Field-moist Soil Research Articles

Assessment of a laboratory method to obtain the equilibrium solution composition of forest soils

SummaryEquilibrium studies on soil require reliable estimates of ion concentrations in the soil solution under field conditions. We evaluated the previously described iterative method to approximate the equilibrium soil solution (ESS) with four acid forest soils. We examined for which ions the ESS is suitable, making use of the fact that concentrations in water extracts are functions of the soil: solution ratio. The electric conductivity, pH, and the concentrations of base cations, Mn2+, NO−3, SO42+, and dissolved organic carbon (DOC) were usually linear with the soil:solution ratio in water extracts, whereas no relation was observed for Al (with one exception) and Fe. Assuming that the ESS can be attributed the soil solution ratio of the field moist soil at the time of sampling, concentrations appeared as the continuation of the linear relation with the soil: solution ratio for base cations, pH, and the electric conductivity. This indicates that the ESS actually represents field conditions for these solution properties. For Al water extracts allowed no evaluation of the ESS result. The ESS underestimated SO42− concentrations under field conditions, presumably because the lack of DOC in the solutions added distorts the balance amongst anions.

Denitrification with and without maize plants (Zea mays L.) under irrigated field conditions

The study was conducted under irrigated field conditions to examine the effect of maize plants on denitrification. Both planted and unplanted field plots received 150kgNha–1 as urea. In a third treatment, which was also planted and received urea at 150kgNha–1, the soil nitrate N content was brought up to equal to that in the unplanted plots by applying additional doses of N as calcium nitrate. Soil cores were collected 24 and 72h after irrigation and the denitrification rate was measured by the acetylene inhibition method. Nitrate-N content, aerobically mineralizable C, microbial biomass carrying capacity and denitrification potential were also studied on field-moist soil. Maize plants grown under field conditions always had the potential to increase denitrification in conditions of both high and low water-filled porosity. When nitrate-N content of the planted soil decreased due to plant uptake, denitrification was reduced in the planted soils. However, when nitrate-N uptake by plants was compensated through additional doses of nitrate fertilizer, denitrification was always higher in planted than unplanted soil. The stimulatory effect of plants on denitrification was observed at both high and low soil nitrate-N concentrations, though it was more pronounced at high nitrate-N levels. The effect of plants on denitrification and related parameters was confined to the root zone.

Seasonal dynamics of nutrients in leaves and xylem sap of coffee plants as related to different soil compartments

The northwestern province of Costa Rica is a marginal coffee growing area. At the onset of the rainy season low redox potentials probably induce the mobilization of soil Mn resulting in enhanced plant uptake of Mn. To test this hypothesis we monitored from April to the end of June 1995 the mobile Mn in the soil and nutrient and Mn concentrations in leaves and xylem sap of coffee plants. Every 2 weeks we took aggregate and bulk soil samples. The aggregates were mechanically separated into interior and exterior, air-dried and all soil samples were extracted with 1 M NH4NO3. We also extracted the field moist soil with distilled water. In addition, the 3rd and the youngest pair of coffee leaves and xylem sap were sampled and analyzed. According to the results of leaf analyses the nutrient supply of the coffee plants in general seemed to be balanced. However, Mn concentrations of 223 mg kg-1 in the 3rd leaf pair at 18 April were above the optimum and the youngest leaves indicated Fe deficiency, but senescent leaves accumulated Fe and overcame the deficiency. Manganese concentrations in the xylem sap showed a pronounced maximum 2 weeks prior to a similar maximum of mobile Mn in the aggregate exterior. But in general the temporal variation of nutrient concentrations (especially Ca and Mg) in the plants are well correlated with the easily extractable nutrient concentrations in bulk soil. Probably due to its specific absorption and high rates of redistribution within the plant, K in the soil extracts did not correlate with plant concentrations. Element concentrations of youngest leaves could not be correlated with soil concentrations and are not considered to be an adequate tool for monitoring current nutrient uptake. Since plant element concentrations did not correlate with the aggregate interior, plants probably cannot use that nutrient source efficiently.

