Microbial responses to fluctuation of soil aeration status and redox conditions

 The response of the microbial community to changes in aeration status, from oxic to anoxic and from anoxic to oxic, was determined in arable soil incubated in a continuous flow incubation apparatus. Soil incubated in permanently oxic (air) and/or anoxic (O₂-free N₂) conditions was used as the control. Before experiments soil was preincubated for 6 days, then aeration status was changed and glucose added. Glucose concentration, extractable C, CO₂ production, microbial biomass, pH and redox potential were determined 0, 4, 8, 12, 16, 24, 36 and 48 h after change of aeration status. If oxic conditions were changed to anoxic, the amount of glucose consumed was reduced by about 60%, and CO₂ production was 10 times lower at the end of incubation compared to the control (permanently oxic conditions). Microbial biomass increased by 114% in glucose-amended soil but did not change in unamended soil. C immobilization prevailed over C mineralization. Redox potential decreased from +627 mV to –306 mV. If anoxic conditions were changed to oxic, consumption of glucose and CO₂ evolution significantly increased, compared to permanently anoxic conditions. Microbial biomass did not change in glucose-amended soil, but decreased by 78% in unamended soil. C mineralization was accelerated. Redox potential increased from +238 to +541 mV. The rate of glucose consumption was low in anoxic conditions if soil was incubated in pure N₂ but increased significantly when incubation was carried out in a CO₂/N₂ mixture. Received: 6 January 1999     

Nitrification and denitrification in the rhizosphere of rice: the detection of processes by a new multi-channel electrode

 N turnover in flooded rice soils is characterized by a tight coupling between nitrification and denitrification. Nitrification is restricted to the millimetre-thin oxic surface layer while denitrification occurs in the adjacent anoxic soil. However, in planted rice soil O₂ released from the rice roots may also support nitrification within the otherwise anoxic bulk soil. To locate root-associated nitrification and denitrification we constructed a new multi-channel microelectrode that measures NH₄ ⁺, NO₂ ^–, and NO₃ ^– at the same point. Unfertilized, unplanted rice microcosms developed an oxic-anoxic interface with nitrification taking place above and denitrification below ca. 1 mm depth. In unfertilized microcosms with rice plants, NH₄ ⁺, NO₂ ^– and NO₃ ^– could not be detected in the rhizosphere. Assimilation by the rice roots reduced the available N to a level where nitrification and denitrification virtually could not occur. However, a few hours after injecting (NH₄)₂HPO₄ or urea, a high nitrification activity could be detected in the surface layer of planted microcosms and in a depth of 20–30 mm in the rooted soil. O₂ concentrations of up to 150 μM were measured at the same depth, indicating O₂ release from the rice roots. Nitrification occurred at a distance of 0–2 mm from the surface around individual roots, and denitrification occurred at a distance of 1.5–5.0 mm. Addition of urea to the floodwater of planted rice microcosms stimulated nitrification. Transpiration of the rice plants caused percolation of water resulting in a mass flow of NH₄ ⁺ towards the roots, thus supporting nitrification. Received: 23 July 1999     

Oxygen and redox potential gradients in the rhizosphere of alfalfa grown on a loamy soil

