Response of Mucuna pruriens to symbiotic nitrogen fixation by rhizobia following inoculation in farmers' fields in the derived savanna of Benin

 Leguminous cover crops such as Mucuna pruriens (mucuna) have the potential to contribute to soil N and increase the yields of subsequent or associated cereal crops through symbiotic N fixation. It has often been assumed that mucuna will freely nodulate, fix N₂ and therefore contribute to soil N. However, results of recent work have indicated mucuna's failure to nodulate in some farmers' fields in the derived savanna in Benin. One of the management practices that can help to improve mucuna establishment and growth is the use of rhizobial inocula to ensure compatibility between the symbiotic partners. Experiments were conducted in 1995 and 1996 on 15 farmers' fields located in three different villages (Eglimé, Zouzouvou and Tchi) in the derived savanna in Benin. The aim was to determine the response of mucuna to inoculation and examine the factors affecting it when grown in relay cropping with maize. The actual amount of N₂ fixed by mucuna in the farmers' fields at 20 weeks after planting (WAP) averaged 60 kg N ha^–1 (range: 41–76 kg N ha^–1) representing 55% (range: 49–58%) of the plant total N. The result suggested that mucuna in these farmers' fields could not meet its total N demand for growth and seed production only by N₂ fixation. It was estimated that after grain removal mucuna led to a net N contribution ranging from –37 to 30 kg N ha^–1. Shoot dry weight at 20 WAP varied between 1.5 and 8.7 t ha^–1 and N accumulation ranged from 22 to 193 kg N ha^–1. Inoculation increased shoot dry matter by an average of 28% above the uninoculated treatments, but the increase depended on the field, location and year. For the combinations of inoculated treatments and farmers' fields, the response frequency was higher in Eglimé and Tchi than in Zouzouvou. The response to inoculated treatments was dependent on the field and inversely related to the numbers of rhizobia in the soil. Soil rhizobial populations ranged from 0 to >188 cells g^–1 soil, and response to inoculation often occurred when numbers of indigenous rhizobia were <5 cells g^–1 soil. In two farmers' fields at Zouzouvou where extractable P was below 10 μg g^–1 soil, mucuna did not respond to rhizobial inoculation despite a higher population of rhizobia. Significant relationships between mycorrhizal colonization, growth and nodulation of mucuna were observed, and inoculated plants with rhizobia had a higher rate of colonization by arbuscular mycorrhizal fungi (%AMF) than uninoculated ones. Therefore, it was shown that mucuna will establish and fix N₂ effectively in those fields where farmer's management practices such as good crop rotation and rhizobial inoculation allow a build up of AMF spores that might lead to a high degree of AMF infection and alleviate P deficiency. Received: 14 June 1999     

Time-course of dinitrogen fixation of promiscuous soybean cultivars measured by the isotope dilution method

 Soybean cultivars capable of nodulating with indigenous Bradyrhizobium spp. have been developed by the International Institute of Tropical Agriculture (IITA) and national programs in Africa in order to avoid artificial inoculation by resource-poor farmers in Africa. The current selection procedure for enhanced N₂ fixation is based on an assessment of nodule formation which does not directly quantify the proportions of crop N derived from the atmosphere. We have monitored N accumulation patterns and N₂ fixation in nine promiscuous soybean cultivars with different maturity periods, using the ¹⁵N dilution technique. Nodule development generally peaked at the early podfill stage for all cultivars except Tgx 1519-1D and Tgx 1447-2D in which it continued to increase. The proportion of crop N derived from fixation (%NDFA) ranged between 51% and 67%, 77% and 84%, and 66% and 73% at full bloom, early podfill, and physiological maturity stages, respectively. Total N accumulation increased in all soybean genotypes with increasing plant age. Significant correlations (P<0.001) were established between nodule weight and %NDFA, even though this did not explain the relationship between nodule development and N₂ fixation in cultivars such as Tgx 1519-1D. Promiscuous soybean cultivars retained between 10% and 19% of total N accumulated at the final harvest, in belowground biomass. Our results indicated that these soybean cultivars can derive substantial proportions of plant N from N₂ fixation in soils where compatible indigenous bradyrhizobia populations are adequate and effective. Also, we have substantiated the claims that qualitative nodulation parameters currently used to select varieties with a high N₂ fixation capacity need to be validated with other measurements of N₂ fixation. Received: 5 November 1998     

