Immediate and adaptational temperature effects on nitric oxide production and nitrous oxide release from nitrification and denitrification in two soils

 Nitrification and denitrification are, like all biological processes, influenced by temperature. We investigated temperature effects on N trace gas turnover by nitrification and denitrification in two soils under two experimental conditions. In the first approach ("temperature shift experiment") soil samples were preincubated at 25  °C and then exposed to gradually increasing temperatures (starting at 4  °C and finishing at 40–45  °C). Under these conditions the immediate effect of temperature change was assessed. In the second approach ("discrete temperature experiment") the soil samples were preincubated at different temperatures (4–35  °C) for 5 days and then tested at the same temperatures. The different experimental conditions affected the results of the study. In the temperature shift experiment the NO release increased steadily with increasing temperature in both soils. In the discrete temperature experiment, however, the production rates of NO and N₂O showed a minimum at intermediate temperatures (13–25  °C). In one of the soils (soil B9), the percent contribution of nitrification to NO production in the discrete temperature experiment reached a maximum (>95% contribution) at 25  °C. In the temperature shift experiment nitrification was always the dominant process for NO release and showed no systematic temperature dependency. In the second soil (soil B14), the percent contribution of nitrification to NO release decreased from 50 to 10% as the temperature was increased from 4  °C to 45  °C, but no differences were evident in the discrete temperature experiment. The N₂O production rates were measured in the discrete temperature experiment only. The contribution of nitrification to N₂O production in soil B9 was considerably higher at 25–35  °C (60–80% contribution) than at 4–13  °C (15–20% contribution). In soil B14 the contribution of nitrification to N₂O production was lowest at 4  °C. The effects of temperature on N trace gas turnover differed between the two soils and incubation conditions. The experimental set-up allowed us to distinguish between immediate effects of short-term changes in temperature on the process rates, and longer-term effects by which preincubation at a particular temperature presumably resulted in the adaptation of the soil microorganisms to this temperature. Both types of effects were important in regulating the release of NO and N₂O from soil. Received: 20 October 1998     

Seasonal changes of microbial biomass carbon related to climatic factors in soils from karst areas of southwest China

 The seasonal responses of soil microbial biomass C to changes in atmospheric temperature, soil moisture and soluble organic C were studied in soils from the karst areas of southwest China. These soils are relatively weathered, leached and impoverished, and have a low input of plant residues. Over 1 year, an inverse relationship between soil microbial biomass C and atmospheric temperature was found. The highest microbial biomass C occurred in winter and the lowest in summer, and ranged from 231–723 μg g^–1 dry soil. Although there was no obvious relationship between microbial biomass C and soil moisture, a negative correlation existed between microbial biomass C and soluble organic C. In the ecosystem studied, the marked changes in soil microbial biomass C at above 20  °C were ascribed to fluctuations of soil moisture, which were controlled by climatic factors and geomorphic conditions. The patterns of soluble organic C turnover were similar to those of soluble carbohydrate C, both of which were controlled by soil drying-rewetting cycles. It was concluded that the lowest amounts of soil microbial biomass C, measured in the summer, resulted in increases in soluble organic C due to higher turnover rates of the former at warmer air temperatures. Thus, there was a marked seasonal change in soil microbial biomass C. Received: 1 November 1998     

Determination of the soil microbial biomass carbon using the method of substrate-induced respiration

Specific features of determining the carbon content in the soil microbial biomass using the method of substrate-induced respiration (MB_SIR) were studied as related to the conditions of the incubation (the glucose concentration and temperature) and pre-incubation (the duration and temperature) of the soil samples collected in the summer (tundra gley and soddy-podzolic soils and chernozems) and in different seasons (for the gray forest soil). The glucose concentration providing the highest substrate-induced respiration (SIR) in the soils studied was shown to be 2–15 mg/g. The MB_SIR in the soil samples collected in summer and in the soils pre-incubated for 10 and 22°C (7 days) did not significantly differ. The MB_SIR in the gray forest soil pre-incubated at 3, 6, and 10°C (winter, spring/autumn, and summer, respectively) and at 22°C (recommended by the authors of the SIR method) was similar for the cropland in all the seasons. For the meadow, it was the same in the winter, summer, and autumn, and, in summer, it did not differ only for the forest. For the comparative assessment of the MB_SIR, soil samples from different ecosystems are recommended to be collected in the autumn or in the summer. Soil samples of 100–500 g should be pre-incubated for 7 days at 22°C and moisture of 60% of the total water capacity; then, 1-2 g soil should be incubated with glucose (10 mg/g) at 22°C for 3–5 hours.     

