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  • 1
    In: Ecology, May 2011, Vol.92(5), pp.1052-1062
    Description: Lignin is a main component of plant litter. Its degradation is thought to be critical for litter decomposition rates and the build‐up of soil organic matter. We studied the relationships between lignin degradation and the production of dissolved organic carbon (DOC) and of CO during litter decomposition. Needle or leaf litter of five species (Norway spruce, Scots pine, mountain ash, European beech, sycamore maple) and of different decomposition stage (freshly fallen and up to 27 months of field exposure) was incubated in the laboratory for two years. Lignin degradation was followed with the CuO method. Strong lignin degradation occurred during the first 200 incubation days, as revealed by decreasing yields of lignin‐derived phenols. Thereafter lignin degradation leveled off. This pattern was similar for fresh and decomposed litter, and it stands in contrast to the common view of limited lignin degradation in fresh litter. Dissolved organic carbon and CO also peaked in the first period of the incubation but were not interrelated. In the later phase of incubation, CO production was positively correlated with DOC amounts, suggesting that bioavailable, soluble compounds became a limiting factor for CO production. Lignin degradation occurred only when CO production was high, and not limited by bioavailable carbon. Thus carbon availability was the most important control on lignin degradation. In turn, lignin degradation could not explain differences in DOC and CO production over the study period. Our results challenge the traditional view regarding the fate and role of lignin during litter decomposition. Lignin degradation is controlled by the availability of easily decomposable carbon sources. Consequently, it occurs particularly in the initial phase of litter decomposition and is hampered at later stages if easily decomposable resources decline.
    Keywords: C Availability ; Dissolved Organic Matter ; Lignin ; Plant Litter Decomposition ; Respiration Rates
    ISSN: 0012-9658
    E-ISSN: 1939-9170
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  • 2
    In: Nature, 2011, Vol.478(7367), p.49
    Description: Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily—and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming. Journal Article.
    Keywords: Environmental Sciences Geosciences;
    ISSN: 0028-0836
    E-ISSN: 14764687
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  • 3
    Language: English
    In: Soil Biology and Biochemistry, 2015, Vol.88, p.390(13)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.soilbio.2015.06.008 Byline: Bernhard Ahrens, Maarten C. Braakhekke, Georg Guggenberger, Marion Schrumpf, Markus Reichstein Abstract: Profiles of soil organic carbon (SOC) are often characterized by a steep increase of.sup.14C age with depth, often leading to subsoil.sup.14C ages of more than 1000 years. These observations have generally been reproduced in SOC models by introducing a SOC pool that decomposes on the time-scale of millennia. The overemphasis of chemical recalcitrance as the major factor for the persistence of SOC was able to provide a mechanistic justification for these very low decomposition rates. The emerging view on SOC persistence, however, stresses that apart from molecular structure a multitude of mechanisms can lead to the long-term persistence of organic carbon in soils. These mechanisms, however, have not been incorporated into most models. Consequently, we developed the SOC profile model COMISSION which simulates vertically resolved SOC concentrations based on representations of microbial interactions, sorption to minerals, and vertical transport. We calibrated COMISSION using published concentrations of SOC, microbial biomass and mineral-associated OC (MOC), and in addition,.sup.14C contents of SOC and MOC of a Haplic Podzol profile in North-Eastern Bavaria, Germany. In order to elucidate the contribution of the implemented processes to the.sup.14C age in different parts of the profile, we performed model-experiments in which we switched off the limitation of SOC decomposition by microbes, sorptive stabilization on soil minerals, and dissolved OC (DOC) transport. By splitting all model pools into directly litter-derived carbon and microbe-derived organic carbon, we investigated the contribution of repeated microbial recycling to.sup.14C ages throughout the profile. The model-experiments for this site lead to the following implications: Without rejuvenation by DOC transport, SOC in the subsoil would be on average 1700.sup.14C years older. Across the profile, SOC from microbial recycling is on average 1400.sup.14C years older than litter-derived SOC. Without microbial limitation of depolymerization, SOC in the subsoil would be on average 610.sup.14C years younger. Sorptive stabilization is responsible for relatively high.sup.14C ages in the topsoil. The model-experiments further indicate that the high SOC concentrations in the Bh horizon are caused by the interplay between sorptive stabilization and microbial dynamics. Overall, the model-experiments demonstrate that the high.sup.14C ages are not solely caused by slow turnover of a single pool, but that the increase of.sup.14C ages along a soil profile up to ages 〉1000 years is the result of different mechanisms contributing to the overall persistence of SOC. The dominant reasons for the persistence of SOC are stabilization processes, followed by repeated microbial processing of SOC. Author Affiliation: (a) Max Planck Institute for Biogeochemistry, Hans-Knoll-Str. 10, 07445 Jena, Germany (b) Copernicus Institute of Sustainable Development, Faculty of Geosciences, Utrecht University, Netherlands (c) Wageningen University, Earth System Science Group, Wageningen, Netherlands (d) Institute of Soil Science, Leibniz Universitat Hannover, Hannover, Germany Article History: Received 15 January 2015; Revised 29 May 2015; Accepted 6 June 2015
    Keywords: Soil Carbon – Analysis ; Soil Carbon – Models
    ISSN: 0038-0717
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Plant and Soil, 2014, Vol.384(1), pp.289-301
    Keywords: Non-cellulosic sugars ; Lignin Microbial biomass ; Mineralization ; Soil organic carbon quality
    ISSN: 0032-079X
    E-ISSN: 1573-5036
    Source: Springer Science & Business Media B.V.
