| 1. |
- Clarholm, Marianne, et al.
(författare)
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Organic acid induced release of nutrients from metal-stabilized soil organic matter - The unbutton model
- 2015
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 84, s. 168-176
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Forskningsöversikt (refereegranskat)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, citric 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 (10(3)-10(5) Da) resolves micro-spatial priming of SOM, where the destabilization step is prerequisite for subsequent release of nutrients. (C) 2015 Elsevier Ltd. All rights reserved.
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| 2. |
- Clarholm, Marianne
(författare)
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Soil fauna : 25.1 Protozoa
- 2012
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Ingår i: Handbook of Soil Sciences, Properties and Processes. - 1439803056 ; , s. 25-1
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Bokkapitel (refereegranskat)
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| 3. |
- Clarholm, Marianne
(författare)
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Soil protistology rebooted: 30 fundamental questions to start with
- 2017
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 111, s. 94-103
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Tidskriftsartikel (refereegranskat)abstract
- Protists are the most diverse eukaryotes. These microbes are keystone organisms of soil ecosystems and regulate essential processes of soil fertility such as nutrient cycling and plant growth. Despite this, protists have received little scientific attention, especially compared to bacteria, fungi and nematodes in soil studies. Recent methodological advances, particularly in molecular biology techniques, have made the study of soil protists more accessible, and have created a resurgence of interest in soil protistology. This ongoing revolution now enables comprehensive investigations of the structure and functioning of soil protist communities, paving the way to a new era in soil biology. Instead of providing an exhaustive review, we provide a synthesis of research gaps that should be prioritized in future studies of soil protistology to guide this rapidly developing research area. Based on a synthesis of expert opinion we propose 30 key questions covering a broad range of topics including evolution, phylogenetics, functional ecology, macroecology, paleoecology, and methodologies. These questions highlight a diversity of topics that will establish soil protistology as a hub discipline connecting different fundamental and applied fields such as ecology, biogeography, evolution, plant-microbe interactions, agronomy, and conservation biology. We are convinced that soil protistology has the potential to be one of the most exciting frontiers in biology. (C) 2017 Elsevier Ltd. All rights reserved.
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| 4. |
- Clarholm, Marianne
(författare)
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Stimulation of Plant Growth through Interactions of Bacteria and Protozoa: Testing the Auxiliary Microbial Loop Hypothesis
- 2012
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Ingår i: Acta Protozoologica. - 0065-1583 .- 1689-0027. ; 51, s. 237-247
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Forskningsöversikt (refereegranskat)abstract
- By feeding on bacterial biomass protozoa play an acknowledged role in the liberation of nutrients in the plant rhizosphere. In addition there are suggestions that plants have mechanisms working through changes in root architecture and initiation of active release from soil organic matter, which are used to improve uptake and recirculation of nutrients in the ecosystem. All processes are carried out on a local scale in soil with roots, bacteria and protozoa interacting. The many actors and the small scale of interactions make experimentation difficult. We discuss mistakes, pitfalls and misinterpretations and provide suggestions for improvement. Recent methodological progress has opened new exciting avenues for protozoan research. New techniques have already helped to reveal protozoan regulation of cooperation as well as conflict in bacterial communities. These mechanisms in turn affect bacterial functioning and target molecular control points in rhizosphere food webs in relation to plants. Integrating nutritional and regulatory aspects into new concepts of protozoan functioning in soil is a challenging frontier in protozoology.
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| 5. |
- Clarholm, Marianne, et al.
(författare)
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Translocation of metals by trees and fungi regulates pH, soil organic matter turnover and nitrogen availability in acidic forest soils
- 2013
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 63, s. 142-153
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Forskningsöversikt (refereegranskat)abstract
- In this paper we provide support for the hypothesis that trees and fungi modify their physical environment in acidic forest soils by actively translocating metal cations from the mineral to the organic horizon. We suggest that pH buffering and soil organic matter (SOM) turnover in organic horizons are regulated by Ca and Mg cycling via the tree canopy and litter fall and by fungal translocation of Al from mineral soil. Fungi in addition translocate Fe from mineral to organic horizons to enhance the degradation of aromatic structures in SOM. Together these processes are in control of N bioavailability, including new inputs via N fixation. Cycling of Ca and Mg via tree canopy typically increase the base saturation and pH towards the surface of organic forest soil horizons. This process is most clearly depicted at sites with laterally moving groundwater. An up-transport of Al from mineral to the organic horizon is most important as a pH buffering process in less productive, acid soils having a pH below approximately 4.5. At this pH, the non-acidic properties of organically complexed Al keep pH at a level sufficiently high for maintained microbial activity. Furthermore, the formation of bridging bonds between major organic functional groups (carboxyls and phenols) and di- and trivalent metal cations of Ca, Al and Fe possesses a strong influence on the tertiary structure of SOM and its persistence to degradation and delivery of N. (C) 2013 Elsevier Ltd. All rights reserved.
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| 6. |
- Gärdenäs, Annemieke, et al.
