Bacterial community structure in fumigated soil

Soil microbial biomass has been determined since the mid 1970's by the chloroform fumigation incubation technique as proposed by Jenkinson and Powlson (1976). The microbial biomass C can be determined by subtracting the CO₂ emitted from an unfumigated soil (mineralization of soil organic matter) from that emitted from a chloroform fumigated inoculated soil (mineralization of soil organic matter and killed soil microorganisms) and dividing the difference by a proportionality factor (k_C = 0.45). The question remained which microorganisms recolonized a fumigated soil. An arable soil was fumigated for one day with ethanol-free chloroform or left unfumigated and incubated aerobically after removal of the chloroform for 10 days. The bacterial population structures were determined in the fumigated and unfumigated soil after 0, 1, 5 and 10 days by means of 454 pyrosequencing of the 16S rRNA gene. Fumigating the arable soil reduced significantly the relative abundance of phylotypes belonging to different groups, but increased the relative abundance of only four genera belonging to two phyla (Actinobacteria and Firmicutes) and two orders (Actinomycetales and Bacillales). The relative abundance of phylotypes belonging to the Micromonospora (Micromonosporaceae) increased significantly from 0.2% in the unfumigated soil to 6.7% in the fumigated soil and that of Bacillus (Bacillaceae) from 3.6% to 40.8%, Cohnella (Paenibacillaceae) from undetectable amounts to 0.6% and Paenibacillus (Paenibacillaceae) from 0.3% to 4.2%. The relative percentage of phylotypes belonging to the Acidobacteria, Bacteroidetes, Chloroflexi, Gemmatimonadetes and Proteobacteria (α- β-, δ- and γ-Proteobacteria) were significantly lower in the fumigated than in the unfumigated soil and in most of them the relative abundance of different bacterial orders (i.e. Gp3, Gp4, Gp6, Sphingobacteriales, Gemmatimonadales, Rhodospirillales, Burkholderiales, Xanthomonadales) was reduced strongly (P < 0.001). It was found that the relative abundance of a wide range of bacteria was reduced shortly after fumigating an arable soil, but only a limited group of bacteria increased in a fumigated arable soil indicating a capacity to metabolize the killed soil microorganisms or recolonize a fumigated soil.     

Microbial activity and hydrolase synthesis in long-term Cd-contaminated soils

Alkaline and acid phosphomonoesterase, β-glucosidase, arylsulfatase, protease and urease activities, CO₂-C evolution and ATP content were monitored in long-term Cd-contaminated (0-40 mg Cd kg⁻¹ dry weight soil) sandy soils, kept under maize or ‘set aside’ regimes, amended with plant residues. The organic matter input increased soil respiration, ATP contents and hydrolase activities in all soils. However, the Cd-contaminated soils had significantly higher metabolic quotients (qCO₂), as calculated by the CO₂-to-ATP ratio, and significantly lower hydrolase activities and hydrolase activity-to-ATP ratios for alkaline phosphomonoesterase, arylsulfatase and protease activities, compared with the respective uncontaminated soils. The ratios between acid phosphomonoesterase, β-glucosidase and urease activities and ATP were unaffected. A significantly higher qCO₂/μ ratio, an expression of maintenance energy, was observed in most of the contaminated soils, indicating that more energy was required for microbial synthesis in the presence of high Cd concentrations. It was concluded that exposure to high Cd concentrations led to a less efficient metabolism, which was responsible for lower enzyme activity and synthesis and lower hydrolase activity-to-ATP ratios observed in these Cd-contaminated soils.     