Soil-released carbon dioxide from microbial biomass carbon in the cultivated soils of karst areas of southwest China

 Soil microbial biomass and the emission of CO₂ from the soil surface were measured in yellow soils (Ultisols) of the karst areas of southwest China. The soils are relatively weathered, leached and impoverished, and have a low input of plant residues. The measurements were made for a 1-year period and show a reciprocal relationship between microbial biomass and surface CO₂ efflux. The highest (42.6±2.8 mg CO₂-C m^–2 h^–1) and lowest (15.6±0.6 mg CO₂-C m^–2 h^–1) CO₂ effluxes are found in the summer and winter, respectively. The cumulative CO₂ efflux is 0.24 kg CO₂-C m^–2 year^–1. There is also a marked seasonal variation in the amount of soil microbial biomass carbon, but with the highest (644±71 μg C g^–1 soil) and lowest (270±24 μg C g^–1 soil) values occurring in the winter and summer, respectively. The cumulative loss of soil microbial biomass carbon in the top 10 cm of the soil was 608 μg C g^–1 year^–1 soil over 17 sampling times. The mean residence time of microbial biomass is estimated at 105 days, suggesting that the carbon in soil microbial biomass may act as a source of the CO₂ released from soils. Received: 13 July 1999     

Effect of plant residue return on the development of surface soil pH gradients

 In the field, surface soil pH gradients were observed under senescing plants over late spring and summer. A soil incubation experiment was conducted (119 days, 20  °C) to provide direct evidence of the influence of plant residue incorporation on soil pH. This was investigated in terms of plant residue type (wheat and subterranean clover) and dry matter addition rate (0, 6.25, 12.5 and 25.0 g kg^–1), as well as the soil layer of incorporation (0–2.5 and 7.5–10 cm) and moisture regime (continuously moist and moist-dry cycles). During incubation, moist unamended soils slowly acidified. In contrast, the addition of plant residue resulted in a rapid (day 0–7) increase of soil pH due to the association, and particularly oxidation, of added organic anions. This was followed by a gradual (day 7–119) pH decline attributed to the mineralization and subsequent nitrification of added organic N. The addition of 12.5–25.0 g kg^–1 of cereal crop residues, and 6.25–25.0 g kg^–1 of legume-based pasture residues, resulted in a net alkalization of the surface 2.5 cm of soil. It was therefore concluded that surface soil pH gradients observed in the field were largely attributable to an increase of pH at the surface 2.5 cm in response to plant residue return. The magnitude of such gradients will be particularly large with the return of large quantities of plant residues of high ash alkalinity in soils of relatively low initial pH and biological activity, and when the surface of the soil is exposed to moist-dry cycles. Received: 11 October 1999     

Thermal analysis of organic matter in cryogenic soils (Central Siberian Plateau)

The thermal degradation of organic matter was studied in cryogenic soils with methods of thermal analysis: differential scanning calorimetry and thermogravimetry (DSC and TG, respectively). The DSC curves of most of the samples within the temperature range from 221–247°C to 600°C were characterized by the presence of one wide exothermic peak (at 311–373°C) with a shoulder (or without it) on the descending branch at a temperature of about 400°C. This was connected mostly with the destruction of thermolabile compounds (oligo- and polysaccharides) and with the oxidation of low-aromatic complexes of plant residues and humus substances. Two exothermic peaks at 337–373°C and 448–492°C were found for some samples from the organic horizons. The high-temperature peaks were caused by the thermal destruction of lignin. The fraction of the thermolabile organic matter of the soil (237–261…331–377°C) reached 59–73% in the organic and 52–59% in the organomineral and mineral horizons.     

