Showing papers by "Karsten Kalbitz published in 2008"

Organo-mineral associations in temperate soils: Integrating biology, mineralogy, and organic matter chemistry

Abstract: We summarize progress with respect to (1) different approaches to isolate, extract, and quantify organo-mineral compounds from soils, (2) types of mineral surfaces and associated interactions, (3) the distribution and function of soil biota at organo-mineral surfaces, (4) the distribution and content of organo-mineral associations, and (5) the factors controlling the turnover of organic matter (OM) in organo-mineral associations from temperate soils. Physical fractionation achieves a rough separation between plant residues and mineral-associated OM, which makes density or particle-size fractionation a useful pretreatment for further differentiation of functional fractions. A part of the OM in organo-mineral associations resists different chemical treatments, but the data obtained cannot readily be compared among each other, and more research is necessary on the processes underlying resistance to treatments for certain OM components. Studies using physical-fractionation procedures followed by soil-microbiological analyses revealed that organo-mineral associations spatially isolate C sources from soil biota, making quantity and quality of OM in microhabitats an important factor controlling community composition. The distribution and activity of soil microorganisms at organo-mineral surfaces can additionally be modified by faunal activities. Composition of OM in organo-mineral associations is highly variable, with loamy soils having generally a higher contribution of polysaccharides, whereas mineral-associated OM in sandy soils is often more aliphatic. Though highly reactive towards Fe oxide surfaces, lignin and phenolic components are usually depleted in organo-mineral associations. Charred OM associated with the mineral surface contributes to a higher aromaticity in heavy fractions. The relative proportion of OC bound in organo-mineral fractions increases with soil depth. Likewise does the strength of the bonding. Organic molecules sorbed to the mineral surfaces or precipitated by Al are effectively stabilized, indicated by reduced susceptibility towards oxidative attack, higher thermal stability, and lower bioavailability. At higher surface loading, organic C is much better bioavailable, also indicated by little 14C age. In the subsurface horizons of the soils investigated in this study, Fe oxides seem to be the most important sorbents, whereas phyllosilicate surfaces may be comparatively more important in topsoils. Specific surface area of soil minerals is not always a good predictor for C-stabilization potentials because surface coverage is discontinuous. Recalcitrance and accessibility/aggregation seem to determine the turnover dynamics in fast and intermediate cycling OM pools, but for long-term OC preservation the interactions with mineral surfaces, and especially with Fe oxide surfaces, are a major control in all soils investigated here.

How relevant is recalcitrance for the stabilization of organic matter in soils

Abstract: Traditionally, the selective preservation of certain recalcitrant organic compounds and the formation of recalcitrant humic substances have been regarded as an important mechanism for soil organic matter (SOM) stabilization. Based on a critical overview of available methods and on results from a cooperative research program, this paper evaluates how relevant recalcitrance is for the long-term stabilization of SOM or its fractions. Methodologically, recalcitrance is difficult to assess, since the persistence of certain SOM fractions or specific compounds may also be caused by other stabilization mechanisms, such as physical protection or chemical interactions with mineral surfaces. If only free particulate SOM obtained from density fractionation is considered, it rarely reaches ages exceeding 50 y. Older light particles have often been identified as charred plant residues or as fossil C. The degradability of the readily bioavailable dissolved or water-extractable OM fraction is often negatively correlated with its content in aromatic compounds, which therefore has been associated with recalcitrance. But in subsoils, dissolved organic matter aromaticity and biodegradability both are very low, indicating that other factors or compounds limit its degradation. Among the investigated specific compounds, lignin, lipids, and their derivatives have mean turnover times faster or similar as that of bulk SOM. Only a small fraction of the lignin inputs seems to persist in soils and is mainly found in the fine textural size fraction ( 40–50 y, unless fossil C was present in substantial amounts, as at a site exposed to lignite inputs in the past. Here, turnover of pyrolysis products seemed to be much longer, even for those attributed to carbohydrates or proteins. Apparently, fossil C from lignite coal is also utilized by soil organisms, which is further evidenced by low 14C concentrations in microbial phospholipid fatty acids from this site. Also, black C from charred plant materials was susceptible to microbial degradation in a short-term (60 d) and a long-term (2 y) incubation experiment. This degradation was enhanced, when glucose was supplied as an easily available microbial substrate. Similarly, SOM mineralization in many soils generally increased after addition of carbohydrates, amino acids, or simple organic acids, thus indicating that stability may also be caused by substrate limitations. It is concluded that the presented results do not provide much evidence that the selective preservation of recalcitrant primary biogenic compounds is a major SOM-stabilization mechanism. Old SOM fractions with slow turnover rates were generally only found in association with soil minerals. The only not mineral-associated SOM components that may be persistent in soils appear to be black and fossil C.

Stabilization mechanisms of organic matter in four temperate soils: Development and application of a conceptual model

Abstract: Based on recent findings in the literature, we developed a process-oriented conceptual model 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.

