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Ethiopia’s economy is dominated by agriculture which is mainly rain-fed and subsistence. Climate change is expected to have an adverse impact particularly on crop production. Previous studies have shown large discrepancies in the magnitude and sometimes in the direction of the impact on crop production. We assessed the impact of climate change on growth and yield of maize and wheat in Ethiopia using a multi-crop model ensemble. The multi-model ensemble (n = 48) was set up using the agroecosystem modelling framework Expert-N. The framework is modular which facilitates combining different submodels for plant growth and soil processes. The multi-model ensemble was driven by climate change projections representing the mid of the century (2021–2050) from ten contrasting climate models downscaled to finer resolution. The contributions of different sources of uncertainty in crop yield prediction were quantified. The sensitivity of crop yield to elevated CO2, increased temperature, changes in precipitations and N fertilizer were also assessed. Our results indicate that grain yields were very sensitive to changes in [CO2], temperature and N fertilizer amounts where the responses were higher for wheat than maize. The response to change in precipitation was weak, which we attribute to the high water holding capacity of the soils due to high organic carbon contents at the study sites. This may provide the sufficient buffering capacity for extended time periods with low amounts of precipitation. Under the changing climate, wheat productivity will be a major challenge with a 36 to 40% reduction in grain yield by 2050 while the impact on maize was modest. A major part of the uncertainty in the projected impact could be attributed to differences in the crop growth models. A considerable fraction of the uncertainty could also be traced back to different soil water dynamics modeling approaches in the model ensemble, which is often ignored. Uncertainties varied among the studied crop species and cultivars as well. The study highlights significant impacts of climate change on wheat yield in Ethiopia whereby differences in crop growth models causes the large part of the uncertainties.

The rhizosphere shows complex spatial and temporal patterns of biophysical and biochemical processes. Process-based modeling that accounts for functional microbial traits provides a tool to gain a better understanding of microbial interactions involved in carbon cycling in the rhizosphere. Here, we present a trait-based rhizosphere model that accounts for microbial life-history strategies (copiotrophs, oligotrophs), microbial physiology (e.g., dormancy), and organic carbon bioaccessibility (small and large polymers). The model reflects the mm-scale microbial and carbon dynamics around a cylindrical root segment and will be linked with a structural-functional soil-plant model (CPlantBox), which enables to connect water, carbon and nitrogen dynamics in the rhizosphere to plant and bulk soil dynamics. We show the concept of trait-based rhizosphere modeling, first simulations, and our model coupling approach to CPlantBox.

Complex interactions between biodegradation and mass transfer of organic compounds drive the fate of pesticides in soil ecosystems. We hypothesized that, at the small-scale, co-location of degraders and pollutants in soils may be a prerequisite for efficient biodegradation of these chemicals. In non-co-localized micro-environments, however, diffusive and advective solute transport as well as active transport of microbial degraders towards their corresponding substrate may improve the accessibility of microbial substrates. The objective of this study was to test whether water flow can accelerate microbial pesticide degradation by facilitating the encounter of spatially separated pesticides and bacterial degraders at the millimeter scale. Combining natural and sterilized soil aggregates, we built soil cores with different spatial localizations of the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) and microbial degraders: (i) homogeneous distribution of microorganisms and 2,4-D throughout the soil core, (ii) co-localized microorganisms and 2,4-D in a mm3 soil location, and iii) separated microorganisms and 2,4-D in two mm3 soil locations spaced 1 cm apart. Following the fate of 14C labelled 2,4-D (mineralization, extractable and non-extractable residues) as well as the abundance of bacterial 2,4-D degraders harboring the tfdA gene over an incubation period of 24 days, we observed decreased biodegradation of 2,4-D with increasing spatial separation between substrate and bacterial degraders. We found evidence that advection is a key process controlling the accessibility of 2,4-D and pesticide degraders. Advective solute transport induced leaching of about 50% of the initially applied 2,4-D regardless of initial spatial distribution patterns. Simultaneously, advective transport of 2,4-D and bacterial degraders triggered their re-encounter and compensated for the leaching-induced separation of initially co-localized microorganisms and 2,4-D. This resulted in effective biodegradation of 2,4-D, comparable to the homogeneous treatment. Similarly, advective transport processes brought substrate and degraders into contact if both were initially separated. Thus, advection more effectively removed bioaccessibility limitations to pesticide degradation than diffusive transport alone. These results emphasize the importance of considering spatial microbial ecology as well as biogeophysics at the mm scale to better understand the fate of pesticides at larger scales in soil.

The barometric process separation (BaPS) technique is a well-established incubation method to simultaneously measure gross nitrification and respiration rates in soil. Its application, however, is still critical in soils with pH above 6.5. Here, a substantial part of microbial CO2 production is retained in soil solution (CO2,aq) due to shifts in the carbonate equilibrium. This may lead to substantial errors in gas balance calculation. Yet, utilization of the BaPS technique is only reliable if the critical term is adequately quantified. We present an easy, inexpensive, and direct method, the sterilization-CO2-injection (SCI) method, to measure CO2 retention during soil incubation. Sterilized soil was incubated in the BaPS system, and defined volumes of CO2 were injected to stepwise increase CO2 partial pressure (pCO2) inside the chamber and to analyse the physicochemical equilibration process. Five exemplary agricultural soils from Northeast China and Southwest Germany were used for method establishment, presenting pH values between 4.4 and 7.6 and carbonate contents between 0% and 3.9%. We observed that in soils with pH > 6.5, 70%–90% of the injected CO2 was taken up by the soil until the equilibrium inside the chamber was re-established. As expected, in soils with low pH (

The mechanistic integration of microbial behavior remains a major challenge in biogeochemical modeling of organic matter turnover in soil. We recently introduced dynamic feedbacks between specific microbial groups and their micro-environment in a biogeochemical model (Pagel et al., 2014). Here, the model was applied in a case study to simulate pesticide degradation coupled to carbon (C) turnover in the detritusphere. We aimed at unravelling the effects of litter-derived substrate supply on the spatiotemporal dynamics of the microbial community and the resulting biogeochemical processes at the mm-scale in soil. We linked genetic information on abundances of bacteria, fungi and specific pesticide degraders to the biogeochemical dynamics of C and a generic model compound (MCPA, 4-chloro-2-methylphenoxyacetic acid) in soil by multiobjective calibration. We observed and simulated increased dissolved organic and microbial C as well as accelerated MCPA degradation in soil up to a 6 mm distance to litter. We found that, whereas transport and sorption processes act as extrinsic control on the encounter of microorganisms and substrates, microbial traits such as substrate preference or metabolic capabilities intrinsically determine turnover rates triggering feedback effects on physicochemical processes such as diffusion. A process analysis revealed that C cycling and pesticide degradation in the detritusphere were strongly controlled by fungal dynamics. Our study demonstrates that integrating mathematical modeling with experiments provides comprehensive insight into the microbial regulation of matter cycling in soil.

Peatlands are interesting as carbon storage option, but are also natural emitters of the greenhouse gas methane (CH4). Phragmites peatlands are particularly interesting due to the global abundancy of this wetland plant (Phragmites australis (Cav.) Trin. ex Steud.) and the highly efficient internal gas transport mechanism, which is called Humidity Induced Convection (HIC). The research aim was to (1) clarify how this plant-mediated gas transport influences the CH4 fluxes, (2) which other environmental variables influence the CO2 and CH4 fluxes, and (3) whether Phragmites peatlands are a net source or sink of greenhouse gases. CO2 and CH4 fluxes were measured with the eddy covariance technique within a Phragmites-dominated fen in Southwest Germany. One year of flux data (March 2013 to February 2014) shows very clear diurnal and seasonal patterns for both CO2 and CH4. The diurnal pattern of CH4 fluxes was only visible when living green reed was present. In August the diurnal cycle of CH4 was most distinct, with 11-times higher midday fluxes (15.7 mg CH4 m−2 h−1) than night fluxes (1.41 mg CH4 m−2 h−1). This diurnal cycle correlates the highest with global radiation, which suggest a high influence of the plants on the CH4 flux. But if the cause would be the HIC, it is expected that relative humidity would correlate stronger with CH4 flux. Therefore, we conclude that in addition to HIC, at least one additional mechanism must be involved in the creation of the convective flow within the Phragmites plants. Overall, the fen was a sink for carbon and greenhouse gases in the measured year, with a total carbon uptake of 221 g C m−2 yr−1 (26 % of the total assimilated carbon). The net uptake of greenhouse gases was 52 g CO2-eq m−2 yr−1, which is summed from an uptake of CO2 of 894 g CO2-eq m−2 yr−1 and a release of CH4 of 842 g CO2-eq m−2 yr−1.

