Root-induced increases in soil pH and nutrient availability to field-grown cereals and legumes on acid sandy soils of Sudano-Sahelian West Africa

A field experiment with millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata L.) and groundnut (Arachnis hypogeae L.) was conducted on severely P-deficient acid sandy soils of Niger, Mali and Burkina Faso to measure changes in pH and nutrient availability as affected by distance from the root surface and by mineral fertiliser application. Treatments included three rates of phosphorus (P) and four levels of nitrogen (N) application. Bulk, rhizosphere and rhizoplane soils were sampled at 35, 45 and 75 DAS in 1997 and at 55 and 65 DAS in 1998. Regardless of the cropping system and level of mineral fertiliser applied, soil pH consistently increased between 0.7 and two units from the bulk soil to the rhizoplane of millet. Similar pH gradients were observed in cowpea, but pH changes were much smaller in sorghum with a difference of only 0.3 units. Shifts in pH led to large increases in nutrient availability close to the roots. Compared with the bulk soil, available P in the rhizoplane was between 190 and 270% higher for P-Bray and between 360 and 600% higher for P-water. Exchangeable calcium (Ca) and magnesium (Mg) levels were also higher in the millet rhizoplane than in the bulk soil, whereas exchangeable aluminium (Al) levels decreased with increasing pH close to the root surface. The results suggest an important role of root-induced pH increases for crops to cope with acidity-induced nutrient deficiency and Al stress of soils in the Sudano-Sahelian zone of West Africa.

Decomposition of and nutrient release from ruminant manure on acid sandy soils in the Sahelian zone of Niger, West Africa

In ago-pastoral systems of the semi-arid West African Sahel, targeted applications of ruminant manure to the cropland is a widespread practice to maintain soil productivity. However, studies exploring the decomposition and mineralisation processes of manure under farmers' conditions are scarce. The present research in south-west Niger was undertaken to examine the role of micro-organisms and meso-fauna on in situ release rates of nitrogen (N), phosphorus (P) and potassium (K) from cattle and sheep-goat manure collected from village corrals during the rainy season. The results show tha (1) macro-organisms played a dominant role in the initial phase of manure decomposition; (2) manure decomposition was faster on crusted than on sandy soils; (3) throughout the study N and P release rates closely followed the dry matter decomposition; (4) during the first 6 weeks after application the K concentration in the manure declined much faster than N or P. At the applied dry matter rate of 18.8 Mg ha^-1, the quantities of N, P and K released from the manure during the rainy season were up to 10-fold larger than the annual nutrient uptake of pearl millet (Pennisetum glaucum L.), the dominant crop in the traditional agro-pastoral systems. The results indicate considerable nutrient losses with the scarce but heavy rainfalls which could be alleviated by smaller rates of manure application. Those, however, would require a more labour intensive system of corralling or manure distribution.

Changes in amino acid enantiomers and microbial performance in soils from a subtropical mountain oasis in Oman abandoned for different periods

An important feature of maintaining the agricultural stability in millennia-old mountain oases of northern Oman is the temporary abandonment of terraces. To analyse the effects of a fallow period on soil microbial performance, i.e. microbial activity and microbial biomass, samples of eight terrace soils abandoned for different periods were collected in situ, assigned to four fallow age classes and incubated for 30 days in the laboratory after rewetting. The younger fallow age classes of 1 and 5 years were based on the records of the farmers’ recollections, the two older fallow age classes of 10–20 and 25–60 years according to the increase in the D -to- L ratio of valine and leucine enantiomers. The increase in these two ratios was in agreement with that of the D -to- L ratio of lysine. The strongest relationship was observed between the increase in the D -to- L ratio of lysine and the decrease in soil microbial biomass C. However, the most stringent coherence between the increase in fallow age and soil properties was revealed by the decreases in cumulative respiration and net N mineralisation rates with decreasing availability of substrate to soil microorganisms. During the 30-day incubation following rewetting, relative changes in microbial activity (respiration and net N mineralisation) and microbial biomass (C and N)indices were similar in the eight terrace soils on a fallow age-class-specific level, indicating that the same basic processes occurred in all of the sandy terrace soils investigated.

Have increased dissolved organic carbon (DOC) losses from UK peat and organo-mineral soils been driven by the decline in acid rain?

