Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

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.

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Persistence of soil organic matter as an ecosystem property

Climate change 2014 - Synthesis Report

Summary
Mechanisms 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 of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review

Instead of containing stable and chemically unique ‘humic substances’, as has been widely accepted, soil organic matter is a mixture of progressively decomposing organic compounds; this has broad implications for soil science and its applications. The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent ‘humic substances’ in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon–climate interactions and land management. Soil organic matter contains a large portion of the world's carbon and plays an important role in maintaining productive soils and water quality. Nevertheless, a consensus on the nature of soil organic matter is lacking. Johannes Lehmann and Markus Kleber argue that soil organic matter should no longer be seen as large and persistent, chemically unique substances, but as a continuum of progressively decomposing organic compounds.

The contentious nature of soil organic matter

Folgewirkungen der Renaturierung eines Niedermoores auf die Stickstoff-, Phosphor- und Kohlenstoffgehalte im Boden- und Grundwasser

An efficient extraction and analysis method was developed for the isolation and quantification of n-alkanes from bell peppers of different geographical locations. Five extraction techniques, i.e., accelerated solvent extraction (ASE), ball mill extraction, ultrasonication, rinsing, and shaking, were quantitatively compared using gas chromatography coupled to mass spectrometry (GC-MS). Rinsing of the surface wax layer of freeze-dried bell peppers with chloroform proved to be a relatively quick and easy method to efficiently extract the main n-alkanes C27, C29, C31, and C33. A combined cleanup and fractionation approach on Teflon-coated silica SPE columns resulted in clean chromatograms and gave reproducible results (recoveries 90-95 %). The GC-MS method was reproducible (R(2) = 0.994-0.997, peak area standard deviation = 2-5%) and sensitive (LODs, S/N = 3, 0.05-0.15 ng/μL). The total main n-alkane concentrations were in the range of 5-50 μg/g dry weight. Seed extractions resulted in much lower total amounts of extracted n-alkanes compared to flesh and surface extractions, demonstrating the need for further improvement of pre-concentration and cleanup. The method was applied to 131 pepper samples from four different countries, and by using the relative n-alkane concentration ratios, Dutch peppers could be discriminated from those of the other countries, with the exception of peppers from the same cultivar. Graphical Abstract Procedure for pepper origin determination.

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Determination of n-alkanes in C. annuum (bell pepper) fruit and seed using GC-MS: comparison of extraction methods and application to samples of different geographical origin

Properties, processes and ecological functions of floodplain, peatland, and paddy soils

The forest floor represents an important compartment for the accumulation of heavy metals in forest soils. To study the mobility of Pb species and their linkage to C cycling in the forest floor, we investigated concentrations of trimethyllead (C 3 H 9 Pb + ) and total Pb (Pb total ) in throughfall and forest floor percolates during 14 snow-free wk. For our study, we used a long-term field experiment with increased input of throughfall and litterfall at a Norway spruce [Picea abies (L.) H. Karst.] site. The concentrations of trimethyllead were highest in throughfall (106 pg Pb L -1 ) and decreased with depth of the forest floor. Strong correlations between trimethyllead concentrations in throughfall and forest floor percolates suggested that trimethyllead in forest floor percolates originates largely from throughfall. The partition coefficients of trimethyllead and Pb total , estimated as ratios of concentrations in percolates to those in forest floors, ranged from 54 to 560 and 870 to 26,400 L kg -1 , respectively, indicating that the mobility of trimethyllead was greater than Pb total . The Pb total concentrations were low in throughfall and often the highest in Oa horizon percolates (15.2-57.5 5 μg L -1 ). They decreased in Oi horizon percolates after increased throughfall addition. Increased litterfall resulted in increased concentrations of Pb total in forest floor percolates. Concurrently, dissolved organic C increased. The tight coupling of Pb total to dissolved organic C was also indicated by correlations (r ≥ 0.48) that were stronger than those between Pb total and pH (r ≤ -0.22). We concluded that the mobilization of Pb total from the large pool in forest floor horizons is strongly linked to the release of dissolved organic matter. Conversely, the mobility of trimethyllead is regulated by throughfall input and not by dissolved organic C.

Mobility of Trimethyllead and Total Lead in the Forest Floor

https://pure.uva.nl/ws/files/2568468/173978_Postprint_2nd_paper_23_05_2016.pdf

Karsten Kalbitz

Papers

Folgewirkungen der Renaturierung eines Niedermoores auf die Stickstoff-, Phosphor- und Kohlenstoffgehalte im Boden- und Grundwasser

Determination of n-alkanes in C. annuum (bell pepper) fruit and seed using GC-MS: comparison of extraction methods and application to samples of different geographical origin

Properties, processes and ecological functions of floodplain, peatland, and paddy soils

Mobility of Trimethyllead and Total Lead in the Forest Floor

Clay minerals of Pliocene deposits and their potential use for the purification of polluted wastewater in the Sohag area, Egypt