Influence of gibberellic acid on the sporulation of Azolla caroliniana, Azolla microphylla and Azolla pinnata

strains used in the study sporulate throughout the year while Azolla caroliniana and Azolla pinnata strains show sporulation only during November–March, the favourable period for sporulation. Application of gibberellic acid (GA) at a concentration of 2.5–100.0 μg ml^–1 substantially enhanced the sporulation frequency and number of sporocarps (mega plus micro) per plant in Azolla microphylla (two strains) during the unfavourable period (September–October) by 77.3–200.0%. The increase in the number of microsporocarps was more than that of the megasporocarps. The sporulation frequency and number of sporocarps in Azolla pinnata (two strains) and Azolla caroliniana during the favourable season also increased considerably due to GA (2.5–10.0 μg ml^–1) application. The optimum GA concentration for the sporulation of the three Azolla species was 2.5 μg ml^–1, which was used in the subsequent investigations. Efficacy of GA for enhancing sporulation was tested across 20 Azolla pinnata strains. A significant positive response was observed in all the strains. Of them, 17 strains for the sporulation frequency and 15 strains for the sporocarp number showed a >25% increase due to GA treatment. Under field conditions, Azolla microphylla (two strains), Azolla caroliniana and Azolla pinnata exhibited 66.0–88.1% higher sporulation frequency and 42.8–52.6% higher sporocarp number due to GA application. However, the proportion of megasporocarps to total sporocarps did not change. Received: 4 August 1998     

Isolation and selection of indigenous Azospirillum spp. from a subtropical island, and effect of inoculation on growth of lowland rice under several levels of N application

 Thirty-five Azospirillum strains (13 strains from plant roots and 22 strains from soils) were isolated from Ishigaki island, Japan, which has a subtropical climate. These strains were different from each other according to polymerase-chain-reaction band patterns obtained by using a random primer (OPT-08). Two Azospirillum strains (AZ43 and AZ92-2) were also examined for use in further experiments. Inoculation of lowland rice with these strains enhanced early growth of rice to various degrees. Inoculation of strains VIII.P1-2, AZ92-2, V.S2-2, and V.P5 in sterilized soil yielded higher shoot dry weights than the application of 90 μg N g^–1 soil without inoculation. Only inoculation with strains AZ92-2 and VIII.P1-2 caused higher N uptake than the application of 90 μg N g^–1 soil. Three strains were selected for the next experiment based on the results of their effect on the early growth of rice. An investigation was conducted to determine the ability of two indigenous Azospirillum strains (V.S2-2 and VIII.P1-2) and one stock strain (AZ92-2) to promote growth and nutrient-uptake of lowland rice in unsterilized soil under several levels of N application (0, 80, 160, and 240 mg N pot^–1). Inoculation with these strains without N application increased shoot dry weight by 12–15% compared to the uninoculated treatment. Inoculation with Azospirillum V.S2-2 together with the application of 160 mg N pot^–1 resulted in a shoot dry weight as high as that obtained in the treatment with 240 mg N pot^–1 without inoculation. Thus, in this former case, the amount of N applied could be reduced by 80 mg pot^–1 due to the effect of the microbial inoculum without a significant change in the high, targeted, yield.     

Solubilization of inorganic phosphates and growth emergence of wheat as affected by Azotobacter chroococcum mutants

 Phosphate-solubilizing strains of A. chroococcum isolated from the wheat rhizosphere were evaluated for their ability to solubilize tricalcium phosphate (TCP), Mussoorie rock phosphate (MRP) and also for indole-acetic-acid (IAA) production. Strains were selected on the basis of the clearance zone on solid agar media of Pikovskaya and Jensen's media containing TCP, and phosphate solubilization in Jensen's liquid culture medium containing both TCP and MRP. Mutants of the best phosphate-solubilizing (TCP 1.52 μg ml^–1 MRP 0.19 μg ml^–1), IAA-producing A. chroococum strain P-4, were developed and screened for P solubilization and phytohormone production. Five mutants solubilized more P (in the range of 1.5–1.7 μg/ml^–1 of TCP and 0.19–0.22 μg ml^–1 of MRP) than the parent strains. In vitro growth emergence studies of three wheat varieties, viz. C-306, WH-542 and HD-2009, showed better performance with phosphate-solubilizing mutants than with the parent strain. Received: 15 October 1997     

