Kinetics of microbial phosphorus uptake in cultivated soils

Knowledge about the role of microorganisms in P cycling at conditions of constant soil respiration rates and constant size of microbially bound P is lacking. To study the kinetics of microbial P uptake and cycling under such conditions, soils differing in biological activity were ³³PO₄ labelled by introducing a carrier-free tracer solution and incubated for 56 days. The ³³PO₄ incorporation into the fraction of microbial P releasable by chloroform treatment (P_chl) was assessed and the isotopic composition [=specific activity (SA); SA=³³PO₄/³¹PO₄] of P_chl and soil solution P compared. Soils were taken from a 20-year-old field experiment including a non-fertilised control (NON), a minerally fertilised conventional (MIN) and two organic farming systems [bio-organic (ORG); bio-dynamic (DYN)]. Tracer P incorporation continuously increased during incubation in DYN, ORG and MIN soils. It decreased in the order DYN>ORG>MIN, with differences in ³³PO₄ uptake between the farming systems being higher than suggested by the differences in the amount of P_chl. In the P-deficient NON soil, the highest initial incorporation of tracer P was found, but no additional uptake could be detected thereafter. In all soils, the SA of P_chl converged to the SA of the soil solution with increasing time. Since P_chl remained almost constant during the experiment, the findings suggest an intensive uptake of P from the soil solution into P_chl and concomitant release of P back to the soil solution and, thus, a rapid cycling through P_chl. Intensive P cycling between P_chl and the soil solution was confirmed in an additional experiment where microbial activity was stimulated by glucose and N additions.     

Dehydrogenase activity, redox potential, and emissions of carbon dioxide and nitrous oxide from Cambisols under flooding conditions

Samples from topsoils (0-10 cm) of 16 Polish arable Cambisols developed from different parent materials (sand, silt, sandy gravel, loess, loam and clay), were incubated under flooded conditions with NO₃^-. Dehydrogenase activity, redox potential (Eh), and emissions of CO₂ and N₂O were measured. According to dehydrogenase activity, the soils were divided into two groups: those of low activity (I), where the final dehydrogenase activity was <0.03 nmol triphenylformazan (TPF) g^-1 min^-1, and those with high final dehydrogenase activity (II), >0.03 nmol TPF g^-1 min^-1. Generation of CO₂ and of N₂O under flooded conditions was shown to be significantly related to dehydrogenase activity. Soil dehydrogenase activity increased curvilinearly with organic matter content, showed a maximum at pH 7.1, and decreased curvilinearly with Eh. The final cumulative CO₂ production increased linearly with soil organic matter content and curvilinearly with dehydrogenase activity and decreased linearly with Eh. The most significant relationship was found with dehydrogenase activity (R²=0.74, P<0.001). The final cumulative N₂O production decreased linearly with Eh and increased curvilinearly with pH and dehydrogenase activity but linearly with organic matter content; the most significant relation being found with dehydrogenase activity (R²=0.69, P<0.001). The CO₂:N₂O ratio in the gases evolved increased curvilinearly with Eh and decreased with dehydrogenase activity and N₂O and CO₂ production.     

Influence of soil parameters on the effect of 3,4-dimethylpyrazole-phosphate as a nitrification inhibitor

Nitrification inhibitors specifically retard the oxidation of NH₄⁺ to NO₂^- during the nitrification process in soil. In this study, the influence of soil properties on the nitrification-inhibiting effect of 3,4-dimethylpyrazole-phosphate (DMPP), a newly developed nitrification inhibitor, has been investigated. Based on short-term incubation experiments, where the degradation of DMPP could be largely disregarded, the oxidation of the applied NH₄⁺ was more inhibited in sandy soils compared with loamy soils. The influence of soil parameters on the relative NO₂^- formation could be described by a multiple regression model including the sand fraction, soil H⁺ concentration and soil catalase activity (R²=0.62). Adsorption studies showed that the binding behaviour of DMPP was influenced markedly by soil textural properties, viz. the clay fraction (r²=0.61). The adsorption of DMPP was found to be an important factor for the inhibitory effect on NH₄⁺ oxidation in a short-term incubation (r²=0.57). It is concluded that the evaluated soil properties can be used to predict the short-term inhibitory effect of DMPP in different soils. The significance of these results for long-term experiments under laboratory and field conditions needs further investigation.     