Examination of ester sulfates in Podzolic and Regosolic soils using an immobilized arylsulfatase reactor

The enzyme kinetics of an immobilized arylsulfatase reactor were examined. We found that the optimum operating conditions for the reactor were pH 7.0 and 25°C, using p-nitrophenyl sulfate in acetate buffer. The Michaelis constant (K _m) of immobilized arylsulfatase was 5.29 mM, compared with a K _m of 2.18 mM for soluble arylsulfatase from the same source (Helix pomatia). Since arylsulfatase hydrolyzes organic ester sulfate linkages, the immobilized arylsulfatase reactor was used to examine ester sulfate compounds in two soils subjected to different fertility management schemes. Soil samples were obtained from the Ap horizons of a Podzol from S-amended wheat plots and a Regosol from dykeland hayfield plots which had received additions of NH₄NO₃ and compost. The distribution of S in these soils was examined in the fall of 1993 and the spring of 1994. Soil organic matter was extracted and separated into three molecular weight fractions (<500, 500–10 000, >10 000). There was no difference in the ester sulfate content for the >10 000 fraction of control and S-amended Podzol soils; however, the S-amended samples had significantly higher quantities of hydrolysable ester sulfates than controls for the 500–10 000 range, indicating that S amendments resulted in the incorporation of ester sulfate into this lower molecular weight fraction. Both control and NH₄NO₃ treatments to the Regosol showed significantly higher quantities of hydrolysable ester sulfates in the >10 000 fraction, while compostamended plots showed no difference between the >10 000 and 500–10 000 fractions due to suspected microbial degradation of high molecular weight organic S compounds in the compost. Since there was no significant effect of sampling time, this study indicated that naturally occurring low molecular weight ester sulfate compounds accumulate in soil and persist during storage. Hydrolysable ester sulfates constituted 35–55% of the hydriodic acid-reducible S in these different soils and probably represent an important and easily mineralizable portion of total ester sulfates.     

Mineralization and immobilization of nitrogen in fumigated soil and the measurement of microbial biomass nitrogen

Immobilization of N was measured in a fumigated and in an unfumigated soil by adding (¹⁵NH₄)₂SO₄ and following the disappearance of inorganic label from the soil solution and its simultaneous conversion to soil organic N. Calculations based on the measurement of organically-bound ¹⁵N gave more consistent values for immobilization than did calculations based on the measurement of the disappearance of label from solution. The fumigated soil immobilized 6.6 μg N g^?1 N g^?1 soil in 10 days at 25°C, the unfumigated control 4.8 μg. The corresponding gross mineralization rates were 34.9 and 5.6 μg N g^?1 soil in 10 days.Addition of 58 μg N as (¹⁵NH₄)₂SO₄ to the fumigated soil increased the quantity of the ynlabelled NH₄-N extracted at the end of 10 days from 33.8 to 37.8 μg Ng^?1 soil, i.e. there was a positive Added Nitrogen Interaction (ANI). The added labelled N produced this ANI, not by increasing the rate of mineralization of organic N, but by standing proxy for unlabelled N that otherwise would have been immobilized.A procedure for calculating biomass N from the size of the flush of mineral N caused by fumigation is proposed. Biomass N (B_N) is calculated from the relationship B_N = F'_N/0.68 where F'_N is [(N in fumigated soil incubated for 10 days — (N in unfumigated soil incubated for 10 days)].     

Chloroform fumigation and the release of soil nitrogen: The effects of fumigation time and temperature

Fumigation with CHC1₃ (24 h, 25°C) increased the amount of NH₄-N and total N extracted by 0.5 M K₂SO₄ from two soils (one arable, one grassland). The amount of N released by CHC1₃ increased with the duration of fumigation up to 5 days, when it levelled off. Between about 10–34% of the total N released by CHC1₃ was in the form of NH₄-N, the proportion increasing with duration of exposure.When a grassland soil that had received a field application of ¹⁵N-labelled fertilizer 1 yr previously was fumigated, the N released by CHC1₃ was 4 times more heavily labelled than the soil N as a whole. Prolonging the exposure of this soil to CHC1₃ increased the amount of total N released, but hardly altered the proportion of labelled N in the CHC1₃-released N, suggesting that N is being released from a single soil fraction. The most likely soil fraction is the soil microbial biomass. It is suggested that CHC1₃ does not alter the K₂SO₄-extractability of soil-N fractions other than microbial N and that the extra N released by CHC1₃ and extracted by K₂SO₄ gives a direct measure of soil microbial biomass N.In contrast to fumigation done at lower temperatures, less total N was released by soil fumigated at 60°C, or above, than was released from unfumigated soil held at the same temperature. The greater release of N in the non-fumigated soils above 60°C could have been due to soil enzymic processes which were inhibited by CHC1₃ in the fumigated soil.     

