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.     

Sulfur K-edge XANES spectroscopy reveals differences in sulfur speciation of bulk soils, humic acid, fulvic acid, and particle size separates

X-ray absorption near edge structure (XANES) spectra at the sulfur (S) K-edge (E=2472 eV) were compared for bulk soil material, humic and fulvic acid fractions, and different particle size separates from Ah horizons of two arable Luvisols, from an O and a Bs horizon of a Podzol under Norway spruce forest, and from an H horizon of a Histosol (peat bog). In the bulk soil samples, the contribution of reduced organic S (organic mono- and disulfides) to total sulfur increased from 27% to 52%, and the contribution of ester sulfate and SO₄²⁻-S decreased from 39% to 14% of total S in the following order: arable Luvisols Ah—forested Podzol O—Histosol H. This sequence reflects the increasing organic carbon content and the decreasing O₂ availability in that order. Neither sulfonate nor inorganic sulfide was detected in any of the bulk soil samples. For all samples except the Podzol Bs, the XANES spectra of the bulk soils differed considerably from the spectra of the humic and acid fractions of the respective soils, with the latter containing less reduced S (16-44% of total S) and more oxidized S (sulfone S: 19-35%; ester sulfate S: 14-38% of total S). Also the S speciation of most particle size fractions extracted from the Ah horizon of the Viehhausen Luvisol and the Bs horizon of the Podzol was different from that of the bulk soil. For both soils, the contribution of oxidized S species to total S increased and the contribution of sulfoxides and organic mono- and disulfides decreased with decreasing particle size. Thus, sulfur K-edge XANES spectra of alkaline soil extracts, including humic and fulvic acids or of particle size separates are not representative for the S speciation of the original soil sample they are derived from. The differences can be attributed to (i) artificial changes of the sulfur speciation during alkaline extraction (conversion of reduced S into oxidized S, loss of SO₄²⁻ during purification of the extracts by dialysis) or particle size separation (carry-over of water-soluble S, such as SO₄²⁻), but also to (ii) preferential enrichment of oxidized S in hydrophilic water-soluble soil organic matter (ester sulfate) and in the clay fraction of soils (ester sulfate, adsorbed SO₄²⁻).     

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).     

Determining soil enzyme location and related kinetics using rapid fumigation and high-yield extraction

Knowledge concerning the location of soil enzymes and their kinetics is necessary for a better understanding of their roles in biogeochemical cycles. A rapid dichloromethane fumigation procedure and a new extraction method giving negligible co-extraction of humic substances were used. Fumigation and consequent cell lysis increased the activity of arylsulfatase, but not that of acid and alkaline phosphomonoesterase, suggesting that only the former enzyme was located both extracellularly and intracellularly. Extracted arylsulfatase displayed a Michaelis–Menten constant (K_m) significantly lower than in fumigated soil, which in turn was significantly lower than in unfumigated soil. In contrast, each phosphomonoesterase displayed the same K_m in extracts, fumigated and unfumigated soils. These results show that activity of soil phosphomonoesterases is not influenced by any direct interaction with soil and, therefore, are more likely to be extracellular enzymes primarily devoted to biogeochemical cycling than are arylsulfatases.     

Arylsulfatase activity of microbial biomass in soils as affected by cropping systems

