Effect of soil solarization on subsequent nitrification activity at elevated temperatures

Soil solarization is a nonchemical method of soil disinfection achieved by covering the soil surface with sheets of vinyl plastic to generate elevated soil temperature, generally over 45°C. Such elevated temperatures may be detrimental to some nitrifying microorganisms and favorable to others. However, little information exists to indicate how nitrification activity in soil is affected after solarization. We performed several experiments to investigate the effects of soil solarization on nitrification activity. We found that: (1) if a soil was subjected to pretreatment of 45 or 50°C for as little as 1 d, nitrification activity in a subsequent incubation at 30°C was less than that of a soil that did not receive any high-temperature pretreatment. However, if a soil received pretreatments of 45 or 50°C for more than 7 d, nitrification activity in a subsequent incubation at 45 or 50°C was greater than that of soil that did not receive high temperature pretreatment. (2) Nitrification activity in three kinds of soil taken from 0–5 cm depth after solarization treatment was greater at 45°C than 30°C. (3) Nitrification activity at 45°C in soil that had received solarization in the preceding year was greater than that in soil that had not been subjected to solarization. This was consistent with the fact that the population densities of ammonia oxidizers were greater in soils that had been subjected to solarization. These results suggest that soil solarization induces nitrifying microorganisms that are more active at 45–50°C than they are at 30°C, and that the effect of solarization on nitrification persists until the next crop season.     

Cellulase complex in a tomato field soil: Induction,localization and some properties

A soil sample from a tomato field showed cellulase activity after 1 week of storage but showed no cellulase activity when stored for 1 yr. Addition of cellulose and inoculation with untreated soil induced cellulase activity in remoistened, heated soil. Selective inhibition of bacteria or fungal growth in the incubated soil indicated that fungi were the more important source of cellulase in this soil. Over 60% of the isolates tested, showed cellulolytic activity on CMC-agar. Although organic debris was a minor component in the soil sample, most of the cellulase activity was localized in this fraction. Cellulase activity was extracted with 0.1 m phosphate buffer (pH 7), but not with distilled water. The activity of the extract ' was lost on keeping at 80°C for 10 min and was optimal at pH 5. The extract hydrolyzed either soluble or insoluble substrate.     

Stability of urease in soils

Studies with surface samples of Iowa soils selected to obtain a wide range in properties showed that the following treatments of field-moist soils had no effect on urease activity: leaching with water ; drying for 24 h at temperatures ranging from 30 to 60°C ; storage for 6 months at temperatures ranging from ?20 to 40°C; incubation under aerobic or waterlogged conditions at 30 or 40°C for 6 months. No loss of urease activity could be detected when field-moist soils were air-dried and stored at 21–23°C for 2yr, but complete loss of urease activity was observed when they were dried at 105°C for 24 h or autoclaved (120°C) for 2h. Inactivation of urease in moist soils was detected at temperatures above 60°C.Treatment of field-moist soils with proteolytic enzymes which cause rapid destruction of jackbean urease did not decrease urease activity, but jackbean urease was destroyed or inactivated when added to sterilized or unsterilized soils.Although no decrease in urease activity could be detected when field-moist soils were air-dried, an appreciable (9–33%) decrease in urease activity was observed when air-dried soils were incubated under aerobic or waterlogged conditions. This decrease occurred within a few days, and prolonged incubation or repetition of the drying-incubation treatment did not lead to a further decrease in urease activity. Treatment of incubated air-dried soil with urease or glucose initially increased urease activity to a level exceeding that of the undried soil, but this activity decreased with time and eventually stabilized at the level observed for the undried soil.The work reported supports the conclusions from previous work that the native urease in Iowa soils is remarkably stable and that different soils have different levels of urease activity determined by the ability of their constituents to protect urease against microbial degradation and other processes leading to inactivation of enzymes.     

Assay of pyrophosphatase activity in soil

A method to assay pyrophosphatase activity in soil is described. It involves the determination of orthophosphate (Pi) released by pyrophosphatase activity when soil is incubated with 70 mm pyrophosphate (PPi) solution, citrate buffer (pH 5.8) and toluene at 37°C for 2 h. The Pi released is determined by a procedure that involves separation of Pi from PPi and formation of a Pi-molybdate complex. The method is precise and is not complicated by fixation of Pi in soil.     