Effects of trefoil cover crop and earthworm inoculation on maize crop and soil organisms in Reunion Island

 Traditional tree fallows have been abandoned on the western coast of the Reunion Island because of the increasing need for cultivated land. Soil fertility is no longer restored and crop yields have decreased drastically. The leguminous plant, Lotus uliginosus (trefoil), used as a cover crop, has made possible the control of erosion, the restoration of soil macrofauna, especially earthworms, and the increase in crop yields. When trefoil was associated with earthworms (Amynthas corticis), the densities of maize, the yields of maize stalk and dry matter, the yield of trefoil fodder dry matter, and the biomass and respiratory activity of soil microflora were considerably increased. The combined effects of their association led to a significant decrease in populations of the plant-parasitic nematode, Pratylenchus vulnus, in maize roots, and in the population of borers. Some soil chemical features were modified. Received: 10 September 1997     

Effects of fungicides on arbuscular mycorrhizal fungi: differential responses in alkaline phosphatase activity of external and internal hyphae

 The effect of four fungicides on alkaline phosphatase (ALP) activity in internal and external hyphae of an arbuscular mycorrhizal (AM) fungus was examined. Four-week-old plants were treated with an aqueous solution of the fungicides and then harvested 3 days later. Fungicides were applied at a low and a high level corresponding to 1 and 100 times the recommended field application dose. The fungicides were the two abundantly used sterol inhibitors, fenpropimorph and propiconazole, as well as a commercial mixture of them. Benomyl served as a positive control, as this fungicide is known for its deleterious effect on AM fungi. At harvest, roots and hyphae extracted from the soil were stained for ALP activity. Benomyl inhibited fungal ALP activity of both internal and external hyphae at the low application level (1 μg g^–1 soil) corresponding to the recommended field dose. Fenpropimorph had an intermediate effect, inhibiting both internal and external hyphae but only at the high application level (125 μg g^–1 soil). Propiconazole decreased the activity of the external hyphae at the low application level (0.21 μg g^–1 soil) but did not affect the internal activity at any application level. The effect of Tilt Top was similar to that of fenpropimorph. The results showed that the external hyphae were more sensitive than internal hyphae to application of fungicides. Received: 26 May 1999     

Response of maize (Zea mays) to the application of foliar fertilizers in the Sudan and Guinea savanna zone of Nigeria

Soil fertility problems resulting in low maize yields in smallholder farms are common in the West African moist savanna. The effectiveness of commercial foliar fertilizers in improving maize growth and yield was evaluated in three savanna agro‐ecological zones of Nigeria in two steps. In step one, eight commercial foliar fertilizers were assessed in a greenhouse study with two soil types using maize (Zea mays L. cv. 2004 TZE‐Y POPDT STR C4). The treatments included a control and a reference that received the optimum concentrations of nutrients. In step 2, three promising products from the greenhouse study (Turbotop, Agroleaf General, and Agroleaf high‐P) were evaluated under field conditions to validate the efficacy of products to enhance crop growth and yield. The foliar products were applied at the rate of 5 kg ha^?1. The treatments also included three rates of P application (0, 30, and 60 kg P ha^?1) as triple super phosphate (TSP) with or without foliar fertilizers. In the greenhouse study, differences in maize shoot dry matter yield and N and P concentrations, attributable to the spraying of the commercial foliar fertilizers, were observed for both soils. Spraying Turbotop, Agroleaf General, and Agroleaf high‐P gave the highest shoot dry biomass and N and P uptake compared to other products. Under field conditions, foliar spraying of Agroleaf high‐P significantly increased the shoot dry biomass of maize compared with the 0 P treatment in all locations. The grain yield of maize ranged from 1 to 4 t ha^?1 with significant differences across sites. Products with high concentrations of P and N in their formulation improved maize yield suggesting that appropriate management of P and N resources is a prerequisite for a sustainable maize intensification in the savanna agro‐ecologies.     