Production of B-group vitamins by plant growth-promoting Pseudomonas fluorescens strain 267 and the importance of vitamins in the colonization and nodulation of red clover

 We have estimated the production of water-soluble B vitamins by plant growth-promoting rhizobacterium Pseudomonas fluorescens strain 267 in a minimal medium with different C sources and at different pH values. In the minimal medium, strain 267 produced large amounts of niacin (0.92 μg ml^–1) and pantothenic acid (0.75 μg ml^–1), but also other vitamins such as biotin, thiamine, cobalamine and pyridoxine. The production of B vitamins was dependent on the C source and pH of the growth medium. By random Tn5 mutagenesis, thiamine and niacin auxotrophs were isolated from P. fluorescens strain 267 and mutants were used to evaluate the vitamin production on colonization of clover roots under controlled conditions. Red clover root colonization decreased by about 1 order of magnitude in the case of the niacin auxotroph. The vitamin auxotrophs of P. fluorescens in a mixed inoculation of clover with R. leguminosarum bv. trifolii strain 24.1 showed no plant growth-promotion activity. Received: 23 May 2000     

Comparison of the difference method and 15N technique for studying the fate of nitrogen from plant residues in soil

We compared the dynamics of net mineralization of nitrogen (N) derived from white clover material (Ndfc) as measured by the difference and the ¹⁵N methods in a pot experiment with a sandy loam (15°C and pF 2.4) planted with Italian ryegrass. On day 22, mineralized Ndfc (soil mineral N plus plant N uptake) was 5.8% and 1.3% of added N for the ¹⁵N and the difference methods, respectively. The discrepancy was reduced on day 43. On day 64, the relationship was reversed, and on day 98 the values given by the two methods were 22.8% and 29.5%, respectively. The results obtained by the two methods were linearly correlated (r = 0.987) and, on average, did not differ significantly. Nevertheless, the different temporal patterns led to appreciably different parameter values as estimated by fitting of a reparameterized Richards model. On day 22, clover amendment reduced mineralized N derived from soil (Ndfs) by 3.4 mg N pot^–1. The reason for this was that the clover amendment led to a reduction in plant growth and uptake of Ndfs, most likely because of allelopathy, while mineral Ndfs did not increase correspondingly. Clover-induced Ndfs in the microbial biomass of 5.1 mg N pot^–1 suggested that the mineral Ndfs not taken up by plants had been reimmobilized. Towards the end of the experiment, clover-induced Ndfs in the biomass declined to 1.5 mg N pot^–1, while mineralized Ndfs due to clover amendment increased to 5.1 mg N pot^–1. The results strongly suggested that this increase was caused by a real stimulation of humus N mineralization by clover amendment rather than by isotope displacement or pool substitution. Received: 5 May 1997     

Development of two culture media for mass cultivation of <Emphasis Type="Italic">Azospirillum</Emphasis> spp. and for production of inoculants to enhance plant growth

High yield culture medium is fundamental for production of inoculants for plant growth-promoting bacteria. Based on substitution of glucose in tryptone–yeast extract–glucose medium by Na-gluconate or glycerol, two new culture media were developed for mass cultivation of the commonly used plant growth-promoting bacterium Azospirillum sp. After 18 h of incubation, these modifications increased populations of different strains of Azospirillum (to ∼10¹¹ cells ml⁻¹ [single cell count] and ∼5 × 10⁹ CFU ml⁻¹ [plate count method]), significantly reduced generation time, and were also suitable for production of common synthetic inoculants.     