Responses of phosphatases and arylsulfatase in soils to liming and tillage systems

This study was carried out to investigate the long‐term influence of lime application and tillage systems (no‐till, ridge‐till, and chisel plow) on the activities of phosphatases and arylsulfatase in soils at four research sites in Iowa, USA. The activities of the following enzymes were studied: acid and alkaline phosphatases, phosphodiesterase, and arylsulfatase at their optimal pH values. With the exception of acid phosphatase, which was significantly (P < 0.001) but negatively correlated with soil pH (r ranged from –0.65** to –0.98***), the activities of other enzymes were significantly (P < 0.001) and positively correlated with soil pH, with r values ranging from 0.65** to 0.99*** for alkaline phosphatase, from 0.79*** to 0.97*** for phosphodiesterase, and from 0.66*** to 0.97*** for arylsulfatase. The Δ activity/Δ pH values were calculated to determine the sensitivity of each enzyme to changes in soil pH. Acid phosphatase was the most sensitive and arylsulfatase the least sensitive to changes in soil pH. Activities of the enzymes were greater in the 0 – 5 cm depth samples than those in 0 – 15 cm samples under no‐till treatment. With the exception of acid phosphatase, enzyme activities were mostly significantly (P < 0.001) and positively correlated with microbial biomass C (C_mic), with r values ranging from 0.28 (not significant) to 0.83*** and with microbial biomass N (N_mic), with r values ranging from 0.31 (not significant) to 0.94***. Liming and tillage systems significantly affected the activities of some enzymes but not others, as was evident from the specific activity values (g of p‐nitrophenol released kg^–1 C_org h^–1).     

Early impacts of cotton and peanut cropping systems on selected soil chemical,physical, microbiological and biochemical properties

This study investigated the impacts of cropping systems of cotton (Gossypium hirsutum L.; Ct) and peanut (Arachis hypogaea L.; Pt) on a Brownfield fine sandy soil (Loamy, mixed, superactive, thermic Arenic Aridic Paleustalfs) in west Texas, United States. Samples (0–12 cm) were taken 2 and 3 years after establishment of the plots from PtPtPt, CtCtPt and PtCtCt in March, June and September 2002, and in March 2003. Soil total N and aggregate stability were generally not different among the cropping systems. The pH of the soils was >8.0. Continuous peanut increased soil organic C, microbial biomass C (C_mic) and the activities of -glucosidase, -glucosaminidase, acid phosphatase, alkaline phosphatase, phosphodiesterase and arylsulfatase compared to the peanut-cotton rotations. The arylsulfatase activity of the fumigated field-moist soil and that resulting from the difference of the fumigated minus non-fumigated soil were greater in PtPtPt, but arylsulfatase activity of non-fumigated soil was unaffected by the cropping systems. Soil C_mic showed a different seasonal variation to enzyme activities during the study. Enzyme activities:microbial biomass ratios indicated that the microbial biomass may not have produced significant amounts of enzymes or that newly released enzymes did not become stabilized in the soil (i.e., due to its low clay and organic matter contents). Fungal (18:26c and 18:19c) and bacterial (15:0, a15:0, and a17:0) FAMEs were higher in PtPtPt than in CtCtPt or PtCtCt cropping systems. Our results suggest that the quality or quantity of residues returned to the soil under a peanut and cotton rotation did not impact the properties of this sandy soil after the first 3 years of this study.     