Influence of organic manures on arbuscular mycorrhizal fungi associated with Vigna unguiculata (L.) Walp. in relation to tissue nutrients and soluble carbohydrate in roots under field conditions

 The interaction of plant nutrients, root-soluble carbohydrate availability and arbuscular mycorrhizal (AM) fungi was examined in field grown cowpea [Vigna unguiculata (L.) Walp.]. Plant nutrients were altered through application of farmyard (cow dung, sheep manure) and green (sunnhemp, pongamia) manures. Organic amendments increased plant growth, AM fungal colonization, soluble carbohydrate concentration in roots, and spore numbers. Percent total colonization, root length with vesicles and spore numbers in soil were negatively correlated with the concentration of soluble carbohydrates within roots, which in turn were related to tissue nutrient levels. However, a positive correlation existed between soluble carbohydrate concentrations within root and root length with arbuscules. But the mycorrhizal parameters were related more to plant nutrient level and their ratios, indicating that tissue nutrients have another level of control in addition to their effect on soluble carbohydrate concentration in roots. Increased AM colonization due to organic amendment significantly reduced nutrient imbalances. The strong relationship between colonization and root-soluble carbohydrate concentration levels validates the basic assumption that mycorrhizal fungi act as a 'strong sink' for photosynthates. This study indicates that the host influences AM colonization by regulating the formation of AM fungal structures and spore formation via availability of root carbohydrates. Received: 15 January 1999     

Arbuscular mycorrhizal fungi improve the growth and phosphorus uptake of mung bean plants fertilized with composted rock phosphate fed dung in alkaline soil environment

Abstract

Inoculation effect of arbuscular mycorrhizal fungi (AMF) on phosphorus (P) transfer from composted dung of cattle with a diet supplemented with powdered rock phosphate (RP) and their successive uptake by mung bean plants was assessed in alkaline soil. The efficacy of composted RP fed dung alone or/and in combination with AMF inoculums containing six different species were compared with SSP in six replicates per treatment in pots. The results showed that the association of AMF with composted RP fed dung had a positive effect on mung bean shoot (3.04?g) and root (2.62?g) biomass, chlorophyll (a, b), carotenoid contents and N (58.38?mg plant^?1) and P (4.61?mg plant^?1) uptake. Similarly, the percent roots colonization (56%) and nodulation of mung bean plant roots and their post-harvest soil properties were also improved by the inoculation of AMF together with composted RP fed dung. It is concluded that the combined application of AMF with composted RP fed dung has almost the same effect as SSP for improving mung bean plants growth and their nutrients uptake. Moreover, AMF inoculants can be used as a suitable biofertilizer in combination with locally available organic sources of fertilizers for improving P status and growth of plants in alkaline soils.     

Urease activity and its relation to soil organic matter, microbial biomass nitrogen and urea-nitrogen assimilation by maize in a Brazilian Oxisol under no-tillage and tillage systems

 We studied the relationship between urease activity (UA) and soil organic matter (SOM), microbial biomass N (N_biom) content, and urea-N fertilizer assimilation by maize in a Dark Red Latosol (Typic Haplustox) cultivated for 9 years under no-tillage (NT), tillage with a disc plough (DP), and tillage with a moldboard plough (MP). Two soil depths were sampled (0–7.5 cm and 7.5–15 cm) at 4 different times during the crop cycle. Urea was applied at four different rates, ranging from 0 to 240 kg N ha^–1. The levels of fertilizer N did not affect the UA, SOM content, and N_biom content. No significant difference between the treatments (NT, DP, and MP) was observed for SOM during the experiment, probably because the major part of the SOM was in recalcitrant pools, since the area was previously cultivated (conventional tillage) for 20 years. The N_biom content explained 97% and 69% of the variation in UA in the upper and deeper soil layer, respectively. UA and biomass N were significantly higher in the NT system compared to the DP and MP systems. The highest maize productivity and urea-N recovery was also observed for the NT system. We observed that the increase in urea-N losses under NT, possibly as a consequence of a higher UA, was compensated for by the increase in N immobilized in the biomass. Received: 2 July 1999     

Arbuscular mycorrhizal fungi inoculation and exogenous indole-3-acetic acid application induce antioxidant defense response to alleviate cadmium toxicity in Broussonetia papyrifera