Response of ATP content, respiration rate and enzyme activities in an arable and a forest soil to nutrient additions

Glucose (C), glucose plus NO₃^- (C+N) or glucose plus NO₃^- plus PO₄^3- (C+N+P) were added to an arable and a forest soil at a single dose, or split into four equal doses over 4 consecutive days, and the response of several enzyme activities, ATP content and respiration rate were monitored for 11 days. ß-Glucosidase activity was reduced in the two soils during the first day by substrate addition. Thereafter, this enzyme activity varied only slightly in the arable soil with reference to the non-amended control, while it increased substantially in the beech forest when C+N and C+N+P were added. Casein-hydrolysing activity increased in the C treatment and decreased after C+N+P addition during the first 4 days in the two soils. After 11 days, protease activity was enhanced in the arable soil when C+N was applied in a split dose. Urease activity decreased during the first 4 h, particularly in the arable soil with the addition of C+N or C+N+P, applied in a single dose, and then continuously increased. Thus, urease responded to high nutrient availability, being firstly repressed or inhibited, and stimulated afterwards. Phosphatase activity was only slightly modified in the arable soil but substantially increased in the beech forest by C+N addition. The presence of P usually decreased phosphatase activity. Arylsulphatase activity was repressed after substrate addition, which was particularly evident in the arable soil. In the beech forest topsoil, C added alone increased this enzyme activity. Significant correlations between ATP content and enzyme activity were only observed for urease in the arable system treated with C+N and in the forest when C was applied in a split dose. The effect of C, C+N and C+N+P addition varied between the arable and the forest soil according to environmental conditions and microbial ecophysiology.     

Microfungal community structure in anthropogenic birch stands in central Finland

We describe the soil microfungal communities in 30-year-old birch (Betula pendula Roth) stands planted either on former spruce forest soil (BS) or on former arable soil (BF) and compare these with the soil microfungal communities in spruce forests (S), arable fields (F) and old deciduous forests (D). Fungi were isolated from 0- to 3-cm and 3- to 6-cm samples collected in September 1997 and May 1998. Principal components analysis differentiated fungal communities in the S and BS sites from those in the other site types. The Morisita-Horn index of similarity indicated that fungal communities in the F and BF sites were less similar to those in the other site types. Fungal communities of the BS and S sites were more similar in the 3- to 6-cm layer than in the 0- to 3-cm layer, suggesting that it takes >30 years to replace the spruce litter and associated fungal community. Similarity between fungal communities in the F and BF sites was low, indicating significant changes in the fungal community composition following afforestation of field soil by birch. Fungal communities in the BS and BF sites were more similar to each other than those of their original site types. Fungal communities of the BS, BF and S sites were more similar to those of the D sites than those of the F sites were to the D sites. Differences between the fungal communities of the different site types were attributed primarily to litter quality, earthworm community, and secondarily to organic matter, pH, KCl-extractable NH₄⁺, and PO₄^3-.     

Arbuscular-mycorrhizal inoculation of five tropical fodder crops and inoculum production in marginal soil amended with organic matter

Five fodder crops, Zea mays, Medicago sativa, Trifolium alexandrinum, Avena sativa, and Sorghum vulgare were inoculated with a consortia of indigenous arbuscular mycorrhizal (AM) fungi in non-sterile PO₄^3- deficient sandy loam soil amended with organic matter under field conditions. Shoot and root dry weights and total uptake of P and N of all the test plants were significantly increased by AM inoculation. Mycorrhizal inoculation increased yield in terms of shoot dry weight by 257% in T. alexandrinum followed by 50% in A. sativa, 28% in Z. mays, 20% in M. sativa and 6% in S. vulgare. Variation in dependence on mycorrhiza was observed among the fodder crops. T. alexandrinum showed a maximum dependence of 72% in contrast to 5.7% dependency in S. vulgare. Plant species showed differences in percentage AM colonization, with a high root infection recorded in Z. mays (76%). Spore production and infectious propagules (IP) were as high as 78 spores/IP g^-1 and 103 spores/IP g^-1 in S. vulgare. This study clearly indicates the potential of using indigenous AM inoculations in fodder crops grown in marginal soils along with in situ large-scale production of AM inocula.     