Contribution of dissolved organic matter to carbon storage in forest mineral soils

Abstract: Dissolved organic matter (DOM) is often considered the most labile portion of organic matter in soil and to be negligible with respect to the accumulation of soil C. In this short review, we present recent evidence that this view is invalid. The stability of DOM from forest floor horizons, peats, and topsoils against microbial degradation increases with advanced decomposition of the parent organic matter (OM). Aromatic compounds, deriving from lignin, likely are the most stable components of DOM while plant-derived carbohydrates seem easily degradable. Carbohydrates and N-rich compounds of microbial origin produced during the degradation of DOM can be relatively stable. Such components contribute much to DOM in the mineral subsoil. Sorption of DOM to soil minerals and (co-)precipitation with Al (and probably also with Fe), especially of the inherently stable aromatic moieties, result in distinct stabilization. In laboratory incubation experiments, the mean residence time of DOM from the Oa horizon of a Haplic Podzol increased from 90 y after sorption to a subsoil. We combined DOM fluxes and mineralization rate constants for DOM sorbed to minerals and a subsoil horizon, and (co-)precipitated with Al to estimate the potential contribution of DOM to total C in the mineral soil of a Haplic Podzol in Germany. The contribution of roots to DOM was not considered because of lack of data. The DOM-derived soil C ranges from 20 to 55 Mg ha–1 in the mineral soil, which represents 19%–50% of the total soil C. The variation of the estimate reflects the variation in mineralization rate constants obtained for sorbed and (co-)precipitated DOM. Nevertheless, the estimates indicate that DOM contributes significantly to the accumulation of stable OM in soil. A more precise estimation of DOM-derived C in soils requires mineralization rate constants for DOM sorbed to all relevant minerals or (co-)precipitated with Fe. Additionally, we need information on the contribution of sorption to distinct minerals as well as of (co-)precipitation with Al and Fe to DOM retention.

Stabilization of dissolved organic matter by aluminium: A toxic effect or stabilization through precipitation?

Abstract: Carbon mineralization in acidic forest soils can be retarded by large concentrations of aluminium (Al). However, it is still unclear whether Al reduces C mineralization by direct toxicity to microorganisms or by decreased bioavailability of organic matter (OM) because dissolved organic matter (DOM) is precipitated by Al. We conducted an incubation experiment (6 weeks) with two DOM solutions (40 mg C litre−1) derived from two acidic forests and possessing large differences in composition. Aluminium was added to the solutions in realistic ranges for acidic soils (1.6-24 mg Al litre−1) at pHs of 3.8 and 4.5, to achieve differences in Al speciation. We determined different Al species, including the potentially toxic Al3+, by Diffusive Gradients in Thin Films (DGT) to evaluate toxic effects on microorganisms. Precipitation of OM increased with larger amounts of added Al and higher pH, and we measured a larger fraction of dissolved 'free' Al at pH 3.8 than at pH 4.5. Organic matter degradation decreased significantly with Al addition, and we found more organic matter degraded at pH 3.8 than at pH 4.5 for the respective Al additions. Consequently, the observed reduction in OM degradation (i.e. stabilization) cannot be explained by toxic effects of 'free' Al. However, C stabilization correlated significantly with C precipitation. The pH did not influence C stabilization directly, but determined the amount of C being precipitated. Phosphorus was removed along with OM by precipitation, which possibly also affected C stabilization. We conclude that C stabilization upon Al addition did not result from toxic effects, but was caused by reduced bioavailability of OM after its precipitation. The reduction in OM degradation by 65% is of great relevance for the overall C stabilization in acidic forest soils. Increasing pH and decreasing Al concentrations upon recovery from acidic deposition should therefore not result in decreased stabilization of precipitated OM.

Precipitation of enzymes and organic matter by aluminum-Impacts on carbon mineralization

Abstract: The precipitation of dissolved organic matter (DOM) by aluminum (Al) results in a stable soil organic matter (OM) fraction. Extracellular enzymes can also be removed from soil solution by sorption or precipitation, but whether this affects their activity and their importance for carbon (C) mineralization is largely unknown. We studied the activity of eight extracellular enzymes, precipitated by Al together with DOM, in relation to C mineralization of the precipitated OM. Dissolved OM was obtained from the Oi and Oa horizon of two forest soils and precipitated at different Al : C ratios and pH values to achieve a large variation in composition and C mineralization of precipitated OM. All eight enzymes were present in a functional state in precipitated OM. On average 53% of DOM was precipitated, containing on average 17%-41% of the enzyme activity (EA) involved in C degradation (chitinase, cellobiohydrolase, p-glucosidase, glucuronidase, lacasse, and xylosidase) previously present in soil solution. In contrast, on average only 4%-7% of leucine-aminopeptidase and acid-phosphatase activity was found in precipitated OM. The EA found in precipitates significantly increased the percentage of C mineralized of precipitated OM, with a stronger influence of C-degrading enzymes than enzymes involved in N and P cycling. However, after 8 weeks of incubation the correlations between EA and C mineralization disappeared, despite substantial EA being still present and only 0.5%-7.7% of C mineralized. Thus, degradation of precipitated OM seems to be governed by EA during the first degradation phase, but the long-term stability of precipitated OM is probably related to its chemical properties.

Mobility of Trimethyllead and Total Lead in the Forest Floor

Abstract: 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.