A study of plant-soil-microbe interactions across contrasting treelines in the Peruvian Andes and sub-arctic Sweden

This dissertation addresses the question of how plant species shifts would impact carbon cycling in ecosystems, which are likely to soon and strongly be affected by climate change. Examples of such ecosystems are the high latitudes and the high altitudes, where the treeline ecotone can be an early indicator of changes in plant community composition. Biotic changes aboveground also modify belowground processes, particularly carbon (C) and nutrient cycling between plant roots and the assembled microbes. Plant-soil-microbe interactions were therefore studied across treelines in the Peruvian Andes and sub-arctic Sweden. The first objective was to determine and compare the present soil C and nitrogen (N) stocks and vegetation characteristics through systematic study of a high altitudinal tropical and a sub-arctic treeline. This revealed higher soil C-stocks in the boreal region with potentially also higher microbial activity in summer. For both countries, organic soils were higher in C and N contents compared to the mineral soils. Soils were sampled from both soil horizons across respective treelines and taken to the laboratory to deepen the question of functionality. Microbial mineralisation of soil organic matter (SOM) was quantified in a microcosm soil incubation with addition of substrates of different C:N ratios. Treatment C:N had negligible effect on SOM-mineralisation, which was reduced following substrate addition in the majority of incubations (negative priming). Mechanistically, this questions the N-mining hypothesis and suggests preferential substrate use. For the final data chapter, efforts were made to bring together all three compartments of soils, plants and microbes in vivo and study how their interactions mediate carbon and nutrient cycling between them. Negative rhizosphere priming was measured in most soils during the course of the late growing season. This consistent result provides new insights to potential mechanisms of the finely tuned synchronisation of plant-soil-microbe interactions. In the final discussion, these results were set into context to anticipate what could be done to further our understanding of ecosystem functioning at appropriate scales. Unravelling the interactions of plants, soils and microbes in more detail could help resolve the mechanisms of nutrient cycling and energy flows in different ecosystems and estimate the impact of climate change on the global carbon cycle with less uncertainty.NER