Nitrous oxide emissions from a fallow and wheat field as affected by increased soil temperatures

 In order to determine the effects of increased soil temperature resulting from global warming on microbiological reactions, a 21-month field experiment was carried out in the Bavarian tertiary hills. The major objective was to focus on N₂O releases as either a positive or negative feedback in response to global warming. The soils of a fallow field and a wheat field were heated 3  °C above ambient temperature and N₂O fluxes were measured weekly from June 1994 to March 1996. During the experimental period, measured temperature differences between the control plots and the heated plots were 2.9±0.3  °C at a depth of 0.01 m and 1.0–1.8  °C at a depth of 1 m. Soil moisture decreased with the elevated soil temperatures of the heated plots. The mean differences in soil moisture between the treatments were 6.4% (fallow field) and 5.2%_DW (wheat field dry weight, DW), respectively. Overall N₂O releases during the experimental period from the fallow field were 4.8 kg N₂O–N ha^–1 in the control plot against 5.0 kg N₂O–N ha^–1 in the heated plot, and releases from the wheat field were 8.0 N₂O–N ha^–1 in the control plot and 7.6 N₂O–N kg ha^–1 in the heated plot. However, on a seasonal basis, cumulated N₂O emissions differed between the plots. During the summer months (May–October), releases from the heated fallow plot were 3 times the rates from the control plot. In the winter months, N₂O releases increased in both the fallow and wheat fields and were related to the number of freezing and thawing cycles. Received: 1 December 1997     

Labile organic C and N mineralization of soil aggregate size classes in semiarid grasslands as affected by grazing management

Soil labile organic carbon (C) oxidation drives the flux of carbon dioxide (CO₂) between soils and the atmosphere. However, the impact of grazing management and the contribution soil aggregate size classes (ASCs) to labile organic C from grassland soils is unclear. We evaluated the effects of grazing intensity and soil ASC on the soil labile organic C, including CO₂ production, microbial biomass C, and dissolved organic C and nitrogen (N) mineralization in topsoils (0–10 cm) in Inner Mongolia, Northern China. Soil samples were separated into ASCs of 0–630 μm [fine ASC (fASC)], 630–2000 μm [medium ASC (mASC)] and >2000 μm [coarse ASC (cASC)]. The results showed that heavy grazing (HG) and continuous grazing (CG) increased soil labile organic C significantly compared to an ungrazed site since 1999 (UG99) and an ungrazed site since 1979 (UG79). For winter grazing site (WG), no significant differences were found. CO₂ production was highest in cASC, while lowest in fASC. Microbial biomass C and dissolved organic C showed the highest values in mASC and were significantly lower in fASC. Grazing increased N mineralization in bulk soils, while it exhibited complex effects in the three ASCs. The results suggest that the rate of C mineralization was related to the rate of N accumulation. To reduce CO₂ emission and nutrient loss, and to improve soil quality and productivity, a grazing system with moderate intensity is suggested.     

Isoproturon mineralization in an agricultural soil

The impact of soil moisture content and temperature on isoproturon (3-(4-isopropylphenyl)-1,1-dimethyl-urea [IPU]) mineralization activity was assessed on an agricultural soil regularly exposed to this herbicide. Mineralization of ¹⁴C-IPU was monitored on soil microcosms incubated at different temperatures (10°C, 20°C, 28°C) and soil moisture contents (9%, 12%, 15, 18%, 21%, 24%). An increase in temperature and/or soil moisture significantly enhanced the maximum rate and percentage of IPU mineralization while it decreased the lag time before mineralization. The maximum rate and percentage of IPU mineralization respectively ranged from 0.18% day⁻¹ and 9% for the lowest temperature and soil moisture content pair (10°C–9%) to 1.51% day⁻¹ and 27.1% for the highest pair (28°C–24%). Statistics revealed a cross interaction of temperature and soil moisture content on the maximum rate of IPU mineralization. The optimum conditions for IPU mineralization, estimated from the double Gaussian model, were 25.8°C and 24% soil moisture content. The influence of fluctuations in soil moisture content on IPU-mineralization was investigated by subjecting the soil microcosms to drought stress. When IPU was added at the end of the drought stress, it had no statistical effect on IPU mineralization. However, when it was added before the drought stress, two mineralization phases were observed: (1) one corresponding to the drought stress for which mineralization was low and (2) another one observed after restoration of soil moisture content characterized by higher mineralization rate. It can be concluded that climatic fluctuations affect the activity of IPU mineralizing microbial community, and may lead to an increase in IPU persistence.     