 The impacts of crop rotations and N fertilization on different pools of arylsulfatase activity (total, intracellular, and extracellular) were studied in soils of two long-term field experiments in Iowa to assess the contibution of the microbial biomass to the activity of this enzyme. Surface-soil samples were taken in 1996 and 1997 in corn, soybeans, oats, or meadow (alfalfa) plots that received 0 or 180 kg N ha^–1 before corn, and an annual application of 20 kg P ha^–1 and 56 kg K ha^–1. The arylsulfatase activity in the soils was assayed at optimal pH (acetate buffer, pH 5.8) before and after chloroform fumigation; microbial biomass C (C_mic) and N (N_mic) were determined by chloroform-fumigation methods. All pools of arylsulfatase activity in soils were significantly affected by crop rotation and plant cover at sampling time, but not by N fertilization. Generally, the highest total, intracellular, and extracellular arylsulfatase activities were obtained in soils under cereal-meadow rotations, taken under oats or meadow, and the lowest under continuous cropping systems.Total, intracellular, and extracellular arylsulfatase activities were significantly correlated with C_mic (r>0.41, P<0.01) and N_mic (r>0.38, P<0.01) in soils. The averages of specific activity values, i.e., of arylsulfatase activity of the microbial biomass, expressed per milligram C_mic, ranged from 315 to 407 μg p-nitrophenol h^–1. The total arylsulfatase activity was significantly correlated with the intracellular activity, with r values >0.79 (P<0.001). In general, about 45% of the total arylsulfatase activity was extracellular, and 55% was associated with the microbial biomass in soils, indicating the importance of the microflora as an enzyme source in soils. Received: 23 April 1998     

Influence of extractable iron,aluminium, and manganese on P-sorption in flooded acid sulfate soils

We evaluated the effect of 1 N NH₄OAc and sodium-citrate dithionite extractable forms of soil Fe, Al, and Mn on P-sorption of a flooded acid sulfate soil (Sulfic Tropaquepts) and a non-acid sulfate soil (Typic Tropaquepts) under different soil oxidation-reduction and pH conditions. We used Maha-Phot soil (Sulfic Tropaquepts) and Bangkok soil (Typic Tropaquepts) from the Bangkok Plain, Thailand, and incubated them with 0.2% rice straw under aerobic (O₂ atmosphere) and anaerobic (N₂ atmosphere) conditions at three different levels of pH (4.0, 5.0, and 6.0) for 6 weeks in stirred soil suspensions with a soil to 0.01 M CaCl₂ solution ratio of 1:7. After the incubation period, the soil suspensions in the first treatment (control) were not washed or pretreated with any extractants. For the second treatment (II), the soil suspensions were treated with 1 N NH₄OAc (buffered to pH 4.0) to remove Fe, Al, and Mn in exchangeable form. In the third treatment (III), the soils suspensions were treated with sodium citrate dithionite solution (20%) to remove Fe, Al, and Mn in the form of free oxides. The soil residues were then equilibrated with KH₂PO₄ ranging from 0 to 500 mg P kg^-1 soil. Sorption isotherms were described by the classical Langmuir equation. The P-sorption parameters under study were standard P requirement (SPR), Langmuir maximum sorption capacity (X _m), Langmuir sorption constant (k), and buffering index (BI). Treating soils with 1 N NH₄OAc reduced X _m by 32–55%, SPR by 68–84%, and also decreased the differences in P-sorption due to the effects of pH and oxidation-reduction conditions. Significant correlations between the P-sorption parameters and the amount of free iron oxides indicated the primary role of iron oxides in P-sorption of acid sulfate soils. Aluminium oxides seemed to play a secondary role in P-sorption of these soils. Manganese also showed an important effect on P-sorption, but the mechanism is ambiguous.This is a contribution from the Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, LA 70803-7511     

Characterization of ester sulphate in a gypsum-amended podzol using an immobilized sulphatase reactor

An immobilized sulphatase reactor column was successfully used to determine the biochemical stability of ester sulphate in soil organic matter extracted from a podzol amended with gypsum. The sulphatase from Helix pomatia was covalently attached to controlled-porosity glass beads, and the immobilized enzyme was packed into a small glass column. The optimum pH, the time required to reach equilibrium, and the percentage of substrate consumed for the enzymatic hydrolysis of soil ester sulphate (pH 7.7, 90 min, 23–59%) were substantially different from those of p-nitrophenyl sulphate at similar concentrations of substrate (pH 7.0, 40 min, 99%). The striking difference in the biochemical stability and kinetic behaviour between soil ester sulphate and the simple synthetic substrate reflected their different chemical nature and structural features. The amounts of enzymatically hydrolysable (labile) ester sulphate in soil organic matter extracted from the podzol amended with gypsum at rates of 0, 50, and 200 kg S ha^-1 were significantly different (P=0.0004), being 0.5, 1.1, and 1.4 g S ml^-1 soil extract (or 5, 11, and 14 g S g^-1 soil), respectively. The labile ester sulphate was not correlated with the total hydriodic acid-reducible organic sulphate with the soil organic matter extracts but with the hydriodic acid-reducible organic sulphate: organic C ratio, which increased as a result of gypsum amendment. This study revealed that input of inorganic sulphate as gypsum substantially increased the accumulation of labile ester sulphate in a podzol.     

