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Dependence on soil depth, root morphology, and environment
No full textForest subsoils may represent an important C sink in a warming world, but rhizodeposition as the key biogeochemical process determining the C sink strength of mature forests has not yet been quantified in subsoils. According to studies conducted in topsoil or laboratory experiments, soil C inputs by root exudation are increasing with increasing temperature and decreasing nutrient availability. We examined whether these relationships apply to forest subsoil by analyzing the response of root exudation to increasing soil depth up to 130 cm in a mature European beech (Fagus sylvatica L.) forest. In two subsequent growing seasons differing in temperature and precipitation, we investigated in situ root exudation with a cuvette-based method and analyzed root morphology, microbial biomass, and soil nutrient availability. We proved that root exudation greatly decreases with soil depth as a consequence of a significant decrease in root-mass specific exudation activity to nearly a fifth of topsoil activity. The decrease in specific metabolic activity from 312 mg C g−1 yr−1 in the topsoil to 80 mg C g−1 yr−1 at 130 cm depth was amplified by an exponential decrease in root biomass per soil volume, leading to a relative decrease in root exudation per volume in the deep subsoil to 2% of topsoil root exudation (1 g C 10 cm−1 m−2 yr−1 at 130 cm depth). Specific root area decreased and mean fine root diameter and root tissue density increased with soil depth, indicating a shift in primary root functionality from fibrous roots in the topsoil to pioneer roots in the subsoil. The decrease in root exudation was accompanied by decreases in soil microbial biomass, extractable organic C (EOC), and N and P availability and increases in the aromatic C portion in SOM, but it did not relate to seasonal differences in climatic conditions. More specifically, it responded positively to an increase in EOC and ETN in the topsoil, but remained at its minimum rate in the SOC-poor subsoil, probably due to a lower organic N and higher mineral N content. The vertical pattern of beech root exudation is in accordance with a strategy to maximize whole-tree carbon-use efficiency, as it reduces C loss by exudation in soil spots where positive priming effects are unlikely, but enhances C exudation where microbes can mine less bioavailable SOM. The exudation patterns further suggest that increased C allocation to root systems as a likely tree response to elevated atmospheric [CO2] may not lead to enhanced soil C input by root exudation to subsoils poor in SOM
Spatial distribution and chemical composition of soil organic matter fractions in rhizosphere and non-rhizosphere soil under European beech (Fagus sylvatica L.)
No full textLittle is known about how trees and their roots may influence the spatial distribution and chemical composition of soil organic matter (SOM) in subsoils with subsequent effects on soil organic carbon (SOC) storage and turnover. The aim of this study was to assess the impact of individual trees and their root system on the spatial distribution and chemical composition of SOM fractions and the storage of SOC in subsoils. A Dystric Cambisol was sampled along three vertical replicate transects (3.15 m in length, 2.00 m in depth) in a regular grid (45 cm horizontal spaces, 25 cm vertical spaces) at increasing distance from three individual mature European beech trees (Fagas sylvatica L.). Soil OM fractions were obtained from rhizosphere soil and bulk soil samples taken at 10 and 85 cm depth increments by a combined density and particle size fractionation. Carbon and nitrogen measurements were performed, and the chemical composition of the SOM fractions was further characterized by solid state cross polarization magic angle spinning C-13 nuclear magnetic resonance spectroscopy. The distance from the individual trees had no influence on the SOC contents and stocks or the chemical composition of the SOM fractions. This was ascribed to the dense and even rooting at 0-40 cm depth across all sampled distances. Instead, the SOC contents and stocks highly differed between 10 cm depth (11.4 g SOC kg(-1)), where particulate organic matter (POM) dominated, and 85 cm depth (0.5 g SOC kg(-1)), where clay associated SOC dominated. These differences seemed to be strongly influenced by the roots of the trees which were almost completely absent from depths >= 60 cm. Elevated SOC contents in the rhizosphere soil (40.1 g SOC kg(-1)) were ascribed to root exudates in the root's vicinity and a very high amount (109.3 g kg(-1)) of fresh POM (allcy1/0/N alkyl C ratio of 0.8). The data revealed that, besides root exudates, also root derived POM contributed significant amounts of SOC to the soil. Although only low amounts of the clay fraction were found at 85 cm depth (22.8 g clay kg(-1)), it accounted for high amounts of SOC and played a crucial role for the storage of SOM. The relatively high SOC stocks at 40-200 cm depth (1.4 kg C m(-2)) compared to the SOC stocks at 0-40 cm depth (3.8 kg C m(-2)) indicate that also sandy forest subsoils with low SOC contents have to be considered in terrestrial carbon inventories. (C) 2015 Elsevier B.V. All rights reserved
Which are important soil parameters influencing the spatial heterogeneity of 14C in soil organic matter?
