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  • 1
    Language: English
    In: Soil Biology and Biochemistry, 2011, Vol.43(8), pp.1738-1741
    Description: Understanding the turnover of organic matter (OM) in soils necessitates information on biological stability and ecological functions. For easy characterization of slowly cycling OM, treatments using oxidants such as sodium hypochlorite (NaOCl) have been applied. The rationale for that approach is, however, questionable and concerns exist to which extent abiotic oxidation can mimic biological mineralization. Here we compare biological mineralization of mineral-bound OM to its resistance to chemical oxidation by 6 mass% NaOCl. Water-extractable OM, sorbed to goethite, vermiculite, and pyrophyllite at pH 4.0 and in different background electrolytes (CaCl , NaCl, NaCl–NaH PO ) to favor or exclude certain binding mechanisms, was subsequently subjected to NaOCl treatment (pH 7, either for 18 or 6 × 6 h). Irrespective of mineral surface properties and mechanisms involved in OM sorption, NaOCl removed a constant portion of the sorbed OC. More OC survived when bound to goethite than to vermiculite, thus confirming previous results on the increase of oxidation-resistant OC with increasing Fe and Al (hydr)oxide contents in different soils. Mineralizable OC (within 90 days) was much smaller than the NaOCl-removable OC and both fractions were negatively correlated (  = 0.90 for the 18 h treatment;  = 0.86 for the 6 × 6 h treatment), suggesting that chemically oxidizable OM does not represent the portion of sorbed OM available to biological consumption. ► Organic matter resistant to wet oxidation has been considered a biologically stable fraction. ► The OC fraction in mineral−organic associations oxidizable by sodium hypochlorite was inversely related to the mineralizable OC fraction. ► Organic matter resistant to chemical oxidation cannot be used as proxy for biologically stable OM.
    Keywords: Sodium Hypochlorite ; Naocl ; Wet Oxidation ; Organic Matter ; Stabilization ; Passive Organic Matter Pool ; Stable Organic Matter ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 2
    Language: English
    In: Soil Biology and Biochemistry, September 2012, Vol.52, pp.29-32
    Description: Dissolved organic matter has been recognized as mobile, thus crucial to translocation of metals, pollutants but also of nutrients in soil. We present a conceptual model of the vertical movement of dissolved organic matter with soil water, which deviates from the view of a chromatographic stripping along the flow path. It assumes temporal immobilization (sorptive or by co-precipitation), followed by microbial processing, and re-release (by desorption or dissolution) into soil water of altered compounds. The proposed scheme explains well depth trends in age and composition of dissolved organic matter as well as of solid-phase organic matter in soil. It resolves the paradox of soil organic matter being oldest in the youngest part of the soil profile – the deep mineral subsoil. ► Improved conceptual model of DOM movement in soil. ► Physico-chemical immobilization by sorption and/or co-precipitation. ► Microbial processing of sorbed/co-precipitated matter, subsequent re-release (desorption/dissolution) of altered compounds. ► DOM mirrors soil organic matter. ► Model explains changes in soil organic matter properties with depth.
    Keywords: Adsorption ; Conceptual Model ; Co-Precipitation ; Dissolved Organic Matter ; Microbial Processing ; Soil Organic Matter ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 3
    In: PLoS ONE, 2014, Vol.9(2)
    Description: Temperate forests are assumed to be organic carbon (OC) sinks, either because of biomass increases upon elevated CO 2 in the atmosphere and large nitrogen deposition, or due to their age structure. Respective changes in soil OC and total nitrogen (TN) storage have rarely been proven. We analysed OC, TN, and bulk densities of 100 soil cores sampled along a regular grid in an old-growth deciduous forest at the Hainich National Park, Germany, in 2004 and again in 2009. Concentrations of OC and TN increased significantly from 2004 to 2009, mostly in the upper 0–20 cm of the mineral soil. Changes in the fine earth masses per soil volume impeded the detection of OC changes based on fixed soil volumes. When calculated on average fine earth masses, OC stocks increased by 323±146 g m −2 and TN stocks by 39±10 g m −2 at 0–20 cm soil depth from 2004 to 2009, giving average annual accumulation rates of 65±29 g OC m −2 yr −1 and 7.8±2 g N m −2 yr −1 . Accumulation rates were largest in the upper part of the B horizon. Regional increases in forest biomass, either due to recovery of forest biomass from previous forest management or to fertilization by elevated CO 2 and N deposition, are likely causes for the gains in soil OC and TN. As TN increased stronger (1.3% yr −1 of existing stocks) than OC (0.9% yr −1 ), the OC-to-TN ratios declined significantly. Results of regression analyses between changes in OC and TN stocks suggest that at no change in OC, still 3.8 g TN m −2 yr −1 accumulated. Potential causes for the increase in TN in excess to OC are fixation of inorganic N by the clay-rich soil or changes in microbial communities. The increase in soil OC corresponded on average to 6–13% of the estimated increase in net biome productivity.