Cadmium (Cd) contamination in soil poses a huge threat to plants even at low concentrations; Broussonetia papyrifera has great potential in remediation of soil heavy metal contamination. However, whether exogenous indole-3-acetic acid (IAA) application and arbuscular mycorrhizal fungi (AMF) have synergistic effects on Cd tolerance of B. papyrifera remains unclear. To investigate the effects of AMF inoculation and IAA application on the tolerance of B. papyrifera to Cd stress, two experiments were conducted: the first to investigate the effect of AMF (Rhizophagus irregularis) inoculation on the tolerance of B. papyrifera to Cd stress and the second to investigate the combined effects of AMF inoculation and IAA application on the tolerance of B. papyrifera to Cd stress. Parameters including endogenous hormone concentration, antioxidant defense response, malondialdehyde (MDA) content, and gene expression related to antioxidant enzyme system and hormone were measured. The results indicated that AMF alleviated Cd toxicity of B. papyrifera by reducing MDA content and improving antioxidant enzyme activities and Cd absorption capacity. Furthermore, the combination of AMF inoculation and IAA application had a synergetic effect on the tolerance of B. papyrifera to Cd stress through upregulating BpAUX1 and BpAUX2, which might contribute to root growth and root xylem synthesis, and by upregulating BpSOD2 and BpPOD34 to enhance the antioxidant enzyme system. This work provides a new insight into the application of IAA in the remediation of soil Cd pollution by mycorrhizal plants.     

Soil microbial biomass and mineralization of carbon and nitrogen in ecological, integrated and conventional forage and arable cropping systems

 In a cropping systems experiment in southeastern Norway, ecological (ECO), integrated (INT) and conventional (CON) forage (FORAGE) and arable (ARABLE) model farms were compared. After 5 experimental years, topsoil was sampled in spring from spring grain plots and incubated for 449 days at controlled temperature (15  °C) and moisture content (50% water-holding capacity). There were no detectable differences between model farms in terms of total soil C or N. For INT and CON, however, values of microbial biomass C and N, microbial quotient (C_mic/C_org), and C and N mineralization were, or tended to be, higher for FORAGE than for ARABLE. For the ECO treatment, values were similar for FORAGE and ARABLE and did not differ significantly from that of CON-FORAGE. For INT and CON, the metabolic quotient (qCO₂) was lower for FORAGE than for ARABLE. Again, for the ECO treatment, values were similar for FORAGE and ARABLE and did not differ significantly from that of CON-FORAGE. We estimated the sizes of conceptual soil organic matter pools by fitting a decomposition model to biomass and mineralization data. This resulted in a 48% larger estimate for CON-FORAGE than for CON-ARABLE of physically protected biomass C. For physically protected organic C the difference was 42%. Moreover, the stability of soil aggregates against artificial rainfall was substantially greater for CON-FORAGE than for CON-ARABLE. On this basis, we hypothesized that the lower qCO₂ values in the FORAGE soils were mainly caused by a smaller proportion of active biomass due to enclosure of microorganisms within aggregates. Altogether, our results indicated a poorer inherent soil fertility in ARABLE than in FORAGE rotations, but the difference was small or absent in the ECO system, probably owing to the use of animal and green manures and reduced tillage intensity in the ECO-ARABLE rotation. Received: 28 October 1998     

The number of bacteria in the rhizosphere during plant development: relating colony-forming units to different reference units

 The number of bacteria was determined during the growth of chrysanthemum plants on young (tip) and old (base) root parts. We assessed if the same conclusions could be drawn on the dynamics of bacterial populations during plant development when different reference units were used to express the bacterial counts. The results indicated that the total number of bacteria on the base decreased significantly during plant development, when expressed per root length, per root fresh weight or per root surface. The number of bacteria on the tip only decreased significantly when expressed per root length. Using the unit of dry weight of adhering soil, contradictory results were obtained for both base and tip; in general, the number of bacteria increased significantly during plant development. Thus, different reference units may lead to different conclusions. Root surface seemed to be the best unit to use, but the use of this unit requires time-consuming measurements. Regression analyses indicated that the reference unit "root surface" was highly correlated with root fresh weight (R ²=93%). Thus, once this relation is determined, the less time-consuming unit can be measured in the experimental work. To analyse the data, the colony-forming units should be expressed per root surface. Besides bacterial numbers during plant development, we assessed whether the bacterial populations collected showed different growth rates on agar plates. The growth rates of bacteria from the tip and base and different development stages of the plants showed differences, indicating differences in the metabolic state of the collected populations. Received: 9 December 1997     