A 56-day aerobic incubation experiment was performed with 15-nitrogen (N) tracer techniques after application of wheat straw to investigate nitrate-N (NO3-N) immobilization in a typical intensively managed calcareous Fluvaquent soil. The dynamics of concentration and isotopic abundance of soil N pools and nitrous oxide (N2O) emission were determined. As the amount of straw increased, the concentration and isotopic abundance of total soil organic N and newly formed labeled particulate organic matter (POM-N) increased while NO3-N decreased. When 15NO3-N was applied combined with a large amount of straw at 5000 mg carbon (C) kg−1 only 1.1 ± 0.4 mg kg−1 NO3-N remained on day 56. The soil microbial biomass N (SMBN) concentration and newly formed labeled SMBN increased significantly (P

Uncertainties associated with pesticide exposure forecasts arise from many sources such as spatial and temporal variability of factors influencing pesticide behavior, inaccuracies in the measurement or estimation of input parameters and deficiencies in model structure. It is well acknowledged that, at least to some extent, simulation uncertainties should be accounted for when technical and political strategies are developed to mitigate pesticide exposure. We monitored the fate of the insecticide imidacloprid in a paddy rice field in Chieng Khoi watershed, Northern Vietnam. Pesticide fate was modeled using a new dynamic model together with the General Likelihood Uncertainty Estimation (GLUE) approach. A 95 % prediction uncertainty (PU) band was computed accounting for uncertainties in pesticide parameters, management practices, and weather conditions. As assessment criterion we used the Nash and Sutcliffe modeling efficiency (NSE). Measured paddy water concentrations of imidacloprid reached up to 53 μg L−1, paddy soil concentrations up to 9 μg kg−1. Calculated PU bands were in good agreement with field measurements. The field-scale model was extended to simulate pesticide fate in connected paddies. Imidacloprid concentrations in the afflux of the stream were estimated to be up to 83 μg L−1 under the current management practice in the research area. The loss of imidacloprid to the stream was assessed to range between 21 and 68 % of applied mass. Future studies, however, should focus on assessing loads of pesticides further widely used in connected paddies to help decision makers and farmers to adjust management strategies of protecting the environment.

Effects of dissolved pesticides on fish are widely described, but little is known about effects of pesticide-contaminated feeds taken up orally by fish. In integrated farms, pesticides used on crops may affect grass carp that feed on plants from these fields. In northern Vietnam, grass carp suffer seasonal mass mortalities which may be caused by pesticide-contaminated plants. To test effects of pesticide-contaminated feeds on health and bioaccumulation in grass carp, a net-cage trial was conducted with 5 differently contaminated grasses. Grass was spiked with 2 levels of trichlorfon/fenitrothion and fenobucarb. Unspiked grass was used as a control. Fish were fed at a daily rate of 20% of body mass for 10 d. The concentrations of fenitrothion and fenobucarb in pond water increased over time. Effects on fish mortality were not found. Fenobucarb in feed showed the strongest effects on fish by lowering feed uptake, deforming the liver, increasing blood glucose and reducing cholinesterase activity in blood serum, depending on feed uptake. Fenobucarb showed increased levels in flesh in all treatments, suggesting bio-concentration. Trichlorfon and fenitrothion did not significantly affect feed uptake but showed concentration-dependent reduction of cholinesterase activity and liver changes. Fenitrothion showed bioaccumulation in flesh which was dependant on feed uptake, whereas trichlorfon was only detected in very low concentrations in all treatments. Pesticide levels were all detected below the maximum residue levels in food. The pesticide-contaminated feeds tested did not cause mortality in grass carp but were associated with negative physiological responses and may increase susceptibility to diseases.

Background In Vietnam, paddy rice fields have been identified as a major non-point source of pesticide pollution of surface- and groundwater which is often directly used for domestic purposes. One strategy to assess the risk of pesticide pollution is to use process-based models. Here, we present a new model developed for simulating short-term pesticide dynamics in combined paddy rice field–fish pond farming systems. The model was calibrated using the Gauss–Marquardt–Levenberg algorithm and validated against measured pesticide concentrations of a paddy field–fish pond system typical for northern Vietnam. Results In the calibration period, model efficiencies were 0.82 for dimethoate and 0.87 for fenitrothion. In the validation period, modelling efficiencies slightly decreased to 0.42 and 0.76 for dimethoate and fenitrothion, respectively. Scenario simulations revealed that a field closure period of 1 day after pesticide application considerably reduces the risk of pond and surface water pollution. Conclusion These results indicate that the proposed model is an effective tool to assess and evaluate management strategies, such as extended field closure periods, aiming to reduce the loss of pesticides from paddy fields. © 2013 Society of Chemical Industry

Microbiologically active biogeochemical interfaces are excellent systems to study soil functions such as pesticide degradation at the micro-scale. In particular, in the detritusphere pesticide degradation is accelerated by input of fresh organic carbon from litter into the adjacent soil. This observed priming effect suggests: (i) pesticide degradation is strongly coupled to carbon turnover, (ii) it is controlled by size and activity of the microbial community and (iii) sorption and transport of dissolved carbonaceous compounds and pesticides might regulate substrate availability and in turn decomposition processes. We present a new mechanistic 1D model (PEsticide degradation Coupled to CArbon turnover in the Detritusphere, PECCAD) which implements these hypotheses. The new model explicitly considers growth and activity of bacteria, fungi and specific pesticide degraders in response to substrate availability. Enhanced pesticide degradation due to availability of a second source of carbon (dissolved organic carbon) is implemented in the model structure via two mechanisms. First, additional substrate is utilized simultaneously with the pesticide by bacterial pesticide degraders resulting in an increase in their size and activity. Second, stimulation of fungal growth and activity by additional substrates leads directly to higher pesticide degradation via co-metabolism. Thus, PECCAD implicitly accounts for litter-stimulated production and activity of unspecific fungal enzymes responsible for co-metabolic pesticide degradation. With a global sensitivity analysis we identified high-leverage model parameters and input. In combination with appropriate experimental data, PECCAD can serve as a tool to elucidate regulation mechanisms of accelerated pesticide degradation in the detritusphere.

Phenoxyacetic acids can be degraded by diverse soil microorganisms. Nevertheless, we miss information about the succession of 4-chloro-2-methylphenoxyacetic acid (MCPA) degraders in micro-environments of soils as well as specific functions of different microbial groups during MCPA degradation. We studied MCPA degradation at the soil-litter interface in a microcosm experiment and followed the succession of different degrader populations by quantifying the abundance of 16S rRNA genes as well as, the fungal ITS fragment and the functional genes tfdA (in total and divided into three classes) and cadA. Adjacent to the litter layer, a dynamic depletion zone of MCPA indicated that the litter effect on MCPA degradation depends on substrate availability and the affected soil volume. The increase of the tfdA class III and cadA genes was linked to MCPA mineralisation. Total abundance of tfdA genes was dominated by class I MCPA degraders and did not reflect MCPA degradation potential of the soil. Litter addition induced the development of pioneer and late-stage fungal communities, which were probably both involved in MCPA degradation. The results underline the importance of the ecological behaviour of different degrader populations for the understanding of herbicide degradation in soils.

Excessive amounts of nitrate have accumulated in many soils on the North China Plain due to the large amounts of chemical N fertilizers or manures used in combination with low carbon inputs. We investigated the potential of different carbon substrates added to transform soil nitrate into soil organic N (SON). A 56-d laboratory incubation experiment using the 15N tracer (K15NO3) technique was carried out to elucidate the proportion of SON derived from accumulated soil nitrate following amendment with glucose or maize straw at controlled soil temperature and moisture. The dynamics and isotopic abundance of mineral N (NO−3 and NH+4) and SON and greenhouse gas (N2O and CO2) emissions during the incubation were investigated. Although carbon amendments markedly stimulated transformation of nitrate to newly formed SON, this was only a substitution effect of the newly formed SON with native SON because SON at the end of the incubation period was not significantly different (P > 0.05) from that in control soil without added C. At the end of the incubation period, amendment with glucose, a readily available C source, increased nitrate immobilization by 2.65 times and total N2O-N emission by 33.7 times, as compared with maize straw amendment. Moreover, the differences in SON and total N2O-N emission between the treatments with glucose and maize straw were significant (P 0.05) greater than that in the control. Straw amendment may be a potential option in agricultural practice for transformation of nitrate N to SON and minimization of N2O emitted as well as restriction of NO3-N leaching.