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  • 5
    Language: English
    In: Forest Ecology and Management, 2011, Vol.261(6), pp.1090-1098
    Description: ▶ Soil moisture is the major environmental factor controlling soil respiration. ▶ Length of dry and wet season has stronger impact on soil CO efflux. ▶ Tree species influence soil CO efflux differently. Variability of soil CO efflux strongly depends on soil temperature, soil moisture and plant phenology. Separating the effects of these factors is critical to understand the belowground carbon dynamics of forest ecosystem. In Ethiopia with its unreliable seasonal rainfall, variability of soil CO efflux may be particularly associated with seasonal variation. In this study, soil respiration was measured in nine plots under the canopies of three indigenous trees ( , and ) growing in an Afromontane forest of south-eastern Ethiopia. Our objectives were to investigate seasonal and diurnal variation in soil CO flux rate as a function of soil temperature and soil moisture, and to investigate the impact of tree species composition on soil respiration. Results showed that soil respiration displayed strong seasonal patterns, being lower during dry periods and higher during wet periods. The dependence of soil respiration on soil moisture under the three tree species explained about 50% of the seasonal variability. The relation followed a Gaussian function, and indicated a decrease in soil respiration at soil volumetric water contents exceeding a threshold of about 30%. Under more moist conditions soil respiration is tentatively limited by low oxygen supply. On a diurnal basis temperature dependency was observed, but not during dry periods when plant and soil microbial activities were restrained by moisture deficiency. Tree species influenced soil respiration, and there was a significant interaction effect of tree species and soil moisture on soil CO efflux variability. During wet (and cloudy) period, when shade tolerant late successional is having a physiological advantage, soil respiration under this tree species exceeded that under the other two species. In contrast, soil CO efflux rates under light demanding pioneer appeared to be least sensitive to dry (but sunny) conditions. This is probably related to the relatively higher carbon assimilation rates and associated root respiration. We conclude that besides the anticipated changes in precipitation pattern in Ethiopia any anthropogenic disturbance fostering the pioneer species may alter the future ecosystem carbon balance by its impact on soil respiration.
    Keywords: Croton Macrostachys ; Ethiopia ; Podocarpus Falcatus ; Prunus Africana ; Soil CO 2 Efflux ; Soil Moisture ; Soil Respiration ; Soil Temperature ; Tropical Forest ; Forestry ; Biology
    ISSN: 0378-1127
    E-ISSN: 1872-7042
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 November 2014, Vol.144, pp.258-276
    Description: Ferric oxyhydroxides play an important role in controlling the bioavailability of oxyanions such as arsenate and phosphate in soil. Despite this, little is known about the properties and reactivity of Fe(III)-organic matter phases derived from adsorption (reaction of organic matter (OM) to post-synthesis Fe oxide) coprecipitation (formation of Fe oxides in presence of OM). Coprecipitates and adsorption complexes were synthesized at pH 4 using two natural organic matter (NOM) types extracted from forest floor layers (Oi and Oa horizon) of a Haplic Podzol. Iron(III) coprecipitates were formed at initial molar metal-to-carbon (M/C) ratios of 1.0 and 0.1 and an aluminum (Al)-to-Fe(III) ratio of 0.2. Sample properties were studied by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), N gas adsorption, dynamic light scattering, and electrophoretic mobility measurements. Arsenic [As(V)] adsorption to Fe-OM phases was studied in batch experiments (168 h, pH 4, 100 μM As). The organic carbon (OC) contents of the coprecipitates (82–339 mg g ) were higher than those of adsorption complexes (31 and 36 mg g ), leading to pronounced variations in specific surface area (9–300 m g ), average pore radii (1–9 nm), and total pore volumes (11–374 mm g ) but being independent of the NOM type or the presence of Al. The occlusion of Fe solids by OM (XPS surface concentrations: 60–82 atom% C) caused comparable pH (1.5–2) of adsorption complexes and coprecipitates. The synthesis conditions resulted in different Fe-OM association modes: Fe oxide particles in ‘M/C 0.1’ coprecipitates covered to a larger extent the outermost aggregate surfaces, for some ‘M/C 1.0’ coprecipitates OM effectively enveloped the Fe oxides, while OM in the adsorption complexes primarily covered the outer aggregate surfaces. Despite of their larger OC contents, adsorption of