Root development of winter wheat in erosion‐affected soils depending on the position in a hummocky ground moraine soil landscape

Agricultural soil landscapes of hummocky ground moraines are characterized by 3D spatial patterns of soil types that result from profile modifications due to the combined effect of water and tillage erosion. We hypothesize that crops reflect such soil landscape patterns by increased or reduced plant and root growth. Root development may depend on the thickness and vertical sequence of soil horizons as well as on the structural development state of these horizons at different landscape positions. The hypotheses were tested using field data of the root density (RD) and the root lengths (RL) of winter wheat using the minirhizotron technique. We compared data from plots at the CarboZALF‐D site (NE Germany) that are representing a non‐eroded reference soil profile (Albic Luvisol) at a plateau position, a strongly eroded profile at steep slope (Calcaric Regosol), and a depositional profile at the footslope (Anocolluvic Regosol). At each of these plots, three Plexiglas access tubes were installed down to approx. 1.5 m soil depth. Root measurements were carried out during the growing season of winter wheat (September 2014–August 2015) on six dates. The root length density (RLD) and the root biomass density were derived from RD values assuming a mean specific root length of 100 m g^?1. Values of RD and RLD were highest for the Anocolluvic Regosol and lowest for the Calcaric Regosol. The maximum root penetration depth was lower in the Anocolluvic Regosol because of a relatively high and fluctuating water table at this landscape position. Results revealed positive relations between below‐ground (root) and above‐ground crop parameters (i.e., leaf area index, plant height, biomass, and yield) for the three soil types. Observed root densities and root lengths in soils at the three landscape positions corroborated the hypothesis that the root system was reflecting erosion‐induced soil profile modifications. Soil landscape position dependent root growth should be considered when attempting to quantify landscape scale water and element balances as well as agricultural productivity.     

Consequences of nitrogen fertilization on soil methane consumption in a productive temperate deciduous forest

To investigate the consequences of long-term N additions on soil CH₄ dynamics, we measured in situ CH₄ uptake rates, soil profiles and kinetics parameters during the growing season in a temperate deciduous forest in northwestern Pennsylvania (Allegheny College Bousson Environmental Forest). Measurements were made in control and adjacent plots amended with 100 kg N ha^–1 year^–1 for 8 years. We found that the in situ consumption rates were 0.19±0.02 (mean±SE) for the control and 0.12±0.01 mg CH₄–C m^–2 h^–1 for the N treatment, indicating that consumption had been reduced by 35% after 8 years of N amendments. Despite the large difference in rates of consumption, there were no differences in the CH₄ concentration profiles between the control and N-amended plots. Laboratory incubations of CH₄ consumption throughout the soil column (organic horizon and mineral soil depths) showed that rates were greatest in the organic horizon of both control and N-amended soils, although consumption was reduced by 42% in the N-amended plot. However, the rate in the organic horizon was only about 50% the rate measured in organic horizons at other temperate forests. The apparent K_m [K_m(app)] value in the organic horizon of the control plot was fourfold less than the K_m(app) value in the organic horizon of another temperate forest, but similar to the K_m(app) values in adjacent plots amended with N for a decade. Unlike results for other temperate forests, K_m(app) values at Bousson generally did not decrease with soil depth. These results indicate that N cycling strongly controls the CH₄-consuming community, and suggest that alterations of the N cycle due to N deposition or addition may alter rates and the location of CH₄ consumption by soils, even in soils with high N content and cycling rates.     

