Rapid Lignin Degradation in a Laboratory Incubation Experiment

Alpine and sub-alpine areas react very sensitive to global climate change and carbon cycling therein has been understudied, so far. A major component of plant litter that is commonly regarded as hardly decomposable is lignin. Consequently, the improved knowledge on degradation of lignin and soil organic carbon in alpine areas is of great importance to better understand their response to climate change. Therefore, we conducted a closed-jar incubation experiment under controlled conditions. ¹³C labelled plant litter (above ground litter from Lolium perenne) was added to two different soils from a sub-alpine area, one pasture soil and one forest soil originating from Jaun, Switzerland. To investigate the effect of increasing temperatures, the incubation was conducted under three different temperature regimes (average growing season temperature of 12.5°C, +4°C (16.5°C) and +8°C (20.5°C)) for the period of one year with five consecutive destructive samplings.

Lignin phenols were extracted using the CuO oxidation method, subsequent sample clean-up and quantification by GC-FID. Compound-specific stable carbon (δ¹³C) isotope composition of the lignin phenols was measured by GC-IRMS.

For all treatment groups, lignin concentrations decreased over the period of one year. The average decrease across all treatment groups was -22.7%. The decrease was slightly higher for the forest soil (-24.9%) than for the pasture site (-20.5%). No significant difference was observed between the control soil with and without added labelled litter. Average lignin decrease for the pasture soil was highest for the lowest temperature (-27.1%). For the two higher temperature treatments the decreases were identical with -17.1% and -17.3%. For the forest soil, the decrease was highest for a temperature of 16.5 °C (26.9%) and slightly lower for 12.5°C (25.7%). Surprisingly, the lowest decrease was observed for 20.5°C (22.1%).

The evolution of the ¹³C labelled litter signal enables the assessment of the degradation of fresh litter in the soils. For all different soils and incubation temperatures, the amount of litter-derived lignin phenols decreased by more than 50% already within two weeks after litter addition. In the further course of time, the ¹³C signal decreased much more slowly but remained considerably different from control soils. A possible explanation for this is a high availability of easily degradable carbon within the litter, providing enough energy to produce enzymes for lignin degradation.

Over the course of a year, also older lignin in the control samples degraded in a similar range as in the samples with litter addition, with a strong decrease in the initial phase and a slower decomposition in the later phase. This can be explained by the better availability of carbon at the beginning of the experiment and missing fresh litter during the later course.

Contrary to expectations, the degradation of lignin did not increase with rising temperature. This could be due to a lower temperature optimum of the current microbial community which is adapted to the current sub-alpine temperature regime. A complementary field incubation will show whether and how the laboratory results can be transferred to field conditions.