Modeling of the fine‐scale temperature response of arylsulfatase activity in soil

Fine‐scale (1.0–2.2 °C) temperature dependence of soil arylsulfatase activity (arylsulfate sulfohydrolase, EC 3.1.6.1) was measured at 0 to 75 °C in a Danish sandy, arable soil. Assays were done with field‐moist soil samples in the absence of toluene as plasmolytic agent – a procedure that primarily measures the extracellular enzymes. The aim was to evaluate the use of temperature models to describe the temperature response of soil arylsulfatase activity. In addition, we searched for increases in activity at high temperatures (e.g., 50–60 °C), which might be associated with unmasking (exposure) of intracellular enzymes. Arylsulfatase activities ranged from 1.1 to 60.3 μg p‐nitrophenol (g dry weight soil)^–1 h^–1, with an optimum temperature at 58.1 °C. The temperature response below 58.1 °C could be described by the Arrhenius equation (r² = 0.978, n = 83) and the simple Ratkowsky equation (r² = 0.977, n = 83). The expanded Ratkowsky equation, which covered the entire temperature range (0–75 °C), was less satisfactory (r² = 0.958, n = 90) because the model underestimated the reaction rates near the optimum temperature. The activation energy (E_a) calculated from the Arrhenius equation was 42.2 kJ mol^–1. This was higher than previously found for other soils (16.5–34.7 kJ mol^–1), possibly due to the use of toluene in these studies. Further analysis of the temperature response showed that no increase in activity occurred due to potential unmasking of intracellular enzymes by disintegration of bacterial cell membranes at high temperatures. Thus, the use of high incubation temperatures did not facilitate the differentiation between intra‐ and extracellular enzyme activity.     

Adenosine Deaminase Activity in Soils

Adenosine deaminase activity in six soil samples was measured. One g of moist soil was incubated with 0.02 M adenosine in 0.1 M phosphate buffer (pH 7.6) and toluene in a shaking water bath (30°C) for 72 h. Inosine and NH₂ ⁺ were detected in the filtrate as degradation products of adenosine; inosine was identified by paper chromatography, and NH₂ ⁺ was determined by Nessler’s method.

The adenosine deaminase activity in soils was completely inhibited by autoclaving (120°C, 15 min).

The relation between substrate concentration and reaction rate followed the Michaelis-Menten equation.

The optimum pH for adenosine deaminase activity in soils ranged from about 7 to 8.

The adenosine deaminase activity in soils ranged from about 1.51 to 4.33 munit (nmol per min) per g of dry soil at 30°C.     

Oxidative coupling activity in soil extracts

An assay was developed in which 50 mm citrate buffer was used to extract catalysts responsible for oxidative coupling reactions in soil. The assay was based on the formation of the dimerized quinone from the oxidative coupling of 2,6-dimethoxyphenol. Oxidative coupling activity was destroyed by heating the extracts for 15 min at 100°C and was inhibited by H₂O₂ (98% at 10 mm), KCN (96% at 10 mm), dithiothreitol (100% at 0.1 mm) and 2,3-dimercapto-1-propanol (100% at 0.1 mm). In the assay, a pH of 7.0 and a temperature of 55°C were determined as optimal. Freezing the soil extracts provided the best protection against activity loss, whereas storage caused a decline in oxidative coupling activity as a function of increasing temperatures (5–50°C). If soil had been supplemented with sucrose before extraction, activity increased. Soil extracts reacted with syringaldazine, benzidine, o-dianisidine, guaiacol, and p-cresol, substrates previously used to detect phenoloxidase enzymes.     

In Situ Digestion of Rock Phosphates to Mobilize Plant-Available Phosphate for Organic Farming

Sustainable phosphorous (P) management is a key problem in organic farming. In situ digestion of naturally occurring rock phosphates (RPs) may be a solution. This would require the application of fertilizers consisting primarily of RP mixed with elemental sulfur (S). Through microbial action, the S is oxidized into sulfuric acid, which then transforms the RP into soluble, plant-available forms. By means of an incubation experiment, this study characterized the in situ digestion of RP and revealed how it is influenced by temperature and microbial action. When either S alone or S together with RP (SP) was added to soil that had been inoculated with S-oxidizing microorganisms, the soil pH decreased rapidly from about 7.3 to 3.2 over 12 weeks of incubation. In soil that had not been inoculated with of S-oxidizing microorganisms, the pH of the soil treated either with S or with SP decreased only slightly. The pH of the inoculated soil to which either S or SP had been added decreased more rapidly at 30 °C than at 23.8 °C during the first 4 weeks. The oxidation rate in inoculated soil was much greater than in noninoculated soil and greater at 30 °C than at 23.8 °C. The S oxidation rate in inoculated soil was significantly greater in the SP treatment than in the S treatment both at 23.8 °C and at 30 °C. After incubation, the amounts of water-soluble P and of CAL P (calcium-acetate-lactate–extracted P) were large only in the SP treatment in inoculated soil.     

