Afforestation on a subalpine pasture does not result in an increase in carbon sequestration but in a change in the soil organic matter (de)composition

Planting trees on non-forested land has the potential to sequester atmospheric CO₂ in biomass and soil. While afforestation on former agricultural land often results in an increased soil organic carbon sequestration, the outcomes of afforestation on pastures vary from carbon sink to source. Alpine soils are characterized by a higher proportion of labile carbon compounds compared to soils in temperate ecosystems, which makes alpine ecosystems more sensitive to environmental changes. The conversion of subalpine pasture to forests thereby might have a substantial effect on the SOC dynamics and  on soil organic matter (SOM) stabilization. In addition, the alteration in the proportion of aboveground biomass- and root-derived organic matter and the associated alterations in the soil microbial community following afforestation on subalpine pastures are not yet fully understood.

In this study, the alteration in SOC stocks as well as in the SOM composition following  afforestation (0 to 130 years) with Norway spruce (Picea abies) on a subalpine pasture is investigated in the Swiss Alps. To determine the alteration of potential sources and decomposition of SOM, a multi-proxy molecular marker approach was applied. Specifically, the combination of n-fatty acids, n-alkanes, and n-alcohols was applied to identify possible sources of plant-derived SOM. For the identification of microorganism-derived SOM, a combination of phospholipid fatty acids and glycerol dialkyl glycerol tetraethers was used.

Afforestation with Norway spruce on a subalpine pasture did not result in any significant change in SOC stocks (Pasture: 11.5 ± 0.5 kg m^-2; 130-year-old forest 11.0 ± 0.3 kg m^-2) after 130-years. The organic matter input, however, changed from grass leaves to spruce needles with increasing forest stand age. Surprisingly, root-derived organic matter seems to play a minor role in the pasture soil as well as in forest soils of all stand ages as one of the predominant sources of SOM. With increasing forest age an increased abundance of Gram+ bacteria as well as arbuscular mycorrhizal fungi was observed. In the pasture soil, a clearly higher abundance of archaea was observed compared with the forest. This shift in the soil microbial community shows its adaptation to the changes in the vegetation cover.  Furthermore, the difference in the soil microbial community structure implies a use of different carbon substrates of the microorganisms between the pasture and forests, which can have substantial effects on soil organic matter stabilization. Conclusively SOC stocks did not change after 130 years of afforestation on a subalpine pasture, but the SOM dynamics has changed due to the changes in the vegetation cover. For a better understanding of the connection between organic matter input and its decomposition, the analysis of plant polymers such as cutin and suberin polymers can help to unravel the difference in shoot- vs. root-derived organic matter and their contribution to the stable SOM pool in subalpine ecosystems.