Changes in soil acidity and the size and activity of the microbial biomass in response to the addition of sugar mill wastes

The effects of additions of three wastes from sugar mills on the properties of two acid soils were investigated. The wastes used were boiler ash and filter cake from a conventional mill and fly ash from a new mill, where filter cake is burnt. Additions of each of the wastes raised soil pH, reduced concentrations of exchangeable Al and total and monomeric Al in soil solution and increased maize yields in a pot experiment. Microbial biomass C and the percentage of organic C present as microbial biomass were decreased by additions of wastes but the effect was less marked at the higher rate of addition. Although basal respiration was decreased or unaffected by the lower rate of addition of wastes it was increased by the higher rate. The metabolic quotient increased in the order: control4⁺ and NO₃^- accumulated during incubation and arginine ammonification rate were all less at the higher rate of addition of each of the wastes. This was attributed to microbial immobilization of mineral N and arginine due to the wide C/N ratio of light fraction organic matter present in the study soil. It was concluded that all three waste materials were effective liming materials but their effect on soil microbial activity was complex.     

Effects of nitrogen fertilization on soil nitrogen pools and microbial properties in a hoop pine (Araucaria cunninghamii) plantation in southeast Queensland, Australia

A field study was conducted to investigate the effects of N fertilization on soil N pools and associated microbial properties in a 13-year-old hoop pine (Araucaria cunninghamii) plantation of southeast Queensland, Australia. The treatments included: (1) control (without N application); (2) 300 kg N ha^-1 applied as NH₄NO₃; and (3) 600 kg N ha^-1 as NH₄NO₃. The experiment employed a randomized complete block design with four replicates. Soil samples were taken approximately 5 years after the N application. The results showed that application of 600 kg N ha^-1 significantly increased concentrations of NH₄⁺-N in 0-10 cm soil compared with the control and application of 300 kg N ha^-1. Concentrations of NO₃^--N in soil (both 0-10 cm and 10-20 cm) with an application rate of 600 kg N ha^-1 were significantly higher compared with the control. Application of 600 kg N ha^-1 significantly increased gross N mineralization and immobilization rates (0-10 cm soil) determined by ¹⁵N isotope dilution techniques under anaerobic incubation, compared with the control. However, N application did not significantly affect the concentrations of soil total C and total N. N application appeared to decrease microbial biomass C and N and respiration, and to increase the metabolic quotient (qCO₂) in 0-10 cm soil, but these effects were not statistically significant. The lack of statistical significance in these microbial properties between the treatments might have been associated with large spatial variability between the replicate plots at this experimental site. Spatial variability in soil microbial biomass C and N was found to relate to soil moisture, total C and total N.     

Naidid worms (Oligochaeta, Naididae) in paddy soils as affected by the application of legume mulch and/or tillage practice

Reproduction, intrinsic rate of natural increase and population density of naidid worms were investigated in submerged paddy fields and the laboratory. No tillage plus legume-mulching increased the population density of naidid worms. Soil treatments with neither tillage nor legume mulch, and tillage practice alone, did not increase the number of worms. Dero dorsalis Ferronnière was dominant in soil of the no-tillage treatment. In laboratory experiments, legume-mulching with the proper amount of dissolved O₂ accelerated asexual reproduction of D. dorsalis through zooid budding. For the legume and aeration treatment, (N_i+1-N_i) N_i^-1 values (where N_i and N_i+1 are the populations at times t=i and t=i+1) were plotted against N_i+1. Utilizing this linear relation, this data fitted the logistic curve (r²=0.885, P<0.05). Based on the linear relation, the intrinsic rate of natural increase (r), carrying capacity (K), and doubling time (T) were calculated as 0.2125 day^-1, 12,666 m^-2, and 3.26 days, respectively. The amounts of legumes applied were highly correlated with the population of D. dorsalis, indicating that the weight of legume is a limiting factor with respect to carrying capacity. A literature review indicated a significant correlation (P<0.01) between intrinsic rate of natural increase and maximum body length of naidids with temperature conversion of the growth rate. Sexually mature worms were rarely found in submerged paddy fields. Sexual reproduction seems to be adopted in response to soil desiccation after paddy field drainage.     