Straw decomposition and nitrogenase activity (C2H2 reduction): Effects of soil moisture and temperature

The effects of moisture and temperature on straw decomposition (CO₂ production) and nitrogenase activity (C₂H₂ reduction) were measured in laboratory experiments to evaluate the potential for nitrogenase activity at various times of the year in soils with added wheat straw. Soils collected from two areas (Gunnedah and Cowra) representative of large areas of the wheat belt of New South Wales, Australia, were examined. Straw decomposed over a wide range of temperatures (20–50°C, Gunnedah; 15–45°C, Cowra) and moistures (0.3–2.0 times ?10 kPa water content). Microbial populations that fix nitrogen were adapted to a broad range of temperatures (10–45°C, Gunnedah; 4–40°C; Cowra). However, nitrogenase activity with added glucose occurred at much lower soil moisture contents in the Gunnedah soil (0.5–1.75 times ?10 kPa water content) than in the Cowra soil (1.0–2.5 times ?10 kPa water content). Despite the differences between the soils the results show that there is potential for straw decomposition and nitrogenase activity throughout most of the year.     

Determination of phosphate desorption characteristics in soils using successive resin extractions

Abstract

An alternative method for examining phosphate desorption characteristics in soil was tested. Five tropical soils with very different phosphate sorption capacities were incubated with added phosphate under three different conditions: 2 days at 25°C, 55 days at 25°C, and 2 days at 50°C. After incubation the phosphate was desorbed from the soil using successive cation‐anion resin extractions. The data from these extractions were fitted to a first order rate equation describing desorption. From the equation, an asymptote (B) was found to represent the ultimate amount of phosphate desorbable from each soil after incubation. It was found that increasing the incubation time or increasing the temperature of the incubation lowered this parameter ‘B’ suggesting that the slow reaction of adsorbed phosphate had reduced the amount of readily desorbable phosphate. Differences between soils as reflected in this parameter may indicate differences in the residual value of added phosphate in the field.     

Immobilization and mineralization of nitrogen in a saline and alkaline soil during microbial use of sugarcane filter cake amended with glucose

A 42-day incubation was conducted to study the effect of glucose and ammonium addition adjusted to a C/N ratio of 12.5 on sugarcane filter cake decomposition and on the release of inorganic N from microbial residues formed initially. The CO₂ evolved increased in comparison with the non-amended control from 35% of the added C with pure +5 mg g⁻¹ soil filter cake amendment to 41% with +5 mg g⁻¹ soil filter cake +2.5 mg g⁻¹ soil glucose amendment to 48% with 5 mg g⁻¹ soil filter cake +5 mg g⁻¹ soil glucose amendment. The different amendments increased microbial biomass C and microbial biomass N within 6 h and such an increase persisted. The fungal cell-membrane component ergosterol initially showed a disproportionate increase in relation to microbial biomass C, which completely disappeared by the end of the incubation. The cellulase activity showed a 5-fold increase after filter cake addition, which was not further increased by the additional glucose amendment. The cellulase activity showed an exponential decline to values around 4% of the initial value in all treatments. The amount of inorganic N immobilized from day 0 to day 14 increased with increasing amount of C added, in contrast to the control treatment. After day 14, the immobilized N was re-mineralized at rates between 1.3 and 1.5 μg N g⁻¹ soil d⁻¹ in the treatments being more than twice as high as in the control treatment. This means that the re-mineralization rate is independent of the actual size of the microbial residues pool and also independent of the size of the soil microbial biomass.     

Organic amendments decomposability influences microbial activity in saline soils

Organic amendments with contrasting biochemical properties were investigated by conducting an incubation experiment in soils irrigated with different levels of saline water. Soil samples were taken from a long-term experimental field plots irrigated with normal water and saline water having electrical conductivity (EC) 6 and 12 dS m^?1, respectively. Finely ground biochar, rice straw (RS), farm yard manure (FYM) and glucose were added at two rates (1% and 2.5% carbon basis) and incubated for 8 weeks at 25°C. Cumulative respiration (CR), microbial biomass carbon and available nutrients (nitrogen and phosphorus) were negatively correlated with EC, irrespective of the source and amount of added carbon (C). Compared with non-saline soil, at EC 12, relative decrease in CR was lowest with glucose (21.0%) followed by RS (32.0%), FYM (46.0%) and biochar (55.0%). Dissolved organic carbon was positively correlated with salinity and its concentration was higher in treatments with higher rate of C addition (2.5% C). This study showed decomposability of organic amendments and their rate of addition determines microbial activity in saline soils. Further, lower nitrogen (N) release from amendments under saline conditions limits microbial ability to utilize available C for satisfying their energy needs.     