The intensification of agriculture in the mountainous regions of northern Thailand has led to an increased input of agrochemicals, which may be lost to streams and contaminate the surface water of the lowlands. The present study quantifies the dynamics of pesticide loads in a tropical river during three runoff events. To elucidate the processes involved in pesticide transport from agricultural fields to the stream water we used a high temporal resolution of sampling (1 h) and applied a time series analysis. Water samples were analyzed for seven pesticides (atrazine, chlorothalonil, chlorpyrifos, cypermethrin, dichlorvos, α- and β-endosulfan). Six of the seven pesticides were detected in the river water. Only dichlorvos was below the detection limit in all samples. In particular, pesticides with low Koc value such as atrazine and dimethoate were transported during the runoff peaks. In case of chlorothalonil, chlorpyrifos, α- and β-endosulfan and cypermethrin, short concentration peaks lasting about 1 h were detected during the falling limbs of the runoff peaks, indicating that a fast and sporadic sub-surface flow component (e.g., preferential interflow) plays an important role as a transport pathway. Our study demonstrates that in tropical areas sampling schemes with a high temporal resolution are needed to adequately assess the pesticide contamination of rivers. Otherwise, extreme situations may remain unsampled.

During the last years, the increased use of pesticides and growing awareness of associated environmental and health problems have led to the implementation of various monitoring programmes in South-East Asia. The introduction of numerous new active ingredients and commercial pesticide formulations in connection with reports on pesticide-related health problems strongly indicate that the analytical procedures should be tested and evaluated for currently used pesticides. Coping with matrix effects and ensuring pesticide stability when samples are taken in remote areas are paramount. In the present study, we tested an analytical method that targets nine currently used pesticides in surface water in northern Vietnam. The method consists of solid phase extraction, storage at −18°C in the adsorbed state, and capillary gas chromatography with nitrogen-phosphorus-detection of five insecticides (dichlorvos, fenobucarb, dimethoate, fenitrothion, and chlorpyrifos), three fungicides (chlorothalonil, metalaxyl, and ed...

In the present study, the temporal and spatial variation of the abundance of the alkane monooxygenase gene alkB and 16S rRNA genes in different soil compartments was analysed in the presence or absence of 2-methyl-4-chlorophenoxyacetic acid (MCPA) after the addition of pea litter to soil in a microcosm study. Samples were analysed shortly after litter addition (T0) and 1 week (T1), 3 weeks (T3) and 6 weeks (T6) after the addition of litter. In addition also, the quantity and quality of litter-derived alkanes was analysed and measured. The results revealed a fast and complete degradation of MCPA in all compartments throughout the experiment. Nevertheless, significant changes in the distribution patterns of short- and middle-chained alkanes suggest an interaction of MCPA and alkane degradation. alkB gene copy numbers were highly influenced by the time point of analysis and by the investigated soil compartment. Overall, an increase in alkB gene copy numbers from T0 to T3 was visible in the upper soil compartments whereas a decrease compared to T0 was measured in the deeper soil compartments. MCPA addition resulted in an increase of alkB abundance at T6. Gene copy numbers of 16S rRNA were not influenced by sampling time and soil compartment. In contrast to the control treatments, a slight increase in 16S rRNA gene copy numbers was visible at T1 and T3 compared to T0 in all soil compartments.

The sorption isotherm is the key to modeling the fate of many environmental chemicals in soil. Iden and Durner (2008) proposed a simple method, the multiple batch extractions test (MuBaX), to simultaneously estimate the isotherm parameters and the total desorbable heavy metal contents. The method essentially consists of equilibrating soil suspensions at varying liquid to soil (L/S) ratios, which induces desorption of heavy metals, measuring the solution phase concentrations at equilibrium, and using a model to evaluate the measured data. This study was carried out to test the capability of the proposed method to estimate Freundlich parameters of eight heavy metals for 49 topsoil samples from nine soil types. Heavy metals studied were Cd, Cr, Cu, Mo, Ni, Sb, Tl and Zn. Five L/S ratios (1, 2, 10, 30, and 100 L kg− 1) were used. Parameter estimation was carried out using a Bayesian approach. Although measured solution phase concentrations could be reproduced very well (modeling efficiency, EF, between 0.89 and 0.99), the agreement between total desorbable heavy metal contents estimated by MuBaX and those measured by isotope dilution was quite poor as indicated by negative values of the modeling efficiency. We conclude that the MuBaX method is not well suited for application to strongly sorbing topsoils with relatively low heavy metal contents. Nevertheless, the Bayesian parameter estimation proved useful because it enabled the reliable quantification of the uncertainties in the parameter estimates and was not negatively affected by the existence of local minima of the objective function.

This study was designed to examine the environmental exposure of surface- and groundwater pollution in remote mountainous regions of northern Vietnam. In 2008, we monitored the loss of four commonly applied pesticides (imidacloprid, fenitrothion, fenobucarb, dichlorvos) from paddy rice farming systems to a receiving stream on the watershed scale and quantified groundwater pollution. For the entire monitoring period, runoff loss of pesticides from the watershed was estimated to range between 0.4% (dichlorvos) and 16% (fenitrothion) of the total applied mass. These losses were correlated well with the octanol-water partition coefficient and water solubility of pesticides (r 2 = 0.78-0.99). In the groundwater collected from eight wells, all target pesticides were frequently detected. Maximum measured concentrations were 0.47, 0.22, 0.17, and 0.07 μg L ―1 for fenitrothion, imidacloprid, fenobucarb, and dichlorvos, respectively. Our results strongly indicate that under the current management practice pesticide use in paddy fields poses a serious environmental problem in mountainous regions of northern Vietnam.

Immobilization of Cd in contaminated soil is a technique to improve soil quality. Zeolites are potentially useful additives to bind heavy metals. This study examines the influence of decreasing the grain size of raw zeolitic tuff to the nano-range on Cd sorption-desorption isotherms in a sandy soil. The aim was to evaluate the feasibility of using nanoparticulate zeolitic tuff as a sequestering agent for Cd in soil. Sorption and desorption of Cd on zeolitic nanotuff was investigated in batch experiments. The sandy soil was amended with zeolitic nanotuff, synthetic Al oxide nanoparticles, or raw zeolitic tuff. After a 48 h sorption step, three desorption steps were induced using the sequential dilution method. The measured sorption-desorption data were adequately described by the Freundlich equation. A nonlinear two-stage one-rate model matched well with measured Cd sorption-desorption isotherms of zeolitic nanotuff, raw zeolitic tuff, Al oxide nanoparticle, and control soil. The batch experiments demonstrated the strong influence of pH on Cd sorption. Zeta-potential measurements of zeolitic nanotuff indicated that electrostatic interaction was important for sorption. Soil amended with zeolitic nanotuff showed the lowest hysteresis index based on the differences obtained from sorption-desorption isotherms regarding the amount of Cd sorbed. The index decreased with increasing metal concentration. For soil amended with nanoparticulate zeolitic tuff, the estimated Freundlich coefficient was 4 orders of magnitude higher than in the control soil and 2 orders of magnitude higher than the soil amended with raw zeolitic tuff. In addition, this sorbent decreased the amount of Cd released after three desorption steps by up to 12 and 7 times compared to the control soil and the soil amended with raw zeolitic tuff, respectively. The effect on Cd sorption of grinding the raw zeolitic tuff to the nanorange was considerably more pronounced than the effect of raw zeolitic tuff.

Much is known about mechanisms and regulation of phenoxy acid herbicide degradation at the organism level, whereas the effects of environmental factors on the performance of the phenoxy acid degrading communities in soils are much less clear. In a microcosm experiment we investigated the small-scale effect of litter addition on the functioning of the MCPA degrading communities. 14 C labelled MCPA was applied and the functional genes tfdA and tfdAα were quantified to characterise bacterial MCPA degradation. We identify the transport of litter compounds as an important process that probably regulates the activity of the MCPA degrading community at the soil–litter interface. Two possible mechanisms can explain the increased tfdA abundance and MCPA degradation below the litter layer: 1) transport of α-ketoglutarate or its metabolic precursors reduces the costs for regenerating this co-substrate and thereby improves growth conditions for the MCPA degrading community; 2) external supply of energy and nutrients changes the internal resource allocation towards enzyme production and/or improves the activity of bacterial consortia involved in MCPA degradation. In addition, the presence of litter compounds might have induced fungal production of litter-decaying enzymes that are able to degrade MCPA as well.