Nitrification and denitrification as sources of atmospheric nitrous oxide – role of oxidizable carbon and applied nitrogen

Laboratory incubation experiments were conducted to study the influence of easily oxidizable C (glucose) and mineral N (NH₄⁺ and NO₃^-) on N₂O emission, evolution of CO₂ and consumption of O₂. A flush of N₂O was always observed during the first few hours after the start of soil incubation, which was significantly higher with NH₄⁺ compared to NO₃^- applications. The increase in N₂O emission was attributed mainly to enhanced soil respiration and subsequent O₂ limitation at the microsite level. Application of NH₄⁺ helped to develop denitrifying populations since subsequent additions of NO₃^- and a C source significantly enhanced N₂O emissions. In soils treated with NH₄⁺, N₂O emissions declined rapidly, which was related to decreasing concentrations of easily oxidizable C. Addition of glucose in different amounts and pre-incubation of soil for different lengths of time (to create variation in the amount of easily oxidizable C) changed the pattern of N₂O emissions, which was ascribed to changes in soil respiration.     

Impact of soil nitrogen concentration on Glomus spp.-Sinorhizobium interactions as affecting growth, nitrate reductase activity and protein content of Medicago sativa

Our objective was to evaluate how increasing levels of N in the medium (0, 4, 8 and 16 mmol N added kg^-1 soil) affect the interaction between Sinorhizobium and arbuscular mycorrhiza (AM) fungi in the tripartite symbiosis with Medicago sativa. Growth response, nutrient acquisition, protein content, and nitrate reductase (NR) activity were measured both in plant shoots and roots. Results showed that N levels in soil did not affect mycorrhizal colonization but they strongly influenced nodulation, particularly of mycorrhizal plants. Mycorrhizal colonization was required for a proper nodulation when no N was applied to soil. In contrast, the addition of 4 mmol N kg^-1 soil reduced nodulation only in mycorrhizal plants and 8 mmol N added kg^-1 soil allowed nodule formation only in non-mycorrhizal plants. Nodulation was totally inhibited in all treatments with the addition of 16 mmol N added kg^-1 soil. N addition enhanced NR activity in all the treatments, while AM colonization increased the proportion of NR allocated to roots. This effect was more pronounced under the lowest N levels in the medium. The two AM fungal species showed different distribution pattern of enzymatic activities in plant tissues indicating specific physiological traits. Protein content as well as the relative proportion of protein in roots were greatly increased after mycorrhizal colonization. Glomus intraradices-colonized plants had the highest protein content in shoot and root. Mycorrhizal effects on growth, N acquisition and biochemical variables cannot be interpreted as an indirect P-mediated effect since P content was lower in mycorrhizal plants than in those which were P fertilized. Mycorrhizal colonization increased the N content in plants irrespective of the N level, but the effectiveness of AM fungi on plant N acquisition depended on the AM fungus involved, G. intraradices being the most effective, particularly at the highest N rate. N₂ fixation, enhanced by AM colonization, contributed to N acquisition when a moderate N quantity was available in the soil. Nevertheless, under a high N amount the nodulating process and/or fixing capacity by Sinorhizobium was reduced in AM plants. In contrast, the AM fungal mycelium from a particular mycorrhizal fungus may continue to contribute efficiently to the N uptake from the soil even at high N levels. These results demonstrate the particular sensitivity of AM fungal species in terms of their growth and/or function to increasing N amounts in the medium. A selection of AM fungi used to address specific environmental conditions, such as N fertilization regimes comparable to those used in agronomic practices, is required for a better use of N applied to soil.     