Dissolved organic carbon (DOC) concentrations have been rising in streams and lakes draining catchments with organic soils across Northern Europe. These increases have shown a correlation with decreased sulphate and chloride concentrations. One hypothesis to explain this phenomenon is that these relationships are due an increased in DOC release from soils to freshwaters, caused by a decline in pollutant sulphur and sea-salt deposition. We carried out controlled deposition experiments in the laboratory on intact peat and organomineral O-horizon cores to test this hypothesis. Preliminary data showed a clear correlation between the change in soil water pH and change in DOC concentrations, however uncertainty still remains about whether this is due to changes in biological activity or chemical solubility.

Effects of acid sulphate on DOC release in mineral soils: the influence of SO42−retention and Al release

Dissolved organic carbon (DOC) in acid-sensitive upland waters is dominated by allochthonous inputs from organic-rich soils, yet inter-site variability in soil DOC release to changes in acidity has received scant attention in spite of the reported differences between locations in surface water DOC trends over the last few decades. In a previous paper, we demonstrated that pH-related retention of DOC in O horizon soils was influenced by acid-base status, particularly the exchangeable Al content. In the present paper, we investigate the effect of sulphate additions (0–437 μeq l−1) on DOC release in the mineral B horizon soils from the same locations. Dissolved organic carbon release decreased with declining pH in all soils, although the shape of the pH-DOC relationships differed between locations, reflecting the multiple factors controlling DOC mobility. The release of DOC decreased by 32–91% in the treatment with the largest acid input (437 μeq l−1), with the greatest decreases occurring in soils with very small % base saturation (BS, <3%) and/or large capacity for sulphate (SO42−) retention (up to 35% of added SO42−). The greatest DOC release occurred in the soil with the largest initial base status (12% BS). These results support our earlier conclusions that differences in acid-base status between soils alter the sensitivity of DOC release to similar sulphur deposition declines. However,superimposed on this is the capacity of mineral soils to sorb DOC and SO42−, and more work is needed to determine the fate of sorbed DOC under conditions of increasing pH and decreasing SO42−.

Microbial formation of nitrous acid and its exchange processes between soils and atmosphere