Atrazine Sorption-Desorption In Field-Moist Soils

Abstract Methods commonly used to obtain sorption coefficients require that the soils be above field capacity moisture so that the aqueous phase containing the pesticide to be quantified can be separated from the soil. We have developed a system using supercritical CO2 that can remove the pesticide from soil solution of unsaturated soil without first requiring the separation of the solution from soil. Sorption coefficients increased with soil organic carbon and clay contents for three field-moist soils. Also, sorption significantly increased in a sand as moisture content increased from 4.0 to 16% and in a silt loam as moisture increased from 9.6 to 27%. Isosteric heats of sorption were easily determined with the supercritical fluid system and ranged from −10 to −12 kcal mol−1 and were correlated to organic carbon and clay contents of the soils. Sorption coefficients in field-moist soils were much greater than are typically obtained with the batch slurry system, while heats of sorption were much more negative, indicating greater sorption at low moisture contents.

Water, NaHCO3−, NaH2PO4− and NaCl‐extractable SO42− in acid forest soils

AbstractA variety of different methods have been used for the determination of inorganic soil SO42− in the past, which makes it difficult to compare SO42− contents of soils.Sulfate was extracted with the four commonly used extraction solutions 0.5 M NaHCO3, 0.02 M NaH2PO4, 0.1 M NaCl and H2O from A‐, Bw‐ and Bs‐horizons of six acid forest soils. 5 g of field moist soil were percolated with a flow rate of 5 ml/h and percolations were repeated as long as SO42− was detectable in the percolate (> 0.5 mg SO4·l−1).NaCl and NaHCO3 extracted highest amounts of total inorganic SO42− in A‐horizons, but NaHCO3 caused analytical problems. NaHCO3 and NaH2PO4 yielded highest amounts in B‐horizons. With the exception of Bs‐horizons more than 70% of the total inorganic SO42− was H2O‐soIuble. Thus, if H2O‐soluble SO42− is defined as reversibly bound, the greater part of the inorganic SO42− in the investigated acid forest soils was reversibly bound. This SO42− fraction can potentially be released, if SO42− deposition decreases.

Supercritical Carbon Dioxide for Determining Atrazine Sorption by Field-Moist Soils

Supercritical fluid extraction (SFE) with carbon dioxide (SF-CO2) has been used effectively as a method for the extraction of pesticides from soils. This study was intended to test its potential as a means of removing atrazine from soil water for quantification, to allow calculation of sorption coefficients (Kds) of atrazine in soils having water contents below field capacity. Low-density SF-CO2 removed atrazine from soil solutions without first requiring separation of the solution from the soil. The Kds obtained by the SF-CO2 method for the topsoil and the lower root zone samples were 1.21 ± 0.04 and 1.14 ± 0.03, respectively, while that of the vadose zone soil was 0.16 ± 0.00. Desorption was rapid; equilibrium was reattained within 7 min. Desorption Kds for the topsoil and lower root zone soil were constant for successive desorption equilibrations, through removal of approximately 25% of the applied atrazine from the system. The SF-CO2 method can be used to determine the effect of changes in water content and temperature on sorption. It was found that little atrazine can be extracted by the SF-CO2 method from desiccated soil though large amounts of water (16%) caused a dramatic increase in the Kd values determined with SF-CO2. The soil solution concentration at 4% soil water content related linearly to the inverse of the temperature (T, K) and the isosteric heat (ΔHi) was determined to be −55.2 ± 1.7 kJ mol-1. The SF-CO2 method is promising as a technique to characterize herbicide sorption-desorption from field-moist soils.