Effects of earthworms on Zn fractionation in soils

Laboratory incubation experiments were conducted to examine the effect of earthworm (Pheretima sp.) activity on soil pH, zinc (Zn) fractionation and N mineralization in three soils. No Zn uptake by earthworms was observed. Zinc addition decreased pH of red soil (soil 1) and hydragric paddy soil (soil 3) by 0.5 and 0.2 unit, respectively, but had no effect on alluvial soil (soil 2). The effect of Zn on soil pH was possibly due to a specific adsorption mechanism between Zn and oxides. Earthworm activity significantly decreased the pH of the red soil, a key factor affecting Zn solubility, but not of the other two soils. Earthworm activity significantly increased DTPA-Zn (DTPA-extractable) and OxFe-Zn (NH₂OH-HCl-extractable) in the red soil, but had little effect on other fractions. In the alluvial soil, earthworm activity significantly increased OxFe-Zn but decreased organic-Zn (organic-associated Zn). In the hydragric paddy soil, earthworm activity significantly increased MgCl₂-Zn (MgCl₂-extractable) and organic-Zn. The level of CaCl₂-extractable Zn in all three soils was not affected by earthworm activity. Nitrogen mineralized as a result of earthworm activity was equivalent to 110, 120 and 30 kg N ha^-1 in soils 1, 2 and 3, respectively. Zinc added at rates less than 400 mg Zn kg^-1 did not seem to affect the activity of N-mineralizing microorganisms. The present results indicated the possibility of increasing the metal bioavailability of relatively low level metal-contaminated soils, with a higher organic matter content, by earthworm inoculation.     

Plant growth enhancement and suppression of Macrophomina phaseolina causing charcoal rot of peanut by fluorescent Pseudomonas

A plant growth-promoting bacterial strain, GRC₂, was isolated from potato rhizosphere and characterized as fluorescent Pseudomonas. It produced a hydroxamate-type siderophore in iron-deficient tryptic soy medium. The production of hydrocyanic acid and indole acetic acid was also recorded under normal growth conditions. The strain showed a strong antagonistic effect against Macrophomina phaseolina, a charcoal rot pathogen of peanut, when co-cultured on tryptic soy agar medium. The growth inhibition of M. phaseolina was 74% after 5 days of incubation. Bacterization of peanut seeds with fluorescent Pseudomonas GRC₂ resulted in increased seed germination, early seedling growth, fresh nodule weight, grain yield and reduced charcoal rot disease of peanut in M. phaseolina-infested soil as compared with control. A streptomycin-resistant marker of the bacterium GRC₂^strep+ was used to monitor root colonization, which positively confirmed the efficient colonization of peanut root. Seed bacterization reduced charcoal rot disease in M. phaseolina-infested soil as compared with the control by 99%, making the organism a potential biocontrol agent against charcoal rot of peanut.     

Changes in sorption/bioavailability of imidacloprid metabolites in soil with incubation time

Changes in sorption/bioavailability of two metabolites, imidacloprid-urea {1-[(6-chloro-3-pyridinyl)methyl]-2-imidazolidinone} and imidacloprid-guanidine {1-[(6-chloro-3-pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine} of the insecticide imidacloprid {1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine} with aging in different soils were determined. Soil moisture was adjusted to -33 kPa and ¹⁴C- and analytical-grade imidacloprid-urea and imidacloprid-guanidine were added to the soil at a rate of 1.0 mg kg^-1. Spiked soils were incubated at 25°C for 8 weeks. Replicate soil samples were periodically extracted successively with 0.01 N CaCl₂, acetonitrile, and 1 N HCl. Imidacloprid-urea sorption, as indicated by sorption coefficient values, was highest in the soil with highest organic C content, and increased by an average factor of 2.6 in three soils during the 8-week incubation period. Imidacloprid-guanidine sorption increased by a factor of 2.3 in the same soils. The increase in sorption was the result of a decrease in the metabolite extractable with CaCl₂ (solution phase); the amount of metabolite extractable with acetonitrile and HCl (sorbed phase) did not significantly change with incubation time. It appears the increase in sorption was because the rate of degradation in solution and on labile sites was faster than the rate of desorption from the soil particles. It may have also been due to metabolite diffusion to less accessible or stronger binding sites with time. Regardless of the mechanism, these results are further evidence that increases in sorption during pesticide aging should be taken into account during characterization of the sorption process for mathematical models of pesticide degradation and transport.     

