1,720,997 research outputs found
Effects of experimental drought on the fine root system of mature Norway spruce
No full textNorway spruce (Picea abies (L) Karst.) is an economically important, but relatively drought-sensitive tree species that might suffer from increasing drought intensities and frequencies, which are predicted to occur in parts of central Europe under future climatic change. In a throughfall exclusion experiment using sub-canopy roofs, we tested the hypothesis that enhanced drought leads to an increase in fine root mortality, and also to a higher, subsequent fine root growth. Fine root production and mortality were assessed using two independent approaches, sequential soil coring (organic layer) and direct observations in minirhizotrons (organic layer plus upper mineral soil). Six weeks of throughfall exclusion resulted in mild drought stress, i.e. a reduction in average soil moisture from 20 to 12 vol.% during the treatment. Based on the sequential coring data, experimental drought did not result in significant changes in fine root biomass during the 6-week treatment period, but caused an increase in fine root mortality by 61% in the 6 weeks following the drought treatment. Remarkably, fine root production showed a synchronous increase in this period, which more than compensated for the loss due to increased mortality. The minirhizotron data confirmed that the drought treatment increased fine root loss in the organic layer. Based on this method, however, root loss occurred during the drought period and was not compensated by increased root production. The mild drought stress was mainly restricted to the organic layer and did not significantly influence fine root dynamics in the mineral soil. We calculated that the drought event resulted in an extra input of about 28 g C m(-2) and 1.1 g N m(-2) to the soil due to increased fine root mortality. We conclude that even periods of mild drought significantly increase fine root mortality and the associated input of root-derived C to the soil organic matter pool in temperate Norway spruce forests. (C) 2008 Elsevier B.V. All rights reserved.DFG (Deutsche Forschungsgemeinschaft
An integrative approach of organic matter stabilization in temperate soils: Linking chemistry, physics, and biology
No full textWith this topical issue, we present the work of the Priority Program 1090 of the German Research Foundation ("Deutsche Forschungsgemeinschaft DFG"): "Soils as a source and sink for CO(2) - mechanisms and regulation of organic matter stabilisation in soils". This introduction gives an overview on the sites investigated and the major research approaches, including a glossary of major terms used in the field of soil organic matter research. We point out the advantages of integration of data from a broad field of different soil-science disciplines and the progress achieved by application and combination of new analytical methods describing the quality and turnover of soil organic matter
SOM fractionation methods: Relevance to functional pools and to stabilization mechanisms
No full textSoil organic matter (SOM) consists of various functional pools that are stabilized by specific mechanisms and have certain turnover rates. For the development of mechanistic models that predict changes in SOM storage, these pools have to be quantified and characterized. In the past, numerous fractionation schemes have been developed to separate and analyse such SOM fractions. In this review, the SOM fractions obtained with such operational fractionation procedures are described in terms of their pool sizes, chemical properties, and turnover rates. The main objective of this review is to evaluate these operationally defined fractions with respect to their suitability to describe functional SOM pools that could be used to parameterize SOM turnover models. Fractionation procedures include (1) physical separation of SOM into aggregate, particle size, and density fractions and fractions according to their magnetic susceptibility, and (2) various wet chemical procedures that fractionate SOM according to solubility, hydrolysability, and resistance to oxidation or by destruction of the mineral phase. Furthermore, combinations of fractionation methods are evaluated. The active SOM pool with turnover rates < 10 years may best be represented by the soil microbial biomass and the light fraction (< 1.6-2 g cm(-3)) obtained by density fractionation (if black carbon contents are considered). Most chemical and physical fractionations as well as combinations of methods yield SOM fractions that are not homogeneous in terms of turnover rates. It has proven to be particularly difficult to isolate functional fractions that represent the passive model pools in which the majority of soil SOM is stabilized. The available fractionation methods do not correspond to specific stabilization mechanisms and hence do not describe functional SOM pools. Another problem is that comprehensive data for turnover rates and data for whole soil profiles are only now becoming available, especially for new fractionation methods. Such information as well as the use of specific markers and compound-specific isotope analysis may be important for future differentiation and evaluation of functional SOM fractions. (C) 2007 Elsevier Ltd. All rights reserved
Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions - a review
No full textMechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. Here we review the mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to organic matter (OM) protection against decomposition in temperate soils: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe-, Al-, Mn-oxides, phyllosilicates) and metal ions. Our goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decomposition and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant organic components that accumulate in proportion to their chemical properties can no longer be accepted. In contrast, our analysis of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) molecular recalcitrance of OM is relative, rather than absolute; (iii) recalcitrance is only important during early decomposition and in active surface soils; while (iv) during late decomposition and in the subsoil, the relevance of spatial inaccessibility and organo-mineral interactions for SOM stabilization increases. We conclude that major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. We discuss knowledge gaps and promising directions of future research
Stabilization mechanisms of organic matter in four temperate soils: Development and application of a conceptual model
No full textBased on recent findings in the literature, we developed a process-oriented conceptual model L that integrates all three process groups of organic matter (OM) stabilization in soils namely (1) selective preservation of recalcitrant compounds, (2) spatial inaccessibility to decomposer organisms, and (3) interactions of OM with minerals and metal ions. The model concept relates the diverse stabilization mechanisms to active, intermediate, and passive pools. The formation of the passive pool is regarded as hierarchical structured co-action of various processes that are active under specific pedogenetic conditions. To evaluate the model, we used data of pool sizes and turnover times of soil OM fractions from horizons of two acid forest and two agricultural soils. Selective preservation of recalcitrant compounds is relevant in the active pool and particularly in soil horizons with high C contents. Biogenic aggregation preserves OM in the intermediate pool and is limited to topsoil horizons. Spatial inaccessibility due to the occlusion of OM in clay microstructures and due to the formation of hydrophobic surfaces stabilizes OM in the passive pool. If present, charcoal contributes to the passive pool mainly in topsoil horizons. The importance of organo-mineral interactions for OM stabilization in the passive pool is well-known and increases with soil depth. Hydrophobicity is particularly relevant in acid soils and in soils with considerable inputs of charcoal. We conclude that the stabilization potentials of soils are site- and horizon-specific. Furthermore, management affects key stabilization mechanisms. Tillage increases the importance of organo-mineral interactions for OM stabilization, and in Ap horizons with high microbial activity and C turnover, organo-mineral interactions can contribute to OM stabilization in the intermediate pool. The application of our model showed that we need a better understanding of processes causing spatial inaccessibility of OM to decomposers in the passive pool
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Get PDFWe 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
Phosphorus in forest ecosystems: New insights from an ecosystem nutrition perspective
No full textPhosphorus is one of the major limiting factors of primary productivity in terrestrial ecosystems and, thus, the P demand of plants might be among the most important drivers of soil and ecosystem development. The P cycling in forest ecosystems seems an ideal example to illustrate the concept of ecosystem nutrition. Ecosystem nutrition combines and extents the traditional concepts of nutrient cycling and ecosystem ecology. The major extension is to consider also the loading and unloading of nutrient cycles and the impact of nutrient acquiring and recycling processes on overall ecosystem properties. Ecosystem nutrition aims to integrate nutrient related aspects at different scales and in different ecosystem compartments including all processes, interactions and feedbacks associated with the nutrition of an ecosystem. We review numerous previous studies dealing with P nutrition from this ecosystem nutrition perspective. The available information contributes to the description of basic ecosystem characteristics such as emergence, hierarchy, and robustness. In result, we were able to refine Odum's hypothesis on P nutrition strategies along ecosystem succession to substrate related ecosystem nutrition and development. We hypothesize that at sites rich in mineral‐bound P, plant and microbial communities tend to introduce P from primary minerals into the biogeochemical P cycle (acquiring systems), and hence the tightness of the P cycle is of minor relevance for ecosystem functioning. In contrast, tight P recycling is a crucial emergent property of forest ecosystems established at sites poor in mineral bound P (recycling systems). We conclude that the integration of knowledge on nutrient cycling, soil science, and ecosystem ecology into holistic ecosystem nutrition will provide an entirely new view on soil–plant–microbe interactions
- …