Spatial changes of soil fungal and bacterial biomass from a sub-alpine coniferous forest to grassland in a humid, sub-tropical region

 Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO₂) were, respectively, 89.1 μl CO₂ g^–1 soil h^–1 and 55.1 μl CO₂ g^–1 soil h^–1 in the forest and 36.7 μl CO₂ g^–1 soil h^–1 and 35.7 μl CO₂ g^–1 soil h^–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g^–1) to the grassland zone (15.9 μg g^–1). A good correlation (R ²=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO₂ production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles. Received: 30 March 2000     

Effect of microbial nitrogen immobilization during the growth period on the availability of nitrogen fertilizer for winter cereals

 Pot and field experiments were conducted to determine microbial immobilization of N fertilizer during growth periods of winter wheat and winter barley. In a pot experiment with winter wheat, Ca(¹⁵NO₃)₂ was applied at tillering [Zadok's growth stage (GS) 25)], stem elongation (GS 31) and ear emergence (GS 49). Rates of 100 mg N pot^–1, 200 mg N pot^–1 or 300 mg N pot^–1 were applied at each N application date. At crop maturity, ¹⁵N-labelled fertilizer N immobilization was highest at the highest N rate (3×300 mg N pot^–1). For each N-rate treatment about 50% of the total immobilized fertilizer N was immobilized from the first N dressing, and 30% and 20% of the total ¹⁵N immobilized was derived from the second and third applications, respectively. In field trials with winter wheat (three sites) and winter barley (one site) N was applied at the same growth stages as for the pot trial. N was also applied to fallow plots, but only at GS 25. N which was not recovered (neither in crops nor in soil mineral N pools) was considered to represent net immobilized N. A clear effect of N rate (51–255 kg N ha^–1) on net N immobilization was not found. The highest net N immobilization was found for the period between GS 25 (March) and GS 31 (late April) which amounted to 54–97% of the total net N immobilized at harvest (July/August). At GS 31, non-recovered N was found to be of similar magnitude for cropped and fallow plots, indicating that C from roots did not affect net N immobilization. Microbial biomass N (N_mic) was determined for cropped plots at GS 31. Although N_mic tended to be higher in fertilized than in unfertilized plots, fertilizer-induced increases in N_mic and net N immobilization were poorly correlated. It can be concluded that microbial immobilization of fertilizer N is particularly high after the first N application when crop growth and N uptake are low. Received: 6 July 1999     

Components of organic phosphorus in soil extracts that are hydrolysed by phytase and acid phosphatase

 Extracts were prepared from soil using water, 50 mM citric acid (pH ∼2.3) or 0.5 M NaHCO₃ (pH 8.5), and were incubated with excess phytase from Aspergillus niger to determine the amounts of labile P. Two A. niger phytase preparations were used: (1) a purified form which exhibited a narrow substrate specificity and high specific activity against phytate; and (2) a commercial preparation (Sigma) with activity against a broad range of P compounds. A comparatively large proportion (up to 79%, or 5.7 μg g^–1 soil) of the organic P (P_o) extracted with citric acid was hydrolysed by the commercial phytase, while between 28% and 40% (up to 3.1 μg g^–1 soil) was hydrolysed using purified phytase. By comparison, only small quantities of the P_o in water and NaHCO₃ soil extracts were enzyme labile. While extractable P_o was increased both with increasing concentrations of citric acid (up to 50 mM) and increasing pH (pH 2.3–6.0), enzyme-labile P increased only with citric acid concentration. The labile component of P_o in citric acid extracts from soils with contrasting fertiliser histories indicated that enzyme-labile P_o is a relatively large soil P pool and is potentially an important source of P for plants. Received: 29 October 1999     

Effects of Azospirillum brasilense on root morphology of common bean (Phaseolus vulgaris L.) under different water regimes

