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Conventional wet chemical methods for the determination of soil phosphorus (P) pools, relevant for environmental and agronomic purposes, are labor-intensive. Therefore, alternative techniques are needed, and a combination of the spectroscopic techniques-in this case, laser-induced breakdown spectroscopy (LIBS)-and visible near-infrared spectroscopy (vis-NIRS) could be relevant. We aimed at exploring LIBS, vis-NIRS and their combination for soil P estimation. We analyzed 147 Danish agricultural soils with LIBS and vis-NIRS. As reference measurements, we analyzed water-extractable P (Pwater), Olsen P (Polsen), oxalate-extractable P (Pox) and total P (TP) by conventional wet chemical protocols, as proxies for respectively leachable, plant-available, adsorbed inorganic P, and TP in soil. Partial least squares regression (PLSR) models combined with interval partial least squares (iPLS) and competitive adaptive reweighted sampling (CARS) variable selection methods were tested, and the relevant wavelengths for soil P determination were identified. LIBS exhibited better results compared to vis-NIRS for all P models, except for Pwater, for which results were comparable. Model performance for both the LIBS and vis-NIRS techniques as well as the combined LIBS-vis-NIR approach was significantly improved when variable selection was applied. CARS performed better than iPLS in almost all cases. Combined LIBS and vis-NIRS models with variable selection showed the best results for all four P pools, except for Pox where the results were comparable to using the LIBS model with CARS. Merging LIBS and vis-NIRS with variable selection showed potential for improving soil P determinations, but larger and independent validation datasets should be tested in future studies.

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The mobilization and transport of colloid particles in soils can have negative agronomic and environmental effects. This work investigates the controls of particle release and transport from undisturbed soil columns sampled from an agricultural, loamy field with clay and silt contents of 0.05 to 0.14 and 0.07 to 0.16 kg kg, respectively. Forty-five soil columns (20 × 20 cm) were collected from the field and exposed to a constant irrigation of 10 mm h for 8 h. The accumulated mass of particles in the outflow from each column was highly correlated ( = 0.88) with the volumetric mass of fines (MF). The MF is defined as the sum of clay and fine silt (<20 µm) multiplied by the soil bulk density and divided by the particle density of the mineral fines. Thereby, MF represents both the particle source available for mobilization and leaching and an indicator of soil structure. The particle release process showed two linear particle release rates. Although the two particle release rates were distinctly different, both were strongly correlated with MF. The difference between the two rates was related to the degree of preferential flow characterized by the 5% arrival time of an applied tracer pulse. Soil columns with a longer 5% arrival time (less preferential flow) showed a distinct difference between the two rates, whereas soil columns with a short 5% arrival time and fast water transport showed resemblance between the two particle release rates. Thus, the combined effects of particle source, type, and pathways (via soil structure and compaction) need consideration to understand and predict particle transport dynamics through intact topsoil.

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The intensification of agricultural production to meet the growing demand for agricultural commodities is increasing the use of chemicals. The ability of soils to transport dissolved chemicals depends on both the soil's texture and structure. Assessment of the transport of dissolved chemicals (solutes) through soils is performed using breakthrough curves (BTCs) where the application of a solute at one site and its appearance over time at another are recorded. Obtaining BTCs from laboratory studies is extremely expensive and time- and labour-consuming. Visible-near-infrared (vis-NIR) spectroscopy is well recognized for its measurement speed and for its low data acquisition cost and can be used for quantitative estimation of basic soil properties such as clay and organic matter. In this study, for the first time ever, vis-NIR spectroscopy was used to predict dissolved chemical breakthrough curves obtained from tritium transport experiments on a large variety of intact soil columns. Averaged across the field, BTCs were estimated with a high degree of accuracy. So, with vis-NIR spectroscopy, the mass transport of dissolved chemicals can be measured, paving the way for next-generation measurements and monitoring of dissolved chemical transport by spectroscopy.

