焦化厂土壤中PAHs的累积、垂向分布特征及来源分析

引言多环芳烃(polycyclic aromatic hydrocarbons,PAHs)在环境中普遍存在,由于具有强烈的诱变、致癌和致畸作用而受到越来越多的关注。以煤为主要原料的焦化行业,是环境中人类活动产生PAHs的主要来源之一,其各个生产车间内化石燃料的不完全燃烧及焦油、煤气等化工产品的加工过程都可能导致PAHs的排放。土壤是PAHs累积和迁移的重要介质,环境中的PAHs可由大气     

Fertilizer nitrogen movement in a Central Amazon Oxisol and Entisol cropped to corn

Nitrogen fertilization experiments were conducted on a Oxisol and Entisol in the Central Amazon to evaluate the influence of soil properties and rainfall distribution on soil inorganic N movement and N recovery by corn (Zea mays L.). One corn crop was grown during the wet season on each site. A second crop was planted in the Oxisol during the dry season. Inorganic N was monitored in urea-N treatments (0 to 160 kg ha^?1) to a depth of 0.60 m by periodic soil sampling during each crop. During the wet season large N losses by leaching occurred in both soils with 120 or 160 kg ha^?1 of applied N. Differences in soil permeability and corn rooting depth between soils contributed to a greater movement of N into subsoil layers in the Oxisol than the Entisol. However, N leaching beyond 0.60 m in the Oxisol was delayed, apparently because of NO ₃ ^? adsorption in the net positively charged subsoil layers. Corn yields and N recovery in the Entisol were higher than in the Oxisol, during the wet season. During the dry season N leaching in the Oxisol was greatly reduced, relative to the previous wet season, by split applications of fertilizer N and lower cumulative rainfall (300 vs. 1012 mm). Management practices which promote root growth into acid subsoil layers of the Oxisol would increase plant access to soil N, improve crop N recovery and reduce fertilizer N requirements.     

Simulating water and nitrogen behaviour in soils cropped with winter wheat

The SWATNIT model [26], predicting water and nitrogen transport in cropped soils, was evaluated on experimental data of winter wheat for different N treatments. The experiments were monitored at three different locations on different soil types in the Netherlands. Crop growth was simulated using the SUCROS model [11] which was integrated in the SWATNIT model. Both water and nitrogen stress were incorporated. Except for the soil hydraulic properties, all model parameters were taken from literature. The model performance was evaluated on its capability to predict soil moisture profiles, nitrate and ammonia profiles, the time course of simulated total dry matter production and LAI; and crop N-uptake. Results for the simulations of the soil moisture profile indicate that the soil hydraulic properties did not reflect the actual physical behaviour of the soil with respect to soil moisture. Good agreement is found between the measured and simulated nitrate and ammonia profiles. The simulation of the nitrate content of the top layer at Bouwing was improved by increasing the NH ₄ ⁺ -N-distribution coefficient thereby improving the simulation of the NH ₄ ⁺ -N-content in this layer. Deviations between simulated and measured nitrate concentrations also occurred in the bottom layers (60–100 cm) of the soil profile. The phreatic ground water might influence the nitrate concentrations in the bottom layers. Concerning crop growth modelling, improvements are needed with respect to the partitioning of total dry matter production over the different plant organs in function of the stress, the calculation of the nitrogen stress and the total nitrogen uptake of the crop through a better estimate of the N-demand of the different plant organs.     

Concentration and vertical distribution of total soil phosphorus in relation to time of abandonment of arable fields

Abandonment of agricultural soils is a common practice in Western Europe to increase the area of nature and to counteract agricultural overproduction. However, it has been suggested that abrupt changes in management of land, such as abandonment of heavily fertilized agricultural fields, could trigger leaching of phosphorus into deeper soil layers and groundwater. In a previous study we observed that total phosphorus (P) in the upper 10 cm of ex-arable soils in the Netherlands was negatively related to the time of abandonment. In a subsequent study in the region reported here, we measured total P concentrations at different soil depths in four ex-agricultural fields that differed in time since abandonment to examine if the decrease in total P with increasing time of abandonment could be due to leaching of P into deeper soil layers. At each site total P concentration decreased with increasing depth, and for each soil profile depth, total P also decreased with increasing years since abandonment. We calculated, based on estimated P fertilizer gifts over the last decades and the regression coefficient of the relation between total P in a core of 95 cm and time of abandonment, the amount of net total P that should have accumulated in the oldest ex-arable field to reach the P level of the most recently abandoned field. The continuation of accumulation of P for a longer period of time in recently abandoned fields appeared to be the most reasonable explanation for the decrease of P with years of abandonment. Therefore, abandonment of agricultural land does not seem to trigger a ‘chemical time bomb’ to explode as no large amounts of P seem to leach into deeper soil layers.     