Abundance of soil mites under four agroforestry tree species with contrasting litter quality

 Populations of soil-dwelling mites were monitored in monoculture plots of four agroforestry tree species, Gliricidia sepium, Leucaena leucocephala, Dactyladenia barteri and Treculia africana, and compared to those in grass and secondary forest plots in the dry season (December 1993 to January 1994) and in the wet season (April to June 1994) in southwest Nigeria. Mite populations were very low in all plots during the dry season (500–3000 m^–2), compared to those during the wet season (10 000–30 000 m^–2). The highest mite population was observed in Gliricidia plots (3 044 m^–2) for the dry season and Leucaena plots (30 240 m^–2) for the wet season. Mite genera that were dominant in all the experimental plots were Annectacarus, Haplozetes, Machadobelba, Scheloribates and members of the Galumnidae, Dermanyssidae and Parasitidae. The community structure of mites was similar in the soil for Treculia and Gliricidia plots and for Leucaena and Dactyladenia plots. There were more taxonomic groups of mites under Leucaena than in the other agroforestry plots. Based on the density, diversity and complexity of the mite communities, Leucaena was considered to be better than other agroforestry species in encouraging the growth of mite populations. Received: 28 April 1998     

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.     

Organic nitrogen compounds extracted from arable and forest soils by electro-ultrafiltration and recovery rates of amino acids

Summary Extraction of synthetic amino acids dissolved in water by means of electro-ultrafiltration (EUF) showed average recovery rates of about 75%. Higher losses were obtained, particularly with cysteine, methionine and NH₄ ⁴; the latter, probably being deprotonated at the cathode, may be lost in form of NH₃. The EUF extracts of three arable and two forest soils were investigated for their N compounds. In the arable soils only about 3% of the total organic N extracted by EUF was free amino acids; about 23%–55% consisted of amino N (hydrolysable N) and the rest was non-hydrolysable N. The two forest soils contained higher amounts of EUF-extractable organic N compared with the arable soils. In the two forest soils the content of free amino-acid N amounted to 8% and 11% of the EUF organic N, and the proportion of hydrolysable N from total EUF-organic N was 41% and 46%. It is suggested that the amino-acid N and the hydrolysable N can be easily mineralized.     

Tillage effects on physical properties of agricultural organic soils of north central Ohio

Drainage, tillage, and intensive land use lead to drastic alterations in physical characteristics of organic soils. As decomposition and soil formation progress, bulk density (ρ_b) increases and total porosity (f_t) decreases due to subsidence, shrinkage, and mineralization of soil organic matter (SOM). However, the rate of subsidence and the changes in soil properties differ among management systems. Thus, the objectives of this study were to determine the effects of different tillage practices on ρ_b and f_t of cultivated peat soils. These experiments were conducted during 2004–2005, on Histosols in north central Ohio. Soil core samples were obtained from experimental plots managed with moldboard plow (MB), no-till (NT), or left bare (B). Conversion of plow tillage to NT increased ρ_b from 0.52 to 0.57 Mg m⁻³, and decreased f_t from 0.72 to 0.70 m³ m⁻³.     

Use of a coupled equilibrium model to describe the buffering of protons and hydroxyl ions in some acid soils

The buffering of protons and hydroxyl ions in acid soils was studied by the addition of small amounts of HCl, H₂SO₄, and NaOH in consecutive batch experiments using surface soils and subsoils from two Cambisols and one Podzol. A chemical equilibrium model was used to study the main buffer processes. The model included inorganic complexation and multiple cation exchange, and also the solubility of jurbanite and Al(OH)₃ for the subsoils. Buffering of protons was predicted quite well by the model for the surface soil of the Spodi-Dystric and Spodic Cambisols, suggesting that multiple cation exchange was the main buffer process. For the Podzol surface soil, however, the model overestimated proton buffering by cation exchange considerably. Hydroxyl buffering in acid surface soils could be described well by the model for the Podzol soil only. For the Cambisols, hydroxyl buffer reactions included not only cation exchange, but also solubilization of large amounts of organic matter and presumably deprotonation of dissolved organic carbon (DOC). Modelling proton and hydroxyl buffering in subsoils suggested that equilibrium with AJ(OH)₃ was not maintained for the Podzol and spodic Cambisol. Sulphate sorption had to be considered to describe titration experiments in all three soils. The assumption of jurbanite being in equilibrium with soil extracts was useful only for the Spodi-Dystric Cambisol.     

