Martin Kaupenjohann

Dynamics and functional relevance of ammonia-oxidizing archaea in two agricultural soils.

Crucial steps in geochemical cycles are in many cases performed by more than one group of microorganisms, but the significance of this functional redundancy with respect to ecosystem functioning is poorly understood. Ammonia-oxidizing archaea (AOA) and their bacterial counterparts (AOB) are a perfect system to address this question: although performing the same transformation step, they belong to well-separated phylogenetic groups. Using pig manure amended with different concentrations of sulfadiazine (SDZ), an antibiotic that is frequently used in veterinary medicine, it was possible to affect AOB and AOA to different degrees. Addition of manure stimulated growth of AOB in both soils and, interestingly, also growth of AOA was considerably stimulated in one of the soils. The antibiotic treatments decreased the manure effect notably on AOB, whereas AOA were affected to a lower extent. Model calculations concerning the respective proportions of AOA and AOB in ammonia oxidation indicate a substantial contribution of AOA in one of the soils that further increased under the influence of SDZ, hence indicating functional redundancy between AOA and AOB.

Carbon mobilization from the forest floor under red spruce in the northeastern U.S.A.

Global climate change may alter soil temperature and moisture conditions, increasing the need to understand how these basic factors affect C dynamics. This is particularly important in boreal forests, which often have large C pools in the forest floor and mineral horizons. We examined the effects of temperature and precipitation frequency on C dynamics in forest floor horizons from eight red spruce sites in the northeastern U.S. using column leaching experiments. Intact and sieved forest floor samples were incubated at 3, 10 or 20°C and leached either daily, once per week, or twice per week during 14 to 39 days using simulated throughfall solutions (pH 2.7 or 4.0). Leachate DOC and CO2 production were measured along with soil C and N concentrations. For intact samples, losses of C as DOC and as CO2 increased with increasing temperature, and the increase (Q10) was usually greater between 3 and 10°C than between 10 and 20°C. There was a greater response of CO2 to temperature than of DOC (e.g. Howland sieved soil Q10s of 1.9 and 7.2 for CO2 and 1.5 and 2.0 for DOC at 3–10 and 10–20°C ranges, respectively). More frequent leaching increased steady state DOC mobilization (e.g. 145 and 58 μg g−1 forest floor d−1 for daily and weekly leachings at 10°C, respectively), but not CO2 evolution (e.g. 79 and 74 μg CO2C g−1 forest floor d−1 for daily and weekly leachings at 10°C, respectively). Across the eight sites DOC loss and CO2 evolution varied by factors of 3.6 and 4.0, respectively. Both CO2 evolution and DOC in leachates calculated as fluxes were correlated (r = 0.73 and 0.87 respectively, n = 8) with the C-to-N ratios of the samples (C-to-N ratios ranged from 27 to 58), which could be explained by N limitations that triggered selective lignin degradation, differences in degree of humification of the material, or position on a west-to-east pollution gradient. Although higher temperatures and more frequent leaching increased DOC mobilization, and higher temperatures increased CO2 evolution, both treatments and site to site variation illustrate the complexity of the response of forest-floor C pools to manipulations.

EFFECTS OF SOIL SOLUTION ON THE DYNAMICS OF N2O EMISSIONS : A REVIEW

In this review, which consists of two parts, major interactions between nitrous oxide (N2>O) and soil solution are described. In the first part, as an introduction, concentrations of dissolved N2>O in different aqueous systems are summarized. An inventory of data on maximal N2>O concentrations in soil solution (up to 9984 μg N2>O-N l−1>) and in soil air (up to 8300 ppm) from literature is presented. The peak N2>O concentrations represent a N2>O supersaturation in the soil solution up to 30000 times with respect to ambient air and a soil air N2>O concentration about 25000 times higher than in the atmosphere. The main physico–chemical parameters (solubility, diffusion) controlling N2>O distribution between soil solution and soil air are outlined. The influences of cultivation practice, nitrogen turnover, water content and temperature on N2>O a ccumulation in soil solution and soil air are reviewed. In the second part some models of N2>O dynamics in soils are discussed with emphasis on N2>O transport processes. A simple qualitative scheme is developed to categorize the effects of the soil solution on N2>O dynamics in soils. In this scheme the temporary, intensive N2>O oversaturation of the soil solution is interpreted as a result of gas diffusion inhibition by water (barrier function of soil solution) resulting in an accumulation of N2>O. In addition, N2>O supersaturation is an indication that transitory much N2>O can be stored in the soil solution (storage function of soil solution). Where the soil solution flows up-, down- or sidewards it can act as a relevant transport medium for dissolved N2>O (transport function of soil solution). This scheme is applied to examples from the literature.