Column experiments on phytolith transport were conducted to assess the partial contributions of water percolation and earthworm activity to phytolith transport in loamy and sandy soils. Six intact cores of a loamy sandy Haplic Cambisol and nine cores of a silty loamy Stagnic Luvisol were excavated. With the Luvisol, three treatments were perfomed: a percolation treatment with periodic irrigation, but without earthworms, a percolation and earthworm treatment with periodic irrigation and earthworms (Aporrectodea caliginosa) and a control. The Cambisol cores did not contain earthworms and hence only percolation and control treatments were tested. The phytoliths of common reed (Phragmites australis) were labelled with the fluorescent dye fluorescein isothiocyanate and applied to the soil surface of each core. Except for the control treatment, 3600 mm of water was applied over 6 months. In the Cambisol, the weighted mean transport distance of phytoliths was significantly greater with percolation (2.2 ± 0.1 cm) than in the control (0.9 ± 0.3 cm), indicating that water percolation is a driving mechanism of phytolith transport. In the Luvisol, the difference in mean transport depth between control and percolation treatments (1.0 ± 0.2 and 1.5 ± 0.3 cm) was not significant. The earthworms did not affect the mean transport distance of phytoliths in the Luvisol, but the phytolith concentrations in the leachates were significantly greater and their size distribution did not change with soil depth as observed in the percolation treatment without earthworms. Further studies are required to quantify the effect of earthworms on phytolith transport.

In recent years large areas of conventional cereal production in China have been transferred to greenhouse production with huge excessive nitrogen (N) fertilizer application and massive irrigation. However, the effects of this change in land use on soil carbon and nitrogen pools remain to be explored. Here we report a comparative study in which paired soil samples were taken from four greenhouses and from adjacent conventional cereal fields. Soil organic carbon (SOC), carbonate carbon (IC), total nitrogen (TN) and mineral nitrogen (N min ) to 100 cm depth and the soil active organic pools, including particulate organic matter (POM), soil microbial biomass (SMB) and dissolved organic matter (DOM), to 0–40 cm depth were determined. The natural isotopic signatures of SOC, TN and POM were also analyzed. In both production systems all of the carbon and nitrogen pools in the surface soil (0–10 cm) were greater than deeper in the soil profile except for dissolved organic nitrogen (DON) and NH 4 -N. SOC and TN and dissolved organic carbon (DOC) concentrations were higher in the greenhouse system than in conventional cereal soils ( P > 0.05). A similar trend was found for POM ( P 3 -N ( P P > 0.05) and IC in the greenhouse system showed a dramatic decline. The SOC/TN ratios of different pools in the greenhouse soils were lower than in the conventional cereal system ( P > 0.05). The SOC/TN ratio ranged from 8.4 to 10.0 in greenhouse soils and 8.5 to 11.7 in the cereal soils. At each depth POM content in the greenhouses (1.5–7.1 g kg −1 ) was significantly greater than that in the field soils (0.8–2.9 g kg −1 ) ( P

A better understanding of the internal N cycle in agriculturally used soils is crucial for developing sustainable and environmentally friendly N fertilizer management and to propose effective N 2 O mitigation strategies. The present laboratory study quantifies gross nitrogen transformations in an intensively used agricultural soil of the North China Plain (NCP). It also elucidates the role of nitrification and denitrification in the emissions of the greenhouse gas N 2 O. In the lab, soil samples adjusted to a water-filled pore space (WFPS) of 40 or 60% were spiked with 15 NH 4 NO 3 , NH 4 15 NO 3 , or 15 NH 4 15 NO 3 and incubated at 20°C for 10 d. One subset of the samples was amended with glucose. The size and 15 N enrichment of the mineral N pools and N 2 O fluxes were determined at intervals of 0 to 10 d. The studied calcareous soil showed a very rapid nitrification of the applied NH 4 + , which disappeared during the first 3 d. Glucose enhanced mineralization of native soil organic matter (SOM), stimulated dissimilatory nitrate reduction to ammonium (DNRA), and promoted the immobilization-remineralization cycle. Throughout the incubation, nitrification, and denitrification occurred simultaneously. Nitrification was the dominant N 2 O-producing process and contributed 83.0 to 95.4% to the totally emitted N 2 O in the non-glucose soils. In contrast, the N 2 O emitted from 15 NO 3 - pool increased after adding glucose, indicating that denitrification was C-limited in the studied soil.

The detritusphere is a very thin but microbiological highly active zone in soil. To trace the fate of litter carbon in the detritusphere we developed a new 1D dynamic mechanistic model. In a microcosm experiment soil cores were incubated with 13 C labelled rye residues ( δ 13 C=299‰), which were placed on the surface. Microcosms were sampled after 3, 7, 14, 28, 56 and 84 days and soil cores were separated into layers of increasing distance to the litter. Gradients in soil organic carbon (TOC), dissolved organic carbon (DOC), microbial biomass and activity were detected over a distance of 3 mm from the litter layer. The newly developed 1D model simulates both the total carbon and the 13 C carbon pools and fluxes, so that it was possible to include the 13 C data in model optimisation. The special feature of the model is that it operates with two decomposer populations; the first one is assumed to be dominated by bacteria (initial-stage decomposer) and second one by fungi (late-stage decomposer). Moreover, in the model the DOC pool is divided into two sub pools. Each DOC pool is consumed by one of the decomposer populations. After parameter optimisation the model was well suited to simulate the experimental data. The model explained 92% of the observed variance. The model output provides a comprehensive insight into the carbon cycling within the detritusphere. The simulation results showed among others that after 84 days about 10% of total litter C was transferred to the soil organic matter (SOM) pool. Only 3% was located in the microbial biomass. From the evolved CO 2 71% was litter-derived and 29% was soil-derived. From the litter-derived CO 2 , 69% was directly formed in the litter layer. The remaining 31% was transported to soil before mineralisation. Our study shows that a combination of experimental work and mathematical modelling is a powerful approach to provide a comprehensive insight into the small-scale carbon turnover in soil.

In a growing number of studies, the barometric process separation (BaPS) method has been applied for measuring gross nitrification rates in soil. In 2000, the company Umweltanalytische Mess-Systeme (UMS) GmbH (Munich, Germany) presented the first and only commercially available automatic BaPS measuring system. In an ongoing project, we have used the UMS BaPS system to measure gross nitrification rates in two alkaline agricultural soils. During data evaluation, we came across certain shortcomings in the UMS data evaluation routine. We identified three problems: (i) a unit error in the calculation of the carbonate equilibrium, (ii) an erroneous calculation when the respiration quotient is unequal to unity, and (iii) an inappropriate procedure for handling a negative rate of N gases produced by denitrification (ΔN x O y ). Particularly the error in calculating the carbonate equilibrium caused a significant overestimation of the gross nitrification rate at pH values >6. A literature review showed that the BaPS method works well in acidic to weakly acidic soils. For soils with higher pH values, its performance remains unclear. More research is needed to test the applicability of the BaPS method in neutral and alkaline soils.

Organic residues can be a major source of nutrients and are valuable fertilizers. But their benefits with regard to soil quality are undisputed. However, only few studies have focused on emissions of greenhouse gases from soil enriched with organic residues. A microcosom approach was employed to investigate the influence of the origin and composition of various organic residues on mineralization and N2O and CO2 emissions in an arable soil. In total, we set up six treatments: control, poultry manure, bio-waste compost, sheep and wheat straw compost, cow manure (CM) and for further comparison, the mineral fertilizer calcium ammonium nitrate. 500 g of sieved and homogenized soil was mixed with the amendments and packed into microcosms. After a pre-incubation period of 10 days, gas concentrations were measured periodically from the headspace of the microcosm by means of an airtight surgical syringe. The measurement period continued for 32 days. Soil amended with CM showed a significantly (α = 0.05) higher cumulative CO2 emission (914 mg kg−1) followed by bio-waste compost than poultry manure, sheep waste compost, control and calcium ammonium nitrate. Amending soil with cow manure and poultry manure led to the highest N2O-N emissions (110 µg kg−1). However, poultry manure and calcium ammonium nitrate significantly enhanced mineralization and net nitrification. Amendment of sheep and wheat straw compost and cow manure led to C sequestration and reduced N2O emission. Soil pH greatly decreased with poultry manure, sheep and wheat straw compost and bio-waste compost. Summing up, the application of organic residues to soil has some disadvantageous environmental effects calling for further research.