Use of tween 20 as a substrate for assay of lipase activity in soils

Abstract

A method for assaying the soil lipase activity is described. It involves the titrimetric estimation of the amount of lauric acid released by the lipase activity when the soil is incubated with Tween 20 in the presence of toluene at 30°C for 18 h under agitation. The method is simple and precise and incubation without agitation is also possible. The method has been applied to six different kinds of soils. The lipase activity in the cultivated soils ranged from 22.5 to 75.5 mmol min^?1 g^?1 of dried soil. The K _m value for Tween 20 was 1.8 × 10^?4 m. The optimum pH was approximately 7.5. The hydrolysis of liveen 20 in soil was inhibited by glycerol which was the essential moiety of glyceride. The inhibition by glycerol was found to be competitive. These results indicate that Tween 20 is a potential substrate for the assay of the glyceride hydrolytic activity in soils.     

The response of cylindrocladium conidia to soil fungistasis

Direct observation of washed conidia of Cylindrocladium scoparium on non-sterile soils, air dried and rewetted immediately before deposition of conidia, indicated that peak germination (33–58%) occurred after 24 h incubation at 26°C. Peak germination on continually moist soils was lower (18–26%) than on rewetted soils. Lysis of germ tubes and germinating conidia on continually moist soils at 26°C was evident with 48 h. Conidia did not germinate on continually moist soils at 6°C and lysis did not become apparent until 168 h. Conidia germinated at a high level (93–99%) in axenic culture in the absence of exogenous C and N sources. The inhibition of conidial germination on soils may be attributed, in part, to the presence of soil volatiles. Germination of conidia placed on washed agar disks and exposed to volatiles from four soils ranged from 51 to 86% of the no-soil controls. Addition of carbon (13 ng C per conidium as glucose) and nitrogen (65 pg N ng^?1 C as NH₄C1) nullified the inhibitory effect of the soil volatiles. Germinability assayed on a selective medium at 26°C of conidia in artificially infested soils (approximately 10⁴ conidia g^?1 soil) decreased progressively during incubation at 26°C from 1 week to 4 months. No germinable conidia were recovered from artificially infested soils after 2 months incubation at 6°C. Conidia of C. floridanum and C. crotalariae responded similarly to C. scoparium in many assays.     

Microbial biomass and activity in soils amended with glucose

Four contrasting soils were amended with glucose at concentrations up to 10 mg g^?1 soil. The soils were incubated at 22°C for 14 days and the biomass determined at various times by chloroform fumigation or substrate-induced respiration. The adenosine triphosphate (ATP) content or the amylase and dehydrogenase activities were also determined. The size of the increases in biomass, ATP content and the enzyme activities was generally related to the amount of glucose added. The initially higher ATP levels quickly declined, and apparent substrate conversion figures up to 84% indicated that substrate-induced respiration overestimated the biomass. There were generally no significant correlations between ATP, biomass or enzyme activities.     

Effect of temperature on EDTA‐extractable copper in soils

Abstract

Two contrasting soils were extracted with 0.05 M EDTA in 1 M CH₃ COONH₄ at pH 6, before and after incubation for 4 weeks at constant (10, 20 or 30°C) or fluctuating (10/30, mean 20°C) temperatures. Less copper was extracted from soils which were incubated at fluctuating temperature than from those maintained at a constant 20 C. Where incubation temperature was constant, extractable copper increased or decreased with increasing temperature depending on the soil and how it was treated. Recovery of added copper was low initially but increased during the incubation. Maximum recovery was associated with low incubation temperature in one soil, but high temperature in the other. The amounts of copper extracted were slightly increased by γ‐irradiation of the soils. Extractable copper was also increased by increasing the temperature at which the extraction was performed.     

