Biologically-induced hydrolysis of parathion in soil: Isolation of hydrolyzing bacteria

Fifty bacterial isolates from a parathion-treated soil (Gilat, Israel) were tested for their ability to hydrolyze the organophosphorus insecticide, parathion in peptone-yeast extract medium. After 5 days 33 isolates had hydrolyzed at least a portion of the added parathion. Eight of these isolates hydrolyzed 75% of the added parathion in 5 days and appeared to be Bacillus strains. Ten of these 33 isolates had hydrolyzed all of the parathion after 5 days and appeared to be Arthrohacter strains. One isolate from each group was tested further. During the logarithmic phase of growth, Bacillus sp., isolate 10, hydrolyzed less than 10% of the parathion added to peptone-yeast extract medium and was not active in parathion hydrolysis when inoculated into sterilized, parathion-treated soil. Arthrobacter sp., isolate 6, hydrolyzed parathion rapidly in peptone-yeast extract medium and in sterilized, parathion-treated soil. It used parathion or its hydrolysis product, p-nitrophenol, as sole carbon source. The parathion hydrolyzing enzyme appeared to be constitutive in isolate 6. Single applications of p-nitrophenol at concentrations greater than 1 mM inhibited growth but successive additions of smaller amounts permitted growth to continue.     

The Influence of Surfactants and Metals on the Mineralization of 14C-Labeled Methyl Parathion in a Mediterranean Soil under Aerobic Conditions

The mineralization of ¹⁴C-labeled methyl parathion, in a soil from a semiarid Mediterranean area, was followed by a simple method. This method consisted in trapping evolved CO₂ in a NaOH trap, using common laboratory glassware and low quantities of reagents. Volatiles were trapped in polyurethane foams, which are cheap and easy to handle. The influence of soil water content (10?C20%) and the addition of surfactants [Aerosol 22 and hexadecyl trimethyl ammonium bromide (HD)] and metals (Cu and Zn) were assessed. Different models were applied to evaluate parathion mineralization kinetics. Mineralization kinetics was initially faster for 20% soil water content, although the final mineralization extent was similar in the range of tested soil water content. Surfactants behaved differently: HD decreased mineralization rate while Aerosol 22 did not affect parathion mineralization. A bimetal Cu and Zn solution led to a significant decrease in methyl parathion mineralization, which persisted when applied together with HD. Results suggest a possible reduction of pesticide availability in soil or a direct effect of the exogenous chemicals on soil microbial community.     

Application of eco-physiological quotients (qCO2 and qD) on microbial biomasses from soils of different cropping histories

Metabolic quotients for CO₂C (qCO₂C) and microbial-C-loss (qD) were studied on soil microbial communities under long-term monoculture (M) or continuous crop rotations (CR). Under defined laboratory conditions the mean qCO₂C (unit CO₂C unit⁻¹ C_mic h⁻¹) of different microbial biomasses from 17 M systems amounted to 1.097 μg CO₂qCO₂CC as compared to 0.645 μg CO₂C of microbial biomasses from 19 CR systems. The 1.7 times higher CO₂C release per unit biomass and time of microbial biomasses from M systems was significantly different at the P =0.001 level.In addition, microbial C-loss in samples from M or CR plots was followed for 5 weeks. Again, mean qD per unit microbial biomass and time was 1.6 times higher (P = 0.01) for microbial biomasses from M systems (0.301 μg C, 14 soils) when compared with CR systems (0.188μg C, 14 soils).These differences were not related to soil texture, C_org or pH of these soils. The effects of environmental influences (soil management) on the microbial pool in terms of a changing energy demand are discussed.     