Long-term tillage,straw and N rate effects on some chemical properties in two contrasting soil types in Western Canada

Long-term use of soil, crop residue and fertilizer management practices may affect some soil properties, but the magnitude of change depends on soil type and climatic conditions. Two field experiments with barley, wheat, or canola in a rotation on Gray Luvisol (Typic Cryoboralf) loam at Breton and Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, were conducted to determine the effects of 19 or 27 years (from 1980 to 1998 or 2006 growing seasons) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S_Rem] and straw retained [S_Ret]) and N fertilizer rate (0, 50 and 100 kg N ha^?1 in S_Ret, and 0 kg N ha^?1 in S_Rem plots) on pH, extractable P, ammonium-N and nitrate–N in the 0–7.5, 7.5–15, 15–30 and 30–40 cm or 0–15, 15–30, 30–60, 60–90 and 90–120 cm soil layers. The effects of tillage, crop residue management and N fertilization on these chemical properties were usually similar for both contrasting soil types. There was no effect of tillage and residue management on soil pH, but application of N fertilizer reduced pH significantly (by up to 0.5 units) in the top 15 cm soil layers. Extractable P in the 0–15 cm soil layer was higher or tended to be higher under ZT than CT, or with S_Ret than S_Rem in many cases, but it decreased significantly with N application (by 18.5 kg P ha^?1 in Gray Luvisol soil and 20.5 kg P ha^?1 in Black Chernozem soil in 2007). Residual nitrate–N (though quite low in the Gray Luvisol soil in 1998) increased with application of N (by 17.8 kg N ha^?1 in the 0–120 cm layer in Gray Luvisol soil and 23.8 kg N ha^?1 in 0–90 cm layer in Black Chernozem soil in 2007) and also indicated some downward movement in the soil profile up to 90 cm depth. There was generally no effect of any treatment on ammonium-N in soil. In conclusion, elimination of tillage and retention of straw increased but N fertilization decreased extractable P in the surface soil. Application of N fertilizer reduced pH in the surface soil, and showed accumulation and downward leaching of nitrate–N in the soil profile.     

Phenolic Compounds in Natural Solutions of a Coniferous Forest

Phenolic compounds have been identified previously as potentially responsible for allelopathic interferences in spruce forest at high altitude. They have now been analyzed in canopy leachates, snow, and soil solutions collected from the three layers of the podsolic soil: OA, E. and B. Leachates were characterized by high tanning capacity and by p-hydroxyacetophenone (found at 10^?6 M) also detected as the major (10^?7 M) monomeric compound in snow. At least 10 phenolic monomers, including vanillic, p-hydroxybenzoic, and protocatechuic acids were identified in capillary waters extracted from the OA layer with their sum reaching 2 × 10^?6 M. These monomers were only a small part (1%) of the total phenolics. In soil solutions, significant decreases in phenolic concentrations with depth were observed between the E and B layers, with qualitative modifications of the phenolic pattern. Spruce leachates and soil solutions exhibited high temporal variability, resulting in transitory allelopathic potential towards both aerial and subterranean parts of spruce seedlings. The occurrence of various and soluble phenolic coumpounds in the whole forest system, including p-hydroxyacetophenone, which persist from green needles to soil solutions, suggests that they could be involved in complex and interactive processes occurring during organic matter accumulation on the coniferous forest floor.     