Vegetation and soil acidity of a mangrove swamp in southeastern Nigeria

Abstract. The relationships between vegetation and acid properties of soils from a mangrove swamp in southeastern Nigeria were examined. Field moist soils were moderately acidic with pH ranging from 5.3 to 6.8. Air dry pH ranged from 2.9 to 4.9 indicating strong acidity; the values increased with profile depth and were related to decreasing fibrous root content. Less acidic conditions (air dry pH 4.3–4.9) were associated with the less fibrous soils under Avicennia africana and Pandanus candelabrum; more acidic conditions (air dry pH 2.9–3.8) occurred in the more fibrous Rhizophora spp. and Nypa fruticans soils. Species occurrences and levels of aluminium and sulphate in the profiles were related, suggesting that hydrogen ions, aluminium and sulphate were the main contributors to exchangeable acidity. Unless crops are known to tolerate very acidic conditions, the fibrous Rhizophora and Nypa soils should not be drained for cultivation.

Interactions between soil properties and moisture content in crust formation, runoff and interrill erosion from tilled loess soils

Soil-surface seals and crusts resulting from aggregate breakdown reduce the soil infiltration rate and may induce erosion by increasing runoff. The cultivated loess areas of northwestern Europe are particularly prone to these processes. Surface samples of ten tilled silty loamy loess soils, ranging in clay content from 120 to 350 g kg −1 and in organic carbon from 10 to 20 g kg −1, were packed into 0.5 m 2 plots with 5% slopes and subjected to simulated rainfall applied at 30 mm h −1. The 120 minutes rainfall events were applied to initially field-moist soil, air-dried soil and rewetted soil to investigate the effect of soil moisture content prior to rainfall. Runoff and eroded sediments were collected at 5 minutes intervals. Aggregate stability of the soils was assessed by measuring particle-size distribution after different treatments. All soils formed seals. Runoff rates were between 70 and 90% by the end of the rainfall event for field-moist plots. There were large differences between soil runoff rates for the air-dried and rewetted plots. Interrill erosion was associated with runoff, and sediment concentration in runoff readily reached a steady-state value. Measurements of aggregate stability for various treatments were in good agreement with sealing, runoff and erosion responses to rainfall. Runoff and erosion were lower for air-dried plots than for field-moist plots, and were either intermediate or lowest for rewetted plots, depending on soil characteristics. Soils with a high clay content had the lowest erosion rate when they were rewetted, whereas the soil with a high organic-carbon content had the lowest erosion rate in air-dry conditions. The results indicate the complexity of the effect of initial moisture content, and the interactions between soil properties and climate.

Effect of depth and lime or phosphorus-fertilizer applications on the soil solution chemistry of some New Zealand pastoral soils

Thirteen trails were sampled to investigate the effects of depth, or the surface application of lime and phosphorus (P) fertilizer, on solution composition. Soil solutions were extracted by centrifuge from field moist soils within 24 h of sampling. Solution Ca, Mg, Na and K, Al, Mn and Fe concentrations generally decreased and Al, Mn and Fe concentrations generally increased with depth; although exceptions occurred. The largest decrease occurred in the first 25-50 mm of soil. Higher solution Al concentrations occurred in a band at a depth of between 50 and 100 mm in some soils. Lime generally increased solution pH and solution Ca, Mg and HCO3 concentrations, and reduced solution Al, Fe and Mn concentrations in the topsoils. In one soil (Matapiro silt loam) 2 years after lime was applied, lime increased solution pH down to a depth of 100 mm, Ca and HCO3 down to 75 mm and Mg down to 50 mm. Lime also decreased solution Al and Mn down to 75 mm and Fe down to 50 mm. In one series of trials, lime increased solution Ca concentrations at a depth of 50-100 mm 4 years after application in six out of the eight sites. In the same trial series, the application of P fertilizer increased solution P concentrations at 0-50 mm from a mean of 5 �M in the no-added P plots up to a mean of 56 �M at the highest P rate. The highest solution P concentration recorded was 194 �M. The increase in solution P concentrations for a given application of fertilizer P varied from 0.05 to 1.03 �M P per kg P ha-1 applied. Maximum pasture yield and 90% maximum yield occurred when solution P concentrations were about 28 and 14 �M respectively. Solution P concentrations determined from P adsorption isotherms were not a good indicator of solution P concentrations measured in soil. Solution pH was higher than soil pH (1:2.5 soil:water ratio, 2 h equilibration) with a solution pH of 6.0 corresponding to a soil pH in water of about 5.2.