Oxygen (O₂) supply and the related redox potential (E_H) are important parameters for interactions between roots and microorganisms in the rhizosphere. Rhizosphere extension in terms of the spatial distribution of O₂ concentration and E_H is poorly documented under aerobic soil conditions. We investigated how far O₂ consumption of roots and microorganisms in the rhizosphere is replenished by O₂ diffusion as a function of water/air‐filled porosity. Oxygen concentration and E_H in the rhizosphere were monitored at a mm‐scale by means of electroreductive Clark‐type sensors and miniaturized E_H electrodes under various matric potential ranges. Respiratory activity of roots and microorganisms was calculated from O₂ profiles and diffusion coefficients. pH profiles were determined in thin soil layers sliced near the root surface. Gradients of O₂ concentration and the extent of anoxic zones depended on the respiratory activity near the root surface. Matric potential, reflecting air‐filled porosity, was found to be the most important factor affecting O₂ transport in the rhizosphere. Under water‐saturated conditions and near field capacity up to –200 hPa, O₂ transport was limited, causing a decline in oxygen partial pressures (pO₂) to values between 0 and 3 kPa at the root surface. Aerobic respiration increased by a factor of 100 when comparing the saturated with the driest status. At an air‐filled porosity of 9% to 12%, diffusion of O₂ increased considerably. This was confirmed by E_H around 300 mV under aerated conditions, while E_H decreased to 100 mV on the root surface under near water‐saturated conditions. Gradients of pO₂ and pH from the root surface indicated an extent of the rhizosphere effect of 10–20 mm. In contrast, E_H gradients were observed from 0 to 2 mm from the root surface. We conclude that the rhizosphere extent differs for various parameters (pH, Eh, pO₂) and is strongly dependent on soil moisture.     

Towards a better understanding of carbon flow in the rhizosphere: a time-dependent approach using carbon-14

 Controversies exist in interpreting rhizosphere C flow obtained by different ¹⁴CO₂ labelling methods. However, there is a need for the standardisation of methods in order to be able to compare values obtained for different plants, different stages of development and different habitats. Perennial bromegrass (Bromus erectus Huds) grown in soils of different fertility was exposed to a ¹⁴CO₂ atmosphere for different periods of time: 1 h, 298 h and 78 days. The evolution of ¹⁴CO₂ in the soil was measured during and after labelling. The ¹⁴C contents of plant and rhizosphere compartments were then estimated. The time-sequence of the rate of ¹⁴CO₂ evolution after 1 h of labelling, indicated a maximum after around 20 h, followed by an exponential decrease. When expressed as a percentage of net ¹⁴C assimilation, root-soil respiration accounted for 14% and 18% in the nutrient-poor and nutrient-rich soils, respectively. Integration of the hourly values over several days showed that the dynamics of the evolution rate were similar for the 298-h and 78-day experiments, thus indicating that rhizosphere C flow was dominated by newly assimilated C. This was confirmed by the proportions of below-ground ¹⁴C, measured for roots, respiration and soil, which were not significantly affected by the labelling regime. The differences were, however, found to be significant between the two types of soils. The conclusion was that the conditions for plant growth during labelling were more important than the length of time of labelling, and that this explained the discrepancies in the literature-cited values. A succession of short-term ¹⁴C labelling of plants at different development stages followed by an allocation period of about 1 week is proposed to give a reliable estimation of the dynamics of C flow in the rhizosphere. Received: 7 June 1999     

The dynamics of oxygen concentration, pH value, and organic acids in the rhizosphere of Juncus spp.

A novel type of planar optodes for simultaneous optical analysis of pH and oxygen dynamics in the rhizosphere is introduced. The combination of the optical, non-invasive measurement of these parameters with sterile sampling of rhizosphere solution across and along growing roots by use of a novel type of rhizobox provides a methodical step forward in the investigation of the physicochemical dynamics of the rhizosphere and its underlying matter fluxes between roots and soil. In this study, this rhizobox was used to investigate the effect of oxygen releasing roots of three Juncus species on the amount and distribution of organic acids in reductive, oxygen-deficient soils of different pH (pH 3.9-pH 5.9). Pronounced diurnal variations of oxygen concentration and pH along the roots, particularly along the elongation zone were observed. Long-term records over more than eight weeks revealed considerable spatial and temporal patterns of oxygen over a range of almost 200 μmol O₂ L⁻¹ and pH dynamics of ±1.4 pH units in the rhizosphere. A strong effect of oxidative acidification due to oxygen release by the plant roots was clearly visible for Juncus effusus, whereas the roots of Juncus articulatus alkalinized the rhizosphere. In contrast, roots of Juncus inflexus induced no effects on rhizospheric pH. Only four different organic acids (oxalate, acetate, formate and lactate) were detectable in all soil solutions. Maximal concentration of all organic acids occurred at pH 3.9, whereas the lowest concentration of each organic acid was found at pH 5.9. Hence, considering the pH-dependence of the redox potential, the acid soil provided increased reductive conditions leading to slower anaerobic degradation of organic acids to CO₂ or methane (CH₄). The concentration of organic acids decreased by up to 58% within a distance of only 4 mm from the bulk soil to the root surface, i.e. reciprocal to the pronounced O₂-gradient. The decreasing presence of organic acids toward the oxygen releasing roots is possibly due to a change in the composition of the microbial community from anaerobic to aerobic conditions. The present study highlights the dynamic interplay between O₂ concentration, pH and organic acids as key parameters of the physicochemical environment of the rhizosphere, particularly for wetland plants growing in oxygen-deficient waterlogged soils.     

