Production of dissolved organic carbon and low-molecular weight organic acids in soil solution driven by recent tree photosynthate
ABSTRACT Dissolved organic carbon (DOC) is an important component in the terrestrial carbon cycle. Yet, the relative importance of
different inputs of DOC to the soil solution remains uncertain. Here, we used a large-scale forest girdling experiment to
examine how the supply of recent photosynthate to tree roots and their mycorrhizal fungi affects DOC, in particular low-molecular
weight organic acids (LMWOA). We also studied effects of tree girdling on non-structural carbohydrates in microorganism, and
examined the effects of freezing of soil and the presence of roots in the soil samples on soil solution DOC and LMWOA in this
The concentration of DOC was reduced by 40%, while citrate was reduced by up to 90% in the soil solution by the girdling treatment.
Other LMWOA such as oxalate, succinate, formate and propionate were unaffected by the girdling. We also found that girdling
reduced the concentrations of trehalose (by 50%), a typical fungal sugar, and of monosaccharides (by 40%) in microorganisms
in root-free soil. The effect of freezing on DOC concentrations was marked in samples from control plots, but insignificant
in samples from girdled plots. Release of DOC from cell lysis after freezing was attributed equally to roots and to microorganisms.
Our observations suggest a direct link from tree photosynthesis through roots and their mycorrhizal fungi to soil solution
chemistry. This direct link should impact solute transport and speciation, mineral weathering and C dynamics in the soil compartment.
Importantly, our finding of a substantial photosynthate driven production of DOC challenges the paradigm that DOC is mainly
the result of decomposition of organic matter.
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ABSTRACT: The relationships between plant carbon resources, soil carbon and nitrogen content, and ectomycorrhizal fungal (EMF) diversity in a monospecific, old-growth beech (Fagus sylvatica) forest were investigated by manipulating carbon flux by girdling. We hypothesized that disruption of the carbon supply would not affect diversity and EMF species numbers if EM fungi can be supplied by plant internal carbohydrate resources or would result in selective disappearance of EMF taxa because of differences in carbon demand of different fungi. Tree carbohydrate status, root demography, EMF colonization, and EMF taxon abundance were measured repeatedly during 1 year after girdling. Girdling did not affect root colonization but decreased EMF species richness of an estimated 79 to 90 taxa to about 40 taxa. Cenococcum geophilum, Lactarius blennius, and Tomentella lapida were dominant, colonizing about 70% of the root tips, and remained unaffected by girdling. Mainly cryptic EMF species disappeared. Therefore, the Shannon-Wiener index (H�) decreased but evenness was unaffected. H� was positively correlated with glucose, fructose, and starch concentrations of fine roots and also with the ratio of dissolved organic carbon to dissolved organic nitrogen (DOC/DON), suggesting that both H� and DOC/DON were governed by changes in belowground carbon allocation. Our results suggest that beech maintains numerous rare EMF species by recent photosynthate. These EM fungi may constitute biological insurance for adaptation to changing environmental conditions. The preservation of taxa previously not known to colonize beech may, thus, form an important reservoir for future forest development.
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ABSTRACT: There is growing evidence of the importance of extramatrical mycelium (EMM) of mycorrhizal fungi in carbon (C) cycling in ecosystems. However, our understanding has until recently been mainly based on laboratory experiments, and knowledge of such basic parameters as variations in mycelial production, standing biomass and turnover as well as the regulatory mechanisms behind such variations in forest soils is limited. Presently, the production of EMM by ectomycorrhizal (EM) fungi has been estimated at ~140 different forest sites to be up to several hundreds of kg per ha per year, but the published data are biased towards Picea abies in Scandinavia. Little is known about the standing biomass and turnover of EMM in other systems, and its influence on the C stored or lost from soils. Here, focussing on ectomycorrhizas, we discuss the factors that regulate the production and turnover of EMM and its role in soil C dynamics, identifying important gaps in this knowledge. C availability seems to be the key factor determining EMM production and possibly its standing biomass in forests but direct effects of mineral nutrient availability on the EMM can be important. There is great uncertainty about the rate of turnover of EMM. There is increasing evidence that residues of EM fungi play a major role in the formation of stable N and C in SOM, which highlights the need to include mycorrhizal effects in models of global soil C stores.Plant and Soil 05/2013; 366:1-27. · 2.64 Impact Factor
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ABSTRACT: Natural organic matter (NOM) is a key component in aquatic ecosystems. It influences for example acidity, mobility and toxicity of metals and organic pollutants, energy input to aquatic food webs, weathering, and water light conditions. There are also considerable costs associated with removing NOM in drinking water production. NOM is also an integral part of the carbon cycle with possible indirect effects on climate. During recent decades, there have been observations of increasing concentrations of NOM in surface waters in parts of North America and Europe. The causes of these trends are not fully understood, but are thought to be related to climate change and recovery from anthropogenic acidification. This thesis presents results from studies on intra-annual NOM cycling in more than 130 boreal streams and rivers. It also presents developments of the Riparian flow-concentration Integration Model (RIM). Detailed studies on five forested headwater catchments revealed that stream discharge and soil temperature were the main drivers of NOM variability. In addition, a small headwater catchment at the Swedish West Coast was substantially influenced by sea-salt deposition, which suppressed NOM mobilization. A modified version of RIM with discharge and soil temperature as variables could successfully simulate NOM dynamics in all five catchments. Riparian soil organic matter content and distribution was hypothesized to be the underlying control on NOM response to discharge and soil temperature. Catchments where NOM was sensitive to discharge displayed stronger gradients in soil NOM concentrations than did catchments with weak discharge sensitivity. A large scale study of 136 streams and rivers indicated common relationships among NOM, discharge and temperature. Conversely, there was no geographical pattern in NOM trends. Relative trends were weakly related to NOM response to flow and temperature. There were also clear relationships among intra-annual NOM dynamics, temperature, flow, and catchment landscape characteristics, indicating that catchments can be classified based on NOM dynamics. Taken together, this implies that NOM dynamics could change in ways not reflected in inter-annual trends due to climate change.01/2013, Degree: PhD, Supervisor: Kevin Bishop