The CLIMEX soil-heating experiment: soil response after 2 years of treatment

 Most model predictions concerning the response of boreal forest ecosystems to climate change are inferred from small-scale experiments on artificial, simplified systems. Whole-ecosystem experiments designed to validate these models are scarce. We experimentally manipulated a small forested catchment in southern Norway by increasing soil temperature (+3  °C in summer to +5  °C in winter) using heating cables installed at 1 cm depth in the litter layer. Especially nitrification in the 0 to 10-cm soil layer increased as a result of the climate manipulation. Betula litter, produced after exposing trees for 2 years to ambient and elevated CO₂ in greenhouses, was incubated for 1 year in the manipulated catchment. Exposure to elevated CO₂ did not affect the C/N ratio or decomposition of the Betula litter, but lignin content decreased by 10%. We found no effect of elevated temperature on litter decomposition, probably due to desiccation of the litter. The heating cables caused a permanent increase in soil temperature in this soil layer, but when soils were dry, the temperature difference between control and heated plots decreased with increasing distance from the cables. When soils were wet, no gradients in temperature increase occurred. Received: 25 November 1997     

Phosphorus mineralization and microbial biomass in a Florida Spodosol: effects of water potential, temperature and fertilizer application

 Phosphorus mineralization and microbial biomass were measured in the surface 5 cm of a Spodosol (sandy, siliceous hyperthermic Ultic Alaquod) from north-central Florida. Soils from fertilized and unfertilized plantations of loblolly pine (Pinus taeda L.) were incubated at a range of water potentials (∼0, –3, –8, –10 and –1500 kPa) and temperatures (15  °C, 25  °C and 38  °C) for 14 days and 42 days. Increasing water potential and temperature increased specific P mineralization (mineralization expressed as a percentage of total P) regardless of fertilizer treatment. An increase in water potential from –10 kPa to –0.1 kPa resulted in an increase of between 38% and 239% in the concentration of KCl-extractable inorganic P, depending on incubation temperature and time. An increase in incubation temperature from 15  °C to 38  °C resulted in an increase of between 13% and 53% in KCl-extractable inorganic P. Changes in specific P mineralization with change in water potential or temperature were not affected by fertilizer application. This suggests that, although specific P mineralization was greater in the fertilized soils, environmental control of P mineralization was the same for both treatments. Specific P mineralization was most sensitive when soils were at higher water potentials, and decreased logarithmically to water potentials of between –3 kPa and –8 kPa. Specific P mineralization was relatively insensitive to changes in water potential when water potential was lower than –8 kPa. Microbial biomass C showed no consistent responses to changes of temperature or water potential and was not significantly correlated with specific P mineralization. Our results suggest that field estimates of P mineralization in these Spodosols may be improved by accounting for changes in soil water potential and temperature. Received: 30 October 1997     

Nitrogen and phosphorus uptake by maize as affected by particulate organic matter quality, soil characteristics, and land-use history for soils from the West African moist savanna zone

 The impact of land use (unfertilized continuous maize cropping, unfertilized and fertilized alley cropping with maize, Gliricidia sepium tree fallow, natural fallow) on the soil organic matter (SOM) status and general soil fertility characteristics were investigated for a series of soils representative for the West African moist savanna zone. Three soils from the humid forest zone were also included. In an associated pot experiment, relationships between maize N and P uptake and SOM and general soil characteristics were developed. Soils under natural fallow contained the highest amount of organic C (1.72%), total N (0.158%), and had the highest effective cation exchange capacity (ECEC) [8.9 mEq 100 g^–1 dry soil], while the Olsen P content was highest in the fertilized alley cropping plots (13.7 mg kg^–1) and lowest under natural fallow (6.3 mg kg^–1). The N concentration of the particulate organic matter (POM) was highest in the unfertilized alley cropping plots (2.4%), while the total POM N content was highest under natural fallow (370 mg N kg^–1) and lowest in continuously cropped plots (107 mg N kg^–1). After addition of all nutrients except N, a highly significant linear relationship (R ²=0.91) was observed between the total N uptake in the shoots and roots of 7-week-old maize and the POM N content for the savanna soils. POM in the humid forest soils was presumably protected from decomposition due to its higher silt and clay content. After addition of all nutrients except P, the total maize P uptake was linearly related to the Olsen P content. R ² increased from 0.56 to 0.67 in a multiple linear regression analysis including the Olsen P content and clay content (which explained 11% of the variation in P uptake). Both the SOM status and N availability were shown to be improved in land-use systems with organic matter additions, while only the addition of P fertilizer could improve P availability. Received: 9 April 1999     

