M. A. Tabatabai

Use of p-nitrophenyl phosphate for assay of soil phosphatase activity

Abstract A simple method of assaying soil phosphatase activity is described. It involves colorimetric estimation of the p-nitrophenol released by phosphatase activity when soil is incubated with buffered (pH 6·5) sodium p-nitrophenyl phosphate solution and toluene at 37° C for 1 hr. The method is rapid and precise, and it has significant advantages over methods previously proposed for assay of soil phosphatase activity.

Phosphatases in soils

Most studies on phosphatase activity in soils have been concerned with acid phosphatase. This study was conducted to determine the activity of phosphomonoesterases (acid and alkaline phosphatases), phosphodiesterase, and “phosphotriesterase”. The results indicate that acid phosphatase is predominant in acid soils and that alkaline phosphatase is predominant in alkaline soils. With universal buffer, the pH optima of phosphodiesterase and phosphotriesterase were at pH 10. The activities of these phosphatases in soils were much lower than those of the acid and alkaline phosphatases. Studies on the effects of various soil treatments on the activity of phosphatases in soils indicated that air-drying increased the activity of acid phosphatase and phosphotriesterase, decreased the activity of alkaline phosphatase, but did not affect the activity of phosphodiesterase. Steam sterilization of soils at 121 C for 1 h inactivated alkaline phosphatase, phosphodiesterase, and phosphotriesterase, but did not completely inactivate acid phosphatase. Addition of toluene to the incubation mixture did not markedly affect the activity of acid phosphatase, alkaline phosphatase, phosphodiesterase, but increased the activity of phosphotriesterase in soils. Studies of the kinetic parameters of phosphatases in the soils studied showed that the Km values ranged from 1.11 to 3.40 mm for acid phosphatase. from 0.44 to 4.94 mm for alkaline phosphatase, and from 0.25 to 1.25 mm for phosphodiesterase. Expressed as μg p-nitrophenol released·h−1·g−1 soil, the Vmax values ranged from 200 to 625 for acid phosphatase, from 124 to 588 for alkaline phosphatase, and from 46 to 127 for phosphodiesterase. The substrate of phosphotriesterase (tris-p-nitrophenyl phosphate) is insoluble in water; hence, the Km and Vmax values of this enzyme in soils could not be determined.

Glucosidases and galactosidases in soils

Abstract An improved method to assay activities of α- and β-glucosidases and α- and β-galactosidases in soils is described. It involves extraction and colorimetric determination of the p-nitrophenol released when 1 g of soil is incubated with 5 ml of buffered p-nitrophenyl glycoside solution at 37°C for 1 h. The reagents [0.5 M CaCl2 and 0.1 M Tris (hydroxymethyl)aminomethan THAM, pH 12] used for extraction of the p-nitrophenol released give quantitative recovery of p-nitrophenol added to soils and do not cause chemical hydrolysis of the substrates. Results showed that these enzymes have their optimum activities at buffer pH 6.0. The initial rates of p-nitrophenol release obeyed zero-order kinetics. β-Glucosidase activity was the most predominant of the four enzymes. The temperature dependence of the rate constant conformed to the Arrhenius equation up to the point of enzyme inactivation (60°C for α- and β-galactosidases and α-glucosidase and 70°C for β-glucosidase). The average activation energy values of these enzymes in three soils were 43.1, 30.8, 57.0 and 32.6 kJmol−1 for α-glucosidase, β-glucosidase, α-galactosidase and β-galactosidase activities, respectively. By using the Lineweaver-Burk plot. the Km values were the lowest for β-glucosidase activity. The Vmax values varied among the four enzymes and soils studied.

Assay of urease activity in soils

Abstract A simple and precise method of assaying urease activity in soils is described. It involves determination of the ammonium released by urease activity when soil is incubated with tris(hydroxymethyl)aminomethane (THAM) buffer, urea solution, and toluene at 37°C for 2 h, ammonium release being determined by a rapid procedure involving treatment of the incubated soil sample with 2.5 M KC1 containing a urease inhibitor (Ag 2 SO 4 ) and steam distillation of an aliquot of the resulting soil suspension with MgO for 3.3 min. Studies reported showed that the optimal buffer pH and substrate (urea) concentration for assay of soil urease activity using THAM buffer are 9.0 and 0.02 M, respectively, and that the method described is satisfactory for assay of urease activity in ammonium-fixing soils.