Turnover of microbial tissue in soil under field conditions

The microbial population of a Brown Chernozemic soil was labelled in situ by adding ¹⁴C-glucose and ¹⁵NH₄¹⁵NO₃ to the plow layer. The loss of ¹⁴C, nitrogen immobilization-mineralization reactions, bacterial numbers (plate count, direct count) and fungal hyphal lengths were determined periodically throughout the growing period in amended and unamended microplots and in the surrounding field soil. After 5 days, 90 per cent of the labelled N occurred in the organic form with little subsequent mineralization. Of the labelled C added, 63, 56 and 39 per cent, remained in the soil after 3, 14 and 104 days, respectively.The ratio of fungal C to bacterial C increased as soil moisture decreased. Viable (plate count) and total numbers of bacteria in samples from unamended plots and field soil were significantly correlated with each other and with soil moisture. Fungal hyphal lengths from amended soil were also significantly related to moisture but the rate of loss of ¹⁴C and mineralization of ¹⁵N were not. The synthesized microbial material (tissue and metabolites) exhibited a high degree of stability throughout the study. The half-life of labelled C remaining in the soil after 30 days was calculated to be 6 months compared to only 4 days for the added glucose C. The amount of energy used for maintenance by the soil population under field conditions was calculated from measurements of biomass C, respired labelled C and respired soil C.     

Decomposition and distribution of residual activity of some 13C-microbial polysaccharides and cells,glucose, cellulose and wheat straw in soil

Studies were made to determine the rate of decomposition of some ¹⁴C-labeled microbial polysaccharides, microbial cells, glucose, cellulose and wheat straw in soil, the distribution of the residual ¹⁴C in various humic fractions and the influence of the microbial products on the decomposition of plant residues in soil. During 16 weeks from 32 to 86 per cent of the C of added bacterial polysaccharides had evolved as ¹⁴CO₂. Chromobacterium violaceum polysaccharide was most resistant and Leuconostoc dextranicus polysaccharide least resistant. In general the polysaccharides, microbial cells, and glucose exerted little effect on the decomposition of the plant products. Upon incubation the ¹⁴C-activity was quickly distributed in the humic. fulvic and extracted soil fractions. The pattern of distribution depended upon the amendment and the degree of decomposition. The distribution was most uniform in the highly decomposed amendments. After 16 weeks the bulk of the residual activity from Azotobacter indicus polysaccharide remained in the NaOH extracted soil. From C. violaceum polysaccharide both the extracted soil and the humic acid fraction contained high activity. About 50–80 per cent of the residual activity from the ¹⁴C-glucose, cellulose and wheat straw amended soils could be removed by hydrolysis with 6 n HCl. The greater part of this activity in the humic acid fraction was associated with the amino acids and that from the fulvic acids and residual soils after NaOH extraction with the carbohydrates. About 8 16 per cent of the activity of the humic acid fraction was present in substances (probably aromatic) extracted by ether after reductive or oxidative degradation.     

Salinity and the persistence of parathion in flooded soils

The persistence of parathion in five coastal saline soils of varying electrical conductivity and in one nonsaline soil sample was studied under flooded conditions. Parathion was decomposed faster in nonsaline soil than in saline soils and its stability increased with increasing electrical conductivity. The addition of salts to the nonsaline soil at 4, 8 and 16 dS^?1 increased the persistence of parathion. Nitro-group reduction, and not hydrolysis, was the major route of parathion degradation in saline and nonsaline soils. The accumulation of aminoparathion was less pronounced in saline soils than in nonsaline soil concomitant with slow degradation of parathion in saline soils. The inhibition of nitro-group reduction in saline soils was related to low microbial activities as reflected in decreased dehydrogenase activity and slow iron reduction.     

Homogeneous inoculation vs. microbial hot spots of isolated strain and microbial community: What is the most promising approach in remediating 1,2,4-TCB contaminated soils?