 The effects of inoculation with Azospirillum brasilense Cd on root morphology and growth of common bean (Phaseolus vulgaris L.) were studied under different growth systems and water regimes. The root systems were evaluated by image analysis. In a PVC-tube growth system, inoculation with A. brasilense at 10⁷ colony forming units (CFU) ml^–1 increased root length, root projection area, specific root length (m g^–1) and specific root area (cm² g^–1), as compared with non-inoculated controls, resulting in root systems with longer and thinner roots. Water stress induced similar root responses to those observed after inoculation with A. brasilense. No increase in plant biomass was observed in inoculated plants, suggesting that under the tested growth conditions, a relatively larger amount of resources is required for the maintenance of the thinner roots. In water-stressed potted plants, the effect of A. brasilense on tap root length was inoculum-concentration dependent. At 10⁷ CFU ml^–1 this effect was significant as compared to non-inoculated controls. In a pouch system without water stress, inoculation with A. brasilense at a concentration of 10⁵–10⁷ CFU ml^–1 2 days after germination resulted initially (2 days after inoculation) in an increase in root length (95%) and root fresh weight (66%), but reduced root diameter (20%), compared to controls. At this early stage of growth the distribution of root length among the different root diameter classes changed: the thinner-root classes had the largest proportion of longer roots. Received: 3 January 2000     

Critical sulphur concentration and sulphur requirement of microbial biomass in a glucose and cellulose-amended regosol

 The critical S concentration and S requirement of the soil microbial biomass of a granitic regosol was examined. S was applied at the rate of 0, 5, 10, 20, 30 and 50 μg S as MgSO₄·7H₂O, together with either 3000 μg glucose-C or 3333 μg cellulose-C, 400 μg N, and 200 μg P g ^–1 soil and 200 μg K g^–1 soil. Microbial biomass, inorganic SO₄ ^2–-S, and CO₂ emission were monitored over 30 days during incubation at 25  °C. Both glucose and cellulose decomposition rates responded positively to the S made available for microbial cell synthesis. The amounts of microbial biomass C and S increased with the level of applied S up to 10 μg S g^–1 soil and 30 μg S g^–1 soil in the glucose- and cellulose-amended soil, respectively, and then declined. Incorporated S was found to be concentrated within the microbial biomass or partially transformed into soil organic matter. The concentration of S in the microbial biomass was higher in the cellulose- (4.8–14.2 mg g^–1) than in the glucose-amended soil (3.7–10.9 mg g^–1). The microbial biomass C:S ratio was higher in the glucose- (46–142 : 1) than in the cellulose-amended soil (36–115 : 1). The critical S concentration in the microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) was estimated to be 5.1 mg g^–1 in the glucose- and 10.9 mg g^–1 in the cellulose-amended soil. The minimum requirement of SO₄ ^2–-S for microbial biomass formation was estimated to be 11 μg S g^–1 soil and 21 μg S g^–1 soil for glucose- and cellulose-amended soil, respectively. The highest levels of activity of the microbial biomass were observed at the SO₄ ^2–-S concentrations of 14 μg S g^–1 soil and 17 μg S g^–1 soil, for the glucose and cellulose amendments, respectively, and were approximately 31–54% higher during glucose than cellulose decomposition. Received: 20 October 1999     

Effectiveness of foreign bradyrhizobia strains in enhancing nodulation, dry matter and seed yield of soybean (Glycine max L.) cultivars in Nigeria