Effect of weed management in coffee plantation on soil chemical properties

Coffee plantation in south Sumatra, Indonesia, is developed on the slopes in hilly areas, where soil erosion is a severe problem. Introduction of weed as cover plant has been found to be effective in the reduction of soil erosion. Here the effect of weed management, weeding and coverage with Paspalum conjugatum Berg. or natural weeds, on soil chemical properties was investigated. After 4 years, contents of total C, total N, available P, and exchangeable (ex.) Mg in the 0–10 cm and 10–20 cm depth layers were significantly greater in the Paspalum conjugatum and natural weed plots than in the weeding plot. In the 0–10 cm layer, ex. K and ex. Ca were also greater in the weed-introduced plots than in the weeding plot. Decrease in soil pH (H₂O) and the increase in ex. Al under coffee plantation were reduced by the coverage with weeds. Thus soil coverage with weeds in coffee plantation on slopes was effective in the maintenance of soil fertility.     

Nutrient concentration in soil water extracts and soybean nutrition in response to lime and gypsum applications to an acid Oxisol under no-till system

No-till system (NTS) occupies 20 million hectares with grain crops in Brazil. However, calcium deficiency and aluminum toxicity can limit crop yields in many soils, and liming, associated to gypsum application, is an option for improving soil management. The objective of this study was to evaluate the effects of lime and gypsum application on the composition of soil water extracts of a clayey Rhodic Hapludox, cultivated with soybean under NTS. The experiment had a randomized complete block design with split-plots. The plots consisted of lime treatments (either a single rate of 4.5 t ha⁻¹ or three annual rates of 1.5 t ha⁻¹) surface-applied or incorporated at 0.2 m depth. The subplots received surface applications of gypsum (3, 6 and 9 t ha⁻¹). Liming increased total calcium and magnesium concentrations and the magnesium free Form activity (aMg²⁺) in the water extracts. The effect of liming on Mg was observed at deeper layers of the soil profile. Gypsum increased total concentration and free forms activities at calcium (aCa²⁺) and sulfate, but decreased to magnesium in the 0.05–0.2 m soil layer. Part of Mg lost from these upper layers probably contributed to increased Mg in the subsoil (0.4–0.8 m). Free forms activities at the aluminum, calcium, magnesium and sulfate were lower than the total concentrations, mainly for aluminum. Ca and Mg concentrations in soybean leaf tissue were positively correlated to the aCa²⁺ and aMg²⁺ in the soil water extract. Soybean grain yield was negatively correlated to both Mn total concentration and activity (free form) in the soil water extract, but it was positively correlated to sulfate (total concentration and free form activity) in the subsoil layer and to the Ca total concentration in the upper layer (0–0.05 m). It is concluded that lime and gypsum ameliorate soybean grain yield under NTS.     

Increasing organic C and N in soil under bromegrass with long-term N fertilization

Two field experiments were conducted on bromegrass (Bromus inermis Leyss.) on a thin Black Chernozem (Typic Boroll) at Crossfield, Alberta, Canada to determine the long-term effects of N fertilization on changes in concentration and mass of organic C and N in soil. In both experiments, bromegrass was harvested for hay each year. In the experiment where ammonium nitrate (AN) was applied annually at 0 to 336 kg N/ha for 27 consecutive years from 1968 to 1994, the concentration of total C in the 0–5 cm soil layer increased from 50.33 g/kg in the zero-N treatment to 61.64 g/kg with 56 kg N/ha and to 64.15 g/kg with the 112 kg N/ha rate. Total C in soil also increased in the 5–10, 10–15 and 15–30 cm layers but to a lesser extent. The mass of total C in the 0–30 cm soil layer was increased by 18.46 Mg/ha with 56 kg N/ha and by 23.38 Mg/ha with the 112 kg N/ha rate as compared to the zero-N treatment. Total N in soil followed a similar trend as total C. In the experiment which received four N sources [ammonium nitrate (AN), urea, calcium nitrate (CN) and ammonium sulphate (AS)] applied annually at 168 and 336 kg N/ha for 15 years from 1979 to 1993, the total C in soil was greater where N fertilizer was applied, but the increase in total C varied with N source. The concentration of total C in soil in the 0–5 cm layer tended to be greater with AN and AS than with CN, with the smallest increase from urea. The mass of total C in soil (average of four N sources) at the 168 kg N/ha rate was increased by 18.98 Mg/ha in 0–30 cm and by 43.48 Mg/ha in the 0–60 cm layer as compared to the check treatment. The concentration of total C in soil also increased in the deeper layers to a depth of 60 cm, but the increases were much smaller than in the 0–5 cm layer. The changes in total N in soil followed a similar pattern as total C. In conclusion, long-term annual additions of fertilizer N to bromegrass resulted in a marked increase in total C and N in soil and the increases were influenced by both rate and source of N fertilizer. The implications of these results are that grasslands can be managed to lessen the increase in atmospheric CO₂ concentration, while also improving fertility (N-supplying capacity) and tilth of soil. This revised version was published online in August 2006 with corrections to the Cover Date.     