不同浓度镉胁迫对玉米幼苗光合作用、脂质过氧化和抗氧化酶活性的影响

以玉米为材料,通过营养液培养试验,研究浓度为5~100 μmol/L的镉胁迫后不同时间内,植株体内活性氧代谢及其抗氧化酶活性的变化特征,探讨镉胁迫导致植物体内活性氧自由基累积的原因及不同程度镉胁迫对植物体内活性氧代谢的影响。随着加镉量的增加,玉米地上部生物量明显降低,而根部生物量未表现出差异。镉处理降低了叶片光合作用速率,高镉处理的影响较早。镉处理4d后,5、20、和100 mol/L Cd2+浓度处理玉米叶片Fv/Fm减小,PSII系统的原初光能转换效率下降,但比光合作用速率下降的时间要晚;镉处理7d的叶片中丙二醛（MDA）含量还没有受到明显影响,但20和100 μmol/L Cd2+处理4d后,根系膜质过氧化增强,MDA含量升高。随着镉浓度升高,处理时间延长,活性氧酶清除系统包括超氧化物歧化酶（SOD）、过氧化物酶（POD）、过氧化氢酶（CAT）和谷胱甘肽还原酶（GR）等酶活性明显增加,受到镉胁迫诱导,高浓度镉处理该现象出现更早。本文试验结果表明,镉胁迫下植物体内活性氧形成增多,诱导活性氧酶清除系统活性升高,其中一个重要原因是与CO2同化受到限制有关。     

Overwinter soil denitrification activity and mineral nitrogen pools as affected by management practices

During freeze-thaw events, biophysical changes occurring in soils can affect processes such as mineralization, nitrification and denitrification which control inorganic N balances in agro-ecosystems. To evaluate the impact of these climatic events on soil biochemical properties, a study was conducted comparing soil denitrification enzyme activity (DEA), dissolved organic C (DOC) and inorganic N levels before and after the winter season in plots under: (1) continuous corn (Zea mays L.) (CC) with annual chisel plow and disking, (2) corn-soybean (Glycine max L.) (CS) rotation with chisel plow every other year prior to planting soybean, and (3) corn-soybean-wheat (Triticum aestivum L.)/hairy vetch (Vicia villosa Roth) (CSW-V) with ridge tillage during the corn and soybean crops, and dairy manure application during the corn year. Soil cores were collected in late autumn and immediately after spring thaw at 0-5, 5-10, 10-15, and 15-30 cm depths. Regardless of management practices, freeze-thaw events resulted in significant (2-10 times) increases in NH₄⁺-N, NO₃^--N (P<0.001) and DOC (P<0.01) levels at all soil depths. Following freeze-thaw, DEA remained unchanged in the 5-30 cm depth but dropped significantly (P<0.01) in the 0-5 cm soil layer. In that layer, soils which had been chisel plowed during the previous growing season lost 78-84% of the DEA recorded during the fall, whereas in the plots amended with manure during the previous season, the loss of activity was 40-45%. These data indicate that frequent tillage, compared with manure additions, is more conducive to overwinter loss of DEA in surface layers of soils subject to freeze-thaw cycles.     

Correlations between pesticide transformation rate and microbial respiration activity in soil of different ecosystems

Cecil sandy loam soils (ultisol) from forest (coniferous and deciduous), pasture, and arable ecosystems were sampled (0-10 cm) in the vicinity of Athens, Georgia, USA. Soil from each site was subdivided into three portions, consisting of untreated soil (control) as well as live and sterile samples treated with the fungicide metalaxyl and the herbicide propachlor at 10 mg kg^-1 soil. Pesticide transformation rate, basal respiration (basal) and substrate-induced respiration (SIR) rates, and microbial metabolic quotient (qCO₂) were measured for the initial application of metalaxyl [methyl-N-(2,6-dimethylphenyl)-N-(metoxyacetyl)-DL-alaninate] or propachlor (2-chloro-N-isopropyl-acetanilide) at 22°C and 60% water holding capacity. Positive correlations were found for the following: metalaxyl transformation rate constant (K_met) and basal (r=0.73); K_met and SIR (r=0.83); propachlor transformation rate constant (K_pr) and basal (r=0.89); and K_pr and SIR (r=0.91). Regression analysis of pesticide transformation rate and soil respiration activity, coupled with specific soil properties (pH, C_org, and clay content), revealed a positive correlation between K and SIR for C_org (r=0.88 and 0.98, for metalaxyl and propachlor, respectively). qCO₂s were not significantly different (P=0.05) in propachlor-amended and pesticide-free soils. Metalaxyl amendment resulted in a change in the ecophysiological status of the soil microbial community as expressed by qCO₂. The qCO₂ values in metalaxyl-amended soils were significantly greater (P=0.05) in pine forest (by 25%) and arable and pasture (by 20%) soils compared to unamended soils. Differences in qCO₂ values may represent the magnitude of pesticide-induced disturbance. The duration of this disturbance was greater in the pine forest soil (48 days) compared to arable and pasture soils (21 and 15 days, respectively).     

