Soil organic matter dynamics along a long-term soil warming gradient in a subarctic forested ecosystem

Global warming will lead to a temperature rise of the soil, with a stronger effect in high latitude areas as compared to the global average. Soil warming could cause positive carbon-climate feedback, but this is still subject to many uncertainties. To enhance the understanding of how warming impacts underlying biogeochemical processes in top – and subsoils, our study makes use of a century-scale geothermal warming gradient. It is located in an aspen-dominated subarctic forest in the southern Yukon-Territory, Canada. A previous study at the site showed the SOC-stock to be reduced with warming, while the N-stock remained mainly unchanged. Thus, the C:N ratio was reduced, which was particularly pronounced in subsoils. Moreover, N shifted from the particulate-organic matter (POM) pool to the mineral-associated organic matter (MAOM) pool. We hypothesize the shift to be related to a higher microbial contribution to the MAOM fraction. In addition, the contribution of plant-derived OM in the subsoil might have decreased. This might be due to a change of root biomass distribution along the soil profile, as it is assumed that warming enhances N availability and the distribution of soil moisture. Furthermore, we hypothesize the soil structure to be a relevant factor for the distribution and loss of SOC, as the fraction of sand and stable aggregates (S+A) might be reduced with warming.

To understand the coupling- or decoupling of C- and N cycles, we will determine biogeochemical parameters at four warming intensities up to a warming of +10 °C to a depth of 80 cm. This design allows us to identify potential non-linear responses and it includes different warming scenarios. We will measure C- and N stocks of the POM, S+A and MAOM fraction. To moreover address the warming effect on the relative organic matter turnover, δ¹³C and δ¹⁵N values will be analyzed. For soil structural changes, the mean weight diameter of water-stable aggregates is evaluated. Furthermore, the coarse- and fine root biomass is assessed along the soil profiles. This comprehensive study will gain valuable biogeochemical insights and first results of ongoing evaluations will be presented.