Salpetrige Säure (HONO) ist eine wichtige Form von reaktivem Stickstoff, die aufgrund ihrer Photolyse zu Stickstoffmonoxid (NO) und dem Hydroxylradikal (OH), sehr kurzlebig ist. Ein genaues Verständnis der Quellen und Senken von HONO ist eine grundlegende Voraussetzung, um dessen Einfluss auf die Umwelt zu beurteilen. Allerdings wird immer noch nach einer starken HONO-Quelle am Tag gesucht und nächtliche HONO-Deposition auf den Boden wurde bisher stets nur postuliert. Diese Dissertation folgt der Zielsetzung die Prozesse der HONO-Aufnahme und Freisetzung von Böden aufzudecken und die zugrunde liegenden Mechanismen zu verstehen.rnUm die Rolle von HONO-Bodenemissionen zu quantifizieren, wurden 17 Böden in einem dynamischen Kammersystem untersucht. Es konnten HONO-Emissionen derselben Größenordnung wie die bereits gut untersuchten NO-Emissionen festgestellt werden. Unerwarteter Weise wurden die stärksten Emissionen bei Böden mit neutralem pH aus ariden und landwirt¬schaftlichen Gebieten beobachtet. Die Temperaturabhängigkeit der Bodenemissionen von HONO und NO führten zu der Annahme einer mikrobiellen Freisetzung von HONO, welche durch Reinkulturexperimente mit dem ammoniumoxidierenden Bakterium Nitrosomonas europaea bestätigt werden konnte. Ein konzeptionelles Model für die Freisetzung reaktiver Stickstoffverbindungen aus Böden in Abhängigkeit des Bodenwassergehaltes wurde um HONO-Emissionen erweitert.rnDurch Nachweise mittels Reinkultur- und Inhibitionsexperimenten konnten weitere Untersuchungen der bakteriellen Freisetzung von HONO aus Böden zeigen, dass innerhalb der bakteriellen Nitrifikation nur ammoniumoxidierende Bakterien zur Emission von HONO fähig sind. Durch kontrolliert initiierte Zelllyse konnte gezeigt werden, dass intrazellulär akkumuliertes Hydroxylamin (NH2OH) für die HONO-Freisetzung verantwortlich sind. Zum ersten Mal wurde NH2OH in der Gasphase nachgewiesen und dass dieses über den gesamten Bodenfeuchtebereich von ammoniumoxidierenden Bakterien freigesetzt wird. Es wurde gezeigt, dass die heterogene Reaktion von NH2OH mit Wasserdampf auf einer Glasperlenoberfläche HONO bildet. Diese Reaktion erklärt die beobachtete Freisetzung von HONO bei niedrigen Bodenfeuchten, da nur dann die Oberfläche zur Reaktion zur Verfügung steht und nicht von Wasser bedeckt ist.rnEine 15N Isotopenmarkierungsmethode wurde entwickelt um isotopenmarkiertes gasförmiges HONO zu messen, was die Untersuchung der Bildungsprozesse von HONO und dessen Rolle in biogeochemischen Zyklen ermöglicht. Die Anwendung dieser neuen Methode auf eine Bodenprobe die mit 15N Harnstoff angereichert und in einem dynamischen Kammersystem untersucht wurde, bestätigt die obigen Ergebnisse einer starken mikrobiellen Beteiligung von Bodenbakterien zur HONO Freisetzung.rnBidirektionale Flüsse von HONO wurden für sechs untersuchte Bodenproben gefunden. Die Richtung der Flüsse hängt dabei vom Umgebungsmischungsverhältnis von HONO und dem Bodenwassergehalt ab. Eine wichtige Größe, die die bidirektionalen Flüsse von HONO beschreibt, ist das „Ökosystem spezifische Kompensationsmischungsverhältnis von HONO“, χcomp. Dieser neue Begriff wurde definiert und eingeführt, da die verschiedenen in den Bodenaustausch von HONO involvierten Prozesse nicht mit dem klassischen Kompensationspunktkonzept kompatibel sind. Die Untersuchungen bestätigen neueste Feldbeobachtungen, dass HONO, welches bei hohen Umgebungsmischungsverhältnissen vom Boden adsorbiert wird, bei niedrigen Mischungsverhält-nissen wieder vom Boden desorbiert wird. Folglich wird nächtlich akkumuliertes HONO tagsüber in eine Quelle für HONO umgewandelt. Vier Prozesse - Verteilung von HONO zwischen Gas- und Flüssigphase nach Henrys Gesetz, bakterielle HONO Bildung aus NH2OH, Adsorption und Desorption von HONO - und deren Dominanz in speziellen Bodenfeuchtebereichen wurden identifiziert. Dadurch wurde ein konzeptionelles Model für die Prozesse, die in Aufnahme und Freisetzung von HONO aus Böden involviert sind, als Funktion der Bodenfeuchte entwickelt.rnZusammenfassend hat diese Dissertation die entscheidenden Prozesse im Austausch von HONO zwischen Boden und Atmosphäre aufgeklärt und den der bakteriellen HONO Bildung zugrunde liegenden Mechanismus aufgedeckt. Es konnte gezeigt werden, dass Böden sowohl eine wichtige Quelle als auch eine Senke für HONO sind und sollten folglich in zukünftigen Feldmessungen stärker berücksichtigt werden.rn

Residual effect of organic zinc complexes in acid and calcareous soils on navy bean (phaseolus vulgaris l.) and flax (linum usitatissimum l.) crops under greenhouse conditions. Influence of the moisture conditions = Efecto residual de complejos orgánicos de zinc en suelos ácido y calizo en cultivos de judía (phaseolus vulgaris l.) y lino (linum usitatissimum l.) Realizados en condiciones de invernadero. Influencia de las diferentes condiciones de humedad