    Keywords: Research Article ; Agriculture ; Biology ; Chemistry ; Earth Sciences
    E-ISSN: 1932-6203
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  • 4
    Language: English
    In: Plant and Soil, 1 June 2012, Vol.355(1/2), pp.407-416
    Description: Background and aims Environmental factors such as climate and atmospheric CO2 control inputs of plant-derived matter into soils, which then determines properties and decomposition of soil organic matter. We studied how dissolved organic matter (DOM) in forest floors responded to six years of litter fall manipulation at a spruce site. Methods Experimental treatments included (i) ambient litter fall, as well as (ii) reduction or (iii) increase of litter fall, each by 80%. Results Reduced litter input did not change fluxes of dissolved organic carbon (DOC), which suggests that much of the mobile DOM in forest floors is not from recent litter but older, partly degraded material. Litter addition increased DOC fluxes over 6 years by 68% (Oi), 23% (Oe) and 12% (Oa). This was mainly due to excessive DOM production in Oi horizons, while net DOM production in Oe and Oa horizons decreased. Upon litter addition, aromaticity and molecule complexity of DOM released from the Oi horizon increased, but decreased for DOM leaving Oa horizons. The results suggest that DOM production by lignin degradation was enhanced in Oi but reduced in Oe and Oa horizons upon litter addition. Conclusions Recent litter is not an important source for DOM entering mineral soils, but increased litter input changes organic matter decomposition, thus affecting fluxes and properties of DOM within forest floors.
    Keywords: Biological sciences -- Biology -- Botany ; Applied sciences -- Materials science -- Materials ; Biological sciences -- Agriculture -- Agricultural sciences ; Biological sciences -- Agriculture -- Agricultural sciences ; Biological sciences -- Agriculture -- Agricultural sciences ; Biological sciences -- Agriculture -- Agricultural sciences ; Biological sciences -- Agriculture -- Agricultural sciences ; Biological sciences -- Agriculture -- Agricultural sciences ; Physical sciences -- Chemistry -- Chemical compounds ; Biological sciences -- Agriculture -- Agricultural sciences
    ISSN: 0032079X
    E-ISSN: 15735036
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  • 5
    Language: English
    In: Plant and Soil, 2012, Vol.355(1), pp.407-416
    Keywords: Litter manipulation ; Dissolved organic matter ; Field experiment ; Soil organic matter
    ISSN: 0032-079X
    E-ISSN: 1573-5036
    Source: Springer Science & Business Media B.V.
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 August 2018, Vol.235, pp.89-102
    Description: Ferric iron (Fe ) solid phases serve many functions in soils and sediments, which include providing sorption sites for soil organic matter, nutrients, and pollutants. The reactivity of Fe solid phases depends on the mineral structure, including the overall crystallinity. In redox-active soils and sediments, repeated reductive dissolution with subsequent exposure to aqueous ferrous iron (Fe ) and oxidative re-precipitation can alter Fe phase crystallinity and reactivity. However, the trajectory of Fe mineral transformation under redox fluctuations is unclear and has been reported to result in both increases and decreases in Fe phase crystallinity. Several factors such as water budget, organic matter input, redox dynamics as well as the initial Fe phase composition might play a role. The objective of our study was to examine if Fe minerals in soils that differ in porosity-dependent water leaching rate and initial Fe phase crystallinity, demonstrate distinct mineral transformations when subjected to redox fluctuations. We sampled paired plots of two soil types under similar management but with different water leaching rates and contrasting Fe oxide crystallinity — an Alisol rich in crystalline Fe phases and an Andosol rich in short-range-ordered (SRO) Fe phases. The two soils were either exposed to several decades of redox fluctuations during rice paddy cultivation (paddy) or to predominantly oxic conditions in neighboring vegetable gardens (non-paddy). Paddy soils are uniquely suited for this type of study because they are regularly submerged and develop regular redox fluctuations. We also incubated the non-paddy soils in the laboratory for one year through eight anoxic/oxic cycles and monitored the aqueous soil geochemistry. Mössbauer spectroscopy was then used to evaluate Fe mineral speciation in field soils (paddy and non-paddy) and laboratory incubations. In the field soils, we found that redox fluctuation had contrasting effects on Fe oxide crystallinity, with crystallinity being lower in the Alisol paddy soil and higher in the Andosol paddy soil than in their corresponding non-paddy controls. In the laboratory incubation experiment, Eh, pH and dissolved Fe responded as anticipated, with elevated Fe concentrations during the anoxic periods as well as low Eh and high pH. Mössbauer measurements suggest the fluctuating redox incubation was beginning to alter Fe oxide crystallinity along the same trajectory as observed in the field, but the changes were within the range of fitting errors. We propose that reductive dissolution of crystalline Fe oxides prevails in the soil rich in crystalline Fe oxides (Alisol) and that re-precipitation as SRO Fe oxides is favored by constrained leaching, which leads to the observed decrease in Fe oxide crystallinity. In the soil rich in SRO Fe phases (Andosol), preferential reductive dissolution of SRO Fe oxides coupled with stronger leaching of dissolved Fe causes the observed relative increase in crystallinity of the remaining Fe oxides. The observed increase in Fe oxide crystallinity may further be a result of Fe(II)-catalyzed re-crystallization of SRO Fe oxides. These findings indicate that, besides other factors, the Fe mineral composition of the initial soil or sediment as well as the leaching rate likely influence the trajectory of Fe oxide evolution under alternating redox-conditions.