Changes in aggregate stability,nutrient status,indigenous microbial populations,and seedling emergence,following inoculation of soil withNostoc muscorum

The potential of the N₂-fixing cyanophyteNostoc muscorum for improving the aggregate stability of a poorly structured silt loam soil was studied in a greenhouse experiment. Inoculum rates were 1.61×10⁵ cells g^-1 soil dry weight (low rate) and 4.04×10⁵ cell g^-1 soil dry weight (high rate), approximately equivalent to a field application of 2 and 5 kg ha^-1 cells dry weight, respectively.N. muscorum numbers had increased 8-fold (low rate) and 10-fold (high rate) by 300 days after inoculation, indicating not only survival but proliferation. Increases in soil polysaccharides, determined as soil carbohydrate C, were 2.96–3.49 time the values in the non-inoculated soils and aggregate stability had incrased by an average of 18% on day 300. Inoculation withN. muscorum also had a pronounced effect on soil chemical and biological properties, with total C increasing by 50–63% and total N increasing by 111–120%. Increases in the soil indigenous microbial population were recorded, with numbers of bacteria 500, fungi 16, and actinomycetes 48 times the non-inoculated values on day 300 in the high-rate soil. The emergence of lettuce seedlings (Lactuca sativa var. Saladin) in undisturbed inoculated 300-day soils was 56% (low rate) and 52% (high rate) higher than in non-inoculated soils. However, homogenising soils and irrigating (to smulate ploughing and surface crusting) significantly reduced this increase in both treatments, although emergence in inoculated soils was still greater by 45% (low) and 24% (high). It is recommended that inoculated soils be left undisturbed prior to planting. The effects ofN. muscorum on soil physical, chemical, and biological properties indicate the possible benefits of cyanobacteria as soil inoculants, not only for the improvement of soil aggregate stability but also as a means of improving seedling emergence.     

Sequential transformation rates of soil organic sulfur fractions in two-step mineralization process

To understand the organic sulfur (S) stabilization in volcanic soils, we investigated organic S transformation rates and their relationships to soil properties in incubation experiments using forest soils from the Nikko volcanic region, central Japan. We hypothesized that carbon (C)-bonded S would first be transformed into ester sulfate-S and then into inorganic sulfate-S. We separately calculated the rates of decrease of C-bonded S (velocity 1, v ₁) and ester sulfate-S (velocity 2, v ₂) concentrations. During incubation, the ester sulfate-S concentration increased in two soils characterized by a high concentration of both ammonium oxalate-extractable aluminum (Al_o) and pyrophosphate-extractable Al (Al_p), whereas the C-bonded S concentration decreased in all soils. A large proportion of the S that was lost in the incubation experiments consisted of C-bonded S rather than ester sulfate-S. Velocity 2 was negatively correlated with both of Al_o and Al_p contents when soils were incubated at 20 °C. These results suggest that when C-bonded S is transformed into ester sulfate-S, complete mineralization to inorganic sulfate is inhibited, because ester sulfate-S is stabilized due to organo–mineral association. Incubation temperatures significantly affected v ₂. Thus, production of inorganic sulfate by mineralization of ester sulfate-S appeared to be regulated by soil Al contents and temperatures. Velocity 1 was proportional to soil pH ranging from 4.5 to 5.5, indicating that the degradation of C-bonded S is pH dependent.     