We investigated the effectiveness of different inoculation approaches in enhancing the mineralization of [U-¹⁴C] labeled 1,2,4-trichlorobenzene (1,2,4-TCB) in soil. Inoculation was conducted with a soil-borne 1,2,4-TCB mineralizing microbial community (MC) as well as the Bordetella sp. strain F2 originally isolated from this community as the key degrader organism (IS). Both were applied either via liquid medium (LM) or attached on clay particles (CP). Fluorescence in-situ hybridization in combination with ¹⁴C-1,2,4-TCB mineralization measurements as well as measurements of ¹⁴C-residues in soil were used to investigate the bioaugmentation efficiency of the different approaches. Bordetella sp. cell numbers increased about 2-5 times during the incubation process, indicating that the bacteria could survive and develop in the new soil habitat. While the native soil showed negligible 1,2,4-TCB mineralization rates, soil inoculated with the MC attached on CP showed the highest 1,2,4-TCB mineralization rate per Bordetella cell, whereas the other inoculum approaches showed an increased but lower contaminant mineralization. Additionally, the MC-CP approach showed the highest cumulative 1,2,4-TCB mineralization as well as the highest formation of bound ¹⁴C-residues which is most likely equivalent to ¹⁴C incorporated into the microbial biomass. Thus, our results allow the conclusion that the application of a specific microbe-clay-particle-complex is the most promising approach for an accelerated in-situ mineralization of chemicals in agricultural soils.     

Methyl parathion degradation in soil: Influence of soil-water tension

Four soil-water tensions ranging from 10 kPa to 1.5 MPa were employed to study the effect of soil-water tension on methyl parathion degradation, metabolism and bound-residue formation in two soils. Uniformly ring-labeled [¹⁴C]methyl parathion was used. Mineralization was rapid in soils at 10 and 33 kPa. Results from the disappearance of ¹⁴C-activity indicated that methyl parathion could be also rapidly mineralized in soils at 100 kPa, while mineralization at 1.5 MPa was slower. Nonextractable ¹⁴C-activity (bound residues) was formed rapidly during 7–14 days in the soils maintained at 100 kPa and lower. The formation of nonextractable ¹⁴C-activity in the 1.5 MPa soils was slower but increased steadily during 28 days. Since no methyl aminoparathion was detected, it was suggested that the insecticide was hydrolyzed initially to p-nitrophenol and subsequently was reduced to p-aminophenol. The reduction was favored in moist soils (10 and 33 kPa) but was resisted in dry soil (1.5 MPa).     

The rapid oxidation of atmospheric CO to CO2 by soils

Gas exchange rates over soils were measured in a closed, flowing-gas system. ¹⁴CO was rapidly oxidized to ¹⁴CO₂ with only a minor loss in atmospheric radioactivity. Incorporation of ¹⁴C into the soil was slight and was via ¹⁴CO₂ rather than ¹⁴CO. CO oxidation was a microbial process and no oxidation occurred when soils had been autoclaved. The rate of CO depletion was concentration dependent and followed Michaelis-Menten kinetics. The rate constants K_m and V_max ranged from 18 to 51 μ 1^?1 CO and from 0.58 to 4.35 mg C kg^?1 dry soil h^?1 respectively. The maximum rate of reaction for Hubbard Brook soil was about an order of magnitude greater than any soil previously reported. The oxidation reaction was accompanied initially by a reduction in net soil respiration. This was then followed by a period of high respiration which continued until CO levels were reduced to about 5μll^?1. Thereafter respiration fell below the pretreatment rate and only returned to that rate 45 min after CO had been depleted from the atmosphere. The data suggest that at high CO concentrations (40–100 μll^?1CO) autotrophic carboxydobacteria comprise the main component of the CO-oxidizing population and, as the concentration declines towards ambient levels they are replaced by heterotrophic microorganisms possessing a cometabolic process.     