Environmental controls of 2-methyl-4-chlorophenoxyacetic acid (MCPA) degradation are poorly understood. We investigated whether microbial MCPA degraders are stimulated by (maize) litter and whether this process depends on concentrations of MCPA and litter. In a microcosm experiment, different amounts of litter (0, 10 and 20 g kg(-1)) were added to soils exposed to three levels of the herbicide (0, 5 and 30 mg kg(-1)). The treated soils were incubated at 20 A degrees C for 6 weeks, and samples were taken after 1, 3 and 6 weeks of incubation. In soils with 5 mg kg(-1) MCPA, about 50 % of the MCPA was dissipated within 1 week of the incubation. Almost complete dissipation of the herbicide had occurred by the end of the incubation with no differences between the three litter amendments. At the higher concentration (30 mg kg(-1)), MCPA endured longer in the soil, with only 31 % of the initial amount being removed at the end of the experiment in the absence of litter. Litter addition greatly increased the dissipation rate with 70 and 80 % of the herbicide being dissipated in the 10 and 20 g kg(-1) litter treatments, respectively. Signs of toxic effects of MCPA on soil bacteria were observed from related phospholipid fatty acid (PLFA) analyses, while fungi showed higher tolerance to the increased MCPA levels. The abundance of bacterial tfdA genes in soil increased with the co-occurrence of litter and high MCPA concentration, indicating the importance of substrate availability in fostering MCPA-degrading bacteria and thereby improving the potential for removal of MCPA in the environment.

In the hill country of northern Thailand use of agrochemicals is increasing steadily and contaminating streams and groundwater. We have measured pesticide leached from a litchi orchard to an adjacent stream and identified the flow components contributing to the pesticide transport after installing two discharge measurement stations with automatic water samplers in the stream. For 2 years, between June and September, we applied methomyl, and in 1 year additionally chlorothalonil, to the 2-ha orchard and monitored water fluxes and pesticide concentrations in the stream water. Pesticide loads ranged from 1.6 to 3% of mass applied for chlorothalonil, a strongly sorbing fungicide, and 6.4-11.4% for the more weakly sorbing insecticide methomyl. Directly after application, pesticide transport was dominated by a fast flow component, reaching the flume before the discharge peak. Later, pesticides were transported mainly by preferential interflow, which peaked about 30 hours after a rain event. The groundwater pathway was not found to contribute to pesticide loss. Antecedent rain conditions proved to be important for the pesticide transport behaviour. While at the beginning of the rainy season large falls of rain did not lead to pesticide contamination, at the end of the season pesticide transport was induced by as little as 0.1 mm of rain. The occurrence of preferential interflow means that the hilly regions in northern Thailand are highly susceptible to contamination by pesticides used by farmers.

During the last decades, decision makers and policy have increasingly demanded for regional and national inventories of greenhouse gas emission, such as nitrous oxide (N2O), to develop appropriate strategies and mitigation options. A potential way to derive large-scale estimates of N2O emission is the use of process-based models, such as PnET-N-DNDC or Wetland-DNDC. While PnET-N-DNDC has been effectively applied for various upland forest ecosystems, the Wetland-DNDC model has not yet been validated with regard to N2O emission. We calibrated and validated the Wetland-DNDC model on the basis of a 4-year field data set of two water-logged soils (Humic Gleysol and Histic Gleysol) of a spruce forest ecosystem. Model calibration by means of the Levenberg–Marquardt algorithm considerably improved the model performance for the period of calibration (2001–2002). The error variance was reduced by up to a factor of two and the modelling efficiency was increased from −1.24 to −0.15 (Humic Gleysol) and from −0.42 to 0.1 (Histic Gleysol). However, the model performance for the period of validation (2003–2004) and particularly for the extreme dry period in summer 2003 was not fully satisfying, notably with regard to the temporal pattern of the N2O emission.

The use of heavy metal hyperaccumulating plants has the potential to become a promising new technique to remediate contaminated sites. We investigated the role of metal mobilization in the Cd hyperaccumulation of Thlaspi caerulescens (J. & C. Presl, 'Ganges'). In a micro-lysimeter experiment we investigated the dynamics of Cd concentration of leachate as well as Cd removal by plant uptake in four treatments: (i) Control (bare soil), (ii) T. caerulescens, (iii) nonhyperaccumulator Brassica juncea (L.) Czern. ('PI 426308'), and (iv) co-cropping of the hyperaccumulator and nonhyperaccumulator. The experimental findings were analyzed using one- and two-site rate-limited desorption models. Co-cropping of T. caerulescens and B. juncea did not enhance metal uptake by B. juncea. Although Cd uptake of T. caerulescens was 10 times higher than that of B. juncea, the Cd concentration of leachate of the T. caerulescens treatment did not decrease below that of the B. juncea treatment. The Cd depletion in leachate was well reproduced by the two-site rate-limited desorption model. The optimized desorption coefficient was three orders of magnitude higher in the rhizosphere than in the bulk soil. Our results indicate that T. caerulescens accelerates the resupply of Cd from soil pointing to an important role of kinetic desorption in the hyperaccumulation by T. caerulescens.

This paper compares the capability of a first-order and a spherical diffusion model to describe and predict long-term sorption and desorption processes of chlortoluron in two soils. Chlortoluron sorption was investigated at different time scales utilizing one rate experiment (120 days) and two sorption/desorption experiments. Experimental periods for sorption and desorption were set to 1 day (five desorption steps) and 30 days (three desorption steps), respectively. Upon fitting, the two models satisfactorily described the whole set of data. The spherical diffusion model performed better than the first-order model. We then tested the predictive capability of the models by predicting 30-day sorption/desorption data using kinetic parameters fitted on 1-day sorption/desorption data only. While the spherical diffusion model was able to predict the 30-day data set, the first-order model failed completely. Fitting both models to subsets of the data corresponding to different experimental time scales revealed that the rate parameter as well as the Freundlich coefficient of the first-order model are strongly time-dependent--a property that is not shared by parameters of the spherical diffusion model. The apparent stability of the spherical diffusion model with regard to time dependency of its parameters indicates that sorptive uptake may be diffusion-controlled. This also explains the models greater predictive power across different time scales compared to the first-order model. Finally, we investigate the suitability of solute class specific log-linear relationships between the first-order rate parameter and the Freundlich coefficient presented by earlier researchers in the light of the time dependency observed for the parameters of the first-order model.