Comparison of sulfur mineralisation in open and closed incubation systems and from field moist and conditioned soils

Abstract The amounts of sulfur (S) mineralised in three soils incubated in both open (i.e. leached) and closed systems increased with time over the period of the incubation. However, the rates and patterns of S release differed with the soil type and incubation technique used. The pattern of S mineralisation during the open incubation was a rapid release of S in the first two weeks, followed by a slightly slower almost linear rate of release for the remainder of the incubation. The pattern of S release during the closed incubation was slightly curvilinear. In the short term, the closed system released similar or greater amounts of S than the open system. However, in the long term, more S would be mineralised in the open than that in the closed system. Conditioning (i.e. pre‐incubation of air‐dried samples) did not significantly affect the amounts of S mineralised in the soils incubated in the open system, but did in two soils incubated in the closed system.

Comparison of immiscibly displaced soil solutions and saturated paste extracts from saline soils

Soil solutions from four salt-affected soils in Alberta were extracted by immiscible displacement from field moist soils and saturated pastes after air-drying to examine the implications of using saturated paste extracts as a proxy of soil solutions. Solutions ranged in pH from 7.8 to 10.6 and in electrical conductivity (EC) from 6 to 45 dS m−1. Sodium dominated the cation suite, with Mg and K concentrations varying by several orders of magnitude between sites. Calcium was the major cation in lowest abundance at all sites. Sulfate was the predominant anion. Phosphorus concentrations ranged from 1 to 247 ppm. Considerable variation was seen between the properties of saturated paste extracts and immiscibly displaced solutions, with the saturated paste extracts having lower conductivities and Na, Mg, K and anion concentrations. The differences in composition between the two extraction methods resulted in differences in ion speciation, SAR and ESP values and mineral stabilities. Saturation indices for evaporite minerals were poor predictors of minerals formed in evaporated soil solutions. It was also found that even in chemically similar solutions, different minerals precipitated, and therefore the prediction of evaporite mineral formation from extracted soil solutions is not recommended. Key words: Salinity, saturated paste extracts, soil solutions, evaporites, mineral stability

Cellulase activity of soils

Cellulases are a group of enzymes that catalyze the degradation of cellulose, a polysaccharide built of β-1,4 linked glucose units. This group consists of endo-1,4-glucanase (EC 3.2.1.4), exo-1,4-glucanase (EC 3.2.1.91), and β-d-glucosidase (EC 3.2.1.21). The products of cellulose degradation are glucose, cellobiose, and higher molecular weight oligosaccharides. An accurate and precise method for the assay of cellulase activity in soil was developed. It involves determination of the reducing sugars produced when a soil sample is incubated with acetate buffer (50 mM, pH 5.5), carboxymethyl cellulose (CMC), and toluene at 30°C for 24 h. Results showed that the optimal pH of cellulase activity was 5.5. Reducing sugars produced from cellulase activity increased linearly up to 7 days, and they increased with increasing amounts of soil up to 7 g. The substrate concentration-activity curves of cellulase activity in soils obeyed the Michaelis-Menten equation. In two soils tested, the Michaelis constant (Km) ranged from 9.7 to 21.4 g CMC 1−1. Cellulase activity in soils was maximal at temperatures ranging from 50 to 60°C. The activation energy values of the reaction catalyzed by cellulase in four soils ranged from 21.7 to 28.0 kJ mol−1. Soil cellulase was denatured at temperatures ranging from 60 to 70°C; from 60 to 80%of total cellulase activity remained after heating air-dried soil samples at 100°C for 2 h. The corresponding values for field-moist soils were 18–23%. Toluene treatment markedly increased the cellulase activity values over the nontreated samples. Reducing sugar values increased significantly when air-dried or field-moist soil samples were incubated as described but without CMC, suggesting a role for cellulase in degradation of the native substrates.