Impact of sodium sulfate on soil frost heaving in an open system

The existence of solutes and their redistribution during freezing have a deep influence on the process of soil freezing. We performed unidirectional freezing experiments in an open system with red clay collected at the Beiluhe test site along the Qinghai–Tibet Railway. The groundwater supply of the soil was simulated in the laboratory experiment by attaching a liquid replenishment system to the bottom of the sample container. In order to see the influence of the salt on the evolution of the sample during unidirectional freezing, two types of experiments were performed. (i) The soil samples were supplied with a sodium sulfate solute of 5% concentration, and (ii) only distilled water without any salty component was added. Based on the freezing temperature measurements of salty soil, migration of sodium sulfate solution towards a lower temperature zone during freezing 0 °C isotherm in the soil moved gently towards deeper layers, but frost depth of the soil ascended slightly with time when the sample was constantly cooled. Compared to the distilled water replenishment, the amount of frost-heaving was smaller in the soil column with sodium sulfate solution replenishment. Based on the frost depth curve, the solubility curve of the Na₂SO₄–H₂O system and the amount of frost heaving and salt expansion in the soil column we have calculated the amount of frost heaving and salt expansion. In an early stage of the experiment deformation of the soil column was mainly caused by frost heaving, while in a later phase crystallization of the sodium sulfate played a lager role. This conclusion is confirmed by the results of the dry density measurement after the experiments.     

Erratum to: Long-term tillage,straw and N rate effects on quantity and quality of organic C and N in a Gray Luvisol soil

Long-term use of soil, crop residue and fertilizer management practices may affect some soil properties, but the magnitude of change depends on soil type and climatic conditions. Two field experiments with barley, wheat, or canola in a rotation on Gray Luvisol (Typic Cryoboralf) loam at Breton and Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, were conducted to determine the effects of 19 or 27 years (from 1980 to 1998 or 2006 growing seasons) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S_Rem] and straw retained [S_Ret]) and N fertilizer rate (0, 50 and 100 kg N ha⁻¹ in S_Ret, and 0 kg N ha⁻¹ in S_Rem plots) on pH, extractable P, ammonium-N and nitrate–N in the 0–7.5, 7.5–15, 15–30 and 30–40 cm or 0–15, 15–30, 30–60, 60–90 and 90–120 cm soil layers. The effects of tillage, crop residue management and N fertilization on these chemical properties were usually similar for both contrasting soil types. There was no effect of tillage and residue management on soil pH, but application of N fertilizer reduced pH significantly (by up to 0.5 units) in the top 15 cm soil layers. Extractable P in the 0–15 cm soil layer was higher or tended to be higher under ZT than CT, or with S_Ret than S_Rem in many cases, but it decreased significantly with N application (by 18.5 kg P ha⁻¹ in Gray Luvisol soil and 20.5 kg P ha⁻¹ in Black Chernozem soil in 2007). Residual nitrate–N (though quite low in the Gray Luvisol soil in 1998) increased with application of N (by 17.8 kg N ha⁻¹ in the 0–120 cm layer in Gray Luvisol soil and 23.8 kg N ha⁻¹ in 0–90 cm layer in Black Chernozem soil in 2007) and also indicated some downward movement in the soil profile up to 90 cm depth. There was generally no effect of any treatment on ammonium-N in soil. In conclusion, elimination of tillage and retention of straw increased but N fertilization decreased extractable P in the surface soil. Application of N fertilizer reduced pH in the surface soil, and showed accumulation and downward leaching of nitrate–N in the soil profile.     

