Relationship between rimsulfuron degradation and microbial biomass content in a clay loam soil

 The present research was conducted to determine the relationship between the degradation of rimsulfuron and soil microbial biomass C in a laboratory-incubated clay loam soil (pH=8.1; organic matter=2.1%) under different conditions and at different initial dosages (field rate, 10 and 100 times the field rate). The half-life values varied between 0.4 and 103.4 days depending on temperature, soil moisture and initial dose. Evidence suggested that rimsulfuron could pose environmental risks in cold and dry climatic conditions. Significant decreases in microbial biomass C content in rimsulfuron-treated soil, compared to untreated soil, were observed initially, especially at higher temperatures and low moisture levels, but never exceeded 20.3% of that in control soil. The microbial biomass C content then returned to initial values at varying times depending on incubation conditions. The relationship between herbicide degradation and microbial biomass C content gave parabolic curves (P<0.005 in all cases) under all conditions tested. Generally, maximum biomass C decrease coincided with the decrease in the concentration of rimsulfuron to about 50% of the initial dose, except at 10  °C and 100×, when biomass began to recover as early as 65–70% of the initial dose. The final equations could be useful to deduce the decrease of soil microbial biomass in relation to herbicide concentration. From the degradation kinetics of the herbicide, the time required to reach this decrease can also be calculated. Received: 19 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     

Biological and chemical properties of arable soils affected by long-term organic and inorganic fertilizer applications

 Using soils from field plots in four different arable crop experiments that have received combinations of manure, lime and inorganic N, P and K for up to 20 years, the effects of these fertilizers on soil chemical properties and estimates of soil microbial community size and activity were studied. The soil pH was increased or unaffected by the addition of organic manure plus inorganic fertilizers applied in conjunction with lime, but decreased in the absence of liming. The soil C and N contents were greater for all fertilized treatments compared to the control, yet in all cases the soil samples from fertilized plots had smaller C:N ratios than soil from the unfertilized plots. The soil concentrations of all the other inorganic nutrients measured were greater following fertilizer applications compared with the unfertilized plots, and this effect was most marked for P and K in soils from plots that had received the largest amounts of these nutrients as fertilizers. Both biomass C determined by chloroform fumigation and glucose-induced respiration tended to increase as a result of manure and inorganic fertilizer applications, although soils which received the largest additions of inorganic fertilizers in the absence of lime contained less biomass C than those to which lime had been added. Dehydrogenase activity was lower in soils that had received the largest amounts of fertilizers, and was further decreased in the absence of lime. This suggests that dehydrogenase activity was highly sensitive to the inhibitory effects associated with large fertilizer additions. Potential denitrification and anaerobic respiration determined in one soil were increased by fertilizer application but, as with both the microbial biomass and dehydrogenase activity, there were significant reductions in both N₂O and CO₂ production in soils which received the largest additions of inorganic fertilizers in the absence of lime. In contrast, the size of the denitrifying component of the soil microbial community, as indicated by denitrifying enzyme activity, was unaffected by the absence of lime at the largest rate of inorganic fertilizer applications. The results indicated differences in the composition or function of microbial communities in the soils in response to long-term organic and inorganic fertilization, especially when the soils were not limited. Received: 10 March 1998     

Influence of cadmium on the metabolic quotient, <Emphasis Type=\"SmallCaps\">l</Emphasis>- : <Emphasis Type=\"SmallCaps\">d</Emphasis>-glutamic acid respiration ratio and enzyme activity : microbial biomass ratio under laboratory conditions

 This study was carried out to investigate the effect of very high cadmium concentrations (50 and 500 μg Cd g^–1 soil) on some biochemical and microbiological measurements under laboratory conditions involving daily soil samplings. The data for both DTPA- and water-soluble Cd showed two distinctive patterns during soil incubation; from 0 to 4 days, values were about 50–500 and 1–100 μg g^–1 dry weight soil, whereas they decreased markedly after 7 days. Both daily respiration and the ATP content but not the microbial biomass C determined by the fumigation–extraction method were lowered by high DTPA- and water-soluble Cd concentrations. Dehydrogenase and phosphatase activities as well as both enzyme activity : microbial biomass ratios were decreased by the high DTPA- and water-soluble Cd concentrations. In the first 2 days of incubation, the metabolic quotient (qCO₂) was also decreased by the highest values of available Cd. The early (after 6 h) mineralization of l- but not d-glutamic acid to CO₂ was inhibited during the 0–4 day incubation period by the highest Cd concentration. Possibly the l-enantiomer was used by a larger fraction of soil microorganisms than the d-enantiomer or, if they were used by the same fraction of soil microorganisms, the d-enantiomer was mineralized at a lower rate. The l- : d-glutamic acid respiration ratio was decreased by the high available Cd content because under polluted conditions soil microorganisms probably discriminated less between the two stereoisomers of glutamic acid. Received: 13 July 1999     