 Field experiments were conducted to investigate the performance of three soybean cultivars with five foreign bradyrhizobia strains in different regions. The experiments at the two sites were designed with soybean (Glycine max L.) cultivars as the main factor and bradyrhizobia strains (USDA 136, TAL 122, USDA 6, TAL 377 and TAL 102) as the sub-factor. The experiments were arranged in randomised complete block design with four replications. Results show that nodule number, nodule dry weight and shoot dry weight, total N and seed yield were significantly increased when soybean cultivars were inoculated with foreign bradyrhizobia in two locations in the south east of Nigeria. At 63 days after planting the percentage increase in nodule number and dry weight after inoculation of soybean cultivars with bradyrhizobia strains ranged from 71 to 486% and from 0 to 200%, respectively. The percentage increase in shoot dry matter, %N and total N after bradyrhizobia inoculation ranged between 2–130%, 18–62% and 35–191%, respectively at Awka, and at the Igbariam site the percentage increase in shoot dry weight, %N and total N ranged between 3–76%, 0–43% and 19–125%, respectively. Seed yields after bradyrhizobia inoculation of soybean cultivar TGX 1485–1D at Igbariam ranged between 1.20 and 2.18 t ha^–1 against the uninoculated plants, which had seed yields of 1.05 t ha^–1. The poorest yield response after inoculation with bradyrhizobia strains was observed in soybean cultivar M-351, with a seed yield ranging from 0.60 to 0.98 t ha^–1. The fact that foreign bradyrhizobia strains were more effective than the indigenous strains for all the parameters studied suggests that there is a need to use bradyrhizobia inoculants for increased soybean production in Nigeria. The variations in the strain performance with the different soybean cultivars at the two sites, emphasises the need for careful Bradyrhizobium spp. strain selection. The fact that inoculation response was cultivar- and site-specific suggests that strategies for improving inoculation response in soybean cultivars should also consider the soil environment where the soybean is to be produced. Received: 25 May 1999     

Solubilization of inorganic phosphates and plant growth promotion by <Emphasis Type="Italic">Aspergillus niger</Emphasis>

Two of 187 fungal isolates (Aspergillus niger 1B and 6A) displaying superior phosphate (P) solubilization and hydrolytic enzyme secretion were studied using P forms of calcium (Ca-P), iron (Fe-P), and aluminum (Al-P). Phosphate solubilization in a sucrose-basal salt (SB) broth was increased and pH decreased by both isolates. In Ca-P medium, solubilization for 6A was approximately 322 μg P mL⁻¹ and pH decreased by 4.2 units to 2.3 in 72 h. However, when pH value of the SB broth was lowered to 2.5 using HCl, 65.3  ±  0.4 μg mL⁻¹ of P was released from Ca-P, whereas trace amounts of P were released from Fe-P and Al-P. Both isolates displayed enhanced Al-P solubilization using NH₄Cl rather than KNO₃ as the N source; final pH values were not significantly different. With Ca-P, gluconic acid was predominantly produced by 1B and 6A, whereas oxalic acid predominated with Fe-P and Al-P. Addition of gluconic acid (final concentration of 8.5 μmol mL⁻¹) to Ca-P-supplemented SB lowered pH (2.9) and solubilized phosphate (146.0 ± 1.0 μg mL⁻¹). Similarly, addition of oxalic acid (final concentration 6.6 μmol mL⁻¹) to Ca-P- and Fe-P-amended media solubilized P (60.2 ± 0.9 and 21.6 ± 2.1 μg mL⁻¹, respectively), although these quantities were significantly lower than those detected in unamended SB. The presence of unidentified P solubilized compound(s) in the dialyzed (MW>500) supernatant warrants further study. In pot experiments, significant increases in plant (Brassica chinensis Linn.) dry weight and N and P contents were observed with the addition of isolate 6A, when a small amount of organic fertilizer together with either rock phosphate (South African apatite) or Ca-P served as the main P sources.     