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The mobility of water-dispersible colloids (WDCs) in soil may be influenced by soil management practices such as organic soil amendments. Biochar has recently been promoted as a useful soil amendment, and extensive research has been devoted to investigating its effects on soil macroscopic properties and functions. However, there is limited understanding of the effects of biochar application on micro-scale particle dynamics. We conducted a field study to investigate the effects of the application of birch ( spp.) wood biochar on colloid dispersibility with respect to application rate, history, and physicochemical soil properties. Undisturbed soil cores (100 cm) were collected from the topsoil of two agricultural sites in Denmark with soils of sandy loam texture. The two sites received biochar at different application rates (0-100 Mg ha) and were sampled 7 to 19 mo later. The WDC content was determined using an end-over-end shaking method on 100-cm intact soil cores, and the colloid solution was analyzed for electrical conductivity, pH, and zeta potential. The WDC content increased with biochar application rate because of biochar-induced changes in soil chemistry and was strongly and positively correlated with the concentration of exchangeable monovalent cations in the soils. Biochar application increased pH and decreased electrical conductivity and zeta potential in the colloid suspension more in the short term (7 mo) than in the long term (19 mo). Thus, there is potential for biochar to induce short-term changes in soil solution chemistry in agricultural soils, which may influence the mobility of soil colloids.

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Solute transport through the soil matrix is non-uniform and greatly affected by soil texture, soil structure, and macropore networks. Attempts have been made in previous studies to use infiltration experiments to identify the degree of preferential flow, but these attempts have often been based on small datasets or data collected from literature with differing initial and boundary conditions. This study examined the relationship between tracer breakthrough characteristics, soil hydraulic properties, and basic soil properties. From six agricultural fields in Denmark, 193 intact surface soil columns 20cm in height and 20cm in diameter were collected. The soils exhibited a wide range in texture, with clay and organic carbon (OC) contents ranging from 0.03 to 0.41 and 0.01 to 0.08kgkg(-1), respectively. All experiments were carried out under the same initial and boundary conditions using tritium as a conservative tracer. The breakthrough characteristics ranged from being near normally distributed to gradually skewed to the right along with an increase in the content of the mineral fines (particles ≤50µm). The results showed that the mineral fines content was strongly correlated to functional soil structure and the derived tracer breakthrough curves (BTCs), whereas the OC content appeared less important for the shape of the BTC. Organic carbon was believed to support the stability of the soil structure rather than the actual formation of macropores causing preferential flow. The arrival times of 5% and up to 50% of the tracer mass were found to be strongly correlated with volumetric fines content. Predicted tracer concentration breakthrough points as a function of time up to 50% of applied tracer mass could be well fitted to an analytical solution to the classical advection-dispersion equation. Both cumulative tracer mass and concentration as a function of time were well predicted from the simple inputs of bulk density, clay and silt contents, and applied tracer mass. The new concept seems promising as a platform towards more accurate proxy functions for dissolved contaminant transport in intact soil.

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In intensely cultivated regions, it is crucial to have knowledge of the leaching potential related to pesticides in agricultural production. This is especially true in countries, like Denmark, that base its drinking water supply on untreated groundwater. Since fluazifop-P-butyl (FPB) is applied to control perennial and annual weed grasses in agricultural fields, the objective of this study was to evaluate leaching of its two degradation products - fluazifop-P (FP; free acid; (R)-2-(4-(5-trifluoromethyl-2-pyridyloxy)phenoxy)propionic acid) and TFMP (5-(trifluoromethyl)-2(1H)-pyridinone) - through an agricultural field consisting of loamy soil. Drainage and groundwater samples were collected over a five-year period following four spring/summer applications of FPB, and analysed for both FP and TFMP. FP was only detected once in groundwater, whereas TFMP within the first year after the first and fourth application was detected in concentrations exceeding the value of 0.1µgL(-1) in 100% and 24% of the drainage samples and 9% and 14% of the groundwater samples, respectively. Detections of TFMP up to 18months after application were obtained both in the drainage and groundwater. What differentiated the first and fourth FPB applications from the two others were heavy precipitation events within one week of FPB application, which resulted in rapid transport of TFMP through the discontinuities in the soil and contributed to relatively high TFMP detections in drainage and groundwater. This study indicated that pesticide degradates like TFMP, often being more soluble than the pesticide, have a relatively high leaching potential especially associated with heavy precipitation events shortly after the application. Hence, such pesticide degradates should like in Denmark be considered "relevant" meaning that the EU value for drinking water applies to them, having its leaching potential regulatory assessed based on high quality estimations of their persistence, and be exposed to an assessment of the risk to consumers of drinking contaminated groundwater.