Enzyme activities in agricultural soils fumigated with methyl bromide alternatives

Pre-plant fumigation of agricultural soils with a combination of methyl bromide (MeBr) and chloropicrin (CP) to control nematodes, soil-borne pathogens and weeds has been a common practice in strawberry (Fragaria X ananassa Duchesne) production since the 1960s. MeBr will be phased out by 2005, but little is known about the impacts of alternative fumigants on soil microbial processes. We investigated the response of microbial biomass and enzyme activities in soils fumigated over two years with MeBr+CP and the alternatives propargyl bromide (PrBr), InLine, Midas and CP. Results were compared to control soils, which were not fumigated for the last 4-5 years for Watsonville and Oxnard, respectively, but had a 10 year history of MeBr+CP fumigation (history soils). Soil samples (0-15 cm) were taken from two sites in the coastal areas of California, USA, in Watsonville and Oxnard, at peak strawberry production after two years of repeated application. In addition to the soil enzymes, the activities of purified reference enzymes of β-glucosidase, acid phosphatase and arylsulfatase were assayed before and after fumigation with MeBr+CP and alternative biocides. At the Oxnard site, microbial respiration significantly decreased in soils fumigated with MeBr+CP (P=0.036), while microbial biomass C and N showed no response to fumigation at both sites. These results may indicate that fumigation promotes the growth of resistant species or that soil microorganisms had recovered at the time of sampling. Repeated soil fumigation with MeBr+CP significantly decreased the activities of β-glucosidase and acid phosphatase at the Watsonville site, and dehydrogenase activity at the Oxnard site. Although, enzyme activities in soils fumigated with PrBr, InLine, Midas and CP were lower compared to the control soil, effects were, in general, not significant. Fumigation with MeBr+CP and alternatives reduced the activities of purified reference enzymes by 13, 76 and 28% for acid phosphatase, β-glucosidase and arylsulfatase, respectively. Mean enzyme protein concentrations in fumigated agricultural soils were 2.93, 0.105, and 2.95 mg protein kg⁻¹ soil for acid phosphatase, β-glucosidase and arylsulfatase, respectively, all lower than in control soils. Organic matter turnover and nutrient cycling, and thus, the long-term productivity of agricultural soils seem unaffected in soils repeatedly fumigated with PrBr, InLine, Midas and CP.     

Effect of tillage and residue management on enzyme activities in soils: III. Phosphatases and arylsulfatase

This study was carried out to investigate the effect of tillage and residue management on activities of phosphatases (acid phosphatase, alkaline phosphatase, phosphodiesterase, and inorganic pyrophosphatase) and arylsulfatase. The land treatments included three tillage systems (no-till, chisel plow, and moldboard plow) in combination with corn residue placements in four replications. The activities of these enzymes in no-till/double mulch were significantly greater than those in the other treatments studied, including no-till/bare, no-till/normal, chisel/normal, chisel/mulch, moldboard/normal, and moldboard/mulch. The effect of mulching on activities of phosphatases was not as significant as on activities of arylsulfatase. The lowest enzyme activities were found in soil samples form no-till/bare and moldboard/normal treatments, with the exception of inorganic pyrophosphatase, which showed the lowest activity in no-till/bare only. Among the same residue placements, no-till and chisel plow showed comparable arylsulfatase activity, whereas the use of moldboard plow resulted in much lower arylsulfatase activity. The activities of phosphatases and arylsulfatase were significantly correlated with organic C in the 40 soil samples studies, with r values ranging from 0.71*** to 0.92***. The activities of alkaline phosphatase, phosphodiesterase, and arylsulfatase were significantly correlated with soil pH, with r values of 0.85***, 0.78***, and 0.77***, respectively, in the 28 surface soil samples studied, but acid phosphatase and inorganic pyrophosphatase activities were not significantly correlated with soil pH. The activities of phosphatases and arylsulfatase decreased markedly with increasing soil depth and this decrease was associated with a decrease in organic C content. The activities of these enzymes were also significantly intercorrelated, with r values ranging from 0.50*** to 0.92***. Received: 4 October 1995     