Organic residues affect phosphorus availability and maize yields in a Nitisol of western Kenya

 The effects of organic residues and inorganic fertilizers on P availability and maize yield were compared in a Nitisol of western Kenya. Leaf biomass of Calliandra calothyrsus, Senna spectabilis, Croton megalocarpus, Lantana camara, Sesbania sesban, and Tithonia diversifolia were incorporated into the soil at 5 Mg ha^–1 for six consecutive seasons in 3 years and responses compared with those following the application of 120 kg N ha^–1, 0 kg P ha^–1 (0P); 120 kg N ha^–1, 10 kg P ha^–1; and 120 kg N ha^–1 25 kg P ha^–1 as urea and triple superphosphate (TSP); K was supplied in all treatments. Addition of Tithonia, Lantana and Croton increased soil resin-extractable P over that of fertilizer-amended soil throughout the first crop, but the amounts in the former treatments became similar to those for soils amended with inorganic fertilizers for subsequent crops. Addition of Sesbania, Calliandra and Senna had a similar effect on resin P as inorganic fertilizers. Total maize yields after six seasons were tripled by the application of Tithonia compared to 0P, and were higher than those of the Calliandra, Senna, Sesbania and Lantana treatments, and similar only to that of the Croton treatment. P recovered in the above-ground biomass and resin P, immediately after the implementation of the treatments, was higher in the Senna, Sesbania, Croton, Lantana and Tithonia (35–77%) treatments than in the inorganic fertilizer treatments (21–27%). The P content of organic residues, and the soluble C:total P ratio, were the main residue parameters predicting soil P availability and maize yield. All organic residues used in this study can replace inorganic fertilizers for the enhancement of P availability and maize production, while an additional benefit could be obtained from the use of Croton, Lantana and Tithonia. Received: 19 January 2000     

Evaluation of the selective respiratory inhibition method for measuring the ratio of fungal:bacterial activity in acid agricultural soils

A procedure for the measurement of the fungal and bacterial contribution to substrate-induced respiration was tested in three arable soils. Glucose and different amounts of cycloheximide (eukaryote inhibitor) and streptomycin sulfate (prokaryote inhibitor) were added to soil suspensions, and respiration (CO₂ evolution) was measured. Streptomycin sulfate concentrations from 10 to 120 mg ml^–1 soil solution caused a stable inhibition of respiration. Amounts of cycloheximide ranging from 5 to 35 mg ml^–1 showed an increasing inhibition. In a test with separate and combined addition of the antibiotics at maximum inhibitory concentrations, inhibition by streptomycin was completely overlapped by cycloheximide. This indicated non-target inhibition which may lead to overestimation of fungal respiration. Experiments with sterilized soils inoculated with either fungi or bacteria confirmed that streptomycin selectively inhibited bacteria. Cycloheximide, however, did not only inhibit fungal respiration already at 2 mg ml^–1, but also increasingly inhibited bacterial respiration at increasing concentrations. Only at less than 5 mg cycloheximide ml^–1 was the condition of selective fungal inhibition fulfilled. When 2 mg cycloheximide and 10 mg streptomycin sulfate ml^–1 were applied, the sum of the separate inhibitions almost equalled the combined inhibition by the mix of both inhibitors in field samples. This method yielded fungal:bacterial respiration ratios of 0.50 to 0.60, and confirmed the dominance of bacteria in Dutch arable soils. The ratios obtained by the selective inhibitors were not correlated with, and were higher than, ratios of fungal:bacterial biovolume (0.19 to 0.46) as determined by microscopy and image analysis. Similar measurements in a forest soil (A-horizon) raised doubts on the reliability of the fungal inhibition by cycloheximide in this soil. It is concluded that the separate:combined inhibition ratio should always be checked, and comparison with other approaches is recommended. Received: 17 September 1996     

Determination of competitive abilities of Bradyrhizobium japonicum strains in soils from soybean production regions in South Africa