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

Abstract 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***.

Decomposition of different organic materials in soils

Laboratory experiments were conducted to evaluate organic C mineralization of various organic materials added to soils. A soil sample was mixed with organic material to approximate a field application of 9 g organic C kg-1 soil (0.9% or 50 Mg ha-1). The organic materials used were four crop residues [corn (Zea mays L.), soybean (Glycine max L. Merr.), sorghum (Sorghum vulgare Pers.), and alfalfa (Medicago sativa L.)], four animal manures [chicken (Gallus domesticus), pig (Sus scrofa), horse (Equus caballus), and cow (Bos taurus)] and four sewage sludges [Correctionville (Imhoff tank), Charles City (holding tank), Davenport (secondary digester), and Keokuk (primary digester)]. The soil-organic material mixture was incubated under aerobic conditions at room temperature (20±2°C) for 30 days. The CO2 evolved was collected in standard KOH solution by continuously passing CO2-free air over the soil. Results showed that, in general, the amounts of CO2-C released mereased rapidly initially, but the pattern differed among the organic materials used. More than 50% of the total CO2 produced in 30 days of incubation was evolved in the first 6 days. Expressed as percentages of organic C added, the amounts of CO2 evolved ranged from 27% with corn to 58% with alfalfa. The corresponding percentages for animal manures ranged from 21 to 62% with horse and pig manures, respectively, and for sewage sludges they ranged from 10 to 39% for Charles City and Keokuk sludges. All CO2 evolution data conformed well to a first-order kinetic model. Potentially, readily mineralizable organic C values and first-order rate constants (k) of the organic matter-treated soils ranged from 1.422 g C kg-1 soil with ak value of 0.0784 day-1 to 6.253 g C kg-1 soil with ak value of 0.0300 day-1. The half-lives of the C remaining in soils ranged from 39 to 54 days for plant materials. The corresponding half-lives for the C remaining from animal manures and sewage sludges ranged from 37 to 169 days and from 39 to 330 days, respectively.

Enzyme activities in a limed agricultural soil

Abstract This study assessed the effect of eight lime application rates, with four field replications, on the activities of 14 enzymes involved in C, N, P, and S cycling in soils. The enzymes were assayed at their optimal pH values. The soil used was a Kenyon loam located at the Northeast Research Center in Nashua, Iowa. Lime was applied in 1984 at rates ranging from 0 to 17,920 kg effective calcium carbonate equivalent (ha–1), and surface samples (0–15 cm) were taken after 7 years. Results showed that organic C and N were not significantly affected by lime application, whereas the soil pH was increased from 4.9 to 6.9. The activities of the following enzymes were assayed: α- and β-glucosidases, α- and β-galactosidases, amidase, arylamidase, urease, l-glutaminase, l-asparaginase, l-aspartase, acid and alkaline phosphatases, phosphodiesterase, and arylsulfatase. With the exception of acid phosphatase, which was significantly (P<0.001) but negatively correlated with soil pH (r=–0.69), the activities of all the other enzymes were significantly (P<0.001)and positively correlated with soil pH, with r values ranging from 0.53 for the activity of α-galactosidase to 0.89 for alkaline phosphatase and phosphodiesterase. The Δ activity/Δ pH values ranged from 4.4 to 38.5 for the activities of the glycosidases, from 1.0 to 107 for amidohydrolases and arylamidase, 97 for alkaline phosphatase, 39.4 for phosphodiesterase, and 11.2 for arylsulfatase. This value for acid phosphatase was –35.0. The results support the view that soil pH is an important indicator of soil health and quality.