Landscape fate of nitrate fluxes and emissions in Central Europe: A critical review of concepts, data, and models for transport and retention

Agroecosystems are leaky systems emitting nutrients like nitrate, which affect ecosystems on a range of scales. This paper examines the fate of nitrate on the landscape level focussing on how landscape components either facilitate or impede N translocation from the field to the stream (headwater). According to their role in landscape metabolism, two categories of landscape components are distinguished, ecotones/retention compartments and conduits/corridors. Conduits such as macropores, preferential interflow-paths, drainage tiles and streams rapidly relocate nitrate to headwaters. Retention compartments like the capillary fringe/saturated zone and riparian vegetation eliminate N through denitrification. The differential role of compartments is illustrated with quantitative examples from the literature. On the landscape level retention potential for N is spatially variable and quantitatively limited, while its realisation is uncertain. Notwithstanding, the literature indicates that on a watershed basis the bulk of total N input is retained; thus the potential is discussed for the retention of nitrate on different scales, i.e. the field, landscape, regional and global scale. The transitory retention of excess nitrate in soil and subsoil solution, soil organic matter, groundwater and riparian vegetation may delay nitrate discharge to the aquatic system for decades, contributing to the low emission factors on basin scale. The adverse effects arising from denitrification are discussed, presenting data on the emission of nitrous oxide from the entirety of the different landscape compartments. It is concluded that reliance on landscape metabolism and self-purification postpones the problem of global N overload and partially transfers it to the atmosphere. An assessment scheme is presented which in the face of the unpredictability of ecosystem and landscape behaviour is risk oriented (instead of impact oriented). The scheme uses a budget approach, which accounts for the critical role of corridors and considers the scale and scope of N emissions. A conceptual framework for the remediation of N overload is presented which rests on the realisation of cycling principles and zero-emission approaches on all scales of agricultural production and which pleads for regional approaches that transcend sectoral boundaries and take account of overall regional N fluxes.

Sorption of dissolved organic carbon in soils: effects of soil sample storage, soil-to-solution ratio, and temperature

Abstract Experiments on the sorption of dissolved organic carbon (DOC) in soils were mainly conducted in batch approaches. Because varying setups were used in these studies, comparison of the results requires knowledge on the effects that different experimental conditions may have on the sorption of DOC. This investigation evaluated the DOC sorption of soils using differently pretreated soil samples (field-fresh (two sampling dates), air-dried, stored at 3°C and −18°C), at different soil-to-solution ratios (1:40, 1:20, 1:10 and 1:5 w/v) and different temperatures (5°C, 15°C, 25°C and 35°C). The sorption of DOC was analyzed using the initial mass (IM) approach, which regressed the initial amount of sorbate (normalized to soil mass) against the sorbed amount (normalized to soil mass). The DOC release — when a solution without DOC was added — strongly increased with temperature and soil-to-solution ratio. Among the different types of sample storage and preparation, air-drying resulted in the largest DOC release. The smallest release was from the field-fresh samples. Freezing and storage at 3°C resulted in intermediate DOC release with freezing having the greater effect. The release from air-dried samples exceeded that of field-fresh samples by a factor of four at maximum. In contrast, none of the experimental setups influenced the slope of the IM isotherms. Thus, it seems possible to compare directly the binding affinity of DOC to different soils as determined at varying experimental conditions.

