Giesler R, Högberg MN, Strobel BW, Richter A, Nordgren A, Högberg P.. Production of dissolved organic carbon and low-molecular weight organic acids in soil solution driven by recent tree photosynthate. Biogeochemistry 84: 1-12

Department of Ecology and Environmental Science, Umeå University, Umeå, Västerbotten, Sweden
Biogeochemistry (Impact Factor: 3.49). 04/2007; 84(1):1-12. DOI: 10.1007/s10533-007-9069-3


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 experiment.
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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    • "Often, coverage of peatlands in the catchment area is the best predictor of terrestrial DOC inputs (Urban et al. 1989; Dillon and Molot 1997; Kortelainen et al. 2006a; Einola et al. 2011) followed by coniferous forests (Mattsson et al. 2003; Humborg et al. 2004; Larsen et al. 2011). In addition, type of vegetation cover may determine the chemical composition of OC inputs (Giesler et al. 2007; Ågren et al. 2008; Berggren et al. 2010). Generally, terrestrial OC loading to lakes increases with drainage area relative to lake size (Rasmussen et al. 1989; D'arcy and Carignan 1997; Einsele et al. 2001). "
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    ABSTRACT: Converging evidence suggests that freshwater systems play an important role in the carbon cycles at both regional and global scales. In addition, there are serious concerns that ongoing and future changes to the environment can alter these dynamics. This is particularly important in the boreal forest biome, which contains a very high density of lakes. In this review, we synthesize the current state of research to provide a critical overview of (1) the role of boreal lakes as emitters vs. sinks of carbon, (2) their contribution to the regional carbon balance, (3) knowledge gaps that may inhibit an accurate evaluation of the role of boreal lakes in a landscape context, and (4) impacts of environmental perturbations on carbon dynamics in boreal lakes. Several recent studies indicate that boreal lakes are actively processing, emitting and storing carbon rather than being passive transport conduits. Yet, generalizing the role of lake ecosystems for the overall carbon balance of the boreal forest biome is challenging due to the scarcity of studies on lake carbon budgets in a landscape context that can capture the potential temporal and spatial variability and uncertainties associated with the available estimates of carbon pools and fluxes. Further, environmental perturbations such as climate change, acidic deposition and nutrient enrichment likely affect both carbon export to lakes and in-lake carbon processing in boreal regions. Predicting their overall impacts on lake carbon budgets is particularly difficult not only because individual environmental stressors likely affect multiple processes involved in carbon cycling, but also because often multiple stressors act synergistically or antagonistically at the landscape level. Accordingly, long-term, system-wide approaches are required to accurately evaluate the importance of lakes for boreal carbon budgets in a changing environment.
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    • "One of the studied sites was later stemgirdled . This treatment led to a 90% reduction of citrate in the soil solution also the concentration of DOC was reduced by 40%, while concentrations of oxalate remained low (Giesler et al., 2007). This observation strongly supports a direct link between plant carbon assimilation and concentrations of LMMOAs in soil solution. "
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    ABSTRACT: Processes of soil organic matter (SOM) stabilization and the reverse, destabilization of SOM resulting in subsequent release and mobilization of nutrients from SOM, remain largely unresolved. The perception of SOM as supramolecular aggregates built of low molecular mass biomolecules is currently emerging. Polyvalent metal cations contribute to SOM tertiary structure by bridging functional groups of such molecules (Simpson et al., 2002). The strong bond to metals protects high quality organic material from being immediately accessed and decomposed. Here we propose a three-step process by which low molecular mass organic acids (LMMOAs) and hydrolytic enzymes act in series to destabilize SOM supramolecules to release organic nitrogen (N) and phosphorus (P) for local hyphal and root uptake. Complexation of the stabilizing metals by fungal-released LMMOA gives fungal-root consortia direct access to organic substrates of good quality. Because of their small sizes and carboxyl group configuration, citratic and oxalic acids are the most effective LMMOAs forming stable complexes with the main SOM bridging metals Ca and Al in SOM. Citrate, forming particularly strong complexes with the trivalent cations Al and Fe, is dominant in soil solutions of low-productive highly acidic boreal forest soils where mycorrhizal associations with roots are formed predominantly by fungi with hydrophobic hyphal surfaces. In these systems mycelia participate in the formation of N-containing SOM with a significant contribution from strong Al bridges. In less acidic soils of temperate forests, including calcareous influenced soils, SOM is stabilized predominantly by Ca bridges. In such systems mycorrhizal fungi with more hydrophilic surfaces dominate, and oxalic acid, forming strong bidentate complexes with Ca, is the most common LMMOA exuded. A plant-fungus driven biotic mechanism at the supramolecular aggregate level (103–105 Da) resolves micro-spatial priming of SOM, where the destabilization step is prerequisite for subsequent release of nutrients.
    Full-text · Article · Mar 2015 · Soil Biology and Biochemistry
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    • "In coniferous forests, these fungi represent up to one-third of total microbial biomass (Högberg and Hö gberg, 2002) and contribute significantly to the production and composition of dissolved organic carbon (DOC) and to C immobilization belowground (Clemmensen et al., 2013). Tree girdling thus decreases DOC production in soil by tens of percents (Giesler et al., 2007); at a later stage, ECM fungi disappear (Yarwood et al., 2009). "
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    ABSTRACT: Coniferous forests cover extensive areas of the boreal and temperate zones. Owing to their primary production and C storage, they have an important role in the global carbon balance. Forest disturbances such as forest fires, windthrows or insect pest outbreaks have a substantial effect on the functioning of these ecosystems. Recent decades have seen an increase in the areas affected by disturbances in both North America and Europe, with indications that this increase is due to both local human activity and global climate change. Here we examine the structural and functional response of the litter and soil microbial community in a Picea abies forest to tree dieback following an invasion of the bark beetle Ips typographus, with a specific focus on the fungal community. The insect-induced disturbance rapidly and profoundly changed vegetation and nutrient availability by killing spruce trees so that the readily available root exudates were replaced by more recalcitrant, polymeric plant biomass components. Owing to the dramatic decrease in photosynthesis, the rate of decomposition processes in the ecosystem decreased as soon as the one-time litter input had been processed. The fungal community showed profound changes, including a decrease in biomass (2.5-fold in the litter and 12-fold in the soil) together with the disappearance of fungi symbiotic with tree roots and a relative increase in saprotrophic taxa. Within the latter group, successive changes reflected the changing availability of needle litter and woody debris. Bacterial biomass appeared to be either unaffected or increased after the disturbance, resulting in a substantial increase in the bacterial/fungal biomass ratio.The ISME Journal advance online publication, 27 March 2014; doi:10.1038/ismej.2014.37.
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