Response of organic N monomers in a sub-alpine soil to a dry–wet cycle

Cycles of soil drying followed by rewetting occur in most terrestrial ecosystems, but there is conflicting evidence as to the role of osmolytes in dry–wet cycles. The broad aim of this experiment was to determine how N-containing osmolytes and other organic N monomers are affected by rewetting of a moderately dry soil. In a sub-alpine grassland, experimental plots were irrigated with 50 mm of water near the conclusion of a typical late-summer drying cycle. Twelve putative osmolytes (proline, 8 quaternary ammonium compounds, trimethylamine N-oxide, ectoine, hydroxyectoine) and 60 other organic N monomers were identified and quantified by capillary electrophoresis-mass spectrometry of the free/exchangeable pool of soil water (0.5 M K₂SO₄ extracts) and microbial biomass (via chloroform fumigation extraction). The total concentration of organic N monomers was 25-times greater in fumigated than unfumigated extracts. Differences in relative abundance of compound classes and compounds between fumigated and unfumigated extracts suggested some compounds were localized to the free/exchangeable pool; others were predominantly microbial, whereas many were shared between pools. A striking feature of the free/exchangeable pool was that on an N-basis alkylamines were the most abundant compound class and accounted for 34% of the pool of organic N monomers. There was no evidence that osmolytes were the primary means soil microbes coped with dry–wet cycles. Instead, the pool of osmolytes was an invariant 4% of the pool of CE-MS detected monomers in K₂SO₄ extracts and 7% of the pool of CE-MS detected monomers in the chloroform-labile (microbial) fraction. The absence of substantial amounts of osmolytes may be because water stress was too mild or brief, or because osmolyte synthesis was limited by availability of energy, N or C and some alternative strategy was used to cope with water deficits.     

连续6年施用有机肥处理对水稻土磷酸单酯酶活性、动力学和热力学特性的影响

Soil phosphomonoesterase plays a critical role in controlling phosphorus(P) cycling for crop nutrition,especially in P-deficient soils.A 6-year field experiment was conducted to evaluate soil phosphomonoesterase activities,kinetics and thermodynamics during rice growth stages after consistent swine manure application,to understand the impacts of swine manure amendment rates on soil chemical and enzymatic properties,and to investigate the correlations between soil enzymatic and chemical variables.The experiment was set out in a randomized complete block design with three replicates and five treatments including three swine manure rates(26,39,and 52 kg P ha~(-1),representing low,middle,and high application rates,respectively) and two controls(no-fertilizer and superphosphate at 26 kg P ha~(-1)).The results indicated that the grain yield and soil chemical properties were significantly improved with the application of P-based swine manure from 0 to 39 kg P ha~(-1);however,the differences between the 39(M_(39)) and 52 kg P ha~(-1) treatments(M_(52)) were not significant.The enzymatic property analysis indicated that acid phosphomonoesterase was the predominant phosphomonoesterase in the tested soil.The M_(39) and M_(52) treatments had relatively high initial velocity(V_0),maximal velocity(V_(max)),and activation grade(lgN_a) but low Michaelis constant(K_m),temperature coefficient(Q_(10)),activation energy(E_a),and activation enthalpy(ΔH),implying that the M_(39) and M_(52) treatments could stimulate the enzyme-catalyzed reactions more easily than all other treatments.The correlation analysis showed that the distribution of soil phosphomonoesterase activities mainly followed the distributions of total C and total N.Based on these results,39 kg P ha~(-1) could be recommended as the most appropriate rate of swine manure amendment.     