Using a boundary line approach to analyze N2O flux data from agricultural soils

Predicting the N2O flux from soils is difficult because of the complex interplay of the various processes involved. In this study a boundary line approach was used to apply results from mechanistic experiments to N2O flux data resulting from measurements on field scale in southern Germany. Boundary lines were fitted to the rim of the data points in scattergrams depicting readily obtainable soil variables against the measured N2O flux. The boundary line approach is based on the hypothesis that this line depicts the functional dependency between the two variables. For determining these boundary lines a novel method was applied. The function best representing the relationship between the N2O flux and soil temperature had a maximum above 23 °C and the one between the N2O flux and the water filled pore space (WFPS, to represent water content) had a maximum at 72% WFPS. In the range of 0–20 mg N kg-1 the relationship between N2O flux and nitrate in the soil was best described by a linear function, whereas in the range of 0–35 mg N kg-1 a Michaelis–Menten function was more appropriate. The boundary lines specified in this study are in agreement with existing theoretical concepts as well as experimental results obtained under controlled and field conditions as reported in the literature. Therefore, the boundary line approach can be used to improve empirical models for predicting the N2O flux in the field.

Parameters, prediction, post-normal science and the precautionary principle—a roadmap for modelling for decision-making

Abstract In the wake of the ‘discovery of complexity’, dynamical simulation models have become widespread, guiding human interaction with complex systems, e.g. in ecosystem management, environmental decision-making and risk assessment. Any model establishes a reading frame for ecological phenomena or systems, determining the parameters that are assumed to be adequate for the encoding of ecological phenomena. Departing from a definition of observation as the operation of distinguishing and designating, and as the application of certain distinctions to complex phenomena, we analyze the construction of reading frames. As dynamical systems are the prevalent paradigm and reading frame for ecosystems, we describe the sequence of distinctions and selections by which scientists encode ecosystems into formal, dynamical system representations. Major shortcomings of the dynamical system paradigm are highlighted: dynamical systems are conceptually closed systems requiring a fixed set of a priori defined parameters, part of which are parameters of convenience satisfying mathematical needs and part of which are residual parameters that account for noise and system background. Ecosystems, in contrast, are conceived as conceptually open, self-modifying systems, which constantly (‘on-line’) produce novelty and new parameters and which cannot be severed from their environment. Although calibration may adapt models to data sets of the past, it does not assure predictive capacity nor validity. While models serve heuristic and theoretical functions and may outline the space of possible behaviour, they may be deficient instruments for the reduction of uncertainty as to future system behaviour. Different forms of uncertainty are at the heart of environmental decision-making, among them epistemic uncertainty, which arises when the normal, disciplinary forms of uncertainty reduction fail and which leads to debate on adequate ways of coping with uncertainty. Epistemic uncertainty in environmental issues may call for a different type of science that differs from normal, positivist science. Such post-normal science is transdisciplinary, participative and context-sensitive in that it aims at the production of knowledge for concrete, real-world problems. New forms of knowledge production such as the concept of post-normal science in conjunction with the precautionary principle challenge the established authority of science and may lead to an institutional split of science into an academic branch and a managerial, public policy branch. Correspondingly, modelling for theoretical scientific purposes and modelling for decision-making may follow separate paths. Modelling for decision-making may have to take into account requests for transparency and participation (‘deliberation frames analysis’) and the validity of model products will be judged according to their capacity of providing context-sensitive knowledge for specific decision problems.

Turnover and distribution of root exudates of Zea mays

Decomposition and distribution of root exudates of Zea mays L. were studied by means of 14CO2 pulse labeling of shoots on a loamy Haplic Luvisol. Plants were grown in two-compartment pots, where the lower part was separated from the roots by monofilament gauze. Root hairs, but not roots, penetrated through the gauze into the lower part of the soil. The root-free soil in the lower compartment was either sterilized with cycloheximide and streptomycin or remained non-sterile. In order to investigate exudate distribution, 3 days after the 14C labeling, the lower soil part was frozen and sliced into 15, one-mm thick layers using a microtome. Cumulative 14CO2 efflux from the soil during the first 3 days after 14C pulse labeling did not change during plant growth and amounted to about 13–20% of the total recovered 14C (41–55% of the carbon translocated below ground). Nighttime rate of total CO2 efflux was 1.5 times lower than during daytime because of tight coupling of exudation with photosynthesis intensity. The average CO2 efflux from the soil with Zea mays was about 74 μg C g−1 day−1 (22 g C m−2 day−1), although, the contribution of plant roots to the total CO2 efflux from the soil was about 78%, and only 22% was respired from the soil organic matter. Zea mays transferred about 4 g m−2 of carbon under ground during 26 days of growth. Three zones of exudate concentrations were identified from the distribution of the 14C-activity in rhizosphere profiles after two labeling periods: (1) 1–2 (3) mm (maximal concentration of exudates) 2) 3–5 mm (presence of exudates is caused by their diffusion from the zone 1); (3) 6–10 mm (very insignificant amounts of exudates diffused from the previous zones). At the distance further than 10 mm no exudates were found. The calculated coefficient of exudate diffusion in the soil was 1.9 × 10−7 cm2 s−1.