Grazing intensity in subarctic tundra affects the temperature adaptation of soil microbial communities

Grazing by large ungulates, such as reindeer (Rangifer tarandus L.), in subarctic tundra exerts a considerable effect on the soil microclimate. Because of higher insulation by the aboveground vegetation in light versus heavily grazed areas, soil temperatures during the growing season are considerably higher under heavy grazing. Here, we hypothesized that these grazer-induced changes in soil microclimate affect the temperature sensitivity of soil microbial activity. To test this hypothesis, we conducted soil incubations at different temperatures (4 °C, 9 °C and 14 °C) for six weeks using soils from sites with contrasting long-term grazing intensities. Microbial respiration at low temperature (4 °C) was significantly higher in soils under light grazing than in soils under heavy grazing; however, grazing intensity did not affect respiration rates at 9 °C and 14 °C. In soils under light grazing, post-incubation β-glucosidase (BG) activity at 4 °C was higher in soils that had been incubated at 4 °C than in soils incubated at 14 °C, suggesting functional adaptation of the soil microbial community to low temperature. Similar adaptation was not detected in soils under heavy grazing. Ion Torrent sequencing of bacterial 16S rRNA genes showed major differences in the bacterial community composition in soils incubated at different temperatures. Overall, our results indicate that tundra soil microorganisms may be more cold-adapted under low than high grazing intensity. Due to this difference in temperature adaptation, the consequences of climate warming on soil microbial processes may be dependent on the grazing intensity.     

Effect of temperature on the respiration rate of forest soil organic layer along an elevation gradient in the Polish Carpathians

The aim of our studies was to determine the relation between temperature and the respiration rate of the forest soil organic layer along an altitudinal gradient while controlling the effects of the soil characteristics. The respiration rate was measured in laboratory conditions at different temperatures, 0, 10, 20, and 30°C, in samples collected in the Polish part of the Western Carpathians at 600, 800, 1,000, and 1,200 m above sea level from four different mountains, which were later treated as replicates. The increase in the average respiration rate between two consecutive temperatures was expressed as Q ₁₀ coefficients. Among the nutrients measured in the soil organic layer, only the total organic N concentration significantly increased with elevation. The temperature effect was significant for both the respiration rate and the Q ₁₀ values. The calculated Q ₁₀ values were highest for the temperature range between 10 and 20°C, and the lowest values were obtained from the highest temperature range (20–30°C). The altitude effect was significant for the respiration rate but not for the Q ₁₀ values, indicating that the temperature sensitivity of the soil respiration did not change much along the studied altitudinal gradient.     

Effect of the temperature and moisture on the N<Subscript>2</Subscript>O emission from some arable soils

The effect of the temperature and moisture on the emission of N₂O from arable soils was studied in model experiments with arable soils at three contrasting levels of wetting and in a wide temperature range (from −5 to +25°C), including freeze-thaw cycles. It was shown that the losses of fertilizer nitrogen from the soils with water contents corresponding to 60 and 75% of the total water capacity (TWC) did not exceed 0.01–0.09% in the entire temperature range. In the soils with an elevated water content (90% of the TWC) at 25°C, the loss of fertilizer nitrogen in the form of N₂O reached 2.35% because of the active denitrification. The extra N₂O flux initiated by the freeze-thaw processes made up 88–98% of the total nitrous oxide flux during the entire experiment.     