Laboratory investigation into the potential effectiveness of soil-based countermeasures for soils contaminated with radiocaesium and radiostrontium

Accidental releases of radioactive material into the environment have prompted the search for effective soil-based chemical treatments to reduce the transfer of radiocaesium and radiostrontium along foodchains. As field studies can be impractical and protracted, a simple laboratory method has been evaluated for use after an accident to predict the effect that different soil treatments may have on radionuclide availability to plants. The study involved a batch equilibrium approach in which in situ conditions were replicated by the use of field moist soils and their associated soil solutions. The technique was used to investigate the effects of common agricultural treatments (potassium as sulphate of potash, calcium as garden lime and ammonium as ammonium sulphate) on the distribution of 137Cs, 90Sr, K and Ca between solid and liquid phases of loam, sand and peat soils. A treatment that decreased the 137Cs +:K + or 90Sr 2+:Ca 2+ quotient in the liquid phase compared with controls could be considered a potentially effective countermeasure for reducing uptake of the appropriate radionuclide by plants. Potassium decreased 137Cs +:K + quotients in the liquid phase of all soil types, and in peat in particular. The potential effectiveness of potassium as a soil based countermeasure for reducing radiocaesium uptake to plants in peat soils was supported by field evidence from highly organic soils in Cumbria. Application of the ammonium treatment increased 137Cs +:K + quotients in the liquid phase of all soil types and in the sand in particular. As an increase in this ratio is associated with an enhanced uptake of radiocaesium by plants, the application of ammonium based fertilisers to soils contaminated with radiocaesium should therefore be considered very carefully. No firm conclusions could be reached regarding the effectiveness of garden lime as a countermeasure because of its low solubility and the very high calcium status of the experimental soils at the start of the experiment.

Colorimetric determination of reducing sugars in soils

Reducing sugars are the end products of many biological processes and enzymatic reactions in soils. They are determined in assay of several soil enzymes, including cellulase activity. Five colorimetric methods [phenol-sulfuric acid, anthrone-sulfuric acid, dinitrosalicylic acid (DNS), reaction with potassium ferric hexa-cyanide reagent (Prussian blue), and the Somogyi-Nelson (molybdenum blue) methods] were evaluated for determination of reducing sugars and total saccharides extracted from soils. Results showed that the Prussian blue and the molybdenum blue methods were the most sensitive and accurate for determination of reducing sugars in soils. Metals extracted from soils interfered with molybdenum blue color development. These metals, however, could be removed by K-saturated resin before analysis. The trace amount of metals extracted from soils did not interfere with the Prussian blue color development, but this method is too sensitive to be useful for determination of reducing sugars in soil extracts. Unlike the Prussian blue method, which is very sensitive and has 1 h color stability, the molybdenum blue method has color stability of at least 24 h. Reducing sugar values in soils increased significantly upon air-drying of field-moist soils or incubation of soils with acetate buffer (50 mM, pH 5.5) at 30°C for 24 h, suggesting enzymatic hydrolysis of the native substrates. Calibration graphs showed that the phenol-sulfuric acid, DNS, and anthrone-sulfuric acid methods are not as sensitive as the other two methods.

Effect of drying of soils on the adsorption and leaching of phosphate and 2,4-dichlorophenoxyacetic acid

Surface samples of an allophanic (Patua silt loam) and a non-allophanic (Tokomaru silt loam) soil were used to examine the effects of drying on the adsorption and leaching of phosphate and 2,4-dichlorophenoxyacetic acid (2,4-D). Phosphate and 2,4-D adsorption isotherms and adsorption kinetics were determined for field-moist, air-dried and oven-dried soil samples using KH2PO4 and 14C labelled 2,4-D. In a separate experiment, the leaching of a pulse of phosphate or 2,4-D was examined using soil columns. The Patua silt loam adsorbed 4-7 times more phosphate and 2,4-D than the Tokomaru silt loam. Compared with field-moist soil, both air-dried and oven-dried soil increased (2-5 times) the adsorption of phosphate, and oven-dried soil decreased the adsorption of 2,4-D. The adsorption kinetics indicated that there was no effect of drying on the rate of adsorption of either phosphate or 2,4-D. In column experiments, there was less leaching of phosphate and 2,4-D from the Patua soil than from the Tokomaru soil. In both soils, there was no effect of drying on the leaching of phosphate, whereas oven-drying increased the leaching of 2,4-D. Solubilization of organic matter during drying increased the accessibility of P to adsorption sites on the mineral surface and thereby increased the adsorption, whereas the water-soluble organic matter bound 2,4-D and thereby decreased the apparent adsorption onto soil and increased leaching