El Zn es un elemento esencial para el crecimiento saludable y reproducción de plantas, animales y humanos. La deficiencia de Zn es una de las carencias de micronutrientes más extendidas en muchos cultivos, afectando a grandes extensiones de suelos en diferentes áreas agrícolas. La biofortificación agronómica de diferentes cultivos, incrementando la concentración de micronutriente Zn en la planta, es un medio para evitar la deficiencia de Zn en animales y humanos. Tradicionalmente se han utilizado fertilizantes de Zn inorgánicos, como el ZnSO4, aunque en los últimos años se están utilizado complejos de Zn como fuentes de este micronutriente, obteniéndose altas concentraciones de Zn soluble y disponible en el suelo. Sin embargo, el envejecimiento de la fuente en el suelo puede causar cambios importantes en su disponibilidad para las plantas. Cuando se añaden al suelo fuentes de Zn inorgánicas, las formas de Zn más solubles pierden actividad y extractabilidad con el paso del tiempo, transformándose a formas más estables y menos biodisponibles. En esta tesis se estudia el efecto residual de diferentes complejos de Zn de origen natural y sintético, aplicados en cultivos previos de judía y lino, bajo dos condiciones de riego distintas (por encima y por debajo de la capacidad de campo, respectivamente) y en dos suelos diferentes (ácido y calizo). Los fertilizantes fueron aplicados al cultivo previo en tres dosis diferentes (0, 5 y 10 mg Zn kg-1 suelo). El Zn fácilmente lixiviable se estimó con la extracción con BaCl2 0,1M. Bajo condiciones de humedad por encima de la capacidad de campo se obtuvieron mayores porcentajes de Zn lixiviado en el suelo calizo que en el suelo ácido. En el caso del cultivo de judía realizado en condiciones de humedad por encima de la capacidad de campo se compararon las cantidades extraídas con el Zn lixiviado real. El análisis de correlación entre el Zn fácilmente lixiviable y el estimado sólo fue válido para complejos con alta movilidad y para cada suelo por separado. Bajo condiciones de humedad por debajo de la capacidad de campo, la concentración de Zn biodisponible fácilmente lixiviable presentó correlaciones positivas y altamente significativas con la concentración de Zn disponible en el suelo. El Zn disponible se estimó con varios métodos de extracción empleados habitualmente: DTPA-TEA, DTPA-AB, Mehlich-3 y LMWOAs. Estas concentraciones fueron mayores en el suelo ácido que en el calizo. Los diferentes métodos utilizados para estimar el Zn disponible presentaron correlaciones positivas y altamente significativas entre sí. La distribución del Zn en las distintas fracciones del suelo fue estimada con diferentes extracciones secuenciales. Las extracciones secuenciales mostraron un descenso entre los dos cultivos (el anterior y el actual) en la fracción de Zn más lábil y un aumento en la concentración de Zn asociado a fracciones menos lábiles, como carbonatos, óxidos y materia orgánica. Se obtuvieron correlaciones positivas y altamente significativas entre las concentraciones de Zn asociado a las fracciones más lábiles (WSEX y WS+EXC, experimento de la judía y lino, respectivamente) y las concentraciones de Zn disponible, estimadas por los diferentes métodos. Con respecto a la planta se determinaron el rendimiento en materia seca y la concentración de Zn en planta. Se observó un aumento del rendimiento y concentraciones con el efecto residual de la dosis mayores (10 mg Zn kg-1) con respecto a la dosis inferior (5 mg Zn 12 kg-1) y de ésta con respecto a la dosis 0 (control). El incremento de la concentración de Zn en todos los tratamientos fertilizantes, respecto al control, fue mayor en el suelo ácido que en el calizo. Las concentraciones de Zn en planta indicaron que, en el suelo calizo, serían convenientes nuevas aplicaciones de Zn en posteriores cultivos para mantener unas adecuadas concentraciones en planta. Las mayores concentraciones de Zn en la planta de judía, cultivada bajo condiciones de humedad por encima de la capacidad de campo, se obtuvieron en el suelo ácido con el efecto residual del Zn-HEDTA a la dosis de 10 mg Zn kg-1 (280,87 mg Zn kg-1) y en el suelo calizo con el efecto residual del Zn-DTPA-HEDTA-EDTA a la dosis de 10 mg Zn kg-1 (49,89 mg Zn kg-1). En el cultivo de lino, cultivado bajo condiciones de humedad por debajo de la capacidad de campo, las mayores concentraciones de Zn en planta ese obtuvieron con el efecto residual del Zn-AML a la dosis de 10 mg Zn kg-1 (224,75 mg Zn kg-1) y en el suelo calizo con el efecto residual del Zn-EDTA a la dosis de 10 mg Zn kg-1 (99,83 mg Zn kg-1). El Zn tomado por la planta fue determinado como combinación del rendimiento y de la concentración en planta. Bajo condiciones de humedad por encima de capacidad de campo, con lixiviación, el Zn tomado por la judía disminuyó en el cultivo actual con respecto al cultivo anterior. Sin embargo, en el cultivo de lino, bajo