Bradyrhizobium japonicum strain CB 1809 was recently chosen to replace strain WB 1 in commercial soybean [Glycine max (L.) Merr.] inoculants in South Africa, the selection criterion being N₂-fixing effectiveness. Nodulation competitiveness is an additional characteristic required of inoculants and was determined for CB 1809 and WB 1 as well as two other strains, USDA 110 and a Brazilian strain 965, using the gusA marker gene to identify strains. Initial experiments with plants grown in sterile sand showed that the competitive index of strain WB 1 was less than that of the other strains. Further comparisons used plants grown in five soils containing established populations of B. japonicum. When strains were applied in peat inoculum to seed at a rate of 1,000 cells per seed in a soil containing 300 rhizobia g^–1, significant differences in nodule occupancy were detected and strains ranked in the order 965>CB 1809>USDA 110>WB 1. The remaining four soils each contained about 10⁶ rhizobia g^–1 and 5×10⁶ cells were applied per seed. Nodule occupancy by inoculant strains ranged from 22% to 81% between soils. In this experiment, WB 1 was consistently the poorest performer and its competitiveness was significantly less than CB 1809. The competition results supported the recent decision to replace WB 1 with CB 1809 in commercial inoculants. Although WB 1 had been used in inoculants over a period of 19 years, this strain was detected in only one soil, where it comprised 8% of isolates. In contrast, a substantial proportion (32–78%) of isolates from the soils corresponded serologically to a former inoculant strain WB 66, which had been discontinued in 1966. This illustrates the difficulty of replacing a resident population with an introduced strain. The effect of naturalized populations on the establishment of CB 1809 in South African soils will need monitoring Received: 23 November 1999     

Influence of N and non-N salts on atmospheric methane oxidation by upland boreal forest and tundra soils

 The short-term (24 h) and medium-term (30 day) influence of N salts (NH₄Cl, NaNO₃ and NaNO₂) and a non-N salt (NaCl) on first-order rate constants, k (h^–1) and thresholds (C_Th) for atmospheric CH₄ oxidation by homogenized composites of upland boreal forest and tundra soils was assessed at salt additions ranging to 20 μmol g^–1 dry weight (dw) soil. Additions of NH₄Cl, NaNO₃ and NaCl to 0.5 μmol g^–1 dw soil did not significantly decrease k relative to watered controls in the short term. Higher concentrations significantly reduced k, with the degree of inhibition increasing with increasing dose. Similar doses of NH₄Cl and NaCl gave comparable decreases in k relative to controls and both soils showed low native concentrations of NH₄ ⁺-N (≤1 μmol g^–1dw soil), suggesting that the reduction in k was due primarily to a salt influence rather than competitive inhibition of CH₄ oxidation by exogenous NH₄ ⁺-N or NH₄ ⁺-N released through cation exchange. The decrease in k was consistently less for NaNO₃ than for NH₄Cl and NaCl at similar doses, pointing to a strong inhibitory effect of the Cl^– counter-anion. Thresholds for CH₄ oxidation were less sensitive to salt addition than k for these three salts, as significant increases in C_Th relative to controls were only observed at concentrations ≥1.0 μmol g^–1 dw soil. Both soils were more sensitive to NaNO₂ than to other salts in the short term, showing a significant decrease in k at an addition of 0.25 μmol NaNO₂ g^–1 dw soil that was clearly attributable to NO₂ ^–. Soils showed no recovery from NaCl, NH₄ ⁺-N or NaNO₃ addition with respect to atmospheric CH₄ oxidation after 30 days. However, soils amended with NaNO₂ to 1.0 μmol NaNO₂ g^–1 dw showed values of k that were not significantly different from controls. Recovery of CH₄-oxidizing ability was due to complete oxidation of NO₂ ^–-N to NO₃ ^–-N. Analysis of soil concentrations of N salts necessary to inhibit atmospheric CH₄ oxidation and regional rates of N deposition suggest that N deposition will not decrease the future sink strength of upland high-latitude soils in the atmospheric CH₄ budget. Received: 30 April 1999     

Methane-oxidizing activities and methanotrophic populations associated with wetland rice plants