Influence of phytoparasitic nematodes on symbiotic N2 fixation in tropical herbaceous legume cover crops

 Populations of plant parasitic nematodes and their effects on symbiotic nitrogen (N) fixation in herbaceous legumes and on some selected characteristics of other plant species associated with such cover crops were studied. Two legume species [mucuna, Mucuna pruriens (L) DC. var. utilis (Wright) Bruck and lablab, Lablab purpureus L. Sweet], one grass/weed species [imperata, Imperata cylindrica (L.) Rauschel] and a cereal (maize, Zea mays L.) were used. There were three soil treatments (fumigation, fumigation plus inoculation with Meloidogyne species, and an untreated control). Plant parasitic nematode populations in soil, roots and nodules were determined at 4, 8 and 12 weeks after planting. The response of the phytoparasitic nematodes to soil treatments varied according to the plant species present. The predominant nematodes in soils, roots and nodules of legumes were of the genus Meloidogyne, whereas other genera of parasitic nematodes dominated the fauna in soils and roots of maize and imperata. Biomass yield of mucuna was not significantly affected by either Meloidogyne spp. or the other genera of phytoparasitic nematodes. In contrast, the dry matter yield of lablab measured at 12 weeks was reduced by 16% in inoculated compared with fumigated soils. Similarly, the biomass yields of maize and imperata were reduced by 10% and 29%, respectively, in unfumigated rather than fumigated soils. The amounts of N accumulated in mucuna, maize and imperata were not significantly affected by the two groups of plant parasitic nematodes. However, at 12 weeks, lablab grown on inoculated soils accumulated only 69% of the N found in plants grown on fumigated soils. Inoculation of soil with Meloidogyne spp. significantly increased the number of nodules on lablab roots compared with the non-inoculated treatments, whereas nodulation in mucuna was not affected by soil treatment. After 12 weeks, the quantity of N₂ derived from symbiotic fixation in mucuna was not significantly affected by soil treatments whereas the amount of fixed N in lablab was 32% lower in inoculated than in fumigated soils. Possible mechanisms for the non-suppressive effect of plant parasitic nematodes on mucuna are discussed. Received: 12 March 1999     

Arylsulfatase activity of soil microbial biomass along a Mediterranean-arid transect

The relationships between arylsulfatase and microbial activity were investigated in regional and microenvironmental scales, at three study sites in Israel, that represent different climatic regions—Mediterranean (sub-humid), mildly arid and arid.Total arylsulfatase activity was divided into extracellular and intracellular (microbial biomass enzyme) activities according to the chloroform-fumigation method. The results show that with increasing aridity, C_org (soil organic carbon), C_mic (soil microbial biomass carbon), N_mic (soil microbial biomass nitrogen) and respiration rate decreased, while C_mic/C_org and metabolic quotient (qCO₂) increased. Total, extracellular and microbial biomass arylsulfatase activities decreased with aridity. Expressed as percentage of total activity, the arylsulfatase activity of microbial biomass in the soil, at 0-2 cm and 5-10 cm depths, accounted for more than 50% of the total, in most measurements. This activity was significantly higher in the arid sites than that found in the Mediterranean one for the 0-2 cm soil. The results indicate the importance of the microflora as an enzyme source in soils, especially in arid climate conditions.Enzyme activity in the different study sites was found to be influenced by microenvironmental conditions. The Mediterranean site showed a much higher enzyme activity under shrubs than that under rock fragments and in bare soil. In the arid site rock fragments created a favorable microenvironment for microbial activity on soil surface, which resulted in a much higher microbial biomass and arylsulfatase activity than that in bare soil.The total, extracellular and intracellular arylsulfatase activities, were significantly correlated with C_org, C_mic, N_mic and respiration rate (p<0.05) at all study sites. The correlation coefficients between microbial biomass and arylsulfatase activity were usually higher than those between organic carbon and enzyme activity, especially in the arid sites. Close relationships between microbial biomass and arylsulfatase activities in all the studied sites supported the hypothesis that C_org content and enzyme activities should be related to each other via microbial biomass. Arylsulfatase activity was found to be a good indicator of microbial one. The regression equations between these factors can be incorporated into models of biogeochemical cycling for their easy method of analysis.     