Organic matter and aggregation in a degraded potato soil as affected by raw and composted pulp residue

Soils under intensive potato (Solanum tuberosum L.) production are often low in organic matter content and microbial activity. Pulp fibre residue addition may restore the quality of these soils. An experiment was initiated in which raw or composted pulp fibre waste was added to a Fredericton sandy loam (Orthic Humo-Ferric Podzol) in New Brunswick (Canada). Each material was applied at 45 Mg (dry weight) ha^-1 and 90 Mg (dry weight) ha^-1 before planting. Soil was under continuous potato (cv. Russet Burbank) for 3 years. The raw and the composted pulp greatly increased soil organic matter (SOM) content, the C/N ratio, macroaggregation, C mineralization, microbial biomass C (MBC) and enzyme activities for the whole duration of the experiment but had no effect on its total N. Most of the SOM increase was found in the macroorganic matter fraction. The SOM, C/N ratio, macroaggregation, C mineralization and MBC decreased from the time of application. No major difference was found between materials except for CO₂-C release in the incubation which was higher for the composted than for the raw pulp. This study indicated that pulp fibre, either applied raw or composted, increased SOM and macroaggregation and promoted microbial growth and activity in this potato soil which was low in C content. The beneficial effects on soil physical and biochemical properties were still present after 3 years of continuous cropping.     

A spontaneous chlorate-resistant mutant of Azospirillum brasilense Sp245 displays defects in nitrate reduction and plant root colonization

Azospirillum, a soil bacterium capable of colonizing plant roots, can reduce NO₃^-. In this work, a spontaneous chlorate-resistant mutant of Azospirillum brasilense Sp245, named Sp245chl1, was phenotypically characterized. The mutant is defective in both assimilatory and periplasmic dissimilatory nitrate reductase activity. Using the gusA reporter gene methodology, Sp245chl1 was found to be significantly affected in its ability to colonize roots of wheat and rice seedlings.     

Different behavioral patterns of the earthworms Octolasion tyrtaeum and Diplocardia spp. in tallgrass prairie soils: potential influences on plant growth

This study addressed differences between Diplocardia spp. (a native North American earthworm) and Octolasion tyrtaeum (an introduced European species), with respect to behavior, influence on soil microbial biomass, and plant uptake of N in tallgrass prairie soils. We manipulated earthworms in PVC-encased soil cores (20 cm diameter) over a 45-day period under field conditions. Treatments included: (1) control with no earthworms, (2) Diplocardia spp. only, and (3) O. tyrtaeum only. Prior to addition of earthworms, seedlings of Andropogon gerardii (a dominant tallgrass) were established in each core, and a dilute solution of ¹³C-labeled glucose and ¹⁵N-labeled (NH₄)₂SO₄ was added to the soil to facilitate examination of earthworm/microbe/plant interactions. We found that Diplocardia spp. were significantly more active than O. tyrtaeum, and quickly assimilated ¹³C and ¹⁵N from the tracer. Individuals of Diplocardia spp. were present at shallower soil depths than O. tyrtaeum throughout the study. Contrary to expectation, this greater activity of Diplocardia spp. did not result in increased plant productivity. Rather, the activity of Diplocardia spp. was associated with less plant growth and smaller amounts of N acquired by A. gerardii seedlings compared to controls or O. tyrtaeum treatments. We observed few significant influences of earthworm treatments on microbial biomass C or N pool sizes, but the microbial C/N ratio was consistently greater in the presence of Diplocardia spp. relative to O. tyrtaeum. Results of this study indicate that activity of earthworms may enhance competition for N between microbes and plants during the growing season in tallgrass prairie.