Nitrate in upper groundwater on farms under tillage as affected by fertilizer use,soil type and groundwater table

Indicators are needed to check whether policies on protection of groundwater are effective and if regulations are complied with. We evaluated various indicators at different scales, both in space and in time, and at different degrees of complexity. Groundwater was sampled on 34 arable farms for 3 years. Nitrate concentration in upper groundwater was low on clay soil. On sandy soil, peat layers reduced the nitrate concentration with about 80 mg/l on average. Sandy soils with high groundwater tables had nitrate concentrations that were less than half of those at sandy soils with low groundwater tables. The relationship between different fertilization variables and nitrate in groundwater was investigated for sandy soils without peat layers. N surplus poorly correlated with nitrate concentrations in groundwater when individual sampling points were studied, but clearly increased when data were averaged at the farm level. Soil mineral nitrogen correlated best with nitrate concentrations in groundwater. The relationships show that especially on well drained soil drastic measures will be inevitable to reach good water quality.     

A comparison of local phosphorescence detection and fluid dynamic gauging methods for studying the removal of cohesive fouling layers: Effect of layer roughness

The performance of industrial cleaning in place (CIP) procedures is critically important for food manufacture. CIP has yet to be optimised for many processes, in part since the mechanisms involved in cleaning are not fully understood. Laboratory tests have an important role in guiding industrial trials, and this paper introduces and compares two experimental techniques developed for studying CIP mechanisms: local phosphorescence detection (LPD), and scanning fluid dynamic gauging (sFDG).To illustrate the comparison, each technique is used to investigate the influence of soil topology on the cleaning of pre-gelatinised starch-based layers from stainless steel (SS 316) substrates by aqueous NaOH solutions at ambient temperature. The roughness of the soil surface is varied by incorporating zinc sulphide particles with different particle size distributions (range 1–80 μm) into the starch suspensions. The soil roughness increased with the use of larger particles, increasing the 3D arithmetic mean roughness (S_a) of the dry layers (range 0.37–3.33 μm). Rough layers were cleaned more readily than those containing small inclusions, with a good correlation between the cleaning rates observed during LPD and FDG measurements. The LPD technique, which is an instrumented CIP test, gives a better indication of the cleaning time, while sFDG measurements provide further insight into the removal mechanisms.     

Simulation of nitrogen dynamics and biomass production in winter wheat using the Danish simulation model DAISY

A dynamic simulation model for the soil plant system is described. The model includes a number of main modules, viz., a hydrological model including a submodel for soil water dynamics, a soil temperature model, a soil nitrogen model including a submodel for soil organic matter dynamics, and a crop model including a submodel for nitrogen uptake. The soil part of the model has a one-dimensional vertical structure. The soil profile is divided into layers on the basis of physical and chemical soil characteristics. The simulation model was used to simulate soil nitrogen dynamics and biomass production in winter wheat grown at two locations at various levels of nitrogen fertilization. The simulated results were compared to experimental data including concentration of inorganic nitrogen in soil, crop yield, and nitrogen accumulated in the aboveground part of the crop. Based on this validation it is concluded that the overall performance of the model is satisfactory although some minor adjustments of the model may prove to be necessary.     

Simulated cereal nitrogen uptake and soil mineral nitrogen after clover-grass leys

Simulations were made to test the effects of age and composition of red clover (Trifolium pratense) based leys on yield of two subsequent spring cereal crops, as well as nitrogen (N) uptake and soil mineral N content. The experimental plots in two trials were cropped for 2–3 years with spring cereals, or 1-, 2- or 3-year-old red clover based leys, followed by spring wheat and subsequent spring oats. CoupModel, a process oriented ecosystem model, was calibrated with measured values of above ground N uptake and soil mineral N contents from plots of cereal monoculture. Cereal N uptake was simulated for a 2 year period in cereals after leys. The calculations of N inputs in incorporated plant material of leys were also tested. Simulated N uptake in the above ground biomass generally agreed with the field data with default values of the model. Some parameters were increased in order to improve plant N uptake and keep the soil mineral N contents at the measured levels. The simulated soil mineral N contents were close to the measured values for surface layers and were more accurate than for deeper layers in the profile. However, the high simulated mineral N increase after harvest in one trial was not seen in field measurements, which remains difficult to explain. Most probably the C:N estimate for crop residues was set too low in the model, but calculated N input was on a reasonable level. These results show that further testing and adjusting of N dynamics in organic farming system using CoupModel should be continued.     