Response of field-grown bean (Phaseolus vulgaris L.) to Rhizobium inoculation and nitrogen fertilization in two Cerrados soils

 Most soils sown with field beans (Phaseolus vulgaris L.) contain indigenous rhizobia which might interfere with the establishment of inoculated strains. As a consequence, the benefits of bean inoculation are usually questioned, and the use of N fertilizer is gradually becoming a common practice. The present study had the objective of evaluating the effectiveness of inoculation and N fertilization in field soil with (site 1) and without (site 2) a previous bean-cropping history. At site 1, which had a rhizobial population of 7×10² cells g^–1 soil, inoculation had no effect on nodulation or yield, whereas at site 2 (<10 cells g^–1 soil) inoculation increased nodulation, nodule occupancy by the inoculated strain and grain yield. N fertilizer decreased nodulation at both sites, but increased grain yield at site 1 but not at site 2, indicating that the response to inoculation and N fertilization depends on the cropping history. When bean was cultivated for the first time, indigenous populations of rhizobia were low and high yields were accomplished solely with seed inoculation, with no further response to N fertilizer. In contrast, previous cultivation of bean increases soil rhizobia, preventing nodule formation by inoculated strains, and N fertilizer may be necessary for maximum yields. A significant interaction effect between N fertilizer and inoculation was detected for serogroup distribution only at site 2, with N fertilizer decreasing nodule occupancy by the inoculated strain and increasing the occurrence of indigenous strains. Consequently, although no benefits were obtained by the combination of inoculation and N fertilizer, this practice may be feasible with the selection of appropriate N-tolerant strains from the indigenous rhizobial population. Received: 26 May 1999     

Arbuscular mycorrhizae in a long-term field trial comparing low-input (organic, biological) and high-input (conventional) farming systems in a crop rotation

 Arbuscular mycorrhizal (AM) root colonization was studied in a long-term field trial in which four farming systems currently in use in Switzerland were continuously applied to a randomized set of plots at a single field site from 1978 till 1993. There were two low-input farming systems (organic and bio-dynamic) and two high-input (conventional) farming systems (according to Swiss guidelines of integrated plant production with and without farmyard manure). The systems had an identical 7-year crop rotation and tillage scheme and differed essentially only in the amount and type of fertilizer supplied and in plant protection management. The percentage of root colonization by AM fungi was determined in field samples 2–3 times over the growing season in crops in the rotation, namely in winter wheat (Triticum aestivum L. cv. Sardona), vetch-rye and grass-clover. We found the percentage of root length colonized by AM fungi to be 30–60% higher (P≤0.05) in the plants grown in soils from the low-input farming systems than in those grown in conventionally farmed soils. Approximately 50% of the variation of AM root colonization was explained by chemical properties of the soils (pH, soluble P and K, exchangeable Mg), the effect of soluble soil P being most pronounced. The potential of the field soils from the differently managed plots to cause symbiosis with AM fungi was tested in a glasshouse experiment, using wheat as a host plant. Soils from the low-input farming systems had a greatly enhanced capacity to initiate AM symbiosis. The relative differences in this capacity remained similar when propagules of the AM fungus Glomus mosseae were experimentally added to the soils, although overall root colonization by AM fungi was 2.8 times higher. Received: 27 August 1999     

Earthworm activity and soil structure changes due to organic enrichments in vineyard systems

 The effect of organic enrichment on earthworm activity and soil structure was studied in two French vineyards, by comparing control and test plots. In each vineyard the organic matter quantitatively increased the abundance and biomass of the earthworm community. These increases were associated with a higher level of species diversity and a higher evenness corresponding to the development of endogeic community. These earthworm community changes were associated with an increase in granular bioturbated areas and in macroporosity in the top soil layer. The micromorphological approach incorporated an original process of image analysis which appeared to be an appropriate method for characterizing pore morphology in this study. The pores when characterized by their size and shape could be related to ecological groups and growth stages of earthworms. Received: 4 August 1997     

Accumulation of zinc and its effects on the growth, reproduction and life cycle of Drawida willsi (Oligochaeta), a dominant earthworm in Indian crop fields