(författare)
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Knowledge gaps in soil carbon and nitrogen interactions - From molecular to global scale
- 2011
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 43, s. 702-717
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this review was to identify, address and rank knowledge gaps in our understanding of five major soil C and N interactions across a range of scales – from molecular to global. The studied five soil C and N interactions are: i) N controls on the soil emissions of greenhouse gases, ii) plant utilisation of organic N, iii) impact of rhizosphere priming on C and N cycling, iv) impact of black N on the stabilisation of soil organic matter (SOM) and v) representation of fractions of SOM in simulation models. We ranked the identified knowledge gaps according to the importance we attached to them for functional descriptions of soil–climate interactions at the global scale, for instance in general circulation models (GCMs). Both the direct and indirect influences on soil–climate interactions were included. We found that the level of understanding declined as the scale increased from molecular to global for four of the five topics. By contrast, the knowledge level for SOM simulation models appeared to be highest when considered at the ecosystem scale. The largest discrepancy between knowledge level and importance was found at the global modelling scale. We concluded that a reliable quantification of greenhouse gas emissions at the ecosystem scale is of utmost importance for improving soil–climate representation in GCMs. We see as key questions the identification of the role of different N species for the temperature sensitivity of SOM decomposition rates and its consequences for plant available N
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| 7. |
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| 8. |
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| 9. |
- Klemendtsson, Leif, et al.
(författare)
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Microbial nitrogen transformations in the root environment of barley
- 1987
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Ingår i: Soil Biology and Biochemistry. - : Elsevier. - 0038-0717 .- 1879-3428. ; 19:5, s. 551-558
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Tidskriftsartikel (refereegranskat)abstract
- To determine the influence of barley roots on microorganisms and N-transfonning processes in soil, numbers of nitrifiers and potential nitrification and denitrification rates were measured every week for 5 wks. The barley plants were grown in growth chambers in which the root-containing soil layer (A) was separated from three outer soil layers (B, C, D). The numbers and biomass of bacteria, numbers of flagellates and amoebae, total and FDA-active hyphal lengths, microbial biomass carbon and respiration were also determined.The numbers of ammonium oxidizers were positively correlated with root biomass but did not differ significantly between soil layers. Potential ammonium oxidation was stimulated in the root-layer, while potential nitrite oxidation was stimulated in the B- and C-layers.The denitrification activity (measured anaerobically in the presence of excess No- 3) was positively correlated with root biomass in the A-layer. Denitrification activity in the B-layer was positively correlated with the water content of the soil. When roots grew near the nets separating the root layer from the other layers, denitrification activity was stimulated in the next layer (B).We propose that nitrite oxidation in the root zone partly depends on the reduction of nitrate. This would explain why nitrite-oxidizer numbers were usually several orders of magnitude higher than ammonium-oxidizer numbers.Bacterial numbers decreased between wks 1 and 5. Increases in bacteria, naked amoebae and flagellates in all layers between wks 2 and 3 indicated that bacteria were produced until wk 3. There were no signs of bacterial production after wk 3.The total length of hyphae and the length of FDA-active hyphae were not significantly different between layers. However, both of these parameters, as well as total microbial biomass carbon and respiration, were consistently highest in the A-layer.
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| 10. |
- Robertson, Kerstin, et al.
(författare)
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Microbial biomass in relation to C and N mineralization during laboratory incubations
- 1988
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Ingår i: Soil Biology and Biochemistry. - : Elsevier. - 0038-0717 .- 1879-3428. ; 20:3, s. 281-286
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Tidskriftsartikel (refereegranskat)abstract
- Net carbon and nitrogen mineralization rates were determined for an arable soil during 12 weeks at 37†C using an aerobic incubation-leaching technique. The amounts of mineralized C and N were compared to changes in the contents of C and N in microbial biomass (as determined by the chloroform fumigation incubation method; CFIM) during the incubation and to amounts of organic C and N in the leachates. Microorganisms were also followed by direct counting of bacteria, measurements of total hyphal lengths and fluorescein diacetate (FDA)-active hyphae, and by most probable number determinations of protozoa (naked amoebae and flagellates).Numbers of naked amoebae increased nearly 10-fold initially and then decreased between weeks 6 and 12. Bacterial numbers and FDA-active hyphae decreased during the incubation, and the relative composition changed slightly in favour of bacteria. Total hyphal lengths remained almost constant.A total of 105 μg N g'- soil dry wt and 1179 μg C g- soil dry wt was mineralized during the incubation, while the microbial N pool decreased by 42 γm- soil dry wt and the microbial C pool decreased by 225μ g- soil dry wt. Soluble organic matter in the leachates amounted to 16 and 31% of mineralized C and N, respectively.The possibility of measuring C mineralization with less frequent teachings and determinations of N mineralization offers an easy method for assessing changes in labile soil organic matter over time or for comparisons between soils. Through the use of appropriate C-to-N ratios, the N-content in the labile pool can be calculated.
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