    Keywords: Fe Oxides ; Redox Fluctuation ; Paddy Soils ; Mössbauer Spectroscopy ; Alisol ; Andosol ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 7
    Language: English
    In: Soil Biology and Biochemistry, December 2013, Vol.67, pp.133-139
    Description: Dissolved organic matter (DOM) plays a fundamental role for many soil processes. For instance, production, transport, and retention of DOM control properties and long-term storage of organic matter in mineral soils. Production of water-soluble compounds during the decomposition of plant litter is a major process providing DOM in soils. Herein, we examine processes causing the commonly observed increase in contribution of aromatic compounds to WSOM during litter decomposition, and unravel the relationship between lignin degradation and the production of aromatic WSOM. We analysed amounts and composition of water-soluble organic matter (WSOM) produced during 27 months of decomposition of leaves and needles (ash, beech, maple, spruce, pine). The contribution of aromatic compounds to WSOM, as indicated by the specific UV absorbance of WSOM, remained constant or increased during decomposition. However, the contribution of lignin-derived compounds to the total phenolic products of C-labelled tetramethylammonium hydroxide ( C-TMAH thermochemolysis increased strongly (by 〉114%) within 27 months of decomposition. Simultaneous changes in contents of lignin phenols in solid litter residues (cupric oxide method as well as C-TMAH thermochemolysis) were comparably small (−39% to +21% within 27 months). This suggests that the increasing contribution of lignin-derived compounds to WSOM during decomposition does not reflect compositional changes of solid litter residues, but rather the course of decomposition processes. In the light of recently published findings, these processes include: (i) progressive oxidative alteration of lignin that results in increasing solubility of lignin, (ii) preferential degradation of soluble, non-lignin compounds that limits their contribution to WSOM during later phases of decomposition.
    Keywords: Dissolved Organic Matter ; Water Soluble Organic Matter ; Litter Decomposition ; Lignin Degradation ; 13c-Tmah Thermochemolysis ; Litter Bag Experiment ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 8
    Language: English
    In: Geoderma, February 2015, Vol.239-240, pp.168-178
    Description: Radiocarbon measurements are frequently used to model the turnover of soil organic carbon (OC) fractions. The assumption of homogeneous turnover in these fractions is typically stated, but consequences of its violation have not been tested. We used samples of a repeated soil inventory of a German beech forest from the litter layer to 50 cm depth in 2004 and 2009 to determine the suitability of short-term repeated radiocarbon inventories for estimating the turnover times of OC in soil fractions. Samples from 0–5 and 10–20 cm soil depth were density separated into a free light fraction (fLF), an occluded light fraction (oLF), and a heavy, mineral-associated fraction (HF). Samples were analysed for radiocarbon (∆ C), OC, and total nitrogen (TN) contents. Similar portions of OC were stored in the HF, but contributions of fLF and oLF varied between study years, probably due to interannual variations or methodological constraints. Following declining atmospheric CO – C, also ∆ C values at 0–5 cm depth declined significantly between 2004 and 2009. Exchange of old for new OC was largest in the fLF and smallest in the HF, which confirms slow turnover of OC associated with minerals. Model results revealed that turnover time estimates based on single-pool models were not in agreement with observed changes in any of the fractions, suggesting all of them to be mixtures, to varying degrees, of fast and slow cycling pools. While single-pool models suggest average turnover times of 115 years for HF-OC at 0–5 cm depth, thus being a stable fraction, fitting a two-pool model to the two-point measurements of radiocarbon suggested the presence of a fast cycling pool of 15–25 years of turnover time. It was however only possible to constrain the portion of this fast pool as being between 50 and 85% of total HF-OC. Increasing ∆ C in bulk soil and density fractions between study years at 10–20 cm depth suggest that OC enters deeper soil layers with a longer time lag than topsoil layers, e.g., by slow transport, and 5 years was not enough to induce significant changes. Even 40 years after the bomb peak, radiocarbon time series are still suitable to detect OC fractions of decadal turnover and hint at time lags and translocation processes. Nevertheless, they do not allow for fixing the portions of fast and slow cycling OC in two-pool models and their turnover times. Repeated radiocarbon inventory in a German beech forest.