No full textRadiocarbon (14C) analysis is an important tool that can provide information on the dynamics of organic matter in soils. Radiocarbon concentrations of soil organic matter (SOM) however, reflect the heterogeneous mixture of various organic compounds and are affected by different chemical, biological, and physical soil parameters. These parameters can vary strongly in soil profiles and thus affect the spatial distribution of the apparent 14C age of SOM considerably. The heterogeneity of SOM and its 14C signature may be even larger in subsoil horizons, which are thought to receive organic carbon inputs following preferential pathways. This will bias conclusions drawn from 14C analyses of individual soil profiles considerably. We thus investigated important soil parameters, which may influence the 14C distribution of SOM as well as the spatial heterogeneity of 14C distributions in soil profiles. The suspected strong heterogeneity and spatial variability, respectively of bulk SOM is confirmed by the variable 14C distribution in three 185 cm deep profiles in a Dystric Cambisol. The 14C contents are most variable in the C horizons because of large differences in the abundance of roots there. The distribution of root biomass and necromass and its organic carbon input is the most important factor affecting the 14C distribution of bulk SOM. The distance of the soil profiles to a beech did not influence the horizontal and vertical distribution of roots and 14C concentrations. Other parameters were found to be of minor importance including microbial biomass-derived carbon and soil texture. The microbial biomass however, may promote a faster turnover of SOM at hot spots resulting in lower 14C concentration there. Soil texture had no statistically significant influence on the spatial 14C distribution of bulk SOM. However, SOM in fine silt and clay sized particles (< 6.3 µm) yields slightly higher 14C concentrations than bulk SOM particularly at greater soil depth, which is in contrast to previous studies where silt and clay fractions contained older SOM stabilized by organo-mineral interaction. 14C contents of fine silt and clay correlate with the microbial biomass-derived carbon suggesting a considerable contribution of microbial-derived organic carbon. In conclusion, 14C analyses of bulk SOM mainly reflect the spatial distribution of roots, which is strongly variable even on a small spatial scale of few meters. This finding should be considered when using 14C analysis to determine SOM
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Quellen, chemische Zusammensetzung und räumliche Verteilung von organischer Bodensubstanz in Ober- und Unterboden unter Rotbuche
No full textThe sources, chemical composition, and spatial distribution of soil organic carbon (SOC) under European beech were investigated. No influence of the distance to the trees was detected. Instead, the soil was vertically divided into a leaf-/root-affected zone and a root-affected zone. The results revealed that leaf-derived SOC may still be important below the A-horizons. Roots were highly relevant for the input of SOC, due to higher amounts of little decomposed particulate organic matter in rhizosphere vs. bulk soil. High 14C contents in the deeper subsoil indicated old SOC, probably inherited from the parent material. Because SOC stocks below 35 cm depth were still high, deep subsoils have to be considered in carbon inventories.Es wurden die Quellen, chemische Zusammensetzung und räumliche Verteilung von organischem Bodenkohlenstoff (SOC) unter Rotbuche untersucht. Einen Einfluss der Entfernung zu den Bäumen wurde nicht beobachtet. Stattdessen wurde der Boden vertikal unterteilt in eine laub-/wurzelbeeinflusste Zone und eine wurzelbeeinflusste Zone. Die Ergebnisse zeigten, dass laubbürtiger SOC auch unterhalb der A-Horizonte von Bedeutung sein kann. Wurzeln waren hochrelevant für den Eintrag von SOC durch eine höhere Menge an kaum abgebauter partikulärer organischer Substanz in Rhizosphären- gegenüber Gesamtboden. Hohe 14C Gehalte im Unterboden deuteten auf alten SOC hin, der wahrscheinlich vom Ausgangssubtrat ererbt war
laska
No full textPermafrost-affected soils of the northern circumpolar region represent 50% of the terrestrial soil organic carbon (SOC) reservoir and are most strongly affected by climatic change. There is growing concern that this vast SOC pool could transition from a net C sink to a source. But so far little is known on how the organic matter (OM) in permafrost soils will respond in a warming future, which is governed by OM composition and possible stabilization mechanisms. To investigate if and how SOC in the active layer and adjacent permafrost is protected against degradation, we employed density fractionation to separate differently stabilized SOM fractions. We studied the quantity and quality of OM in different compartments using elemental analysis, C-13 solid-phase nuclear magnetic resonance (C-13-NMR) spectroscopy, and C-14 analyses. The soil samples were derived from 16 cores from drained thaw lake basins, ranging from 0 to 5500years of age, representing a unique series of developing Arctic soils over time. The normalized SOC stocks ranged between 35.5 and 86.2kgSOCm(-3), with the major amount of SOC located in the active layers. The SOC stock is dominated by large amounts of particulate organic matter (POM), whereas mineral-associated OM especially in older soils is of minor importance on a mass basis. We show that tremendous amounts of over 25kgOC per square meter are stored as presumably easily degradable OM rich in carbohydrates. Only about 10kgOC per square meter is present as presumably more stable, mineral-associated OC. Significant amounts of the easily degradable, carbohydrate-rich OM are preserved in the yet permanently frozen soil below the permafrost table. Forced by global warming, this vast labile OM pool could soon become available for microbial degradation due to the continuous deepening of the annually thawing active layer
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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