Discrete functional pools of soil organic matter in a UK grassland soil are differentially affected by temperature and priming

We show that both temperature and priming act differently on distinct C pools in a temperate grassland soil. We used SOM which was ¹⁴C-labelled in four different ways: by labelling soil with ¹⁴C-glucose, by adding leaf litter from plants pre-labelled with ¹⁴CO₂, and by labelling in situ with ¹⁴CO₂ applied to the ryegrass canopy either 6 or 18 months earlier. Samples of each type of ¹⁴C labelled soil were incubated at either 4, 10, 15, or 20 °C and the exponential loss of ¹⁴CO₂ used to characterise treatment effects. ¹⁴C allocation to microbial fractions was greater, and so overall mineralization by microbes was greater, as temperature rose, but turnover of the microbial labile pool was temperature-insensitive, and the turnover of microbial structural material was reduced as temperature rose. The ability of the microbial population to degrade just one fraction of plant litter was increased greatly by temperature. A pool of SOM with a half-life of about 70 d was degraded faster at higher temperatures. Less tractable but abundant pools of SOM were not accessed more readily at higher temperatures by the microbial population. Priming with glucose or amino-acids only speeded the mineralization of recent SOM (probably from the living microbial biomass), and was not altered by temperature. These results have implications for the impacts of climate change on soil C cycling.     

A field study of gross rates of N mineralization and nitrification and their relationships to microbial biomass and enzyme activities in soils treated with dairy effluent and ammonium fertilizer

Abstract. Gross N mineralization and nitrification rates were measured in soils treated with dairy shed effluent (DSE) (i.e. effluent from the dairy milking shed, comprising dung, urine and water) or ammonium fertilizer (NH₄Cl) under field conditions, by injecting ¹⁵N-solution into intact soil cores. The relationships between gross mineralization rate, microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) as affected by the application of DSE and NH₄Cl were also determined. During the first 16 days, gross mineralization rate in the DSE treated soil (4.3–6.1 μg N g^?1 soil day^?1) were significantly (P 14;< 14;0.05) higher than those in the NH₄Cl treated soil (2.6–3.4 μg N g^?1 soil day^?1). The higher mineralization rate was probably due to the presence of readily mineralizable organic substrates in the DSE, accompanied by stimulated microbial and extracellular enzyme activities. The stable organic N compounds in the DSE were slow to mineralize and contributed little to the mineral N pool during the period of the experiment. Nitrification rates during the first 16 days were higher in the NH₄Cl treated soil (1.7–1.2 μg N g^?1 soil day^?1) compared to the DSE treated soil (0.97–1.5 μg N g^?1 soil day^?1). Soil microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) increased after the application of the DSE due to the organic substrates and nutrients applied, but declined with time, probably because of the exhaustion of the readily available substrates. The NH₄Cl application did not result in any significant increases in microbial biomass C, protease or urease activities due to the lack of carbonaceous materials in the ammonium fertilizer. However, it did increase microbial biomass N and deaminase activity. Significant positive correlations were found between gross N mineralization rate and soil microbial biomass, protease, deaminase and urease activities. Nitrification rate was significantly correlated to biomass N but not to the microbial biomass C or the enzyme activities. Stepwise regression analysis showed that the variations of gross N mineralization rate was best described by the microbial biomass C and N.     

清水灌溉在消除土壤多环芳烃中的作用及对土壤微生物特征的影响: 以沈抚灌区为例

To evaluate the effect of groundwater irrigation on the polycyclic aromatic hydrocarbons(PAHs) pollution abatement and soil microbial characteristics,a case study was performed in the Shenfu irrigation area of Shenyang,Northeast China,where the irrigation with petroleum wastewater had lasted for more than fifty years,and then groundwater irrigation instead of wastewater irrigation was applied due to the gradually serious PAHs pollution in soil.Soil chemical properties,including PAHs and nutrients contents,and soil microbial characteristics,including microbial biomass carbon,substrateinduced respiration,microbial quotient(qM),metabolic quotient(qCO2),dehydrogenase(DH),polyphenol oxidase(PO),urease(UR) and cellulase(CE) in surface and subsurface were determined.Total organic C,total N,total P,and available K were significantly different between the sites studied.The PAHs concentrations ranged from 610.9 to 6362.8 μg kg-1 in the surface layers(0-20 cm) and from 404.6 to 4318.5 μg kg-1 in the subsurface layers(20-40 cm).From the principal component analysis,the first principal component was primarily weighed by total PAHs,total organic C,total N,total P and available K,and it was the main factor that influencing the soil microbial characteristics.Among the tested microbial characteristics,DH,PO,UR,CE,qM and qCO2 were more sensitive to the PAHs stress than the others,thus they could serve as useful ecological assessment indicators for soil PAHs pollution.     