As(V) was fastest to coprecipitates formed at low Fe availability (M/C 0.1) and facilitated by desorption of weakly bonded OC and disaggregation. In contrast, ‘M/C 1.0’ coprecipitates showed a comparable rate of As uptake as the adsorption complexes. While small mesopores (2–10 nm) promoted the fast As uptake particularly to ‘M/C 0.1’ coprecipitates, the presence of micropores (〈2 nm) appeared to impair As desorption. This study shows that the environmental reactivity of poorly crystalline Fe(III) oxides in terrestrial and aquatic systems can largely vary depending on the formation conditions. Carbon-rich Fe phases precipitated at low M/C ratios may play a more important role in oxyanion immobilization and Fe and C cycling than phases formed at higher M/C ratios or respective adsorption complexes.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 March 2018, Vol.224, pp.223-248
    Description: Iron (Fe) oxyhydroxides are important constituents of the soil mineral phase known to stabilize organic matter (OM) under oxic conditions. In an anoxic milieu, however, these Fe-organic associations are exposed to microbial reduction, releasing OM into soil solution. At present, only few studies have addressed the influence of adsorbed natural OM (NOM) on the reductive dissolution of Fe oxyhydroxides. This study therefore examined the impact of both the composition and concentration of adsorbed NOM on microbial Fe reduction with regard to (i) electron shuttling, (ii) complexation of Fe(II,III), (iii) surface site coverage and/or pore blockage, and (iv) aggregation. Adsorption complexes with varying carbon loadings were synthesized using different Fe oxyhydroxides (ferrihydrite, lepidocrocite, goethite, hematite, magnetite) and NOM of different origin (extracellular polymeric substances from OM extracted from soil Oi and Oa horizons). The adsorption complexes were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), N gas adsorption, electrophoretic mobility and particle size measurements, and OM desorption. Incubation experiments under anaerobic conditions were conducted for 16 days comparing two different strains of dissimilatory Fe(III)-reducing bacteria ( , ). Mineral transformation during reduction was assessed via XRD and FTIR. Microbial reduction of the pure Fe oxyhydroxides was controlled by the specific surface area (SSA) and solubility of the minerals. For , the Fe reduction of adsorption complexes strongly correlated with the concentration of potentially usable electron-shuttling molecules for NOM concentrations 〈2 mg C L , whereas for , Fe reduction depended on the particle size and thus aggregation of the adsorption complexes. These diverging results suggest that the influence of NOM on the stability of Fe-organic associations in soils cannot easily be assessed without considering the composition of the microbial soil community.
    Keywords: Microbial Reduction ; Iron Oxyhydroxides ; Natural Organic Matter ; Extracellular Polymeric Substances ; Shewanella Putrefaciens ; Geobacter Metallireducens ; Mineral-Organic Associations ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 8
    In: Global Change Biology, November 2010, Vol.16(11), pp.2990-3003
    Description: Global nitrogen (N) deposition rates in terrestrial environments have quadrupled since preindustrial times, causing structural and functional changes of ecosystems. Different emission reduction policies were therefore devised. The aim of our study was to investigate if, and over what timescale, processes of soil organic matter (OM) transformation respond to a decline in atmospheric N deposition. A N‐saturated spruce forest (current N deposition: 34 kg ha yr; critical N load: 14 kg ha yr), where N deposition has been reduced to 11.5 kg ha yr since 1991, was studied. Besides organic C and organic and inorganic N, noncellulosic carbohydrates, amino sugars and amino acids were determined. A decline in organic N in litter indicated initial effects at plant level. However, there were no changes in biomarkers upon the reduction in N deposition. In addition, inorganic N was not affected by reduced N deposition. The results showed that OM cycling and transformation processes have not responded so far. It was concluded that no direct N deposition effects have occurred due to the large amount of stored organic N, which seems to compensate for the reduction in deposited N. Obviously, the time span of atmospheric N reduction (about 14.5 years) is too short compared with the mean turnover time of litter to cause indirect effects on the composition of organic C and N compounds. It is assumed that ecological processes, such as microbial decomposition or recycling of organic N and C, react slowly, but may start within the next decade with the incorporation of the new litter.