Identification and characterization of salt- and thermo-tolerant Leucaena-nodulating Rhizobium strains

 Rhizobium-legume symbioses are important for their nitrogen input, but salinity and elevated temperature in arid and semi-arid areas limit their effectiveness, and therefore plant growth and productivity. Sixteen Rhizobium strains isolated from root nodules of Leucaena trees grown in different geographical areas of Egypt varied in their degree of tolerance to salinity and in their symbiotic effectiveness with Leucaena leucocephala under saline conditions. Three strains were tolerant to >3% NaCl. L. leucocephala grown in the greenhouse at concentrations of NaCl up to 1.0% and inoculated either with strain DS 78 or strain DS 158 displayed significantly better growth than those plants grown at the same levels of salinity and inoculated with reference strain TAL 583. Although nine of the Rhizobium strains grew at 42  °C, their mean generation times were lengthened two- to fourfold. When daylight growth temperatures were elevated from 30  °C to 42  °C, nodule number and mass, nitrogenase activities and shoot top dry weight of plants inoculated with strains DS 78, DS 157 and DS 158 significantly increased, whereas these parameters decreased in plants inoculated with strain TAL 583. Rhizobium strains that effectively nodulate Leucaena under adverse saline conditions and at high temperatures were thus isolated, identified and characterized. Received: 12 September 1997     

Inhibitory effects of the total and water-soluble concentrations of nine different metals on the dehydrogenase activity of a loess soil

 This study focuses on a comparison of the microbial toxicity of nine metals, including As as a metalloid and two species of Cr. A loess soil [Ap horizon, clay 15.2%, organic C 1.12%, pH(CaCl₂) 7.02] was spiked with 8–12 geometrically increasing doses of the metals. The dehydrogenase assay (2-p-iodophenyl-3-p-nitrophenyl-5-phenyltetrazoliumchloride method) was combined with sorption and solubility experiments. The resulting dose-response curves and sorption isotherms were used to derive total doses that caused definite percentage inhibitions [i.e. effective doses (ED) causing a 10–90% reduction in dehydrogenase activity (dha)] as well as the corresponding toxic solution concentrations causing the same reductions in dha (i.e. effective concentrations; EC₁₀–EC₉₀). Based on total doses, the toxicity decreased in the following order with ED₅₀ values (mg kg^–1) given in brackets: Hg (2.0)>Cu (35)>Cr(VI) (71)>Cr(III) (75)>Cd (90)>Ni (100)>Zn (115)>As (168)>Co (582)>Pb (652). With regard to solution concentrations, toxicity decreased in the order (EC₅₀ in mg l^–1): Hg (0.003)>Pb (0.04)>Cu (0.05)>Cd (0.14)>Zn (0.19)>Cr(III) (0.62)>Ni (0.69)>Co (30.6)>As (55.5)>Cr(VI) (78.1). The retention of the metals by the soil differed strongly. Pb, Cu, and Hg exhibited the highest and Ni, As, and Cr(VI) the lowest sorption constants (Freundlich K values: 2455, 724, 348, 93, 13, and 0.06 mg kg^–1, respectively). The sorptivity of the metals and their microbial toxicity in the aqueous phase were characteristically related: metals with a strong toxic action in the soil solution were adsorbed by the soil to a high degree and vice versa. Therefore, especially for metals with a high inherent toxicity, sorption is an effective way of immobilizing them and temporarily detoxifying soil. Received: 2 July 1998     

Regulation of potential denitrification by soil pH in long-term fertilized arable soils

 The denitrifying enzyme activity (DEA), denitrification potential (DP) and anaerobic respiration (RESP) together with chemical characteristics were measured in three contrasting soils collected from experimental arable plots that had been subjected to long-term (21–23 years) fertilizer treatments. The plots sampled were either unfertilized or had received either annual inorganic NPK, manure and lime, or inorganic NPK and manure treatments. Addition of inorganic NPK, manure and lime led to large increases in the DEA for two of the three soils, but in the absence of lime, inorganic NPK and manure caused only small increases in DEA compared to unfertilized soils. Both DP and RESP were increased by the addition of inorganic NPK, manure and lime, but were substantially decreased by fertilizer treatments without lime. In most cases there was a simple relationship between soil pH and either DEA and DP, with those treatments that reduced soil pH also leading to reduced denitrification and vice versa. The effects of artificially increasing the pH to a value close to the pH in unfertilized soils (6.3) by addition of NaOH to the soils that had received inorganic NPK, and which had the lowest soil pH values, were to increase substantially DEA, DP and RESP. In soil from one of the sites that had been stored for 5 weeks, the DP values responded differently between the fertilizer treatments. The DP value was lowest in the soil that had inorganic NPK and manure, higher in the soil that received inorganic NPK, manure and lime and it was the highest in unfertilized (control) soil. The soil pH values for these treatments were 4.47, 5.79 and 6.58, respectively. However, when the soil pHs were adjusted by addition of either H₂SO₄ or NaOH to give a range between pH 2 and 12, the DP values from all three fertilizer treatments showed almost identical responses. The optimum pH value for DP was between 7 and 8 for all three fertilizer treatments. Substrate-induced respiration values from all fertilizer treatments showed a similar trend to DP when the soil pHs were modified. The results show that soil pH was an important factor which in the studied soils controls the microbial community in general and the community of denitrifiers in particular. However, denitrifiers showed a high pH resilience leading to no marked change of the pH optimum for potential denitrification. Received: 10 September 1998     