Total and light fraction organic C in a thin Black Chernozemic grassland soil as affected by 27 annual applications of six rates of fertilizer N

Maintenance and sequestration of C is important to sustain and improve the quality and productivity of soils. The objective of this study was to determine the effects of 27 annual applications of six N rates (0, 56, 112, 168, 224 and 336 kg N ha^–1 yr^–1) on total organic C (TOC) and light fraction organic C (LFOC) in a thin Black Chernozemic loam soil. Nitrogen (ammonium nitrate) was surface-applied to bromegrass (Bromus inermis Leyss) managed as hay near Crossfield, Alberta, Canada. The concentration and mass of TOC and LFOC in the 0–5, 5–10, 10–15 and 15–30 cm soil layers increased with N rate and showed a quadratic response to N rate with significant R² values, with their maximum values at 336 kg N ha^–1 in the 0–5 cm layer and at 224 kg N ha^–1 in other layers. But the increase in TOC and LFOC per kg of N addition was maximized at 56 kg N ha^–1 and declined with further increase in N rate. These trends indicated that higher N rates would cause a faster build up of soil C, whereas lower N rates would achieve a greater increase in soil C per unit of N addition. Response of C mass to N application was much greater for LFOC (range of 697 to 156% increase) than for TOC (range of 67 to 17% increase). Percentage of LFOC in TOC mass increased with N rate. At the 168 to 336 kg N ha^–1 rates, almost all of the increase in TOC in the surface 10 cm soil occurred as LFOC. Thus, LFOC was more responsive to N application and was a good indicator of N effect on soil C. The trend of change in soil TOC and LFOC was similar to hay yield and C removal in hay, which suggests that increasing hay yield with N application concurrently also increases soil organic C. In conclusion, long-term annual applications of N fertilizer to bromegrass resulted in a substantial increase in TOC and LFOC in the soil, thereby indicating that N fertilization can be used to sequester more atmospheric C in prairie grassland soils.     

Leaching of nitrate,calcium and magnesium under maize cultivation on an oxisol in Togo

In tropical regions, crop yields generally decrease with time, partly due to a decline in the levels of exchangeable bases linked to acidification of the upper layers of the soil. Nitrogen, calcium and magnesium balances were studied on an oxisol in southern Togo under continuous maize cropping with mineral fertilization and crop residue return, by measuring inputs and outputs.The balance was near equilibrium for nitrogen (urea dressing, 120 kg N ha^–1 y^–1) and negative for calcium and magnesium. Leaching represented between 29% and 85% of the total output for nitrogen and accounted for practically all the calcium and magnesium outputs.In the upper horizons, nitrate, calcium and magnesium contents in the soil solution were correlated. This was interpreted as an exchange between the calcium and magnesium ions adsorbed on the soil and the protons released by the transformation of ammonium into nitrate. The nutrient content of the soil solution increased in the case of no potassium fertilization, probably because of a lower nitrogen immobilization by microorganisms, the crop residue yield being reduced by the induced deficiency. An insufficient potassium fertilization increases leaching losses of calcium, magnesium and nitrogen.In the deeper horizons, the nitrate content of the soil solution was not correlated with the calcium and magnesium contents. Nitrogen was probably taken up by roots and some calcium and magnesium ions exchanged during their transport through the soil.     