Factors controlling the water repellency induced by fire in calcareous Mediterranean forest soils

Water repellency (WR) is a property affected by fire and of crucial importance in the hydrological behaviour of soils after burning. In dry Mediterranean areas knowledge of the factors that control the development of water repellency by fire is of particular interest. We examined such factors in two calcareous soils, a Regosol and a Luvisol, representative of forest areas of southeast Spain. Heating temperature (200–500°C), vegetation type (Rosmarinus officinalis, Pinus halepensis and Brachypodium retusum), quantity of vegetation litter (control, low and high) and type of soil were selected as factors for assessing the WR induced by fire. The two soils exhibited markedly different WR responses after heating, the Regosol being much more susceptible than the Luvisol. Characteristics such as organic matter and clay content seem to determine the different WR responses to heating. We found that the type and quantity of vegetation litter also control the persistence of induced WR. In general, the order of increasing WR was Brachypodium < Pinus < Rosmarinus, and larger amounts of litter induced more WR. Maximum values of WR, most of them classified as severe (901−3600 s), were found in the range of 300−350°C, whereas beyond this temperature WR was destroyed. These results show that water repellency induced by combustion could be limited by environmental factors such as vegetation type and availability of litter, and that soil type and its characteristics also play a decisive role.     

Quantification and potential availability of non-symbiotically fixed 15N in soil

Summary Non-symbiotic N₂ fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a ¹⁵N-enriched atmosphere. N₂ fixation was greatest with the Drummer soil (18–122 g g^–1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 g g^–1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N₂ fixed in the three soils (r = 0.96). Efficiency of N₂ fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N₂ fixation in all the soils. The percentage of non-symbiotically fixed ¹⁵N in potentially mineralizable form (NH ₄ ⁺ -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.     

Importance of biological processes in the sulfur budget of a northern hardwood ecosystem

Summary Total S, organic S and sulfate were measured in foliage, litter, roots, soil and solutions at a hardwood site within the Adirondack Mountains of New York. Sulfate as a percentage of total S was similar in foliage and litter (10%), but was greater in roots (30%). Sulfur constituents in the hardwood forest ecosystem were dominated by C-bonded S (60 g m^–2) and ester sulfate (16 g m^–2) which are formed by biological processes. Because sulfur mineralization (1.42 g m^–2 yr^–1) was greater than wet precipitation inputs (0.82 g m^–2 yr^–1), those factors that influence mineralization-immobilization processes are important in evaluating S cycling and sulfate fluxes in this ecosystem. Ester sulfate was formed within the forest floor by the soil biota and was leached to mineral horizons. Annual turnover of this pool was high (25%) within the mineral forest floor. Forest-floor C-bonded S was derived from root and above-ground litter, and substantial amounts were leached to mineral horizons. Calculated storage + outputs (1.64 g m^–2 yr^–1) was much greater than wet inputs (0.82 g m^–2 yr^–1).     

Effect of long-term liquid pig manure application on atrazine mineralization in a soil cultivated with maize

Soil samples were collected in plots from a field experiment in maize monoculture receiving 0, 60 and 120 m³ ha^-1 liquid pig manure (LPM) for 19 years. Soils were sampled from the 0- to 20-cm layer in August and October 1997 and in June, July and September 1998. Subsurface samples were also evaluated in September 1998. Laboratory soil radiorespirometry was used to evaluate atrazine mineralization using [U-ring-¹⁴C]-atrazine mixed with commercially available product. The effect of atrazine dose (50, 100 and 500 mg atrazine kg^-1 soil) was evaluated on soils sampled in August 1997. For the other sampling dates, the soils were spiked with 50 mg atrazine kg^-1 soil. No LPM dose effect on atrazine mineralization was obtained in the different experiments. Increasing atrazine dose to 500 mg kg^-1 decreased significantly the mineralization rate (R_i) and the maximum of atrazine mineralized (MAX), while the time needed to mineralize 50% of MAX (DT-50%) was not significantly affected. Sampling time had a significant effect on atrazine mineralization. Atrazine mineralization in the soils sampled in June 1998 showed lower R_i and MAX than in the soils sampled at the other dates. Atrazine mineralization in subsurface soils (20–60 cm) was very variable and quite high in some samples. This may be due to atrazine pre-exposure in subsoils resulting from atrazine deep movement by preferential flow.