than for samples without labelled litter for bulk C, lignin as well as for free extractable lipid fractions. The strongest degradation was observed already during the initial phase of the incubation experiment. E.g., a decrease of more than 50% of the ¹³C signal of individual lignin phenols could be observed already during the first 14 days, which indicates a fast degradation mainly of the added litter.

In general, the degradation of individual compounds increased with increasing temperature with the highest degradation being observed for the highest temperature treatment.

Higher temperatures have led to increased degradation of SOM during the laboratory incubation experiment, even in seemingly more recalcitrant compounds as lignin. In alpine regions, an expected rise in temperature can therefore lead to increased decomposition of recalcitrant components of SOM. In addition, increased degradation in forest compared to pasture soils indicates a higher vulnerability of forest than pasture soils in alpine regions, which points to complex responses of SOM cycling following land-use changes such as afforestation.