Growth,P uptake in grain legumes and changes in rhizosphere soil P pools

In soils with low P availability, several legumes have been shown to mobilise less labile P pools and a greater capacity to take up P than cereals. But there is little information about the size of various soil P pools in the rhizosphere of legumes in soil fertilised with P although P fertiliser is often added to legumes to improve N₂ fixation. The aim of this study was to compare the growth, P uptake and the changes in rhizosphere soil P pools in five grain legumes in a soil with added P. Nodulated chickpea (Cicer arietinum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.), yellow lupin (Lupinus luteus L.) and narrow-leafed lupin (Lupinus angustifolius L.) were grown in a loamy sand soil low in available P to which 80 mg P kg⁻¹ was added and harvested at flowering and maturity. At maturity, growth and P uptake decreased in the following order: faba bean > chickpea > narrow-leafed lupin > yellow lupin > white lupin. Compared to the unplanted soil, the depletion of labile P pools (resin P and NaHCO₃-P inorganic) was greatest in the rhizosphere of faba bean (54% and 39%). Of the less labile P pools, NaOH-P inorganic was depleted in the rhizosphere of faba bean while NaOH-P organic and residual P were most strongly depleted in the rhizosphere of white lupin. The results suggest that even in the presence of labile P, less labile P pools may be depleted in the rhizosphere of some legumes.     

Effects of algae and fertilizer-nitrogen on pH, Eh and depth of aerobic soil in laboratory columns of a flooded sandy loam

 A sandy loam was incubated under floodwater in the laboratory either in the dark or in the light (7 h day, 20  °C; 17 h night, 15  °C) and with four N sources [control, ammonium carbonate [(NH₄)₂CO₃], ammonium chloride (NH₄Cl), potassium nitrate (KNO₃)]. In the dark, floodwater pH rose steadily from 6.4 to about 7.5 over 60 days in the control, KNO₃ and (NH₄)₂CO₃ treatments, but with NH₄Cl pH decreased to 5.8. In the light, algal growth began to affect the floodwater pH after 9 days. At the end of the night, pH values were similar for all treatments to those kept in the dark. During the day, pH changes depended on the morning pH value: the daily increase was zero at pH 5.6 rising to a maximum of about 2 units at pH 6.3 and falling again at higher pH values. Changes in the carbonate equilibria in response to CO₂ removal by algal photosynthesis partly explain the results, but increasing inputs of acid are also implicated below pH 6.3 possibly due to reduced volatilization and increased nitrification. Redox potential (Eh) in the floodwater was little affected by N treatment until algal growth began. Eh then decreased each day as pH rose and recovered during the night. The daily decrease in Eh per unit increase in pH rose from about 10 to 90 mV pH^–1 over the incubation period. Initially, therefore, O₂ concentration must have been increased during the day by algal photosynthesis (values <59 mV pH^–1), but later O₂ concentration must have fallen, due possibly to the decomposition of algal cells. The presence of algae initially increased the depth of the aerobic soil layer, but eventually an algal mat settled on the soil surface acting as a zone of O₂ demand as the algae decomposed. Received: 7 July 1997     

Emissions of nitrous oxide from a constructed wetland using a groundfilter and macrophytes in waste-water purification of a dairy farm

