Anders Lindblad Vendelboe

Comparative Mapping of Soil Physical–Chemical and Structural Parameters at Field Scale to Identify Zones of Enhanced Leaching Risk

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.

Colloid and Phosphorus Leaching From Undisturbed Soil Cores Sampled Along a Natural Clay Gradient

The presence of strongly sorbing compounds in groundwater and tile drains can be a result of colloid-facilitated transport. Colloid and phosphorus leaching from macropores in undisturbed soil cores sampled across a natural clay gradient at Aarup, Denmark, were studied. The aim of the study was to correlate easily measurable soil properties, such as clay content and water-dispersible colloids, to colloid and phosphorus leaching. The clay contents across the gradient ranged from 0.11 to 0.23 kg kg−1. Irrigating with artificial rainwater, all samples showed a high first flush of colloids and phosphorus followed by lower and stable colloid and phosphorus concentrations. The mass of particles leached at first flush was independent of clay content and was attributed to the instant release of particles associated with the macropore walls and released upon contact with flowing water. Below a clay content of ∼0.15 kg kg−1, the later leaching (after the first flush) of particles was independent of the clay content. Above this threshold, there was a positive relationship between the mass of leached particles after the first flush and the clay content. Particle release after the first flush was linearly correlated to the accumulated outflow and was described as a diffusion controlled process, using √(accumulated outflow). The mass of leached particles was positively correlated to the clay content as well as to water-dispersible colloids. Particulate phosphorus (P) was linearly correlated to concentration of leached particles and accounted for ∼70% of the total mass of leached P. Approximately 50% of particulate P was associated with the first flush. The P concentration on leached particles was negatively correlated to clay content (R2 = 0.89) and followed the same trend as the P concentration on soil clay and the so-called degree of P saturation (oxalate-extractable P on iron and aluminum minerals). Because higher colloidal P concentration was countered by a lower colloidal leaching, the total amount of leached P stayed remarkably constant along the natural clay gradient.

Colloid Release From Differently Managed Loess Soil

Abstract The content of water-dispersible colloids (WDC) in a soil can have a major impact on soil functions, such as permeability to water and air, and on soil strength, which can impair soil fertility and workability. In addition, the content of WDC in the soil may increase the risk of nutrient loss and of colloid-facilitated transport of strongly sorbing compounds. In the present study, soils from the Bad Lauchstädt long-term static fertilizer experiment with different management histories were investigated to relate basic soil properties to the content of WDC, the content of water-stable aggregates (WSA), and aggregate tensile strength. Our studies were carried out on soils on identical parent material under controlled management conditions, enabling us to study the long-term effects on soil physical properties with few explanatory variables in play. The content of WDC and the amount of WSA were measured at a series of time steps giving a colloid release and aggregate disaggregation rate and a quantification of the content of WDC and WSA at a given time for each of the six investigated experimental field plots. The content of WDC in the moist soil was linearly correlated (r = 0.82* [P < 0.05]) to the part of the total clay not associated with organic matter. No significant difference in release rate was found for air-dry aggregates. The low-carbon soils initially had a higher content of WSA but were more susceptible to disaggregation than the high-carbon soils. Furthermore, the application of NPK fertilizer had a destabilizing effect on the WSA and also caused a decrease in the cation exchange capacity of the soils. The mean tensile strength was positively correlated to the colloid release rate and the content of WDC after 2 min of shaking and therefore to the amount of clay not associated with organic carbon.

Effects of soil compaction and organic carbon content on preferential flow in loamy field soils

Abstract Preferential flow and transport through macropores affect plant water use efficiency and enhance leaching of agrochemicals and the transport of colloids, thereby increasing the risk for contamination of groundwater resources. The effects of soil compaction, expressed in terms of bulk density (BD), and organic carbon (OC) content on preferential flow and transport were investigated using 150 undisturbed soil cores sampled from 15 × 15–m grids on two field sites. Both fields had loamy textures, but one site had significantly higher OC content. Leaching experiments were conducted in each core by applying a constant irrigation rate of 10 mm h−1 with a pulse application of tritium tracer. Five percent tritium mass arrival times and apparent dispersivities were derived from each of the tracer breakthrough curves and correlated with texture, OC content, and BD to assess the spatial distribution of preferential flow and transport across the investigated fields. Soils from both fields showed strong positive correlations between BD and preferential flow. Interestingly, the relationships between BD and tracer transport characteristics were markedly different for the two fields, although the relationship between BD and macroporosity was nearly identical. The difference was likely caused by the higher contents of fines and OC at one of the fields leading to stronger aggregation, smaller matrix permeability, and a more pronounced pipe-like pore system with well-aligned macropores.

Long-Term Effect of Different Carbon Management Strategies on Water Flow and Related Processes for Three Loamy Soils

Abstract The decline in organic matter of arable land, induced and accelerated by modern agriculture, has been identified as a threat to sustained soil quality. In this article, we studied strategies to counter this decrease by building up soil organic carbon (SOC) levels in the soils using several approaches. They included return of organic matter, such as straw, animal manure, and slurry, to the soil and diverse crop rotations with, for example, grass-clover leys. We used three field sites in Denmark with different C repletion strategies to assess and quantify the effect of these approaches on preferential flow and loss of colloids during heavy irrigation events. The field sites were all under long-term management and therefore represent up to 30 years of pairwise different management strategies. One field in each field pair was managed with a more C-repleting strategy (HighC) than the other (LowC). Only small differences in SOC contents were identified, and none of the management strategies had succeeded in building up SOC pools large enough to saturate the soil with C. Only at one field site was the content of water-dispersible colloids lower in the HighC than the LowC treatment. Preferential flow patterns showed a rapid breakthrough of irrigation water but only little or negligible effect in reducing the risk of colloid loss and chemicals leaching from the root zone, despite 20 to 30 years of different management strategies with this aim. Previous studies reported in the literature have emphasized high SOC contents and grass ley soils to significantly reduce nonequilibrium water flow. Our results thus indicate the need for more effective management options than those addressed in this study.

Continuous ‘Passive’ flow-proportional monitoring of drainage using a new modified Sutro weir (MSW) unit

In view of their crucial role in water and solute transport, enhanced monitoring of agricultural subsurface drain tile systems is important for adequate water quality management. However, existing monitoring techniques for flow and contaminant loads from tile drains are expensive and labour intensive. The aim of this study was to develop a cost-effective and simple method for monitoring loads from tile drains. The Flowcap is a modified Sutro weir (MSW) unit that can be attached to the outlet of tile drains. It is capable of registering total flow, contaminant loads and flow-averaged concentrations. The MSW builds on a modern passive sampling technique that responds to hydraulic pressure and measures average concentrations over time (days to months) for various substances. Mounting the samplers in the MSW allowed a flow-proportional part of the drainage to be sampled. Laboratory testing yielded high linear correlation between the accumulated sampler flow, qtotal, and accumulated drainage flow, Qtotal (r2 > 0.96). The slope of these correlations was used to calculate the total drainage discharge from the sampled volume, and therefore contaminant load. A calibration of the MSW under controlled laboratory condition was needed before interpretation of the monitoring results was possible. The MSW does not require a shed, electricity, or maintenance. This enables large-scale monitoring of contaminant loads via tile drains, which can improve contaminant transport models and yield valuable information for the selection and evaluation of mitigation options to improve water quality. Results from this type of monitoring can provide data for the evaluation and optimisation of best management practices in agriculture in order to produce the highest yield without water quality and recipient surface waters being compromised.