Soil aeration - The relationship between redox potential and air-filled pores

Soil water contents are variable with impact on oxygen diffusion rates and redox potentials (EH). When water saturated soils become aerated, a switch from reducing to oxidizing conditions occurs. However, only limited information are available at which air-filled pore volume (ε) this dramatic shift happens. Therefore, undisturbed soil cores were taken by steel cylinders from the topsoils of a Fluvisol and a Gleysol that differed in soil structure and clay content. After submergence in the laboratory, the samples were sealed by a glass dome to exclude oxygen and to achieve strongly reducing conditions (EH < -100 mV). We aerated the sample on demand by removal of glass plugs in the dome and consecutively measured EH by platinum-(Pt) tipped electrodes and ε by pressure head readings on hourly basis. We propose to use the terms: i) εPt reaction, to indicate the air-filled pore volume at which a response of the Pt-tipped electrode due to contact with oxygen occurs (i.e., EH increase > 5 mV h-1), and ii) εPt aeration, to indicate when oxidizing soil conditions are present (i.e., EH > 300 mV at pH 7). These characteristic εPt reaction values were at 0.036±0.013 cm3 cm–3 for the Fluvisol and at 0.048±0.017 cm3 cm–3 for the Gleysol whereas εPt aeration values were at 0.047±0.005 and at 0.085±0.002 cm3 cm–3, respectively. This study provided important information to determine the aeration status of a soil when, e.g., ε is known but EH measurements are absent