 In less populated rural areas constructed wetlands with a groundfilter made out of the local soil mixed with peat and planted with common reed (Phragmites australis) are increasingly used to purify waste water. Particularly in the rhizosphere of the reed, nitrification and denitrification processes take place varying locally and temporally, and the question arises to what extent this type of waste-water treatment plant may contribute to the release of N₂O. In situ N₂O measurements were carried out in the two reed beds of the Friedelhausen dairy farm, Hesse, Germany, irrigated with the waste water from a cheese dairy and 70 local inhabitants (12 m³ waste water or 6 kg BOD₅ or 11 kg chemical O₂ demand (COD_Mn) day^–1). During November 1995 to March 1996, the release of N₂O was measured weekly at 1 m distances using eight open chambers and molecular-sieve traps to collect and absorb the emitted N₂O. Simultanously, the N₂O trapped in the soil, the soil temperature, as well as the concentrations of NH₄ ⁺-N, NO₃ ^–-N, NO₂ ^–-N, water-soluble C and the pH were determined at depths of 0–20, 20–40 and 40–60 cm. In the waste water from the in- and outflow the concentrations of COD_Mn, BOD₅, NH₄ ⁺-N, NO₃ ^–-N, NO₂ ^–-N, as well as the pH, were determined weekly. Highly varying amounts of N₂O were emitted at all measuring dates during the winter. Even at soil temperatures of –1.5  °C in 10 cm depth of soil or 2  °C at a depth of 50 cm, N₂O was released. The highest organic matter and N transformation rates were recorded in the upper 20 cm of soil and in the region closest to the outflow of the constructed wetland. Not until a freezing period of several weeks did the N₂O emissions drop drastically. During the period of decreasing temperatures less NO₃ ^–-N was formed in the soil, but the NH₄ ⁺-N concentrations increased. On average the constructed wetlands of Friedelhausen emitted about 15 mg N₂O-N inhabitant equivalent^–1 day^–1 during the winter period. Nitrification-denitrification processes rather than heterotrophic denitrification are assumed to be responsible for the N₂O production. Received: 28 October 1998     

Effects of growing roots of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) on rhizosphere soil solution chemistry

The chemical conditions of the rhizosphere can be very different from that of bulk soil. Up to now, little attention has been given to the problem of spatial heterogeneity and temporal dynamics of rhizosphere soil solution and little is known about the influence of different tree species on rhizosphere chemistry. In the present study, we used micro suction cups to collect soil solution from the rhizosphere of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) seedlings in high spatial resolution and capillary electrophoresis for the determination of major cations and anions. The results indicate, that in a soil with a base saturation of about 20—25% and a pH of 6.5, growing roots of beech and spruce lower the concentrations of nutrient cations and nitrate in the rhizosphere soil solution and decrease significantly the pH. The H⁺ release leads to an enhanced mineral weathering as indicated by an increase of CEC and base saturation and to a mobilization of soluble Al, however, on a very low concentration level. In our experiment rhizosphere effects of spruce have been more pronounced than those of beech, indicating, that with respect to below ground activity young spruce trees have a better competitive power than beech.     

Effect of low pO2 on colonization of maize roots by a genetically altered Pseudomonas putida [PH6(L1019)]

Among the factors which may affect colonization of roots by soil bacteria is that of rhizosphere oxygen partial pressure (pO₂). The oxygen concentration in the root zone influences both microbes and roots. Roots exposed to low pO₂, as might occur during flooding and waterlogging of the soil, become more leaky and loss of soluble carbon increases. To determine whether periods of low pO₂ increased root colonization by a genetically altered pseudomonad we inoculated 3- to 4-week-old maize plants, grown in soil and transferred to a hydroponic system or grown in fritted clay, with Pseudomonas putida PH6(L1019)(lacZY+) following exposure of the roots to air or cylinder N₂. Numbers of heterotrophs and the marked pseudomonad were determined by dilution plating. Low pO₂ generally increased the numbers of bacteria associated with roots exposed to the treatments in solution or in undisturbed fritted clay rooting medium. Under low pO₂ in a hydroponic system, roots of intact maize plants tended also to have higher soluble organic C and hexose (anthrone-detectable sugars) than roots exposed to air. The effect of low pO₂ was most pronounced in the fritted clay where low pO₂ favored colonization by the marked strain; numbers were 3- to 96-fold greater than those on roots flushed with air but accounted for only 0.06–0.61% of the total population. Roots exposed to low pO₂ tended to accumulate more C. Results suggest that in the fritted clay, the pseudomonad was able to exploit the increased C supply and to achieve greater numbers on roots exposed to low pO₂, whereas the dilution of carbon released from roots in the hydroponic apparatus did not allow for the same magnitude of increase on roots. Received: 2 December 1996     