Acetylene up to 500 μl l^–1 did not affect methane formation in anoxic soil up to 12 h, but further incubation for 1 week showed strong inhibition of methanogenesis. To ascertain the extent of the oxidation of methane produced from rice-planted pots, the effect of acetylene on methane emission was studied. Two rice varieties (Toyohatamochi and Yamahikari) were grown in a greenhouse in submerged soil in pots. At about maximum tillering, heading, and grain-forming stages, methane fluxes were measured. Flux measurement was performed for 3 h from 6 pm, then acetylene at 100 μl l^–1 was added to some of the pots. At 6 a.m. the following day, methane fluxes were again measured for 3 h. Only at maximum tillering stage of the variety Toyohatamochi was a significant increase (1.4 times) in methane flux caused by acetylene observed, whereas in the other treatments no significant increase in methane fluxes by acetylene could be defected. To ascertain the activity of methane monooxygenase (MMO), propylene oxide (PPO) formation from propylene was measured with excised roots and a basal portion of stems of the rice plants grown on the submerged soil. A level of 0.1–0.2 μmol PPO h^–1 plant^–1 was recorded. The roots showed the highest PPO formation per gram dry matter, followed by basal stems. Methane oxidation was roughly proportional to PPO formation. Soluble MMO-positive methanotroph populations were measured by plate counts. The number of colony-forming units per gram dry matter was 10⁶–10⁵ in roots, and 10⁴–10³ in basal stems. These results indicate the possibility of methane oxidation in association with wetland rice plants. Received: 26 October 1995     

Phosphate solubilization potentials of soil Acinetobacter strains

Many phosphate solubilizing microorganisms (PSM) require external pyrroloquinoline quinone (PQQ) for strong phosphorus (P) solubilization in vitro. The objective of this study was to isolate efficient and PQQ-independent PSM. A total of 21 PSM were isolated from the rhizosphere soil of wheat and maize grown in the pots. Acinetobacter strains were the only PQQ-independent and most effective solubilizers of tricalcium phosphate containing agar. The mean P dissolved in liquid cultures of Acinetobacter strains in a 5-day incubation ranged from 167 to 888 μg/ml P. The pH dropped to below 4.7 from 7.8 in six isolates, which produced gluconic acid in concentrations ranging between 27.5 and 37.5 mM. There was a linear regression between soluble P and gluconic acid concentrations in the bacterial cultures (P < 0.05; R ² = 0.59). Inoculation with Acinetobacter sp. WR922 significantly (P < 0.05) increased wheat (Triticum aestivum L.) P content by 27% at 15 days after emergence (DAE) and dry matter by 15% at 30 DAE compared to the control. The plant P content in inoculated plants at 30 DAE was linearly correlated with soluble P of the bacterial cultures (P < 0.05; R ² = 0.69). Gluconic acid production directly affected phosphate solubilization in vitro, which in turn influenced plant P content of inoculated plants in PQQ-independent P-solubilizing Acinetobacter strains.     

Methane fluxes and nitrogen dynamics from a drained fenland peat

 Rates of methane uptake were measured in incubation studies with intact cores from adjacent fenland peats that have been under arable management and woodland management for at least the past 30 years. On two separate occasions the woodland peat showed greater rates of uptake than the arable peat. These rates ranged from 23.1 to 223.3 μg CH₄ m^–2 day^–1 for the woodland peat and from 29.6 to 157.6 μg CH₄ m^–2 day^–1 for the arable peat. When the peats were artificially flooded there was a decrease in the rate of methane oxidation, but neither site showed any net efflux of methane. ¹⁵N isotopic dilution was used to characterise nitrogen cycling within the two peats. Both showed similar rates of gross nitrogen mineralisation (3.58 mg N kg^–1 day^–1, arable peat; 3.54 N kg^–1 day^–1, woodland peat) and ammonium consumption (4.19 arable peat and 4.70 mg N kg^–1 day^–1 woodland peat). There were significant differences in their inorganic ammonium and nitrate pool sizes, and the rate of gross nitrification was significantly higher in the woodland peat (4.90 mg N kg^–1 day^–1) compared to the arable peat (1.90 mg N kg^–1 day^–1). These results are discussed in the light of high atmospheric nitrogen deposition. Received: 1 December 1997