Wood-ash fertilization and fire treatments in a Scots pine forest stand: Effects on the organic layer, microbial biomass, and microbial activity

We studied the reactions of humus layer (F/H) microbial respiratory activity, microbial biomass C, and the fungal biomass, measured as the soil ergosterol content, to the application of three levels of wood ash (1000, 2500, and 5000 kg ha-1) and to fire treatment in a Scots pine (Pinus sylvestris L.) stand. Physicochemical measurements (pH, organic matter content, extractable and total C content, NH 4 + and total N content, cation-exchange capacity, base saturation) showed similarity between the fire-treated plots and those treated with the lowest dose of wood ash (1000 kg ha-1). The ash application did not change the level of microbial biomass C or fungal ergosterol when compared to the control, being around 7500 and 350 μg g-1 organic matter for the biomass C and ergosterol, respectively. The fire treatment lowered the values of both biomass measurements to about half that of the control values. The fire treatment caused a sevenfold fall in the respiration rate of fieldmoist soil to 1.8 μl h-1 g-1 organic matter compared to the values of the control or ash treatments. However, in the same soils adjusted to a water-holding capacity of 60%, the differences between the fire treatment and the control were diminished, and the ash-fertilized plots were characterized by a higher respiration rate compared to the control plots. The glucose-induced respiration reacted in the same way as the water-adjusted soil respiration. The metabolic quotient, qCO2, gradually increased from the control level with increasing applications of ash, reaching a maximum in the fire treatment. Nitrification was not observed in the treatment plots.

Forms of sulfur extracted from soils after different methods of sample preparation

Three forms of sulfur (S) were determined in extracts from field-moist, air-dried and conditioned (i.e. maintained at 20�C and 75% field moisture capacity for 2 weeks) soils by using four extractants-water, CaCl2, KH2PO4 and Ca(H2PO4)2. The amounts and proportions of inorganic sulfate (SO2-4-S), organic HI-reducible S (HI-S) and carbon bonded S (C-S) in extracts varied with soil type, method of sample preparation and the extractant used. Substantially less S overall was extracted from field-moist soil compared with air-dried and conditioned soils. For soils with pH values close to or above 6, water and phosphate extractants removed similar amounts of SO2-4-S. However, phosphate solutions removed greater amounts of SO2-4-S than water from soils with relatively low pH or with a large capacity for sulfate adsorption. The CaCl2 solution extracted the smallest amounts of sulfate from the soils studied. There was no consistent ratio between extractable sulfate-S and extractable organic S. Extracts from air-dried soil contained the greatest amounts and proportions of organic S, particularly in phosphate extracts. The conditioning of soils decreased the extractable organic S and increased the sulfate-S, indicating that extractable organic S is readily mineralizable. The relative proportions of extractable organic S present as HI-S and C-S varied considerably. Comparison of the conditioned and air-dried soils indicated there was no difference between the two forms of organic S in their ease of mineralization.