Kinetics of nitric oxide consumption in tropical soils under oxic and anoxic conditions

The kinetics of nitric oxide consumption in four tropical soils were studied under oxic and anoxic conditions in a flow-through system in the laboratory. Under anoxic conditions the soils had a very high affinity for NO, resulting in K _M values of 0.02–0.27 ppmv NO (equivalent to 0.04–0.50 nM NO in the aqueous phase). These K _M values were lower than literature values for NO consumption by denitrifying bacteria. Under oxic conditions the kinetics of NO consumption in the tropical soils were completely different, exhibiting K _M values higher than 1.7 ppmv. These higher K _M values were similar to literature values for NO consumption by aerobic heterotrophic bacteria. Thus, the tropical soils studied seem to contain two different NO consumption activities which can be distinguished by their kinetics and which predominate under aerobic and anaerobic conditions, respectively. However, it was not possible to quantify the contribution of each process to total NO consumption under natural conditions. Under aerobic conditions NO turnover kinetics were positively correlated with soil respiration, N mineralisation and soil organic carbon, whereas under anaerobic conditions they were positively correlated with potential and actual denitrification rates and pH. Received: 26 September 1996     

Phosphorus uptake and rhizosphere properties of intercropped and monocropped maize, faba bean, and white lupin in acidic soil

Little information is available on phosphorus (P) uptake and rhizosphere processes in maize (Zea mays L.), faba bean (Vicia faba L.), and white lupin (Lupinus albus L.) when intercropped or grown alone in acidic soil. We studied P uptake and soil pH, carboxylate concentration, and microbial community structure in the rhizosphere of maize, faba bean, and white lupin in an acidic soil with 0–250 mg P (kg⁻¹ soil) as KH₂PO₄ (KP) or FePO₄ (FeP) with species grown alone or intercropped. All plant species increased the pH compared to unplanted control, particularly faba bean. High KP supply (>100 mg P kg⁻¹) significantly increased carboxylate concentration in the rhizosphere of maize. The carboxylate composition of the rhizosphere soil of maize and white lupin was significantly affected by P form (KP or FeP), whereas, this was not the case for faba bean. In maize, the carboxylate composition of the rhizosphere soil differed significantly between intercropping and monocropping. Yield and P uptake were similar in monocropping and intercropping. Monocropped faba bean had a greater concentration of phospholipid fatty acids in the rhizosphere than that in intercropping. Intercropping changed the microbial community structure in faba bean but not in the other corps. The results show that P supply and P form, as well as intercropping can affect carboxylate concentration and microbial community composition in the rhizosphere, but that the effect is plant species-specific. In contrast to previous studies in alkaline soils, intercropping of maize with legumes did not result in increased maize growth suggesting that the legumes did not increase P availability to maize in this acidic soil.     