Soil microbial biomass carbon and nitrogen as affected by cropping systems

Abstract The impacts of crop rotations and N fertilization on microbial biomass C (Cmic) and N (Nmic) were studied in soils of two long-term field experiments initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil samples were taken in 1996 and 1997 from plots of corn (Zea mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha–1 before corn and an annual application of 20 kg P and 56 kg K ha–1. The Cmic and Nmic values were determined by the chloroform-fumigation-extraction method and the chloroform-fumigation-incubation method, respectively. The Cmic and Nmic values were significantly affected (P<0.05) by crop rotation and plant cover at time of sampling, but not by N fertilization. In general, the highest Cmic and Nmic contents were found in the multicropping systems (4-year rotations) taken in oats or meadow plots, and the lowest values were found in continuous corn and soybean systems. On average, Cmic made up about 1.0% of the organic C (Corg), and Nmic contributed about 2.4% of the total N (Ntot) in soils at both sites and years of sampling. The Cmic values were significantly correlated with Corg contents (r≥0.41**), whereas the relationship between Cmic and Ntot was significant (r≤0.53***) only for the samples taken in 1996 at the NERC site. The Cmic : Nmic ratios were, on average, 4.3 and 6.4 in 1996, and 7.6 and 11.4 in 1997 at the NERC and CWRC sites, respectively. Crop rotation significantly (P<0.05) affected this ratio only at the NERC site, and N fertilization showed no effect at either site. In general, multicropping systems resulted in greater Cmic : Corg (1.1%) and Nmic : Ntot (2.6%) ratios than monocropping systems (0.8% and 2.1%, respectively).

Effect Of Cropping Systems On Adsorption Of Metals By Soils: Ii. Effect Of ph

The effect of pH on adsorption of Cd, Cu, Ni, Pb, and Zn by soils under different cropping systems was investigated. Plots of metal adsorption vs. pH (unadjusted) were generated for 24 soils, 12 from each of two long-term cropping systems. Two soils, one from each site, were selected to study the metal adsorption over a range of adjusted pH values. Results showed that differences in metal adsorption were dependent on the initial heavy-metal concentration. At low concentrations, all the added metals were adsorbed regardless of the solution pH. At high concentrations, however, metal adsorption by soils was strongly related to solution pH; metal adsorption increased with increasing solution pH. A decrease in solution pH with increasing initial metal concentration was observed for all soils and metals. In general, a decrease in solution pH for a given amount of metal added followed the sequence: Cu = Pb > Cd = Ni = Zn. Metal solubility diagrams showed that, in general, metal adsorption by soils could not be attributed to precipitation. Although some of the adsorption behavior of these metals was consistent with that based on metal hydrolysis, other results showed that this mechanism could not explain metal adsorption. Metal distribution coefficients, Kd, decreased sharply with increasing initial metal concentration and followed the sequence: Pb ≥ Cu [U00BB] Cd = Ni = Zn. Also, Kd values were related to solution pH. At high solution pH, part of the Pb and Cu adsorption could be explained by adsorption of hydroxocomplexes from hydrolysis reactions of these metals. Plots of Kd vs. initial metal concentration suggested that Cd, Ni, and Zn adsorption could be attributed to ion-exchange reactions.

Effects of trace elements on urease activity in soils

Abstract Disposal of sewage sludges and effluents on agricultural land is becoming a widespread practice. Most sludge samples disposed on soils contain large quantities of various trace elements. Studies of 20 trace elements commonly found in sludge samples showed that they inhibit the activity of urease in soils and that their order of effectiveness as inhibitors of urease depends on the soil. When the trace elements were compared by using 5 μmiol·g −1 soil, however, some of them showed the same order of effectiveness as urease inhibitors in the six soils studied i.e., for the monovalent and divalent ions, Ag + ≥ Hg 2+ > Cu 2+ > Cd 2+ > Zn 2+ > Sn 2+ > Mn 2+ , and generally, Fe 2+ > Fe 2+ and Cu 2+ > Cu + . Other trace element ions that inhibited urease were Ni 2+ , Co 2+ , Pb 2+ , Ba 2+ , As 3+ B 3+ , Cr 3+ , Al 3+ . V 4+ Se 4+ and Mo 6+ . Of the trace element ions studied, only As 5+ and W 6+ did not inhibit urease activity in soils. Studies on the distribution of urease activity showed that it is concentrated in surface soils and decreases with depth. Urease activity was proportional to organic C distribution in each soil profile and was significantly correlated with organic C in the surface soils studied.