Nuyoricans

Die Dissertation ist ein Beitrag zur Debatte um die Revision des amerikanistischen Lektürekanons. Ihre drei Schwerpunkte sind die Geschichte und Mythologie der Karibikinsel Puerto Rico, die soziale Lage und das Image der Puertoricaner in den USA sowie die auf Englisch erschienene Erzählliteratur von Autoren puertoricanischer Herkunft. (1) Die spanische Kolonie Puerto Rico kam 1898 in den Besitz der USA und erlebte in der zweiten Hälfte des 20. Jahrhunderts einen rasanten ökonomischen Aufstieg, der mit wachsender Abhängigkeit vom amerikanischen Wohlfahrtsstaat erkauft ist. In freien Referenden bejahte jeweils eine knappe Mehrheit den Zwitterstatus ihrer Insel, die bis heute weder ein Staat der USA noch ein souveränes Land ist. Da jedoch der Kongress in Washington über die Zukunft des Commonwealth of Puerto Rico zu bestimmen hat, bleibt die Insel eine Kolonie der USA. Puerto Ricos Mythologie ist von Stereotypen geprägt, die sich zu einem negativen Klischee vom Nationalcharakter des Landes verdichtet haben. Die amerikanische Dominanz in Politik, Wirtschaft und Kultur verstärkt die von vielen beklagte Schizophrenie Puerto Ricos. Die Chance einer Lösung des puertoricanischen Syndroms verspricht nur die nationale Unabhängigkeit. (2) Stereotype bestimmen auch das Bild von den übergesiedelten Puertoricanern, den Nuyoricans, in der Öffentlichkeit der USA. Das Negativimage der kaum assimilierten Gruppe wird von den Massenmedien verfestigt, obwohl seriöse Studien zeigen, dass frühere Immigranten ähnliche Probleme mit sich brachten. Die Mehrheit der US-Experten propagiert nach wie vor die allmähliche Assimilation der Übersiedler im Rahmen eines kulturellen Pluralismus. Bei den Puertoricanern geht der Trend seit dem Aufkommen des Multikulturalismus in Richtung einer hybriden, bikulturellen Identität, in ein Wort gefasst mit dem Begriff Nuyorican. Die Dissertation bietet eine Auswertung der auf Englisch erschienenen Literatur über die Puertoricaner in den USA unter 15 Aspekten: Kultur, Religion, Bildung, Sprache, Politik, Arbeit, welfare, Wohnverhältnisse, ethnicity, race, class, gender, Familie, Law and order und Migration. Am Beispiel von belletristischen Texten, Spielfilmen und Musicals wird gezeigt, dass die Puertoricaner in diesen Genres größtenteils wohlwollend dargestellt erscheinen. Das gilt auch für das Musical West Side Story, dem viele Kritiker zu Unrecht vorwerfen, die puertoricanischen Jugendlichen als Gangster zu stigmatisieren. (3) Die puertoricanische Prosa in englischer Sprache hat im Kanon der amerikanischen Literatur und in der Kritik bisher kaum eine Rolle gespielt. Vor allem die zahlreichen Neuerscheinungen der achtziger und neunziger Jahre werden hier erstmals auf historisch-soziologischer Basis analysiert. Allgemeine Trends der neueren Nuyorican-Literatur sind ihre Diversifizierung und Feminisierung. Größere Vielfalt gibt es heute bei den Schauplätzen, den Textsorten und den Themen. Das Thema gender steht nicht nur bei den Frauen, sondern auch bei männlichen Autoren oft im Mittelpunkt. Von den Autoren, die New York zum Schauplatz gewählt haben, ist Abraham Rodriguez, Jr. der bedeutendste. Rodriguez erzählt von Teenagern in der South Bronx, deren puertoricanische Ethnizität kein bestimmender Faktor mehr ist. So ist er der am weitesten amerikanisierte Autor der Nuyoricans. Die überzeugendste Interpretation des Migrationsprozesses bietet Esmeralda Santiago. Bei ihr steht die Kritik am traditionellen puertoricanischen Sexismus im Zentrum. Eine feministische Grundtendenz haben auch die in Puerto Rico angesiedelten, zum Teil magisch-realistischen Werke von Rosario Ferré. Die besten Werke von Rodriguez, Santiago, Ferré und weiteren Puertoricanern verdienen Anerkennung als wertvoller und zukunftweisender Beitrag zur amerikanischen Literatur.The thesis contributes to the debate about the revision of the American literary canon. Its first focus is on the history and mythology of Puerto Rico, the second on the social situation and image of the Puerto Ricans in the U.S., and the third on the prose literature by authors of Puerto Rican descent published in English. (1) The Spanish colony of Puerto Rico became a possession of the U.S. in 1898 and experienced a rapid economic rise in the second half of the 20th century, at the expence of growing dependence on the American welfare state. In free referendums the people of Puerto Rico have so far condoned the intermediate status of their island, which still is neither a state of the union nor an independent nation. But in fact the Commonwealth of Puerto Rico remains a colony of the U.S., as the real power to decide about its status lies with Congress. Puerto Rico's mythology has from the start been dominated by stereotypes, which have resulted in a negative cliché of its national character. The United States' political, economic and cultural hegemony has reinforced the alleged schizophrenic state of Puerto Rico. The only chance of healing this Puerto Rican syndrome is the island's national independence. (2) Stereotypes have also determined the image in the American public of the Puerto Ricans who have migrated to the mainland. The bad reputation of this hardly assimilated group is constantly being confirmed by the mass media, although serious studies prove that earlier immigrants had similar problems. Most U.S. experts still advocate the migrants' gradual assimilitation according to the ideal of cultural pluralism. Since the rise of multiculturalism, within the U.S. Puerto Rican community the trend has been going towards a hybrid, bicultural, Nuyorican identity. The dissertation assesses the literature about Puerto Ricans in the U.S. published in English from 15 key aspects: culture, religion, education, language, politics, work, welfare, housing, ethnicity, race, class, gender, family, law and order und migration. A critical look at books, movies and musicals by non-Puerto Ricans shows that in these genres migrants from Puerto Rico have by and large been portrayed benevolenty. This is true even for the musical West Side Story, which has often been wrongly blamed for stigmatizing Puerto Rican youngsters as gangsters. (3) Puerto Rican prose literature in English has so far played little role in the American canon and in criticism. This thesis offers the first analysis of the many new volumes from the eighties and nineties on a socio-historical basis. The overall trends of recent Nuyorican literature are its diversification und feminization. There is a greater diversity of settings, text types and themes. Gender is a central issue not only with the women, but also with a number of male authors. Of those writers who take New York as the setting, Abraham Rodriguez, Jr. is the most important. Rodriguez's books are about teenagers in the South Bronx whose Puerto Rican ethnicity is no crucial factor any more. Thus he is the most Americanised author among the Nuyoricans. The most convincing interpretation of the migratory process is Esmeralda Santiago's. Her main thrust is against the Puerto Rican tradition of sexism. The works of Rosario Ferré, some told in magic realism and all set in Puerto Rico, also have a feminist tendency. The best books by Rodriguez, Santiago, Ferré and a few more Puerto Ricans deserve to be recognised as a valuable and visionary contribution to American literature

Microbial residence time is a controlling parameter of the taxonomic composition and functional profile of microbial communities.