Temperature dependence of the activity of polyphenol peroxidases and polyphenol oxidases in modern and buried soils

Under conditions of the global climate warming, the changes in the reserves of soil humus depend on the temperature sensitivities of polyphenol peroxidases (PPPOs) and polyphenol oxidases (PPOs). They play an important role in lignin decomposition, mineralization, and humus formation. The temperature dependence of the potential enzyme activity in modern and buried soils has been studied during incubation at 10 or 20°C. The experimental results indicate that it depends on the availability of the substrate and the presence of oxygen. The activity of PPOs during incubation in the absence of oxygen for two months decreases by 2–2.5 times, which is balanced by an increase in the activity of PPPOs by 2–3 times. The increase in the incubation temperature to 20°C and the addition of glucose accelerates this transition due to the more abrupt decrease in the activity of PPOs. The preincubation of the soil with glucose doubles the activity of PPPOs but has no significant effect on the activity of PPOs. The different effects of temperature on two groups of the studied oxidases and the possibility of substituting enzymes by those of another type under changing aeration conditions should be taken into consideration in predicting the effect of the climate warming on the mineralization of the soil organic matter. The absence of statistically significant differences in the enzymatic activity between the buried and modern soil horizons indicates the retention by the buried soil of some of its properties (soil memory) and the rapid restoration of high enzymatic activity during the preincubation.     

A method for the determination of β-glucosidase activity in soil

A method is described for the rapid and simple assay of soil β-glucosidase activity. It involves colorimetric estimation of ρ-nitrophenol released by β-glucosidase activity when soil is incubated in McIlvaine buffer (pH 4.8) with ρnitrophenyl βd-glucoside and toluene at 30°C for 1 hr. The method has been applied to three different soils. The range of β-glucosidase activity in cultivated soils was from 10.1 to 15.2 mµ mole per min per gram of dried soil. K_m value for ρ-nitrophenyl β-d-glucoside was 3.3 × 10^-4 M. Optimum pH was 4.8.     

Seasonal variation of phosphatase activity in woodland soils

Seasonal variation of phosphatase activity in 0–5 cm soils from 1.6 m² plots in 48 English Lake District woodlands has been studied. Six per cent or 21% of the total variation in phosphatase activity at the assay temperature of 13°C was seasonal if activity was expressed respectively as phenol liberated g^?1 soil or cm^?3 soil. No seasonal pattern of activity at 13°C was detectable within individual plots due to high within-plot spatial variation. By averaging the results of all 48 plots, a seasonal pattern was identified but this did not resemble the seasonal pattern of soil available P content. Two, possibly three, peaks of activity occurred during the year, one in summer, a second after leaf-fall in autumn and possibly a minor peak in spring. The highest activity occurred in mid-winter.When adjusted to field temperature, 19 or 37% of the total variation in phosphatase activity, expressed respectively in terms of g^?1 soil or cm^?3 soil, was seasonal. The same three peaks of activity were still apparent, but their relative heights were altered, with maximum activity occurring in summer. After adjustment to field temperature, the seasonal pattern could be detected in many of the individual plots, if activity was expressed in terms of cm^?3 soil, and reflected the seasonal pattern of soil available P.Soil phosphatase data should be expressed in terms of activity per unit soil volume and adjusted to field temperatures, if the biological significance of the seasonal variation in activity is to be appreciated.     

Persistence of the inhibitory effects of phosphoroamides on urea hydrolysis in soils

Abstract

The persistence of the inhibitory effects of three phosphoroamides [N‐(n‐butyl) thiophosphoric triamide (NBPT), phenylphosphorodiamidate (PPD), and thiophosphoryl triamide (TPT)] on urea hydrolysis in soils was assessed by measuring the ability of four soils to hydrolyze urea after they had been treated with 5 μg phosphoroamide/g soil and incubated at 15°C or 30°C for 0, 3, 7, 14, or 28 days. The soils used differed markedly in pH, texture, and organic‐matter content. The data obtained showed that the persistence of the effects of the phosphoroamides studied decreased with increase in soil temperature from 15°C to 30°C and that whereas the effect of PPD decreased with increase in the time of incubation, the effects of NBPT and TPT sometimes increased before decreasing with increased time of incubation. These observations are in harmony with the recent findings that PPD is a potent inhibitor of urease activity, but decomposes in soils with formation of phenol, which is a relatively weak inhibitor of urease activity, whereas NBPT and TPT do not inhibit urease activity but decompose in soil with formation of their oxon analogs, which are potent inhibitors of urease activity. The inhibitory effect of NBPT on urea hydrolysis was considerably more persistent than that of PPD or TPT and was significant even after incubation of NBPT‐treated soil at 15°C or 30°C for 28 days.