Preferential substrate use decreases priming effects in contrasting treeline soils

Abstract In high altitudes and high latitudes climate change manifests in upward and northward shifting treelines. This entails changes to the carbon (C) and nitrogen (N) composition of organic inputs to soils and can increase or decrease microbial mineralisation of native soil organic matter (positive or negative priming effect). It is currently unknown whether the observed treeline shifts will lead to carbon loss or storage in the soils of these ecosystems. We therefore investigated priming effects in treeline soils from high altitudes (Peruvian Andes) and high latitudes (subarctic Sweden). In a laboratory soil incubation, we added realistic amounts of organic carbon at a constant ratio of 30% substrate-C to each soils’ microbial biomass C. Substrate additions were neutral in pH and covered different C:N ratios, representing naturally occurring substances of the studied ecosystems. We observed a clear pattern linking the direction of priming (positive or negative) to land cover (boreal or tropical forest, tundra heath, Puna grassland) and soil horizon (organic or mineral). Mechanistically, no support was found for N-mining to induce positive priming, indeed half of all priming effects were negative. Whenever negative priming prevailed, the decrease in SOM-mineralisation was consistently linked to increased microbial substrate utilisation. In line with other recent studies, our results suggest preferential substrate use is pervasive, linked to negative priming and a promising tool to increase soil carbon storage. Yet, the positive priming observed alerts climate change-induced upwards shifts in high altitudes and deeper rooting plants in high latitudes may cause C-loss via positive priming.