Effects of enhanced supplies of nitrogen and sulphur on rhizosphere and soil chemistry in a Norway spruce stand in SW Sweden

Rhizophere and bulk soil chemistry were investigated in a Norway spruce stand in SW Sweden. The rhizosphere and bulk soil chemistry in water extracts in control plots (C) and plots repeatedly treated with ammonium sulphate (NS) were compared. Treatment regime was started in 1988. Cylindrical core samples of the LFH-layer and mineral soil layers were collected in 1992 and used for water extract analyses. Samples of soil from LFH-layer and mineral soil layers were taken in 1991 and 1993 for determination of CEC and base saturation. Soil pH and NH₄-N, NO₃-N and SO₄-S, Al, Ca, K and Mg concentrations in water extracts were measured for rhizosphere and bulk soils. The pH-values of bulk and rhizosphere soils in NS plots decreased compared with those in control plots, whereas concentrations of NH₄-N, NO₃-N, SO₄-S, base cations and Al in water extract increased. In both bulk and rhizosphere soils the concentration of NH₄-N was much higher than that of NO₃-N. A significant difference in the pH and Mg concentration of bulk and rhizosphere soil between the treated and control plots was found only in the 0–10 cm layer. For all layers, there was a significant difference in NH₄-N concentrations in the bulk and rhizosphere soil between the NS treatment and control plots. Concentrations of exchangeable base cations and the base saturation level in the LFH-layer decreased in the NS plots. The concentration of extractable SO₄-S increased in the NS plots. The NS treatment enhanced the amount of litter in L-layer, owing to increases in needle biomass and litterfall but led to losses of base cations, mainly K and Mg, from LFH-layer. It was concluded that the NS treatment displaced cations from exchangeable sites in the LFH-layer leading to higher concentrations of these elements in both rhizosphere and bulk soil.     

The effect of soil liming on shoot development, root growth, and cluster root activity of white lupin

 The effects of a limed soil upon root and shoot growth of white lupin (Lupinus albus L.) were investigated using soil tubes and pots. After 75 days in the soil tubes, the combined taproot and lateral root dry weight in limed soil (2.5% CaO w/w) was significantly less than in neutral pH soil (by 57%). However, the dry weight and numbers of cluster roots remained comparable between the treatments, demonstrating for the first time that the cluster roots respond differently to the rest of the root system. Cluster roots accounted for 17% of the total root biomass in neutral soil, increasing significantly to over 30% in limed soil. When grown for 43 days in pots containing soil with different additions of lime (0.5–2.5% CaO w/w), soil citrate concentrations were higher than in the neutral pH soil treatment in all except the 2.5% lime treatment, in which they were lower. In both experiments, shoot dry weights were lower in the presence of the limed soil compared with those in the neutral pH soil. Although a reduction in shoot dry weight was not apparent at 21 days in the limed-soil tubes, the initiation of fewer mainstem leaf primordia indicated a slower shoot development than occurred in the neutral soil. Plants grown in the limed-soil tubes showed leaf yellowing and some chlorosis within 9 days. At the final harvest, the shoot phosphorus and manganese concentrations were significantly lower in plants grown in limed soil compared with those in the neutral pH soil, whereas the concentration of calcium was higher. Received: 11 October 1999     

Survival and cell culturability of biocontrol Pseudomonas fluorescens CHA0 in the rhizosphere of cucumber grown in two soils of contrasting fertility status

 The effect of cucumber roots on survival patterns of the biocontrol soil inoculant Pseudomonas fluorescens CHA0-Rif was assessed for 22 days in two non-sterile soils, using a combination of total immunofluorescence cell counts, Kogure's direct viable counts and colony counts on plates containing rifampicin. In Eschikon soil (high fertility status for cucumber), CHA0-Rif persisted as culturable cells in bulk soil and in the rhizosphere, but colony counts were lower than viable counts and total cell counts inside root tissues. The occurrence of viable but non-culturable (VBNC) cells inside root tissues (5 log cells g^–1 root) was unlikely to have resulted from the hydrogen peroxide treatment used to disinfect the root surface, as hydrogen peroxide caused the death of CHA0-Rif cells in vitro. In Siglistorf soil (low fertility status for cucumber), the inoculant was found mostly as non-culturable cells. Colony counts and viable counts of CHA0-Rif were similar, both in bulk soil and inside root tissues, whereas in the rhizosphere viable counts exceeded colony counts at the last two samplings (giving about 7 log VBNC cells g^–1). In conclusion, soil type had a significant influence on the occurrence of VBNC cells of CHA0-Rif, although these cells were found in root-associated habitats (i.e. rhizosphere and root tissues) and not in bulk soil. Received: 12 November 1999     