Antagonistic effect of Pseudomonas spp. on pathogenic fungi and enhancement of growth of green gram (Vigna radiata)

species were isolated from the rhizosphere of green gram [Vigna radiata (L.) Wilczek] and some of the rhizobacterial isolates were found to have a wide range of antifungal activity inhibiting growth of the phytopathogenic fungi Aspergillus sp., Curvularia sp., Fusarium oxysporum and Rhizoctonia solani in culture. These isolates also showed slight inhibition of the growth of a Bradyrhizobium strain (Vigna) in a spot test which was mainly a result of nutrient competition as culture supernatants of the Pseudomonas isolates did not inhibit the growth of bradyrhizobia but inhibited the growth of fungi. The rhizobacterial isolates produced siderophores in Fe-deficient succinate medium. However, the inhibition of fungal growth by different Pseudomonas isolates in Luria Bertani and King's medium B which were not limiting in Fe³⁺ ions suggested that, besides siderophores, other antifungal compounds (antibiotics) produced by these rhizobacteria were involved in antagonism. On coinoculation of green gram with Pseudomonas strains MRS13 and MRS16 and Bradyrhizobium sp. (Vigna) strain S24, there was a significant increase in nodule weight, plant dry weight and total plant N as compared to inoculation with Bradyrhizobium strain S24 alone, suggesting that the nodule-promoting effects of Pseudomonas sp. lead to an increase in symbiotic N fixation and plant growth. Received: 27 October 1997     

Influence of stock camping behaviour on the soil microbiological and biochemical properties of grazed pastoral soils

 The size and activity of the soil microbial biomass in grazed pastures was compared on the main grazing area and on stock camp areas where animals congregate. Two sites were on hill country and three on gently sloping border-dyke irrigated land. Due to the transfer of nutrients and organic matter to the camp areas via dung and urine there was an accumulation of soil organic C, organic and inorganic P and S and soluble salts in the camp areas. Soil pH also tended to be higher in camp areas due to transfer of alkalinity by the grazing animals. Water soluble organic C, microbial biomass C and basal respiration were all higher in soils from camp areas but the proportion of organic C present as microbial C and the microbial respiratory quotient were unaffected. Microbial activity as quantified by arginine ammonification rate and fluorescein diacetate (FDA) hydrolysis was higher in camp than non-camp soils but dehydrogenase activity remained unaffected. Activities of protease, histidase, urease, acid phosphatase and aryl-sulphatase were all higher in stock camp soils. The activities of both histidase and aryl-sulphatase were also higher when expressed per unit of microbial biomass C, indicating that the increased activity was the result of increased enzyme production by the microbial community. Prolonged regular applications of dairy shed effluent (diluted dung and urine from cattle) to a field had a similar effect to stock camping in increasing soil organic matter content, nutrient accumulation and soil biological activity. It was concluded that the stock camping activity of grazing animals results in an increase in both the fertility and biological activity in soils from camp areas at the expense of these properties on the main grazing areas. Received: 20 October 1997     

The use of GUS-reporter gene to study the effect of Azospirillum-Rhizobium coinoculation on nodulation of white clover

 The gusA-marked Azospirillum lipoferum T1371, constructed by inserting transposon mTn5SSgusA20 from Escherichia coli S17-1 λ-pir into the genome of Azospirillum lipoferum 137, was used to evaluate its effect on nodulation of white clover with and without Rhizobium inoculation. When inoculated alone, Azospirillum colonized the tap roots, secondary roots and root hairs. The combined inoculation of white clover with Rhizobium leguminosarum bv trifolii and A. lipoferum enhanced the number of nodules by 2–3 times from 5 to 20 days after inoculation (DAI). The combined inoculation also enhanced acetylene reduction activity by 2.3–2.7 times at 20 DAI. Moreover, Azospirillum was observed colonizing the tap root, root hairs and sites near or on the nodules. These results suggest that the formation of additional infection sites by A. lipoferum, with a combined inoculation, may be the mechanism that will enhance nodulation and nitrogen fixation of white clover. Received: 23 July 1997