Studies of nitrogen immobilization and mineralization in calcareous soils—V. Formation and distribution of isotope-labelled biomass during decomposition of 14C- and 15N-labelled plant material

Medicago littoralis leaf material, labelled with ¹⁴C and ¹⁵N, and of C:N ratio 8.7:1, decomposed rapidly in a calcareous soil. One half of the plant-C and two thirds of the plant-N remained in the soil as organic residues after 34 days. The rates of decomposition and the changes in the distribution of organic-¹⁴C and -¹⁵N residues followed similar patterns.Incorporation of ¹⁴C and ¹⁵N into microbial cells, formed during plant breakdown, reached a maximum after 62 days. At this time the microbial biomass accounted for 21.9 and 23.3%, respectively, of residual organic-¹⁴C and -¹⁵N. Thereafter, the amounts of isotope-labelled biomass decreased with the percentage decrease slightly exceeding that of the total labelled soil residues.During plant decomposition, changes occurred in the concentrations of organic-¹⁴C and -¹⁵N in some of the soil components, these having been fractionated according to density and particle size. Especially evident was the rapid and extensive decrease of labelled material from the fine clay-size components. This was partly due to the decrease in the biomass-¹⁴C of this fraction. Changes in biomass-¹⁴C of some physical fractions were approximately reflected by changes in their numbers of viable microorganisms.     

Budgets for root-derived C and litter-derived C: comparison between conventional tillage and no tillage soils

Placement of plant residues in conventional tillage (CT) and no-tillage (NT) soils affects organic matter accumulation and the organization of the associated soil food webs. Root-derived C inputs can be considerable and may also influence soil organic matter dynamics and soil food web organization. In order to differentiate and quantify C contributions from either roots or litter in CT and NT soils, a ¹⁴C tracer method was used.To follow root-derived C, maize plants growing in the field were ¹⁴C pulse-labeled, while the plant litter in those plots remained unlabeled. The ¹⁴C was measured in NT and CT soils for the different C pools (shoots, roots, soil, soil respiration, microbial biomass). Litter-derived C was followed by applying ¹⁴C labeled maize litter to plots which had previously grown unlabeled maize plants. The ¹⁴C pools measured for the litter-derived CT and NT plots included organic matter, microbial biomass, soil respiration, and soil organic C.Of the applied label in the root-derived C plots, 35–55, 6–8, 3, 1.6, and 0.4–2.4% was recovered in the shoots, roots, soil, cumulative soil respiration, and microbial biomass, respectively. The ¹⁴C recovered in these pools did not differ between CT and NT treatments, supporting the hypothesis that the rhizosphere microbial biomass in NT and CT may be similar in utilization of root-derived C. Root exudates were estimated to be 8–13% of the applied label. In litter-derived C plots, the percentage of applied label recovered in the particulate organic matter (3.2–82%), microbial biomass (4–6%), or cumulative soil respiration (12.5–14.7%) was the same for CT and NT soils. But the percentage of ¹⁴C recovered in CT soil organic C (18–69%) was higher than that in NT (12–43%), suggesting that particulate organic matter (POM) leaching and decomposition occurred at a higher rate in CT than in NT. Results indicate faster turnover of litter-derived C in the CT plots.     