    Keywords: Organic Carbon Turnover ; Modelling ; Density Fractionation ; Radiocarbon ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 9
    Language: English
    In: Agriculture, Ecosystems and Environment, 01 October 2018, Vol.265, pp.340-349
    Description: Reduced tillage and cover cropping are common measures to minimize leaching losses of nutrients from cropland soils. While the efficiency of these measures for reducing inorganic N leaching has been studied intensively, their effect on dissolved, organically-bound nitrogen (DON) remains unclear. In this study, leaching of nitrate, ammonium and DON from spring barley-based cropping systems, subject to either conventional management (CT = inversion tillage with a winter fallow period), or non-inversion tillage with a winter mustard cover crop (NIT + CC), were assessed using suction cup sampling and modelled drainage volumes. Total dissolved nitrogen losses with drainage from the NIT + CC treatment (2.5 ± 0.2 g N m yr ) were considerably smaller than those from the conventional treatment with fallow (13.9 ± 0.7 g N m yr ). As drainage volumes were similar between treatments, differences in total N leaching were mainly associated with larger nitrate concentrations under CT (23.0 ± 1.1 mg N L ) than under the NIT + CC treatment (5.1 ± 0.3 mg N L ). The average contribution of DON to total dissolved nitrogen concentration was 3% within the CT treatment, but rose to 19% within the NIT + CC treatment, which was primarily due to the strong reduction in nitrate and to a lesser extent due to the higher concentrations of DON within the NIT + CC treatment (NIT + CC: 0.52 ± 0.04, CT: 0.33 ± 0.04 mg N L ). Averaged over the two-year study period, the CT system showed a net loss of 9.4 g N m yr 1 whilst an N surplus of 1.7 g N m yr was observed for the NIT + CC system. Here DON accounted for 11% of total N leaching, supporting the notion that it can be an important component of dissolved N losses in agroecosystems. By neglecting DON leaching the N-surplus under NIT + CC would have been overestimated by 18%. In conclusion, our results show that the capacity of winter cover cropping in combination with non-inversion tillage to reduce nitrate leaching far outweighed the higher leaching losses of DON observed. The quantification of DON losses, however, may be essential for a complete picture of the N balance of these and similar cropping systems.
    Keywords: Dissolved Organic Nitrogen ; Nitrogen Leaching ; Nitrogen Balance ; Tillage ; Cover Cropping ; Management Practices ; Agriculture ; Environmental Sciences
    ISSN: 0167-8809
    E-ISSN: 1873-2305
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  • 10
    Language: English
    In: Agriculture, Ecosystems and Environment, 2011, Vol.142(3), pp.393-402
    Description: • We assessed dissolved carbon leaching from an Irish arable soil. • Conventional tillage was compared to non-inversion tillage + cover cropping. • Carbon leaching was twice as large from the non-inversion tillage + cover crop treatment. • The leaching increased because of increased dissolved inorganic carbon concentrations. • This was primarily due to tillage-induced changes in the soil pore system. Reduced tillage and cover cropping are often considered as measures to increase carbon sequestration in cropland soils. We hypothesized that these management practices could result in an increase in carbon leaching. To examine this possibility we assessed carbon leaching from an Irish arable soil under spring barley, either with conventional management or non-inversion tillage plus cover cropping. Concentrations of biogenic dissolved inorganic carbon (DIC) were considerably higher under reduced tillage probably due to a higher supersaturation of soil solution with respect to partial pressures of CO in soil air resulting from a reduced abundance of tillage-induced macropores. Leaching losses of DOC equalled 3 ± 0.3 g m yr for conventional management as well as for non-inversion tillage plus cover cropping. Losses of biogenic DIC were 14.5 ± 4.4 g m yr for conventional tillage and 34.0 ± 4.7 g m yr for non-inversion tillage plus cover crop. Higher leaching losses from the non-inversion tillage plus cover crop plot thereby reduced potential soil carbon gains by 20 g m yr compared to the conventionally managed treatment highlighting the need to consider leaching losses in estimates of carbon sequestration, especially if these are deduced from balancing carbon inputs and outputs.
    Keywords: Doc ; Dic ; Carbon Sequestration ; Tillage ; Net Ecosystem Carbon Balance ; Conservation Agriculture ; Agriculture ; Environmental Sciences
    ISSN: 0167-8809
    E-ISSN: 1873-2305
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