The proper selection of the background electrolyte is of special importance studying element availability and mobility in the laboratory. The determination of the background solution composition can be done with the equilibrium soil solution method ESS (Matschonat and Vogt, 1997). The ESS method is a procedure to find out a salt solution of major cations and anions that does not undergo changes in its composition when brought into contact with a specific soil sample. This composition is experimentally approximated in an iterative procedure until certain quality criteria are fulfilled. We tested if the ESS method, which was developed for forest soils, can successfully be applied also to agricultural soils. The solution composition was confirmed by an independent method. We used samples of a glasshouse and an arable loess soil. Because the ESS method is relatively time and work consuming, we tested modifications which should simplify the procedure: the use of dried and frozen samples instead of field fresh soil, an approximation according to the solution's electric conductivity only, and the modeling of cation exchange to omit iteration steps. The ESS method caused slight overestimation (10–25 %) in cation concentrations, but in general, these were well met. Individual anion concentrations, however, were not buffered in this soil and could not unequivocally be determined. We recommend to adjust the anion concentrations in the ESS procedure according to their proportions in an initial water extract. As ion concentrations are functions of the soil : solution ratio, any method based on addition of water to the soil, like the so-called soil saturation extract (Germany: Bodensattigungsextrakt), runs a risk to seriously underestimate ion concentrations at field conditions. The ESS method may therefore be especially well suited for soils with a low water content. The use of frozen soil gave good results and the omission of iteration steps by cation exchange modeling was promising and will be explored with the aim to operationalize it in forthcoming work. Eignung der ESS-Labormethode zur Bestimmung der Gleichgewichts-Bodenlosung landwirtschaftlicher Boden und Vorschlage zur Vereinfachung des experimentellen Verfahrens Fur Laboruntersuchungen zur Verfugbarkeit und Mobilitat von Stoffen in Boden ist die Auswahl der Hintergrund-Elektrolytlosung von besonderer Bedeutung. Zur Bestimmung der Hintergrundlosung kann z. B. die Gleichgewichts-Bodenlosungsmethode ESS (Matschonat and Vogt, 1997) herangezogen werden. Die ESS-Methode ist eine Prozedur zur Bestimmung einer Salzlosung der wichtigsten Kationen und Anionen, welche keine Anderung ihrer Zusammensetzung erfahrt, wenn sie mit einer bestimmten Bodenprobe in Kontakt gebracht wird. Die Zusammensetzung der Losung wird durch ein iteratives Verfahren experimentell angenahert, bis bestimmte Qualitatskriterien erfullt sind. Wir untersuchten, ob die ESS-Methode, die fur Waldboden entwickelt wurde, auch auf landwirtschaftliche Boden erfolgreich angewendet werden kann. Zur Uberprufung der ermittelten Losungszusammensetzung wurde eine unabhangige Methode herangezogen. Die verwendeten Proben entstammten einem Losboden und wurden innerhalb eines Gewachshauses und dem benachbarten Ackerboden entnommen. Da die ESS-Methode vergleichsweise zeit- und arbeitsaufwandig ist, untersuchten wir auserdem Modifikationsmoglichkeiten zur Vereinfachung der Prozedur: die Verwendung getrockneter oder tiefgekuhlter Bodenproben anstelle von feldfrischem Boden, eine Annaherung allein anhand der elektrischen Leitfahigkeit der Losung, sowie die Modellierung des Kationen-Austauschprozesses mit dem Ziel, Iterationsschritte zu uberspringen. Wahrend die Kationen-Konzentrationen durch die ESS-Methode im allgemeinen gut getroffen, wenngleich etwas (10–25 %) uberschatzt wurden, konnten die Konzentrationen der Anionen nicht eindeutig bestimmt werden; der verwendete Boden pufferte die Anionen-Konzentrationen kaum. Wir empfehlen, stattdessen die Verhaltnisse der Konzentrationen einzelner Anionen in einem ersten Wasserextrakt, mit dem die ESS-Methode startet, heranzuziehen und auf die Konzentrationen der Anionen in der ESS zu ubertragen. Da Ionenkonzentrationen Funktionen des Boden:Losung-Verhaltnisses sind, lauft jede Methode, die auf Zugabe von Wasser zur Bodenprobe beruht (wie der so genannte Bodensattigungsextrakt), Gefahr, die Ionenkonzentrationen unter Feldbedingungen deutlich zu unterschatzen. Die ESS-Methode eignet sich somit insbesondere fur Bodenproben mit niedrigem Wassergehalt. Die Verwendung von tiefgekuhlten Proben fuhrte zu guten Ergebnissen und das Uberspringen von Iterationsschritten durch Modellierung des Kationenaustauschs war viel versprechend und soll zukunftig operationalisiert werden.

This study reconciles an apparent inconsistency between the dimethylphthalate (DMP) batch and column data reported in the literature. Some researchers found that retardation coefficients obtained by fitting a linear two-stage model to column data were about 50 % smaller than those calculated from the distribution coefficient of a 14-d isotherm. In this study, sorption parameters were derived independently by fitting a linear and a nonlinear two-stage model simultaneously to the 3-and 14-d isotherms as well as to a sorption parameters and the dispersion coefficient obtained from a tritium tracer experiment, the column data were adequately predicted. A major part of the differences in the retardation coefficients observed difficulties in detecting the tailing of nonequilibrium breakthrough curves

Nonsingular sorption-desorption behavior of organic compounds has been frequently observed, but it remained unexplained in many cases. In this study we investigate the role slow kinetics may play in nonsingular sorption. Sorption-desorption experiments were carried out with the herbicide chortoluron (3-(3-chloro-p-tolyl)-1,1-dimethylurea) and two soils—a silt loam and a loamy sand—using three different batch techniques. The first technique is the classic decantrefill method where part of the supernatant is replaced with solute-free solution. The second technique is similar;, however, part of the supernatant is replaced with solute containing solution instead. In the third technique desorption is induced by successive dilution of the supernatant with time. Nonsingular sorption of chlortoluron was observed independent of the method. We fitted a kinetic two-stage model to the data of the first technique to estimate sorption rate parameters. With this set of parameters, the model predicted the data measured with the second and third technique well. The agreement indicates that nonsingularity phenomena observed by means of the different techniques may be explained by slow sorption kinetics. The ability of the model to describe nonsingular sorption-desorption data was further tested on data sets taken from the literature where nonsingularity remained unexplained or was explained by other causes different from slow kinetics. All phenomena observed in the different studies could readily be explained by the two-stage model.

Taking Cd as an example we introduce a procedure to estimate tolerable total content of heavy metals in soils with regard to a specific ground water quality criterion. Furthermore, we present a piston-flow approach to estimate breakthrough times of a sorptive solute to the ground water. Both procedures are applied to the sandy soils in the 4300 ha wastewater irrigation area Braunschweig, Germany. Applicability of these procedures is tested by numerical simulations. The calculated breakthrough times of Cd for an input concentration of 3 μg L−1 and a mean water flux density of 570 mm yr−1 varies, as a function of depth of water table and sorption characteristics, between 10 and 805 years (mean = 141 years). The deviation between the piston-flow approach and the numerical simulation is on the average 1.6%. We determined a mean tolerable total Cd content of 0.61 mg kg−1 with regard to a ground water quality criterion of 3 μg L−1. The limit of the German sewage sludge regulation (AbfKlarV, 1992) of 1 mg Cd kg−1 exceeds the calculated tolerable total content in 90% of the investigated Ap horizons. Moreover, the results of the numerical simulations show that the limit of 1 mg Cd kg−1 would lead to a concentration in seepage water significantly above 8 μg Cd L−1. We conclude that in the sandy soils of the wastewater irrigation area the current limit of 1 mg Cd kg−1 is not sufficient to keep the Cd concentration in seepage water below 3 μg L−1 and, thus, to ensure ground water protection in the long run. Grundwasserschutz durch Bodenschutz: Ermittlung von tolerierbaren Cd-Gesamtgehalten und Cd-Durchbruchszeiten Am Beispiel des Schwermetalls Cd wird ein Verfahren vorgestellt, um fur ein definiertes Grundwasserqualitatsziel tolerierbare Gesamtgehalte eines Schwermetalls im Boden zu berechnen. Weiterhin wird ein „piston-flow”-Ansatz beschrieben, der die Berechnung der Durchbruchszeit eines sorptiven Stoffes in das Grundwasser erlaubt. Die Verfahren werden exemplarisch auf die Sandboden des 4300 ha grosen Abwasserverregnungsgebietes Braunschweig angewendet und die Qualitat der Aussagen durch numerische Simulationen getestet. Die berechneten Durchbruchszeiten von Cd bei einer Infiltrationskonzentration von 3 μg L−1 und einer Wasserflussdichte von 570 mm a−1 betragen im Untersuchungsgebiet im Mittel 141 Jahre und variieren, in Abhangigkeit vom Grundwasserflurabstand und den Sorptionseigenschaften, zwischen 10 und 805 Jahren. Die Abweichung zwischen dem „piston-flow”-Ansatz und der numeri schen Simulation betragt im Mittel 1.6%. Fur das Grundwasserqualitatsziel 3 μg L−1 wird ein mittlerer tolerierbarer Cd-Gesamtgehalt von 0.61 mg kg−1 bestimmt. Der Grenzwert der Klarschlammverordnung (AbfKlarV, 1992) von 1 mg kg−1 wurde bei 90% der untersuchten Ap-Horizonte den berechneten tole rierbaren Cd-Gesamtgehalt uberschreiten. Nach den Ergebnissen der numerischen Simulation wurden Cd-Gesamtgehalte in Hohe des Grenzwertes der AbfKlarV (1992) im Untersuchungsgebiet zu Sickerwasserkonzentrationen deutlich oberhalb von 8 μg Cd L−1 fuhren. Der Grenzwert der AbfKlarV (1992) ist folglich fur die Sandboden des Abwasserverregnungsgebietes unter der Masgabe eines Qualitatskriteriums von 3 μg Cd L−1 fur einen langfristigen Grundwasserschutz unzureichend.