Effect of differently post-treated dewatered sewage sludge on β-glucosidase activity,microbial biomass carbon,basal respiration and carbohydrates contents of soils from limestone quarries

This work has evaluated the effects of thermally dried (TDS) or composted (CDS) dewatered sewage sludge on β-glucosidase activity, total (TCH) and extractable (ECH) carbohydrate content, microbial biomass carbon and basal respiration of soils from limestone quarries under laboratory conditions. Two doses (low and high) of the dewatered sludge (DS) or of the respective TDS or CDS were applied to a clayey and a sandy soil, both coming from working quarries. The soil mixtures and the controls (soils with no added sludge) were incubated for 9 months at 25°C and 30% of field capacity. The addition of sludge increased all the studied soil parameters, and the increase depended on the amount of sludge. Except in the case of TCH and ECH, the enhancing effect decreased with time, but at the end of incubation, parameters of the treated soils were higher than those of the control. The rank order of the initial stimulating effect was soil–TDS ≥ soil–DS ≥ soil–CDS, and probably, this order depended on the proportion of stable organic matter, which was the lowest in the TDS. Values of metabolic quotient (qCO₂) were higher at the lower dose, and they did not change during incubation in the CDS-treated soils. Both TCH and ECH were the parameters with the greatest significant sludge and dose effects. Basal respiration, microbial biomass carbon and β-glucosidase activity were the best measured parameters in distinguishing the long-term effects of the three sludge types over the soils.     

Assessment and prediction of changes in the reserves of organic carbon in abandoned soils of European Russia in 1990–2020

Experimental studies and the analysis of published data have shown that carbon reserves in soils generally increase upon soil exclusion from agricultural use. The rate of carbon accumulation in the abandoned soils depends on the soil type, the time elapsed since the soil abandoning (the restoration period), and the thickness of the layer for which the rate of carbon accumulation is determined. For the upper 20-cm-thick layer, it varies from 66 to 175 g C/m² per year in dependence on the type of soil and averages 111 g C/m² per year. The highest rate is typical of the first 10–15 years of soil restoration. According to our calculations, the carbon sequestration in the upper 20-cm-thick layer of Russian soils due to changes in land use was 184–673 Mt C in 1990–2005 and may reach 282–1030 Mt C by 2020.     

Quality Criteria for Re-Use of Organotin-Containing Sediments on Land (7 pp)

Background, Aim and Scope   The use of organotin compounds as antifouling agents on ships is prohibited at EU level since 1 July 2003. Because of its persistence, the presence of organotin compounds in harbour sediment will however remain a problem for years. Dumping of dredged sediment in sea is subject to very strict quality criteria, stimulating the exploration of re-use alternatives, such as re-use on land. Within the TBT Clean project (EU LIFE Project 02/ENV/B/341) an assessment framework for re-use of organotin containing treated sediment on land as secondary granular building material was developed. Three scenarios were considered: free re-use on land, re-use above groundwater level with cover layer, and re-use under groundwater level (the latter two scenarios are referred to as restricted re-use). Receptors considered were humans, ecosystem and groundwater. Generic upper concentration limits and sediment leaching limits were calculated. Materials and Methods: Upper concentration limits were calculated with the Vlier-Humaan model. This model allows to calculate soil remediation values according to the Flemish legal framework. The focus of the methodology is the protection of human health, although a check for ecotoxicity was included in the project. The soil remediation value for residential land-use was selected within the scenario for free re-use; for restricted re-use (no direct contact possible), the soil remediation value for industrial land-use was selected. Leaching values were calculated with an analytical soil and groundwater transport model. The reference scenario behind the leaching criteria of the European Landfill Directive was modified to fit the project objectives. Default values for application height and length were used. The point-of-compliance was situated at 20 m distance in the groundwater. Physicochemical properties were taken from literature; sorption characteristics were taken from literature and were measured on 6 treated sediment samples during the project. Plant-uptake values were taken from the literature. Toxicological criteria were taken from EFSA. Results: The assessment framework provided an upper limit (SedUL) and an leaching value (SedLV) for each scenario, expressed as mg/kg dm in the sediment. Criteria were calculated for tributyltin (TBT) and dibutyltin (DBT); too few data were available for monobutyltin (MBT). The SedUL equalled 0.51 mg TBT/kg dm and 0.07 mg DBT/kg dm for free re-use and 195 mg TBT/kg dm and 205 mg DBT/kg dm for restricted re-use (two scenarios). For free re-use the SedLV was only limiting for TBT at Kd of < 2000 l/kg in the sediment. Under re-use above groundwater level with coverage SedLV values ranged from 6.9 – 29 mg TBT/kg dm and from 12 – 33.3 mg DBT/kg dm (Kd ranging from 100 – 10000 l/kg); under re-use below groundwater level SedLV values ranged from 0.007 – 0.77 mg TBT or DBT/kg dm (Kd ranging from 100 – 10000 l/kg). Discussion: Results are subject to large uncertainties because of variation in input data; the model output is sensitive to variation in plant uptake (SedUL for free re-use), Koc or Kd (SedUL for restricted re-use, SedLV for re-use with coverage), Henry's law coefficient (SedUL for restricted re-use); all these parameters show orders of magnitude variation. Conclusions: A feasible and consistent framework for evaluation of the re-use of treated organotin containing sediment was developed. However, the resulting quality criteria are still subject to large uncertainties, due to uncertainties in input data. Recommendations and Perspectives: High-quality data on plant uptake and soil sorption of organotin compounds, the influence of soil properties on these processes, and long-term terrestrial toxicity data are needed to refine the calculations. The quality criteria should be reviewed when these data become available.     