Comparison of chloroform fumigation-extraction, phospholipid fatty acid, and DNA methods to determine microbial biomass in forest humus

Techniques developed to measure microbial biomass in mineral soils may not give reliable results in humus. We evaluated the relationships between three techniques to estimate microbial biomass in forest humus: chloroform fumigation-extraction (CFE), total extractable phospholipid fatty acids (PLFA), and extractable DNA. There was a good relationship between PLFA and CFE (R²=0.96), with a slope slightly different from that previously reported for mineral soils (1 nmol PLFA corresponded to a flush of 3.2 μg C released by fumigation in humus cf. 2.4 μg C in mineral soil). There was no relationship between DNA concentration and the other two measurements of microbial biomass. This may be due, in part, to the high fungal biomass in forest humus, as DNA concentration per unit biomass is much more variable for fungi than bacteria.     

Significance of microbial biomass for carbon and nitrogen mineralization in soil

C and N mineralization was quantified in an incubation experiment with two samples containing different amounts of microbial biomass. The samples from two layers (0–20, 20–30 cm) of an arable luvisol from loess were fertilized with nitrate, mixed with ¹⁴C-labelled straw and incubated for 52 days at different O₂ levels. Decreasing O₂ concentrations (21, 2, 1 and 0% O₂) in soil conducted a decrease in C and N mineralization. More C and N were mineralized in samples with a higher initial microbial biomass. The differences in microbial biomass were still present at the end of the experiment, but more proliferation was detected in samples with the lower initial microbial biomass, leading to equal ratios between microbial biomass-C and soil organic C in both soils.     

Interactions between 14C-labelled atrazine and the soil microbial biomass in relation to herbicide degradation

A laboratory incubation experiment was set up to determine the effects of atrazine herbicide on the size and activity of the soil microbial biomass. This experiment was of a factorial design (0, 5, and 50 g g^–1 soil of non-labelled atrazine and 6.6×10³ Bq g^–1 soil of ¹⁴C-labelled atrazine) x (0, 20, and 100 g g^–1 soil of urea-N) x (pasture or arable soil with a previous history of atrazine application). Microbial biomass, measured by substrate-induced respiration and the fumigation-incubation method, basal respiration, incorporation of ¹⁴C into the microbial biomass, degradation of atrazine, and ¹⁴C remaining in soil were monitored over 81 days. The amount of microbial biomass was unaffected by atrazine although atrazine caused a significant enhancement of CO₂ release in the non-fumigated controls. Generally, the amounts of atrazine incorporated into the microbial biomass were negligible, indicating that microbial incorporation of C from atrazine is not an important mechanism of herbicide breakdown. Depending on the type of soil and the rate of atrazine application, 18–65% of atrazine was degraded by the end of the experiment. Although the pasture soil had twice the amount of microbial biomass as the arable soil, and the addition of urea approximately doubled the microbial biomass, this did not significantly enhance the degradation of atrazine. This suggests that degradation of atrazine is largely independent of the size of the microbial biomass and suggests that other factors (e.g., solubility, chemical hydrolysis) regulate atrazine breakdown. A separate experiment conducted to determine total amounts of ¹⁴C-labelled atrazine converted into CO₂ by pasture and arable soils showed that less than 25% of the added ¹⁴C-labelled atrazine was oxidised to ¹⁴CO₂ during a 15-week period. The rate of degradation was significantly greater in the arable soil at 24%, compared to 18% in the pasture soil. This indicates that soil microbes with previous exposure to atrazine can degrade the applied atrazine at a faster rate.     

