| Export of dissolved organic carbon (DOC) and iron (Fe) from soils to freshwaters has increased strongly in the last decades. This has resulted in rising concentrations of DOC and Fe, causing increases in water color of a vast number of northern freshwaters. This browning, or brownification, and has received ample attention both by the scientific and the water management community, as it has far-reaching ecological and societal consequences. There are still uncertainties regarding the driver/s behind browning, and probably several mechanisms contribute. Some studies emphasize factors associated to climate change, such as rising temperature and precipitation, which enhance terrestrial productivity and hydrological transport of DOC. It has also been proposed that the observed browning is a return to water color levels of pre-industrial times. The underlying reasoning is that browning represents a recovery from acidification, which made surface waters unnaturally clear during the 70s and 80s, by suppressing solubility of DOC. If climate change and recovery from acidification are the sole drivers of browning, we stand without measures to counteract it.
However, recent work from our group, which make use of long-term historical records extending back to the 1930s, demonstrate that acid deposition has not been the primary driver of centennial water color trends. Instead, the data indicates that browning is to a large extent driven by the transition from an open landscape to forestry. This is due to the buildup of organic soil layers under coniferous forests, which sustain high DOC and Fe export, but may also be linked to the parallel practice of peat ditching to promote forestry. The aims of the proposed project are therefore to:
• Determine the importance of peatland ditching, and subsequent afforestation, as a driver of browning.
• Unravel the role of ectomycorrhizal (EM) fungi - which become more active in peatland soils after ditching - for the mobilization of DOC and Fe.
To reach these aims, the PhD project will span over several scales of investigation. On the largest scale, information on land-cover change in individual lake catchments, based on interpretation of aerial photographs from the 1940s and onwards, will be combined with historical records of water color in corresponding lakes, to test how ditching of peatlands has contributed to browning. At the intermediate scale, field measurements along peatland gradients where the groundwater level differs due to the influence of ditching, will reveal how drainage affects the composition of EM fungal communities, and subsequently processes that promote DOC and Fe mobilization. At the finest scale, controlled mesocosm experiments will provide mechanistic understanding of how specific EM fungi affect DOC and Fe mobilization.
The project addresses novel aspects of browning, and will make a significant contribution by elucidating where in the landscape browning originates, which is critical for our understanding on how and where local measures can be implemented to mitigate browning.
|