 Accumulation of Zn and its effects on the growth, reproduction and life cycle of the earthworm Drawida willsi were determined. D. willsi did not reveal any significant changes in their mass at any of the concentrations of Zn (50, 200 and 400 mg kg^–1) compared to in untreated soils. The Zn concentrations in the exposed earthworms were significantly increased, but they were able to regulate their body content of Zn within a range of 116–125 mg kg^–1 (dry wt) in 200–400 mg kg^–1 Zn-treated soil. Reproduction was significantly reduced when the Zn concentration in soil exceeded 200 mg kg^–1. The drop in reproduction at elevated concentrations of Zn apparently resulted in a delay in completion of the life cycle and a decline in the total population. Received: 9 September 1998     

Microbial and soil parameters in relation to N mineralization in soils of diverse genesis under differing management systems

 Oregon soils from various management and genetic histories were used in a greenhouse study to determine the relationships between soil chemical and biological parameters and the uptake of soil mineralized nitrogen (N) by ryegrass (Lolium perenne L.). The soils were tested for asparaginase, amidase, urease, β-glucosidase, and dipeptidase activities and fluorescein diacetate hydrolysis. Microbial biomass carbon (C) and N as well as metabolic diversity using Biolog GN plates were measured, as were total soil N and C, pH, and absorbance of soil extracts at 270 nm and 210 nm. Potentially mineralizable N (N₀) and the mineralization rate constant (k) were calculated using a first order nonlinear regression model and these coefficients were used to calculate the initial potential rate of N mineralization (N₀ k). Except for Biolog GN plates, the other parameters were highly correlated to mineralized N uptake and each other. A model using total soil N and β-glucosidase as parameters provided the best predictor of mineralized N uptake by ryegrass (R ² =0.83). Chemical and biological parameters of soils with the same history of formation but under different management systems differed significantly from each other in most cases. The calculated values of the initial potential rate of mineralization in some cases revealed management differences within the same soil types. The results showed that management of soils is readily reflected in certain soil chemical and biological indicators and that some biological tests may be useful in predicting N mineralization in soils. Received: 31 January 1997     

The effect of urea and pig slurry fertilization on denitrification, direct nitrous oxide emission, volatile fatty acids, water-soluble carbon and anthrone-reactive carbon in maize-cropped soil from the Po plain (Modena, Italy)

 The use of zootechnical slurries in agriculture can increase N losses as N₂O by direct emission and by denitrification. The aim of this research was to determine the influence of pig slurry, as well as its combination with mineral N, on N₂O emissions in the field and their relationships with some fractions of soil organic matter, with soil moisture and with rainfall. In spite of varying amounts of organic substance applied, the diverse agronomic treatments did not produce substantial differences in N losses due to denitrification. Wide variations between the slurry fertilized and the urea-fertilized plots were not found, whereas the combination of pig slurry with urea usually produced an increase both in N₂O emissions due to denitrification and in direct N₂O emissions (N losses corresponding to about 50% of those due to denitrification). The greatest losses of N₂O-N occurred in the first month following fertilizer administration. N₂O emissions due to denitrification were highest in the days immediately following the administration of fertilizers and lowest in a later period. N₂O emissions due to nitrification occurred later. Therefore, N₂O emission via nitrification differed from N₂O losses via denitrification which, under optimal conditions, presented peaks of activity during the whole growth cycle. The N₂O-N losses were highly influenced by physical parameters, particularly rain. An increase in micropore water creates conditions of scarce oxygenation or of anaerobiosis which influence oxidation-reduction processes and, at the same time, can limit the diffusion of bacteria-produced gas towards the soil surface. Received: 14 January 1998     

Influence of the presence of nitrite and nitrate in soil on maize biomass production, nitrogen immobilization and nitrogen recovery

