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: 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.Soil Biology and Biochemistry 03/2015; 84. DOI:10.1016/j.soilbio.2015.02.019 · 4.41 Impact Factor
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ABSTRACT: Climatic change is currently enhancing permafrost thawing and the flow of water through the landscape in subarctic and arctic catchments, with major consequences for the carbon export to aquatic ecosystems. We studied stream water carbon export in several tundra-dominated catchments in northern Sweden. There were clear seasonal differences in both dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations. The highest DOC concentrations occurred during the spring freshet while the highest DIC concentrations were always observed during winter baseflow conditions for the six catchments considered in this study. Long-term trends for the period 1982 to 2010 for one of the streams showed that DIC concentrations has increased by 9% during the 28 yr of measurement while no clear trend was found for DOC. Similar increasing trends were also found for conductivity, Ca and Mg. When trends were discretized into individual months, we found a significant linear increase in DIC concentrations with time for September, November and December. In these subarctic catchments, the annual mass of C exported as DIC was in the same order of magnitude as DOC; the average proportion of DIC to the total dissolved C exported was 61% for the six streams. Furthermore, there was a direct relationship between total runoff and annual dissolved carbon fluxes for these six catchments. These relationships were more prevalent for annual DIC exports than annual DOC exports in this region. Our results also highlight that both DOC and DIC can be important in high-latitude ecosystems. This is particularly relevant in environments where thawing permafrost and changes to subsurface ice due to global warming can influence stream water fluxes of C. The large proportion of stream water DIC flux also has implications on regional C budgets and needs to be considered in order to understand climate-induced feedback mechanisms across the landscape.Biogeosciences 12/2013; 11(2). DOI:10.5194/bg-11-525-2014 · 3.75 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