    Keywords: Amino Acid Enantiomers ; Amino Acids ; Amino Sugars ; Biomarker ; N Deposition ; Non‐Cellulosic Carbohydrates ; Soil Organic Nitrogen ; Solling Roof Project
    ISSN: 1354-1013
    E-ISSN: 1365-2486
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  • 9
    Language: English
    In: 2012, Vol.7(9), p.e45540
    Description: Anthropogenic disturbance of old-growth tropical forests increases the abundance of early successional tree species at the cost of late successional ones. Quantifying differences in terms of carbon allocation and the proportion of recently fixed carbon in soil CO 2 efflux is crucial for addressing the carbon footprint of creeping degradation. ; We compared the carbon allocation pattern of the late successional gymnosperm (Thunb.) Mirb. and the early successional (gap filling) angiosperm Hochst. es Del. in an Ethiopian Afromontane forest by whole tree CO pulse labeling. Over a one-year period we monitored the temporal resolution of the label in the foliage, the phloem sap, the arbuscular mycorrhiza, and in soil-derived CO. Further, we quantified the overall losses of assimilated C with soil CO efflux. ; C in leaves of declined more rapidly with a larger size of a fast pool (64% . 50% of the assimilated carbon), having a shorter mean residence time (14 h . 55 h) as in leaves of . Phloem sap velocity was about 4 times higher for . Likewise, the label appeared earlier in the arbuscular mycorrhiza of and in the soil CO efflux as in case of (24 h . 72 h). Within one year soil CO efflux amounted to a loss of 32% of assimilated carbon for the gap filling tree and to 15% for the late successional one. ; Our results showed clear differences in carbon allocation patterns between tree species, although we caution that this experiment was unreplicated. A shift in tree species composition of tropical montane forests (e.g., by degradation) accelerates carbon allocation belowground and increases respiratory carbon losses by the autotrophic community. If ongoing disturbance keeps early successional species in dominance, the larger allocation to fast cycling compartments may deplete soil organic carbon in the long run.
    Keywords: Research Article ; Biology ; Earth Sciences ; Chemistry
    E-ISSN: 1932-6203
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  • 10
    Language: English
    In: Soil Biology and Biochemistry, September 2015, Vol.88, pp.390-402
    Description: Profiles of soil organic carbon (SOC) are often characterized by a steep increase of C age with depth, often leading to subsoil C ages of more than 1000 years. These observations have generally been reproduced in SOC models by introducing a SOC pool that decomposes on the time-scale of millennia. The overemphasis of chemical recalcitrance as the major factor for the persistence of SOC was able to provide a mechanistic justification for these very low decomposition rates. The emerging view on SOC persistence, however, stresses that apart from molecular structure a multitude of mechanisms can lead to the long-term persistence of organic carbon in soils. These mechanisms, however, have not been incorporated into most models. Consequently, we developed the SOC profile model COMISSION which simulates vertically resolved SOC concentrations based on representations of microbial interactions, sorption to minerals, and vertical transport. We calibrated COMISSION using published concentrations of SOC, microbial biomass and mineral-associated OC (MOC), and in addition, C contents of SOC and MOC of a Haplic Podzol profile in North-Eastern Bavaria, Germany. In order to elucidate the contribution of the implemented processes to the C age in different parts of the profile, we performed model-experiments in which we switched off the limitation of SOC decomposition by microbes, sorptive stabilization on soil minerals, and dissolved OC (DOC) transport. By splitting all model pools into directly litter-derived carbon and microbe-derived organic carbon, we investigated the contribution of repeated microbial recycling to C ages throughout the profile. The model-experiments for this site lead to the following implications: Without rejuvenation by DOC transport, SOC in the subsoil would be on average 1700 C years older. Across the profile, SOC from microbial recycling is on average 1400 C years older than litter-derived SOC. Without microbial limitation of depolymerization, SOC in the subsoil would be on average 610 C years younger. Sorptive stabilization is responsible for relatively high C ages in the topsoil. The model-experiments further indicate that the high SOC concentrations in the Bh horizon are caused by the interplay between sorptive stabilization and microbial dynamics. Overall, the model-experiments demonstrate that the high C ages are not solely caused by slow turnover of a single pool, but that the increase of C ages along a soil profile up to ages 〉1000 years is the result of different mechanisms contributing to the overall persistence of SOC. The dominant reasons for the persistence of SOC are stabilization processes, followed by repeated microbial processing of SOC.
    Keywords: Stabilization Mechanisms ; Sorptive Stabilization ; Microbial Interaction ; Transport Model ; Radiocarbon Profile ; Soil Organic Carbon ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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