Synthesis of 15N-labelled microbial biomass in soil in situ and extraction of biomass N

Summary A Pakistani soil (Hafizabad silt loam) was incubated at 30°C with varying levels of ¹⁵N-labelled ammonium sulphate and glucose (C/N ratio of 30 at each addition rate) in order to generate different insitu levels of ¹⁵N-labelled microbial biomass. At a stage when all of the applied ¹⁵N was in organic forms, as biomass and products, the soil samples were analysed for biomass N by the chloroform (CHCl₃) fumigation-extraction method, which involves exposure of the soil to CHCl₃ vapour for 24 h followed by extraction with 500 mM K₂SO₄. A correction is made for inorganic and organic N in 500 mM K₂SO₄ extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized ¹⁵N extracted by 500 mM K₂SO₄ containing different amounts of CHCl₃. The extraction time varied from 0.5 to 4 h.The amount of N extracted ranged from 27 to 270 g g^–1, the minimum occurring at the lowest (67 g g^–1) and the maximum at the highest (333 g g^–1) N-addition rate. Extractability of biomass ¹⁵N ranged from 25% at the lowest N-addition rate to 65%a for the highest rate and increased consistently with an increase in the amount of ¹⁵N and glucose added. The amounts of both soil N and immobilized ¹⁵N extracted with 500 mM K₂SO₄ containing CHCl₃ increased with an increase in extraction time and in concentration of CHCl₃. The chloroform fumigation-extraction method gives low estimates for biomass N because some of the organic N in K₂SO₄ extracts of unfumigated soil is derived from biomass.     

Contparison of the influence of Cu,Zn, and Cd on the activity and kinetics of free and intntobilized acid phosphatase

The influence of Cu, Zn, and Cd on the activity and kinetics of acid phosphatase immobilized by two soil clays, kaolin or goethite was investigated. The ability of Cu to inhibit the enzyme activity was higher than that of Zn in all the enzyme complexes examined. The ability of Cd was negligible. The inhibitory effects of Cu and Zn on the two soil clay- and kaolin-enzyme complexes were much stronger than those on the goethite-enzyme complex. The V _max and K _m values of the enzyme complexes indicated that both Cu and Zn decreased the maximum reaction velocity of the enzymes, but increased the affinity of the enzymes for the substrate. The degree of the decrease and increase was higher in the Cu systems than in the Zn systems.     

Arylsulfatase activity in soil and soil extracts using natural and artificial substrates

The arylsulfatase activity of soil and humic arylsulfatase complexes extracted from soil were measured using the substrates p-nitrophenyl sulfate and low molecular weight (500–10000) soil ester sulfate compounds. Soil samples from the Aphorizon of a Podzol from S-amended wheat plots and a Regosol from dykeland hayfield plots were investigated. Soil arylsulfatase activity (assayed with p-nitrophenyl sulfate) in the fall was significantly higher than spring samples; however, no seasonal differences were observed when humic-arylsulfatase complexes were assayed with p-nitrophenyl sulfate. The discrepancy between arylsulfatase activity in soil and soil extracts was probably due to inhibitors which were found in soil materials. These results appear to support the theory that abiotic arylsulfatase is a relatively stable and persistent component of soil. There was a marked difference in the response by humic-arylsulfatase complexes to the artificial substrate p-nitrophenyl sulfate and natural low molecular weight soil substrates. Humic-arylsulfatase complexes hydrolysed 35–80% of added low molecular weight substrates depending on the treatment. The molecular size, concentration, and chemical composition of the low molecular weight ester sulfate compounds affected hydrolysis of the low molecular weight substrates. The response by humic-arylsulfatase complexes to the chromogenic ester sulfate, p-nitrophenyl sulfate did not reflect the ability of these complexes to hydrolyse natural soil substrates. In an experiments we examined arylsulfatase activity and soil S status in relation to the total S in plant tissue and grain from wheat plants grown in the Podzol. Tissue S was more strongly associated with soil S than the wheat grain. Hydriodic acid-S, Ca(H₂PO₄)₂-extractable sulfate, and hydrolysable ester sulfates in the high molecular weight (>10000) and low molecular weight (500–10000) fractions of soil organic matter extracts were strongly positively correlated with tissue S. Arylsulfatase activity in soil and humic-arylsulfatase extracts assayed with p-nitrophenyl sulfate were also strongly correlated with tissue S, while humic-arylsulfatase activity assayed with the low molecular weight substrate was negatively correlated with tissue S.     