The transport of the herbicide chlortoluron (3-(3-chlor-p-tolyl)-1,1-dimethylurea) was monitored in Northern Germany in a sandy soil between November 1994 and March 1995. On a 0.86-ha grid, soil cores were taken down to a depth of 0.3 m and analyzed for total chlortoluron concentrations at 0.03-m intervals. A bromide (Br - ) transport experiment was carried out at the same site. Thirty-three, 61, and 136 d after application, corresponding to 54, 115, and 305 mm of precipitation, chlortoluron was found almost exclusively in the upper two, four, and six layers, respectively. In these layers, coefficients of variation (CVs) varied from about 50 to >200%. At the first sampling, about 1% of applied chlortoluron was detected in the layer just above the plow-sole. Mean travel depths of chlortoluron were distinctly more variable than those of Br - , showing no spatial correlation. The retardation factor of chlortoluron, expressing its displacement relative to that of Br - , considerably increased during the experiment, on the average from 11.5 at the first sampling to 26.3 at the third sampling. Only then, the local mean travel depths of chlortoluron showed a significant (positive) correlation with those of Br and a significant (negative) correlation with organic C contents. Sorption kinetics of chlortoluron were investigated in laboratory sorption-desorption (SD) experiments at 3 and 20°C. Sorption-desorption isotherms showed pronounced hysteresis. The influence of experimental temperature was small. The collected data are intended as a basis to evaluate the ability of different modeling approaches to represent pesticide displacement at the field scale.

This study was conducted to evaluate different modeling approaches for their ability to predict chlortoluron movement observed in a 4-mo field study. The data base includes the results of a Br - transport experiment performed at the same site and of laboratory sorption-desorption (SD) experiments. The approaches tested are based on the one-dimensional convection dispersion equation (CDE) with two-stage or equilibrium sorption. The CDE was applied either at the field scale or, to account for parameter variability, within a parallel soil columns (PSC) approach. Parallel soil column simulations were realized by the Monte Carlo (MC) method. Transport and sorption parameters and their distributions were estimated from Br - concentration profiles and SD isotherms, respectively. Chlortoluron degradation had been investigated in a joint project. The CDE with two-stage sorption represented the movement of the chlortoluron peak well, but not its spreading. Similar results were obtained assuming equilibrium sorption, provided the same sorption parameters were used. If the sorption parameters were derived from the 1-d sorption isotherm instead, displacement was overpredicted. Accounting for the variability of sorption and transport parameters through the PSC approach affected the simulation results only slightly. All simulations substantially underpredicted chlortoluron spreading, probably because the time scales of the laboratory and the field experiments were too different. To convert the estimated rate parameter to the appropriate time scale, we derived a simple formula that is based on the hypothesis of intrasorbent diffusion. Application of the CDE with the rescaled rate parameter distinctly improved the agreement between modeled and measured chlortoluron concentration profiles.

A small percolation chamber was constructed that allows the investigation of cation exchange kinetics on clay minerals under temperature controlled conditions. A mathematical model for the kinetics of cation exchange was developed that accounts for either linear or Gapon-type exchange isotherms on external (quick) and interlayer (slow) sorption sites, respectively. With montmorillonite, as indicated by the course of the potassium percolation curve, almost the whole of the cation exchange capacity could be attributed to slow sorption sites. Potassium percolation curves could best be modeled assuming that no quick sorption took place. Only the mass exchange rate coefficient was estimated. A linear exchange isotherm was assumed, and the cation exchange capacity was set to the value obtained from batch exchange isotherms. At longer times, however, the simulation deviated considerably from the experiment. We concluded that the presence of easily expandable interlayers added effects to the cation exchange that are not considered by the model. With vermiculite, potassium percolation curves needed three parameters to be estimated: mass exchange rate coefficient, partitioning factor, and Gapon coefficient for quick sorption sites. Cation exchange capacity was set to the value obtained in batch experiments. A linear exchange isotherm was assumed for interlayer sorption sites. According to the model about 64% of the cation exchange capacity (CEC) could be attributed to slow sorption sites. Modeled and observed results are in good agreement. A comparison of parameter estimation results revealed that a Gapon coefficient for slow sorption sites is redundant, whereas the assumption of a linear exchange isotherm for quick sorption sites always leads to erroneous results. A faster mass transfer from slow sorption sites into solution has been observed with increasing temperature.

The influence of pH on cadmium (Cd) sorption and desorption isotherms was studied in sandy and clayey soils of northern Germany. Seventeen topsoils from four long-term lime amendment experiment plots ranging in pH from 4.1 to 6.9 were collected. Batch adsorption isotherms demonstrated the strong influence of pH on Cd sorption behavior. Cadmium sorption increased with pH, even within a very narrow range of pH values (ΔpH = 0.3). The experimental desorption isotherms, which were determined by widening the soil/solution ratio, differed markedly from the adsorption isotherms. Differences decreased when reaction time for desorption was prolonged. The data for Cd adsorption and desorption were pooled to derive the coefficients of a generalized Freundlich-type expression S = S(C M ) including soil pH in 0.01 M CaCl 2 and organic C content (oc) (% by mass) (S = k * (H + ) a oc b C M ). Sorption isotherms were used for simulations to predict vertical displacement of Cd in the field. The simulations using measured field mean concentration profiles S(z) as initial values, showed Cd adsorption isotherms to be appropriate for describing longer-term desorption equilibria.

Column flow experiments under saturated conditions were conducted to investigate the sorption behavior of simazine on a Tujunga sandy loam. The flow interruption technique was used to test whether rate-limited processes were present. The outflow results showed that the simazine sorption was not at equilibrium during the time of the experiment. This non-equilibrium was due to the sorption process, as the bromide tracer concentration did not change during flow interruption. Simazine breakthrough data could be best described by a two-stage, two-rate process, where the first rate is considerably faster than the second. The model predicts slow reaction kinetics, so that equilibrium was not reached even after 185 h of flow interruption. Outflow data of the first simazine breakthrough only could be described well with a one-stage, rate-limited sorption model; flow interruption was necessary to reveal the second sorption stage.

Downward displacement of cadmium (Cd) and zinc (Zn) at the field scale was investigated in a sandy soil irrigated under controlled conditions for 29 yr with a total of 12.8 m{sup 3} m{sup -2} of municipal wastewater. On a 0.86 ha grid a total of 720 samples was taken from 0 to 1.2 m depth. Cadmium and Zn contents were determined in all samples by extraction with 0.025 M ethylene-diamine-tetra-acetic acid (EDTA) on the assumption that this fraction represents the solution and sorbed phases in soil. Solution phase concentrations of 480 samples were estimated by equilibrating the samples with 0.0025 M CaCl{sub 2}. Heavy metal downward displacement is spatially highly variable. Five percent of EDTA-extractable Cd and Zn were found below 0.7 and 0.9 m depth, respectively. Spatial variability of heavy metal load as measured by recovered mass per unit area reflects the geometry of irrigation systems employed. Measured sorption equilibria could be well described as a function of organic C content and pH by extending the Freundlich equation. 36 refs., 8 figs., 2 tabs.

The application of agrochemicals in Southeast Asia is increasing in rate, variety and toxicity with alarming speed. Understanding the behavior of these different contaminants within the environment require comprehensive monitoring programs as well as accurate simulations with hydrological models. We used the SWAT hydrological model to simulate the fate of three different pesticides, one of each usage type (herbicide, fungicide and insecticide) in a mountainous catchment in Northern Thailand. Three key parameters were identified: the sorption coefficient, the decay coefficient and the coefficient controlling pesticide percolation. We yielded satisfactory results simulating pesticide load dynamics during the calibration period (NSE: 0.92–0.67); the results during the validation period were also acceptable (NSE: 0.61–0.28). The results of this study are an important step in understanding the modeling behavior of these pesticides in SWAT and will help to identify thresholds of worst-case scenarios in order to assess the risk for the environment.

The increasing application of pesticides in the uplands of northern Thailand has increased the transfer of pesticides to surface water. To assess the risk of pesticide use for stream water quality, we monitored the concentrations of seven pesticides (atrazine, dichlorvos, chlorpyrifos, dimethoate, chlorothalonil, (α-, β-) endosulfan, cypermethrin) frequently used in the Mae Sa watershed (77 km2) in water and sediment samples over a period of one and a half years (2007–2008). All investigated pesticides were recorded in the river. Chlorpyrifos was detected most often in water samples (75 % at the headwater station), while cypermethrin was most often found in riverbed (86 %) and in all suspended sediment samples. The highest concentrations of the pesticides were detected during the rainy season. About 0.002 to 4.1 % by mass of the applied pesticides was lost to surface water. The risk assessment was based on the risk characterization ratio (RCR). The RCRs of dichlorvos in water, (α-, β-) endosulfan, and cypermethrin in water and sediments were higher than unity indicating that they are likely to pose a threat to aquatic ecosystem. Finally, we discuss the role of sampling design on ecotoxicological risk assessment. Our study shows that pesticide contamination of surface waters is an environmental issue in the Mae Sa watershed and that measures need to be undertaken to reduce the loss of pesticides from soil to surface waters.