condiciones de humedad por debajo de la capacidad de campo, se obtuvieron cantidades de Zn tomado superiores en el cultivo actual con respecto al anterior. Esta tendencia también se observó, en ambos casos, con el porcentaje de Zn usado por la planta. Summary Zinc is essential for healthy growth and reproduction of plants, animals and humans. Zinc deficiency is one of the most widespread micronutrient deficiency in different crops, and affect different agricultural areas. Agronomic biofortification of crops produced by an increased of Zn in plant, is one way to avoid Zn deficiency in animals and humans Sources with inorganic Zn, such as ZnSO4, have been used traditionally. Although, in recent years, Zn complexes are used as sources of this micronutrient, the provide high concentrations of soluble and available Zn in soil. However, the aging of the source in the soil could cause significant changes in their availability to plants. When an inorganic source of Zn is added to soil, Zn forms more soluble and extractability lose activity over time, transforming into forms more stable and less bioavailable. This study examines the residual effect of different natural and synthetic Zn complexes on navy bean and flax crops, under two different moisture conditions (above and below field capacity, respectively) and in two different soils (acid and calcareous). Fertilizers were applied to the previous crop in three different doses (0, 5 y 10 mg Zn kg-1 soil). The easily leachable Zn was estimated by extraction with 0.1 M BaCl2. Under conditions of moisture above field capacity, the percentage of leachable Zn in the calcareous soil was higher than in acid soil. In the case of navy bean experiment, performed in moisture conditions of above field capacity, amounts extracted of easily leachable Zn were compared with the real leachable Zn. Correlation analysis between the leachable Zn and the estimate was only valid for complex with high mobility and for each soil separately. Under moisture conditions below field capacity, the concentration of bioavailable easily leachable Zn showed highly significant positive correlations with the concentration of available soil Zn. The available Zn was estimated with several commonly used extraction methods: DTPA-TEA, AB-DTPA, Mehlich-3 and LMWOAs. These concentrations were higher in acidic soil than in the calcareous. The different methods used to estimate the available Zn showed highly significant positive correlations with each other. The distribution of Zn in the different fractions of soil was estimated with different sequential extractions. The sequential extractions showed a decrease between the two crops (the previous and current) at the most labile Zn fraction and an increase in the concentration of Zn associated with the less labile fractions, such as carbonates, oxides and organic matter. A positive and highly significant correlation was obtained between the concentrations of Zn associated with more labile fractions (WSEX and WS + EXC, navy bean and flax experiments, respectively) and available Zn concentrations determined by the different methods. Dry matter yield and Zn concentration in plants were determined in plant. Yield and Zn concentration in plant were higher with the residual concentrations of the higher dose applied (10 mg Zn kg-1) than with the lower dose (5 mg Zn kg-1), also these parameters showed higher values with application of this dose than with not Zn application. The increase of Zn concentration in plant with Zn treatments, respect to the control, was greater in the acid soil than in the calcareous. The Zn concentrations in plant indicated that in the calcareous soil, new applications of Zn are desirable in subsequent crops to maintain suitable concentrations in plant. 15 The highest concentrations of Zn in navy bean plant, performed under moisture conditions above the field capacity, were obtained with the residual effect of Zn-HEDTA at the dose of 10 mg Zn kg-1 (280.87 mg Zn kg-1) in the acid soil, and with the residual effect of Zn- DTPA-HEDTA-EDTA at a dose of 10 mg Zn kg-1 (49.89 mg Zn kg-1) in the calcareous soil. In the flax crop, performed under moisture conditions below field capacity, the highest Zn concentrations in plant were obtained with the residual effect of Zn-AML at the dose of 10 mg Zn kg-1 (224.75 Zn mg kg-1) and with the residual effect of Zn-EDTA at a dose of 10 mg Zn kg-1 (99.83 mg Zn kg-1) in the calcareous soil. The Zn uptake was determined as a combination of yield and Zn concentration in plant. Under moisture conditions above field capacity, with leaching, Zn uptake by navy bean decreased in the current crop, respect to the previous crop. However, in the flax crop, under moisture conditions below field capacity, Zn uptake was higher in the current crop than in the previous. This trend is also observed in both cases, with the percentage of Zn used by the plant