The project on below ground carbon cycling DEEP C aims to answer the fundamental question what the role of soils will be in terrestrial feedbacks to warming until 2100 and beyond. The warming of planet Earth will be accelerated if soil organic carbon (SOC) is lost to the atmosphere as greenhouse gas. Representations of this positive carbon-cycle-climate feedback are part of many climate projections, but there is little experimental evidence.

The project takes advantage of multi-year deep soil warming field experiments representing two biomes: Temperate forest and boreal forested peat. For the first time, we combine multi-year, deep soil warming, molecular markers and isotopic labeling in functionally different SOC pools. We explore how the soil-plant system responds to a +4°C warmer world and want to gain insights into how the physicochemical properties of SOC are affected by warming. Underlying processes, such as the dynamics of the root-microbial-mineral interactions are assessed. Ultimately, we want to integrate our results into the next generation of vertically-resolved SOC models as tools for understanding and predicting soil biogeochemical response to global change.

Soil microbial communities mediate the decomposition of SOC, making them key players in the global carbon cycle. It is known that the fungal abundance decreases with depth and bacterial communities in subsoils are better adapted to the prevalent substrate limited environment. Thus, how will warming affect the communities at different depth? Will the abundance of fungi decrease with warming, changing the ratio in fungi/bacteria in the upper horizons? And will nutrient limitation increase at depth due to the higher microbial activity with warming? This could lead to a further increase of bacteria which thrive at substrate limitation. If so, will these microbes at depth feed on stable carbon compounds?

Carbon compounds from plant above- and belowground sources differ in degradability. Warming will affect degradation and change the relative amount of above- and belowground inputs. How will the allocation between above- and belowground sources change with warming in the so far neglected subsoils? Increasing temperatures are predicted to accelerate the decomposition of labile soil organic compounds like carbohydrates, whereas biochemically resistant compounds, such as lipids from leaf cuticles and roots may remain stable for longer time spans. We hypothesise that warming will reduce root inputs and increase decomposition rates.

Our toolbox to answer these questions consists of molecular markers. We do phospholipid fatty acid analysis, extraction of free and hydrolysable lipids, lignin and pyrogenic carbon. Complementary we do a general characterization of various soil parameters.