Michaelis constants of soil enzymes

Studies to determine the Michaelis constants (k_m values) for the arylsulfatase and phosphatase activity in Iowa surface soils showed that the value obtained for either activity was different for different soils. When the incubation technique used to determine k_m did not involve shaking of the soil-substrate mixture, the k_m value for arylsulfatase activity in nine soils studied ranged from 1·37 × 10⁻³m to 5·69 × 10⁻³m, and the k_m value for phosphatase activity ranged from 1·26 × 10⁻³m to 4·58 × 10⁻³m. Shaking the soil-substrate mixture during incubation decreased the k_m value obtained for arylsulfatase or phosphatase activity and reduced the variation in k_m among soils. The maximum enzyme reaction velocity (V_max value) for soil arylsulfatase or soil phosphatase activity was markedly different for different soils and usually increased when the soil-substrate mixture was shaken during incubation. The k_m value for soil arylsulfatase or soil phosphatase activity was not significantly correlated with other soil properties studied (pH, cation-exchange capacity, percentage organic carbon, percentage clay, percentage sand).     

Mineralization of bacteria and fungi in chloroform-fumigated soils

It has been suggested by others that the size of the flush of mineralization caused by CHC1₃ fumigation can be used to estimate the amount of microbial biomass in soils. Calculation of biomass from the flush requires that the proportion of CHCl₃-killed cell C mineralized be known. To determine this proportion, 15 species of [¹⁴C]labelled fungi and 12 species of [¹⁴C]labelled bacteria were added to four types of soil and these were fumigated for 24 h with CHC1₃, reinoculated with unfumigated soil, and incubated at 22°C for 10 days. The average percentage mineralization of the fungi was 43.7 ± 5.3, while the average for the bacteria was 33.3 ± 9.9. Using a 1:3 ratio for distribution of total biomass between the bacterial and fungal populations, respectively, it was calculated that the average mineralization of both types of cells was 41.1%. In experiments conducted to determine if CHC1₃ vapour alters stabilized microbial metabolites or dead microbial cells in a manner which makes them more susceptible to degradation, it was found that both fumigated and unfumigated dead fungal materials mineralized to the same extent in soil during 10 days of incubation.     

Effects of de-icing salt on soil enzyme activity

Effects of de-icing salt on dehydrogenase, urease, alkalihephosphatase and arylsulfatase activity ofO _L andA _h-horizons of a moder and a mull soil were investigated using a field experiment. Additions of 2.5 kg m^?2 and 5.0 kg m^?2 of de-icing salt reduced activities of most enzymes within four weeks. Eleven months after salt addition there was nearly no reduction of enzyme activity to be measured on salt treated soils. The percentage of reduced enzyme activity was generally higher in the moder soil. It was concluded that reductions of enzyme activity were due to decreases of microbial activity and not to inactivation of enzymes.     

The effects of biocidal treatments on metabolism in soil—V: A method for measuring soil biomass

A new method for the determination of biomass in soil is described. Soil is fumigated with CHCl₃ vapour, the CHCl₃ removed and the soil then incubated. The biomass is calculated from the difference between the amounts of CO₂ evolved during incubation by fumigated and unfumigated soil. The method was tested on a set of nine soils from long-term field experiments. The amounts of biomass C ha^?1 in the top 23 cm of soil from plots on the Broadbalk continuous wheat experiment were 530 kg (unmanured plot), 590 (plot receiving inorganic fertilizers) and 1160 (plot receiving farmyard manure). Soils that had been fallowed for 1 year contained less biomass than soils carrying a crop. A calcareous woodland soil contained 1960 kg biomass C ha^?1, and an unmanured soil under permanent grass 2020. The arable soils contained about 2% of their organic C in the biomass; uncultivated soils a little more—about 3%.