 When comparing nitrite (NO₂ ^–) and nitrate (NO₃ ^–) toxicity to maize (Zea mays L.) growth, it is important to know the fate of applied nitrogen (N). A pot experiment, using potassium nitrite (K¹⁵NO₂) and potassium nitrate (K¹⁵NO₃) was conducted to determine the fate of N (0, 75, 150, and 225 mg N kg^–1 soil) applied to a sandy loam soil collected from Gistel (Belgium). The total dry weight of the plants treated with NO₂ ^– was lower than that of the plants treated with NO₃ ^– at 15 and 26 days after N application (harvest 1 and harvest 2, respectively). Shoot and root biomass reduction started at a relatively low NO₂ ^– application rate (75 mg NO₂ ^–-N kg^–1). Biomass reduction increased, at both harvests with increasing amounts of NO₂ ^– to more than 55% at the highest application rate (225 mg NO₃ ^–-N kg^–1). In the NO₃ ^– treatment, a reduction of 16% in total plant dry biomass was recorded only at the highest application rate (225 mg NO₂ ^–-N kg^–1), at both harvest times. The ¹⁵N plant uptake (shoots plus roots) at harvest 1 decreased with increasing N application rates of both N forms (KNO₂ and KNO₃). Twenty-six days after the N application, the total ¹⁵N taken up by the plant increased in all treatments in comparison with 15 days after the N application. However, only at higher rates of N application (150 and 225 mg N kg^–1) was the ¹⁵N uptake by the NO₂ ^– fed plants significantly lower than by the NO₃ ^– fed plants. The percentage of immobilized N from the applied N was low (0–17.7%) at both harvests, irrespective of the N source. However, with relatively low N application rates (75 mg N kg^–1), the immobilized N in the soil decreased with time. This may be due to the re-mineralization of the applied N. The percentage of inorganic ¹⁵N in the soil in NO₂ ^– treatments was slightly lower than in equivalent doses of NO₃ ^–. This might be due to higher losses of N as N-oxides. Unaccounted for N from the applied N ranged from 21% to 52% for the NO₂ ^– treatments and from 3% to 38% for the NO₃ ^– treatments. Received: 17 July 1997     

High performance liquid chromatography studies on the polysaccharides in the walls of the mounds of two species of termite in Senegal,Cubitermes oculatus and Macrotermes subhyalinus: their origin and contribution to structural stability

 The origin, nature and quantity of polysaccharides in the walls of the epigeal mounds of a species of soil-feeding termite, Cubitermes oculatus, and a fungus-growing termite, Macrotermes subhyalinus, found in Senegal, and of soil not considered to be under the influence of termites, were studied to obtain a clearer picture of the structural stability of these materials. The compounds were extractedand analysed by high performance liquid chromatography. We found that the walls of mounds made by soil-feeding species were very rich in sugars soluble in aqueous acid or hot water. Most of the sugars originated from cellulose and hemicellulose, and only a small proportion from microorganisms. There were also significant amounts of stachyose in the mound walls and in the reference soil. This sugar was probably formed by the surrounding vegetation, which was mainly leguminous crops. Comparison of the mineral and organic-mineral particle sizes of samples confirmed that the walls of soil-feeding termite mounds where there is the greatest redistribution of clay have the best aggregating capacity. The results therefore show that the polysaccharides in mound walls of soil-feeding termites are mostly of plant origin. Their influence on the stability of these structure is discussed. The walls of fungus-growing termite mounds contain little organic matter and hence low levels of polysaccharides, which are mainly of plant origin. Received: 19 July 1999     

Response of the soil microbial biomass and nematode population to a wetting event in nitrogen-amended Negev desert plots

 Water and N availability are the major limiting factors of primary production in desert ecosystems, and the response of soil biota to these two factors is of great importance. We examined the immediate response of soil nematodes and the microbial biomass to a single pulse of water amendment in N-treated plots in the Israeli Negev desert. Plots were treated with 0, 50 and 100 kg NH₄NO₃ ha^–1 in December 1992, and at the end of the summer period (August 1993) the plots were exposed to a 15 mm water. Soil samples from the 0–10 cm layer were collected daily and analysed soil moisture, total soluble N, nematode populations and microbial biomass. Soil moisture increased to 8.5%, then gradually decreased to 2% during the 11 days of the study. Microbial biomass, soil respiration and metabolic quotient values did not exhibit any significant correlation with soil N levels. Free-living nematode population levels in the different plots were found to increase from a mean level of 45 500 to a mean level of 92 300 individuals m^–2. N treatment was found to affect the patterns of free-living nematode population dynamics. The results of this study demonstrated the importance of moisture availability levels and the ability to mobilize previous N inputs into available N which, occurring in pulses, can affect the microbial ecophysiological status, nematode population dynamics and the interrelationship between these two important components in the desert soil milieu. Received: 5 November 1998