Der Einfluß von Bodenart,Durchlüftung des Bodens,N-Ernährung und Rhizosphärenflora auf die Morphologie des seminalen Wurzelsystems von Mais

Influence of soil type, soil aeration, nitrogen supply and rhizosphere flora on the morphology of the seminal root system of maize The influence of the soil type (quartz sand – humous loamy sandy soil), soil aeration, nitrogen supply and rhizosphere flora on the morphology of the seminal root system of maize plants grown in pot culture was investigated. The morphological parameters of number, length, diameter and root hair formation (both length and density) of the main and the lateral roots were determined in addition to the total root length and number and the lateral root density. 1. The biomass production of the shoot and root system was nearly identical in both soils. The total root length growth, however, was enhanced in the sandy soil due to the stimulated formation of first order lateral roots. This increase was correlated with a decrease in the mean diameter and root hair length of the main and lateral roots. 2. A decreased O₂-supply to the soil resulted in a drastic reduction of root biomass, which was correlated, however, with a (relative) increase in total root length (due to the stimulation of the length growth of the first order lateral roots). The root hair length, on the other hand, was reduced under O₂-deficiency. 3. Reduced N-supply resulted in a decrease of the shoot/root-ratio with both substrates which could be ascribed to the enhanced formation and length of the first order lateral roots. 4. The presence of soil microorganisms in quartz sand culture resulted in a reduction of shoot biomass. In comparison with the sterile control culture the total length of the main roots was retarded, the main and lateral roots were more slender and root hair formation was reduced. 5. The experimental results show that the lateral root system demonstrates a significantly greater plasticity than does the main root system.     

Chemical and biological characteristics of alkaline saline soils from the former Lake Texcoco as affected by artificial drainage

 Soils from the former Lake Texcoco are alkaline saline and were artificially drained and irrigated with sewage effluents since the late 1980s. Undrained soil and soil drained for 1, 5 and 8 years were sampled, characterized and incubated aerobically for 90 days at 22±1  °C while production of CO₂, available P and concentrations of NH₄ ⁺, NO₂ ^– and NO₃ ^– were monitored. Artificial drainage decreased pH_H2O, water holding capacity, organic C, total N, and Na⁺, K⁺, Mg²⁺, B, Cl^– and SO₄ ^2– concentrations, increased inorganic C and Ca²⁺ concentrations more than 5-fold while total P was not affected. Microbial biomass C decreased with increased length of drainage but bacteria, actinomycetes, denitrifiers and cellulose-utilizing bacteria tended to show opposite trends. CO₂ production was less in soils drained ≥5 years compared to undrained soil but more than in soils drained for 1 year. Emission of NH₃ was negligible and concentrations of NH₄ ⁺ remained constant over time in each soil. Nitrification, as witnessed by increases in NO₃ ^– concentrations, occurred in soil drained for 8 years. NO₂ ^– concentrations decreased in soils drained ≤1 year in the first 7 days of the incubation and remained constant thereafter. It was found that artificial drainage of soils from the former Lake Texcoco profoundly affected soil characteristics. Decreases in pH and Na⁺, K⁺, Cl^– and SO₄ ^2– concentrations made conditions more favourable for plant growth, although low concentrations of inorganic N and available P might be limiting factors. Received: 1 December 1999     

Methanotrophic bacteria in the rhizosphere of rice microcosms and their effect on porewater methane concentration and methane emission