A remaining challenge within microbial ecology is to understand the determinants of richness and diversity observed in environmental microbial communities. In a range of systems, including activated sludge bioreactors, the microbial residence time (MRT) has been previously shown to shape the microbial community composition. However, the physiological and ecological mechanisms driving this influence have remained unclear. Here, this relationship is explored by analyzing an activated sludge system fed with municipal wastewater. Using a model designed in this study based on Monod-growth kinetics, longer MRTs were shown to increase the range of growth parameters that enable persistence, resulting in increased richness and diversity in the modeled community. In laboratory experiments, six sequencing batch reactors treating domestic wastewater were operated in parallel at MRTs between 1 and 15 days. The communities were characterized using both 16S ribosomal RNA and non-target messenger RNA sequencing (metatranscriptomic analysis), and model-predicted monotonic increases in richness were confirmed in both profiles. Accordingly, taxonomic Shannon diversity also increased with MRT. In contrast, the diversity in enzyme class annotations resulting from the metatranscriptomic analysis displayed a non-monotonic trend over the MRT gradient. Disproportionately high abundances of transcripts encoding for rarer enzymes occur at longer MRTs and lead to the disconnect between taxonomic and functional diversity profiles

Redoxtrons – An experimental system to study redox processes within the capillary fringe

Spatiotemporal characterisation of the soil redox status within the capillary fringe (CF) is a challenging task. Air-filled porosities (ε), oxygen concentration (O2) and soil redox potential (EH) are interrelated soil variables within active biogeochemical domains such as the CF. We investigated the impact of water table (WT) rise and drainage in an undisturbed topsoil and subsoil sample taken from a Calcaric Gleysol for a period of 46 days. We merged 1D (EH and matric potential) and 2D (O2) systems to monitor at high spatiotemporal resolution redox dynamics within self-constructed redoxtron housings and complemented the data set by a 3D pore network characterization using X-ray microtomography (X-ray μCT). Depletion of O2 was faster in the organic matter- and clay-rich aggregated topsoil and the CF extended >10 cm above the artificial WT. The homogeneous and less-aggregated subsoil extended only 4 cm above the WT as indicated by ε–O2–EH data during saturation. After drainage, 2D O2 imaging revealed a fast aeration towards the lower depths of the topsoil, which agrees with the connected ε derived by X-ray μCT (εCT_conn) of 14.9% of the total porosity. However, small-scaled anoxic domains with O2 saturation <5% were apparent even after lowering the WT (down to 0.25 cm2 in size) for 23 days. These domains remained a nucleus for reducing soil conditions (EH < −100 mV), which made it challenging to characterise the soil redox status in the CF. In contrast, the subsoil aeration reached O2 saturation after 8 days for the complete soil volume. Values of εCT_conn around zero in the subsoil highlighted that soil aeration was independent of this parameter suggesting that other variables such as microbial activity must be considered when predicting the soil redox status from ε alone. The use of redoxtrons in combination with localised redox-measurements and image based pore space analysis resulted in a better 2D/3D characterisation of the pore system and related O2 transport properties. This allowed us to analyse the distribution and activity of microbiological niches highly associated with the spatiotemporal variable redox dynamics in soil environments

Soil aeration and redox potential as function of pore connectivity unravelled by X-ray microtomography imaging