Dissolved organic matter induced denitrification in subsoils and aquifers

Nitrate inputs into aquifers may be attenuated by denitrification. This study aims to evaluate the relevance of dissolved organic matter (DOM) as a substrate for denitrifiers. Seepage water and groundwater were sampled using suction plates, suction cups and multilevel wells and were analyzed for dissolved organic carbon (DOC) and for NO3−. DOM was fractionated and subjected to batch incubations. The investigated soils were three well-drained Plaggic Anthrosols, one Gleyic Podzol and one Eutric Gleysol. Concentrations of DOM ranged from 9 to 32 mg C l−1 at 90 cm depth. Fluxes of DOM at this depth were between 60 and 90 kg DOC ha−1 year−1. Concentrations decreased to 4–7 mg C l−1 at depths of 3–5.6 m at the well-drained sites. Corresponding DOC fluxes at these depths were between 10 and 20 kg C ha−1 year−1. No change in DOC concentrations and fluxes with increasing depth was observed at the poorly drained sites. Because DOM fractions did not change along the flowpath, retention by sorption was probably negligible at these sites. Decreasing NO3− concentrations, which indicate denitrification, were observed at the poorly drained sites and 9–13 m below the groundwater table. During incubation studies, DOC and NO3− concentrations changed little (mean decrease: <1 mg C l−1, 0.9 mg N l−1). We conclude that DOM leached from soils does not contribute significantly to the natural attenuation of NO3− leached to aquifers because (a) the zones where DOM concentrations and NO3− concentrations decrease are separated spatially and (b) the bioavailability of leached DOM is low.

Immobilisation of molybdate by iron oxides: effects of organic coatings

Abstract It has been suggested that molybdate penetrates into micropores and interdomains of iron oxides. This process will cause immobilisation of molybdate. The influence of organic matter that may occlude the pores by adsorptive cover has not yet been examined. Thus, the aim of our study is to elucidate the role of organic coatings around iron oxides for intraparticular molybdate diffusion. We used two synthetic goethites of different crystallinity (specific surface G13: 13 m 2 g −1 and G83: 83 m 2 g −1 ), in pure form and equilibrated with dissolved organic matter (DOM), that has been extracted from forest floor samples. The iron oxide samples that were characterised by N 2 adsorption, X-ray diffraction analysis and scanning electron microscopy were preincubated with molybdate solution (5 g iron oxide l −1 , 0.2 mM molybdate, pH 4) for 12, 24 and 48 h. To follow the molybdate immobilisation, molybdate desorption kinetics (desorption periods 0.5–48 h) were determined with ion exchange resins in batch systems after the different incubation times. In addition, the preincubated iron oxides were examined by XPS. Fractional coverage of DOM-treated iron oxides estimated according to the enthalpy of N 2 adsorption was 0.33 m 2 m −2 for G83 and 1.19 m 2 m −2 for G13. The pore volume of G13 decreased after DOM treatment. Furthermore, SEM images show that DOM treatment results in microaggregation of the iron oxides. A combination of the first-order equation and a diffusion term was applicable to the Mo desorption data of both, the pure and the DOM-treated iron oxides. Desorbability and apparent diffusion constants of molybdate decreased with increasing residence time. However, the decrease was less distinct for the DOM treated than for the pure goethites. The Mo/Fe XPS ratios of the iron oxides indicate that in the presence of organic matter a higher percentage of molybdate is sorbed to outer surfaces. The results confirm the hypothesis that molybdate diffuses into the pores of iron oxides. Organic coatings slow down the molybdate immobilisation probably by decreasing the accessibility of diffusion pathways. This mechanism may be relevant even at low molybdate and C concentrations, where no competition effect of sorbed organic molecules can be observed.