CH₄ emission from irrigated rice field is one of the major sources in the global budget of atmoshperic CH₄. Rates of CH₄ emission depend on both CH₄ production in anoxic parts of the soil and on CH₄ oxidation at oxic-anoxic interfaces. In the present study we used planted and unplanted rice microcosms and characterized them by numbers of CH₄-oxidizing bacteria (MOB), porewater CH₄ and O₂ concentrations and CH₄ fluxes. Plant roots had a stimulating effect on both the number of total soil bacteria and CH₄-oxidizing bacteria as determined by fluorescein isothiocyanate fluorescent staining and the most probable number technique, respectively. In the rhizosphere and on the root surface CH₄-oxidizing bacteria were enriched during the growth period of tice, while their numbers remained constant in unplanted soils. In the presence of rice plants, the porewater CH₄ concentration was significantly lower, with 0.1–0.4mM CH₄, than in unplanted microcosms, with 0.5–0.7mM CH₄. O₂ was detected at depths of up to 16 mm in planted microcosms, whereas it had disappeared at a depth of 2 mm in the unplanted experiments. CH₄ oxidation was determined as the difference between the CH₄ emission rates under oxic (air) and anoxic (N₂) headspace, and by inhibition experiments with C₂H₂. Flux measurements showed varying oxic emission rates of between 2.5 and 29.0 mmol CH₄m^-2 day^-1. An average of 34% of the anoxically emitted CH₄ was oxidized in the planted microcosms, which was surprisingly constant. The rice rhizosphere appeared to be an important oxic-anoxic interface, significantly reducing CH₄ emission.     

Isoelectric focusing of β-glucosidase humic-bound activity in semi-arid Mediterranean soils under management practices

Changes in β-glucosidase enzyme–humic complexes and conventional parameters (pH, total organic C, total N, water-soluble C, and bulk density) were studied in an almond-cropped soil prone to erosion under a rehabilitation practice. The experimental plan included three soil slopes (0%, 2%, and 6%) and two type of fertilization (organic and mineral), with sampling of rhizosphere and inter-row soils. The enzyme humic complexes were extracted by pyrophosphate, purified by ultrafiltration of the organic extracts on molecular mass exclusion membranes (mol wt > 10⁴) and fractionated by isoelectric focusing technique (IEF). The IEF on polyacrylamide rod gels with a restricted pH gradient ranging between 6.0 and 4.0 gave five humic bands on the basis of the little differences of their electric charges (pI). Under both organic and mineral fertilization, β-glucosidase activity bound to the fractionated humic substances, especially in the pH range 4.5–4.2 of the rhizosphere soil, was higher than that of the inter-row soil. This also occurred in 6% slope where the enzyme activity was lower than in soil with lower slopes. The higher number of the β-glucosidase active humic bands in rhizosphere than inter-row soil, particularly for the 0% slope, may be due to the presence of humic molecules capable of preserving the enzyme molecules in the active form, other than to the higher microbial activity synthesizing and releasing the tested enzymes.     

The short-term cover crops increase soil labile organic carbon in southeastern Australia

Little information is available about the effects of cover crops on soil labile organic carbon (C), especially in Australia. In this study, two cover crop species, i.e., wheat and Saia oat, were broadcast-seeded in May 2009 and then crop biomass was crimp-rolled onto the soil surface at anthesis in October 2009 in southeastern Australia. Soil and crop residue samples were taken in December 2009 to investigate the short-term effects of cover crops on soil pH, moisture, NH₄⁺–N, NO₃⁻–N, soluble organic C and nitrogen (N), total organic C and N, and C mineralization in comparison with a nil-crop control (CK). The soil is a Chromic Luvisol according to the FAO classification with 48.4 ± 2.2% sand, 19.5 ± 2.1% silt, and 32.1 ± 2.1% clay. An exponential model fitting was employed to assess soil potentially labile organic C (C ₀) and easily decomposable organic C for all treatments based on 46-day incubations. The results showed that crop residue biomass significantly decreased over the course of 2-month decomposition. The cover crop treatments had significantly higher soil pH, soluble organic C and N, cumulative CO₂–C, C ₀, and easily decomposable organic C, but significantly lower NO₃⁻–N than the CK. However, no significant differences were found in soil moisture, NH₄⁺–N, and total organic C and N contents among the treatments. Our results indicated that the short-term cover crops increased soil labile organic C pools, which might have implications for local agricultural ecosystem managements in this region.