Platinum (Pt)-tipped electrodes are frequently employed to measure the soil redox potential (EH). Thereby, the timely transition from reducing towards oxidising soil conditions is one of the most important biogeochemical changes that can occur in soil. This condition is mainly linked to the air-filled pore volume (ε) and pore geometries. However, even when the Pt electrodes are located in close vicinity to each other, EH readings behave non-uniformly, presumably due to the millimetre scaled heterogeneity of pore spaces controlling oxygen (O2) availability and transport. In this study, we examined the ε distribution and pore connectivity in the close vicinity of a Pt electrode during an artificial evaporation experiment using an undisturbed soil sample (Ah-horizon, Calcaric Gleysol). We combined physio-chemical methods with non-destructive X-ray computed microtomography (μCT) and 3D-image analysis. μCT scans were conducted at three-time points, that is, reducing conditions with EH &lt; −100 mV (CT-1), the transition from reducing towards oxidising conditions with an EH increase &gt; 5 mV h−1 (CT-2), and oxidising conditions with EH &gt; 300 mV (CT-3). We observed that the shift from reducing towards oxidising conditions took place at an air-filled porosity (εCT) of ~0.03 cm3 cm−3, which matches very with gravimetrically calculated data obtained by tensiometry of ε ~0.05 cm3 cm−3. Besides the relation of EH and ε, image analysis revealed that a connected εCT (εCT_conn) of ~0.02 cm3 cm−3 is needed to enable enhanced O2 diffusion from the soil surface towards the Pt surface and facilitate a straightforward EH response. We conclude that εCT_conn is a critical parameter to assess aeration processes in temporarily water-saturated soils to characterise a switch in redox conditions. Highlights: Usually, soil redox dynamics are related to the air-filled porosity (εCT) but here its connected portion (εCT_conn) was found more relevant. 3D X-ray computed microtomography imaging close to a redox electrode enabled us to understand the soil aeration process. Connected εCT (εCT_conn) of ~0.02 cm3 cm−3 facilitated oxidising soil conditions. εCT_conn is a critical parameter to assess the aeration process in temporarily water-saturated soils. © 2021 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science

Wer bremst verliert - Antimon in Straßenrandböden

Seit dem weitreichenden Verbot von Asbest in den 1980er Jahren wurde verstärkt das Halbmetall Antimon (Sb) in Bremsbelägen von PKW und LKW verwendet. Im Zuge des Bremsvorganges wird dieses in Form von Feinstaub freigesetzt und straßennah in die Böden eingetragen. Grundsätzlich sind Antimoneinträge in Böden negativ zu bewerten, da das Halbmetall toxische Wirkungen hervorrufen kann. Um die räumliche Verteilung von Antimon und seinen Bindungsformen in Straßenrandböden zu untersuchen, wurden 5 parallele Transekte mit jeweils 7 Entfernungen und 4 Probentiefen entlang einer Kölner Kreisstraße (Verkehrsaufkommen ca. 9.600 Fahrzeuge pro Tag) beprobt. Neben einer allgemeinen physikochemischen Charakterisierung der Bodenproben wurden ihre königswasser-löslichen Antimongehalte bestimmt. Zusätzlich wurde an einem Transekt eine sequentielle Extraktion nach Wenzel et al. 2001 durchgeführt, um die Bindungsformen von Antimon zu ermitteln. Die Antimongehalte nahmen von bis zu 6,18 g kg–1 unmittelbar am Straßenrand bis auf 0,513 g kg–1 in 10 m Entfernung ab. Ebenfalls konnte eine deutliche Abnahme mit der Bodentiefe (0 bis 30 cm) festgestellt werden. Dieses Verteilungsmuster deutet auf den Verkehr als Antimoneintragsquelle hin. Als Hinweis auf einen durch den Bremsabrieb metallener Bremsscheiben dominierten Eintrag wurde die magnetische Suszeptibilität gemessen. Allerdings lieferte der Korrelationskoeffizient (Spearman) von Antimon gegenüber der magnetischen Suszeptibilität 0,423 (p = 0,023) nur einen eher schwachen Beleg. Die ursprünglich für Arsen entwickelte sequentielle Extraktion ließ sich reproduzierbar ebenfalls für Antimon anwenden. Anhand dieser konnten im Wesentlichen drei Trends der Bindungsformen erfasst werden: i) spezifisch gebundenes Antimon (Fraktion 2: 0,05 mol l–1 (NH4)H2P2O4) konnte lediglich innerhalb 1 m Entfernung zur Straße und mit der Bodentiefen abnehmenden Gehalten nachgewiesen werden, ii) der Anteil an schwachkristallinen Eisenoxiden gebundenem Antimon (Fraktion 3: 0,2 mol l–1 (NH4)2-Oxalatpuffer; pH 3,25) nahm signifikant mit steigender Entfernung zur Straße ab, während iii) die prozentualen Gehalte in den starkkristallinen Eisenoxiden (Fraktion 4: 0,2 mol l–1 (NH4)2-Oxalatpuffer + 0,1 mol l–1 Ascorbinsäure; pH 3,25) und in der Residualphase (Fraktion 5: Königswasser) zunahmen