Freeze–thaw cycle amplitude and freezing rate effects on extractable nitrogen in a temperate old field soil

Freeze–thaw cycles can promote soil N losses as a result of microbial and root cell lysis; however, minimal freeze–thaw effects have typically been observed in studies that have imposed moderate temperature cycles. We conducted laboratory incubations on surface soil (top 3 cm) collected in a temperate old field from late fall through mid-winter to examine how variation in freeze–thaw amplitude, number, timing of collection, and freezing rate altered soil extractable N. We varied freeze–thaw amplitude by imposing minimum cycle temperatures of 0, −1, −2, −5, and −10°C for a series of either one or two cycles and held control samples constant at 3°C. We also examined the effects of freezing rates of 1, 3, and 30°C h⁻¹. We hypothesized that extractable N would be highest for both the maximum freezing amplitudes and rates. While multiple freeze–thaw cycles at −10°C and freeze–thaw cycles associated with artificially high freezing rates increased extractable N, freeze–thaw cycles representative of field conditions at our site had no effect on extractable N in late fall and early winter. By mid-winter there was a significant freeze–thaw cycle effect but, contrary to our prediction, less N was extracted from freeze–thaw treated samples than from the control samples, which remained thawed over the treatment period. Increased extractable N in control samples was driven by increased organic N rather than increased inorganic N. Our results suggest that freeze–thaw damage to soil organisms does not contribute substantially to N release in our system. Instead, soil extractable N may increase during mid-winter thaws as a result of increased soil proteolytic activity above freezing temperatures.     

Nitrous oxide emissions under different soil and land management conditions

Nitrous oxide (N₂O) emissions of three different soils – a rendzina on cryoturbed soil, a hydromorphic leached brown soil and a superficial soil on a calcareous plateau – were measured using the chamber method. Each site included four types of land management: bare soil, seeded unfertilized soil, a suboptimally fertilized rapeseed crop and an overfertilized rapeseed crop. Fluxes varied from –1g to 100g N₂O-nitrogen ha^–1 day^–1. The highest rates of N₂O emissions were measured during spring on the hydromorphic leached brown soil which had been fertilized with nitrogen (N); the total emissions during a 5-month period exceeded 3500gNha^–1. Significant fluxes were also observed during the summer. Very marked effects of soil type and management were observed. Two factors – the soil hydraulic behaviour and the ability of the microbial population to reduce N₂O – appear to be essential in determining emissions of N₂O by soils. In fact, the hydromorphic leached brown soil showed the highest emissions, despite having the lowest denitrification potential because of its water-filled pore space and low N₂O reductase activity. Soil management also appears to affect both soil nitrate content and N₂O emissions. Received: 4 April 1997