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Application of biochar to agricultural fields to improve soil quality has increased in popularity in recent years, but limited attention is generally paid to existing field conditions before biochar application. This study examined the short-term physicochemical effects of biochar amendment in an agricultural field in Denmark with a calcium carbonate (CaCO) gradient. The field comprised four reference plots and four plots to which biochar (birch wood pyrolyzed at 500°C) was applied at a rate of 20 t ha. Five undisturbed soil columns (10 cm diam., 8 cm height) were sampled from each plot 7 mo after biochar application, and a series of leaching experiments was conducted. The leachate was analyzed for tritium (used as a tracer), colloids, and phosphorus concentration. The results revealed that the presence of CaCO has resulted in marked changes in soil structure (bulk density) and soil chemical properties (e.g., pH and ionic strength), which significantly affected air and water transport and colloid and phosphorous leaching. In denser soils (bulk density, 1.57-1.69 g cm) preferential flow dominated the transport and caused an enhanced movement of air and water, whereas in less dense soils (bulk density, 1.38-1.52 g cm) matrix flow predominated the transport. Compared with reference soils, biochar-amended soils showed slightly lower air permeability and a shorter travel time for 5% of the applied tracer (tritium) to leach through the soil columns. Colloid and phosphorus leaching was observed to be time dependent in soils with low CaCO. Biochar-amended soils showed higher colloid and P release than reference soils. Field-scale variations in total colloid and P leaching reflected clear effects of changes in pH and ionic strength due to the presence of CaCO. There was a linear relationship between colloid and P concentrations in the leachate, suggesting that colloid-facilitated P leaching was the dominant P transport mechanism.

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Preferential flow and particle-facilitated transport through macropores contributes significantly to the transport of strongly sorbing substances such as pesticides and phosphorus. The aim of this study was to perform a field-scale characterization of basic soil physical properties like clay and organic carbon content and investigate whether it was possible to relate these to derived structural parameters such as bulk density and conservative tracer parameters and to actual particle and phosphorus leaching patterns obtained from laboratory leaching experiments. Sixty-five cylindrical soil columns of 20-cm height and 20-cm diameter and bulk soil were sampled from the topsoil in a 15-m × 15-m grid in an agricultural loamy field. Highest clay contents and highest bulk densities were found in the northern part of the field. Leaching experiments with a conservative tracer showed fast 5% tracer arrival times and high tracer recovery percentages from columns sampled from the northern part of the field, and the leached mass of particles and particulate phosphorus was also largest from this area. Strong correlations were obtained between 5% tracer arrival time, tracer recovery, and bulk density, indicating that a few well-aligned and better connected macropores might change the hydraulic conductivity between the macropores and the soil matrix, triggering an onset of preferential flow at lower rain intensities compared with less compacted soil. Overall, a comparison mapping of basic and structural characteristics including soil texture, bulk density, dissolved tracer, particle and phosphorus transport parameters identified the northern one-third of the field as a zone with higher leaching risk. This risk assessment based on parameter mapping from measurements on intact samples was in good agreement with 9 yr of pesticide detections in two horizontal wells and with particle and phosphorus leaching patterns from a distributed, shallow drainage pipe system across the field.

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Copper contamination affects biological, chemical, and physical soil properties and associated ecological functions. Changes in soil pore organization as a result of Cu contamination can dramatically affect flow and contaminant transport in polluted soils. This study assessed the influence of soil structure on the movement of water and Cu in a long-term polluted soil. Undisturbed soil cores collected along a Cu gradient (from about 20 to about 3800 mg Cu kg soil) were scanned using X-ray computed tomography (CT). Leaching experiments were performed to analyze tracer transport, colloid leaching, and dissolved organic carbon (DOC) and Cu losses. The 5% arrival time () and apparent dispersivity (λ) for tracer breakthrough were calculated by fitting the experimental data to a nonparametric, double-lognormal probability density function. Soil bulk density, which did not follow the Cu gradient, was the main driver of preferential flow, while macroporosity determined by X-ray CT (for pores >180 µm) proved the best predictor of solute transport. Higher preferential flow due to the presence of well-aligned pores and small cracks controlled water movement in compacted soil. Transport of Cu was rapid during the first flush (≈1 pore volume) in association with the movement of colloid particles, followed by slower transport in association with the movement of DOC in the soil solution. The relative amount of Cu released was strongly correlated with macroporosity as determined by X-ray CT, indicating the promising potential of this visualization technique for predicting contaminant transport through soil.