This study was conducted to reconcile an apparent inconsistency between the simazine laboratory sorption isotherm data and the field lysimeter transport experiment reported by Poletika et al. (this issue). In this investigation, linear and nonlinear one- and two-stage simazine sorption models were fitted to the sorption and desorption isotherm laboratory data to obtain parameter estimates for use in the transport model. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate-limited sorption and transport of the simazine added simultaneously with the Br. The calibrated model did an excellent job of representing the final simazine profile in the soil, particularly with the nonlinear model. This is in contrast to a single-stage adsorption model tested by Poletika et al. (this issue), which reached poor agreement with the field profile when laboratory-measured sorption parameters were used. The results demonstrate the compatibility of field and laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibrium than is customarily allowed in current protocols.

This study was performed to identify the transport pathways of pesticides from a sloped litchi (Litchi chinensis Sonn.) orchard to a nearby stream based on a three-component hydrograph separation (baseflow, interflow, surface runoff). Dissolved silica and electrical conductivity were chosen as representative tracers. During the study period (30 d), 0.4 and 0.01% of the applied mass of atrazine and chlorpyrifos, respectively, were detected in the stream after 151 mm of rainfall. Baseflow (80–96%) was the dominant hydrological flow component, followed by interflow (3–18%) and surface runoff (1–7%). Despite its small contribution to total discharge, surface runoff was the dominant atrazine transport pathway during the first days after application because pesticide concentrations in the surface runoff flow component declined quickly within several days. Preferential transport with interflow became the dominant pathway of atrazine. Because chlorpyrifos was detected in the stream water only twice, it was not included in the hydrograph separation. A feature of the surface runoff pathway was the coincidence of pesticide and discharge peaks. In contrast, peak concentrations of pesticides transported by interflow occurred during the hydrograph recession phases. Stormflow generation and pesticide transport depended on antecedent rainfall. The combination of high-resolution pesticide concentration measurements with a three-component hydrograph separation has been shown to be a suitable method to identify pesticide transport pathways under tropical conditions.

During the last decades, high population growth and export-oriented economics in Vietnam have led to a tremendous intensification of rice production, which in turn has significantly increased the amount of pesticides applied in rice cropping systems. Since pesticides are toxic by design, there is a natural concern on the impact of their presence in the environment on human health and environmental quality. The present study was designed to examine the water regime and fate of pesticides (fenitrothion, dimethoate) during two consecutive rice crop seasons in combined paddy rice-fish pond farming systems in northern Vietnam. Major results revealed that 5 and 41% (dimethoate), and 1 and 17% (fenitrothion) of the applied mass of pesticides were lost from the paddy field to the adjacent fish pond during spring and summer crop seasons, respectively. The decrease of pesticide concentration in paddy surface water was very rapid with dissipation half-life values of 0.3 to 0.8 and 0.2 d for dimethoate and fenitrothion, respectively. Key factors controlling the transport of pesticides were water solubility and paddy water management parameters, such as hydraulic residence time and water holding period. Risk assessment indicates that the exposure to toxic levels of pesticides for aquaculture (, ) is significant, at least shortly after pesticide application.

Degradation of volatile organic chemicals during transport has received little attention in the past. In this study we report the results of a series of experiments on toluene movement through soil columns of different length in sterilized, pre-exposed and unexposed soil. Toluene was added to 25-cm-diameter soil columns through an inlet chamber that maintained a constant concentration throughout the experiment. The toluene diffused through the soil to an outlet chamber at the top which was continuously swept with humidified air and samples were periodically analysed to determine toluene flux. The first experiment, conducted under sterilized conditions, was used to measure the soil gas diffusion coefficient, and subsequent experiments in which biodegradation occurred were used to estimate the degradation rate coefficient by fitting the outflow to a mathematical model. The degradation rate was very rapid under both pre-exposed and unexposed soil conditions, corresponding to a half-life of ∼ 2 h when bacterial activity reached high levels. Prior to this stage, the volatilization flux was very erratic, implying that growth rates of the bacteria were out of phase with the transport process. Overall, the degradation process was not well described by a first-order model until the population stabilized. Pre-exposure of the soil to the substrate prior to the transport experiment greatly increased the rate of removal of toluene during transport. Under such conditions, a 30-cm cover could virtually stop volatization losses of the compound when the inlet concentration was well below saturation, and could decrease it by a factor of ⩾30 even when the inlet concentration was saturated.

Nonequilibrium sorption plays an active role in the transport of organic contaminants in soil. We applied a two-stage, one-rate model (2S1R) and a new, nonlinear variant (2S1RN) of this model to examine the effects of wastewater irrigation on the sorption kinetics of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) in soil. The models were applied to previously published sorption-desorption data sets, which showed pronounced deviations between sorption curves and desorption curves (sorption-desorption hysteresis). Moreover, the slopes of the desorption curves decreased with decreasing concentration. Different treatments had been used, and two experimental time steps (2 and 14 d) were used. Treatments considered were lipid removal, fulvic and humic acid removal, and untreated soil. The 2S1R model was unable to reproduce the observed type of hysteresis, but the 2S1RN model, which assumes that the sorption-desorption process follows a power function relationship, was able to reproduce the observed type of hysteresis. Visually, applying the new model improved the model fits in all test cases. Statistically, as tested by an extra sum of squares analysis, the new model performed significantly better in 50% of all test cases. According to an example simulation, the choice of the sorption model has a considerable impact on the prediction of atrazine transport in soil.

Antibiotics, such as sulfadiazine (SDZ), may enter arable soil by spreading of manure of medicated husbandry or directly by the excrement of grazing animals. Knowledge of the fate of antibiotics in soils is crucial for assessing the environmental risk of these compounds, including possible transport to ground water. Kinetic sorption of (14)C-labeled SDZ (4-amino-N-pyrimidin-2-yl-benzenesulfonamide) was investigated using the batch technique. The batch sorption-desorption experiments were conducted at various concentration levels (0.044-13 mg L(-1) initial solute concentration) and time scales (0.75-272 d). Sorption of (14)C-SDZ in the investigated silty loam was time dependent and strongly nonlinear in the solution phase concentration. The time to reach an apparent sorption equilibrium was about 20 d. However, desorption was very slow, and 41 d were insufficient to reach the desorption equilibrium. An inverse modeling technique was used to identify relevant sorption processes of (14)C-SDZ during the batch experiments. Among the investigated two- and three-domain sorption models, adsorption and desorption of (14)C-SDZ were best described with a new model defining two sorption domains and four parameters. Whereas sorption in the first sorption domain was nonlinear and instantaneous, solute uptake in the second sorption domain was rate limited following first-order kinetics. Desorption followed the same rate law until an equilibrium distribution was reached. After that, desorption was assumed to be impossible due to partly irreversible sorption. Although the proposed model needs further validation, it contributes to the discussion on complex sorption processes of organic chemicals in soils.

Plant uptake is one of the major pathways by which cadmium (Cd) in soils enters the human food chain. This study was conducted to investigate the uptake of Cd by crops from soils within the wastewater irrigation area (WIA) of Braunschweig (Germany) and to develop a simple process-oriented model that is suited to predict Cd uptake at the regional scale. The sandy soils within the WIA (4300 ha) have received considerable loads of heavy metals by irrigation using municipal wastewater for up to 40 years. In 1998 and 1999, we sampled soil and plant material at 40 potato (Solanum tuberosum L.), 40 sugar beet (Beta vulgaris L.), and 32 winter wheat (Triticum aestivum L.) fields. In both years and for all three crops, we found close linear relationships between the Cd content of plant material and the Cd concentration in soil solution. For all three crops, we observed a trend of relatively increased Cd uptake in the year with the higher saturation deficit of the atmosphere. We interpret this to indicate that transpiration plays an important role in the Cd uptake of crops under the conditions of the WIA. In modeling the uptake of Cd by crops, we assume that uptake is proportional to mass flow, that is, the product of water transpired, Cd concentration in soil solution, and a plant-specific empirical parameter. The simulations agreed well with the observed Cd contents in crops. Our model explained between 66 and 87% of the observed variance.