Nitrification and nitrous oxide production potentials in aerobic soil samples from the soil profile of a Finnish coniferous site receiving high ammonium deposition

Using aerobic soil slurry technique nitrification and nitrous oxide production were studied in samples from a pine site in Western Finland. The site received atmospheric ammonium deposition of 7–33 kg N ha −1 a −1 from a mink farm. The experiments with soil slurries showed that the nitrification potential in the litter layer was higher at pH 6 than at pH 4. However, the nitrification potentials in the samples from the organic and mineral horizons at pH 6 and 4 were almost equal. Also N 2O was produced at a higher rate at pH 6 than at pH 4 in slurries of the litter layer samples. The reverse was true for samples from the organic and mineral horizons. The highest N 2O production and nitrification rates were measured in the suspensions of litter layer samples. Nitrification activity in field-moist soil samples was lower than the activity in the slurries indicating that the availability of ammonium limited nitrification in these soils. Acetylene (2.5 kPa) retarded nitrification activity (70-–100%) and N 2O production (40 – 90%) in soil slurries. Acetylene inhibited the N 2O production by 40–60% during the first 3 days after its addition to field-moist samples incubated in aerobic atmosphere. After 3 days the inhibition became much lower (4–5%). The results indicate that, in soil profiles of boreal coniferous forests receiving ammonium deposition, chemolithotrophic nitrification may have importance in the N 2O production, and that changes in soil pH affect differently nitrification as well as N 2O production in litter and deeper soil layers.

Nitrification and nitrous oxide production potentials in aerobic soil samples from the soil profile of a Finnish coniferous site receiving high ammonium deposition

Using aerobic soil slurry technique nitrification and nitrous oxide production were studied in samples from a pine site in Western Finland. The site received atmospheric ammonium deposition of 7–33 kg N ha−1 a−1 from a mink farm. The experiments with soil slurries showed that the nitrification potential in the litter layer was higher at pH 6 than at pH 4. However, the nitrification potentials in the samples from the organic and mineral horizons at pH 6 and 4 were almost equal. Also N2O was produced at a higher rate at pH 6 than at pH 4 in slurries of the litter layer samples. The reverse was true for samples from the organic and mineral horizons. The highest N2O production and nitrification rates were measured in the suspensions of litter layer samples. Nitrification activity in field-moist soil samples was lower than the activity in the slurries indicating that the availability of ammonium limited nitrification in these soils. Acetylene (2.5 kPa) retarded nitrification activity (70-–100%) and N2O production (40 – 90%) in soil slurries. Acetylene inhibited the N2O production by 40–60% during the first 3 days after its addition to field-moist samples incubated in aerobic atmosphere. After 3 days the inhibition became much lower (4–5%). The results indicate that, in soil profiles of boreal coniferous forests receiving ammonium deposition, chemolithotrophic nitrification may have importance in the N2O production, and that changes in soil pH affect differently nitrification as well as N2O production in litter and deeper soil layers.

Mineralization of nitrogen in fertilizer-acidified lime-amended soils

The application of NHinf4su+-based fertilizers to soils slowly lowers soil pH, which in turn decreases nitrification rates. Under these conditions nitrification and N mineralization may be reduced. We therefore investigated the impact of liming fertilizer-acidified soils on nitrification and N mineralization. Soil samples were collected in the spring of 1987 from a field experiment, initiated in 1980, investigating N, tillage, and residue management under continuous corn (Zea mays L.). The pH values (CaCl2) in the surface soil originally ranged from 6.0 to 6.5. After 6 years the N fertilizer and tillage treatments had reduced the soil pH to values that ranged between 3.7 and 6.2. Incubation treatments included two liming rates (unlimed or SMP-determined lime requirement), two 15N-labeled fertilizer rates (0 or 20 g N m-2), and three replicates. Field-moist soil was mixed with lime and packed by original depth into columns. Labeled-15N ammonium sulfate in solution was surface-applied and columns were leached with 1.5 pore volumes of deionized water every 7 days over a 70-day period. Nitrification occurred in all pH treatments, suggesting that a ferilizer-acidified soil must contain a low-pH tolerant nitrifier population. Liming increased soil pH values (CaCl2) from 3.7 to 6.2, and increased by 10% (1.5 g N m-2) the amount of soil-derived NO3--N that moved through the columns. This increase was the result of enhanced movement of soil-derived NO3--N through the columns during the first 14 days of incubation. After the initial 14-day period, the limed and unlimed treatments had similar amounts of soil N leaching through the soil columns. Lime increased the nitrification rates and stimulated the early movement of fertilizer-derived NO3--N through the soil.