Fate of bound methyl parathion residues in soils as affected by agronomic practices

The fate of [ring-¹⁴C]methyl parathion in a silt loam soil was monitored during a 49-day incubation. After this period, 54% of the initial ¹⁴C remained in the soil; of this, 13% was soxhletextractable with methanol and 87% was bound residue. Soils were then treated with inorganic and organic amendments and incubated for an additional 70 days. Release of methyl parathion bound residues could not be demonstrated, but both bound and extractable ¹⁴C were mineralized to ¹⁴CO₂, CO₂ was evolved slowly and continuously by the controls and where soil was amended with H₂SO₄, (NH₄)₂SO₄, NH₄OH, chitin, oat seedlings or oat straw. Glucose and asparagine caused higher rates of ¹⁴CO₂ production. HgCl₂ gave very high initial rates of ¹⁴CO₂ loss; the rate declined to that of the control only after 9–10 weeks. The lime treatment exceeded the controls after 1 week, declining only slightly with time. The effects of sewage sludge and dairy manure were similar to the controls except that: sludge caused a very high initial release of ¹⁴CO₂, and both treatments gave an unaccountable loss of ¹⁴C, perhaps as ¹⁴CH₄ resulting from the formation of anaerobic conditions. By 70 days, amounts of extractable ¹⁴C and bound ¹⁴C had both declined twice as rapidly in certain soils as in unamended controls.Studies carried out with soxhlet-extracted soils, containing only bound residues, indicated that the soil microflora able to mineralize bound residues without any appreciable buildup of ¹⁴C activity in the extractable phase.     

Turnover of root-derived material and related microbial biomass formation in soils of different texture

Wheat plants were grown on two soils of different texture, a sandy soil and a silty clay loam, in an atmosphere containing ¹⁴CO₂. The ¹⁴C and total C content of the shoots, roots, soil rhizosphere CO₂ and soil microbial biomass were measured 21, 28, 35 and 42 days after germination. There was a pronounced effect of soil texture on the turnover of root-derived C through the microbial biomass. Turnover was relatively fast and at a constant rate in the sandy soil but slowed down in the clay soil, following an initial high assimilation of root products into the microbial biomass.Four percent of the total fixed ¹⁴C was retained in the clay loam after 6 weeks compared with a corresponding value of 1.2% for the sandy soil. The proportion of fixed ¹⁴C recovered as rhizosphere CO₂ at each of the sampling times was relatively constant for the sandy soil (ca 19%) but decreased from 17% at day 28 to 11% at day 42 in the clay soil. The proportion of total fixed ¹⁴C in the soil biomass as measured by a fumigation technique increased to a maximum value of 20% after 6 weeks in the sandy soil but decreased in the clay soil from 86% at day 21 to 26% after 42 days plant growth.     

Production of root-derived material and associated microbial growth in soil at different nutrient levels

Summary Maize plants were grown for 42 days in a sandy soil at two different mineral nutrient levels, in an atmosphere containing ¹⁴CO₂. The ¹⁴C and total carbon contents of shoots, roots, soil and soil microbial biomass were measured 28, 35 and 42 days after germination. Relative growth rates of shoots and roots decreased after 35 days at the lower nutrient level, but were relatively constant at the higher nutrient level. In the former treatment, 2% of the total ¹⁴C fixed was retained as a residue in soil at all harvests while at the higher nutrient level up to 4% was retained after 42 days. Incorporation of ¹⁴C into the soil microbial biomass was close to its maximum after 35 days at the lower nutrient level, but continued to increase at the higher level. Generally a good agreement existed between microbial biomass, ¹⁴C contents and numbers of fluorescent pseudomonads in the rhizosphere. Numbers of fluorescent pseudomonads in the rhizosphere were maximal after 35 days at the lower nutrient level and continued to increase at the higher nutrient level. The proportions of the residual ¹⁴C in soil, incorporated in the soil microbial biomass, were 28% to 41% at the lower nutrient level and 20%6 – 30% at the higher nutrient level. From the lower nutrient soil 18%6 – 52%6 of the residual soil ¹⁴C could be extracted with 0.5 N K₂SO₄, versus 14%6 – 16% from the higher nutrient soil.Microbial growth in the rhizosphere seemed directly affected by the depletion of mineral nutrients while plant growth and the related production of root-derived materials continued.