Phosphate adsorption and precipitation in calcareous soils: the role of calcium ions in solution and carbonate minerals

P-removal from soil solution is mainly due to adsorption and precipitation reactions. For calcareous soils two pathways have been proposed as being relevant: partitioning on soil surfaces and precipitation induced by Ca2+ ions in solution. To define P-speciation in soil and reduce P-immobilisation following fertilisation, the relative importance of these two reactions needs to be quantitatively established. This investigation, conducted on two calcareous soils, suggests that Ca-ion activity in the liquid phase is mainly responsible for the formation of insoluble Ca-P phases. Our study was carried out by determining: a) batch sorption isotherms at different slurry concentrations, times of contact, pH and indifferent electrolyte concentrations; b) supernatant isotherms on soil suspensions; c) insolubilisation kinetics of P added to soil columns. The shape of the sorption isotherms indicated that adsorption predominated at low concentrations (below approximately 0.5 mM); above this level precipitation became predominant. Precipitation from solution was demonstrated by adding increasing amounts of phosphate to soil suspension supernatants and precipitation levels comparable to those observed in sorption isotherms were obtained. Thus, carbonate mineral surfaces were not necessary for the induction of P precipitation. The formation of Ca-P mineral phases was increased with reaction time and was governed by the concentration of Ca-ions, pH and indifferent electrolyte concentration. P added at the top of soil columns was rapidly insolubilised: after 5 weeks the P-Olsen value was reduced to about 60% and P was not transported to the deepest layers but remained in the surface ones. These results suggest that, for soils with a high reservoir of exchangeable cations able to form insoluble P phases, precipitation is the predominant mechanism which reduces P availability for plants.     

The contribution of dairy farming on peat soil to N and P loading of surface water

In agriculturally used peat land areas, surface water quality standards for nitrogen (N) and phosphorus (P) are frequently exceeded, but it is unclear to what extent agriculture is responsible for nutrient loading of the surface water. We quantified the contribution of different sources to the N and P loading of a ditch draining a grassland on peat soil (Terric Histosol) used for dairy farming in the Netherlands. Measurements were performed on N and P discharge at the end of the ditch, supply of N and P via inlet water, mineralization of soil organic matter, slush application, composition of the soil solution, and on N losses through denitrification in the ditch for 2 years (September 2000 to September 2002). Discharge rates at the end of the ditch were 32 kg N ha^–1y^–1 and 4.7 kg P ha^–1y^–1. For N, 43 to 50% of the discharge was accounted for by applications of fertilizers, manure and cattle droppings, 17 to 31% by mineralization of soil organic matter, 8 to 27% by nutrient-rich deeper peat layers, 8 to 9% by atmospheric deposition and 3 to 4% by inlet water. For P, these numbers were 10 to 48% for applications of fertilizers, manure and cattle droppings, 2 to 14% mineralization of soil organic matter, 33 to 82% nutrient-rich peat layers and 5 to 6% inlet water. The results of this paper demonstrate that nutrient loading of surface water in peat land areas involves several sources of nutrients, and therefore, reducing one source to reduce nutrient inputs to surface water is likely to result in modest effectiveness.     

Soil fertility effects of repeated application of sewage sludge in two 30-year-old field experiments

Recirculation of plant nutrients from waste materials back to agriculture is necessary in a sustainable food production system. In this study we investigated the long-term effect of direct land application of sewage sludge (SS) on soil fertility on two Swedish farms, where field experiments were set up in 1981 with three rates of SS (0, 4 and 12 Mg dry matter ha^?1 every 4 years) and three rates of mineral fertilisation, in a factorial design with four replicates. At one site, SS application tended to increase crop yield over time but, when plant nutrients were not limiting, the effect was only significant for spring barley. We deduced that improved soil structure was the main driver for this fertility enhancement after SS application, as indicated by lower soil bulk density and higher soil carbon concentration. After 30 years, soil organic carbon stocks to 0.40 m depth differed by up to 17 Mg ha^?1 between treatments. According to carbon balance calculations, retention of carbon derived from SS ranged between 18 and 20% for the soil layers analysed at the two sites. Soluble phosphorus (P) increased with sludge application rate, but represented only around 2% of the expected residual P according to P-balance calculations. The fertiliser value of nitrogen was also low, with only 3–8% nitrogen use efficiency. Heavy metals such as copper, zinc and mercury showed moderate accumulation in the soil, but elevated levels of metals were not detected in crops, even at the highest dose of SS. These results show that SS represents a valuable resource for improving soil fertility in terms of soil organic matter and soil structure, but its efficiency for nutrient cycling is very low within the time frame considered in the study.