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

Abstract

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.

Effects of Soil Compaction and Organic Carbon Content on Preferential Flow in Loamy Field Soils

“…Motivated by the Danish Pesticide Leaching Assessment Program (PLAP), the main focus of this study was to investigate the variation of preferential flow across two agricultural fields, one in Faardrup with experimental data collected in the course of this project and the second in Silstrup with measurements previously presented in Norgaard et al (2012). Within this context, the specific objectives were (i) to measure basic and functional parameters such as organic carbon (OC), texture, BD, volumetric content of pores more than 150 μm (ε−20), and saturated hydraulic conductivity (KSAT) and (ii) to derive structural characteristics from tracer BTC (5% tritium mass arrival time, mass recovery, and apparent dispersivity) to generate maps for the two field sites depicting the spatial distribution of preferential flow and transport parameters.…”

“…Down to a depth of 1.5 m, a significant number of vertical clay (CL)-coated fractures (desiccation cracks) and numerous wormholes and root channels that decreased in number with depth were present (Lindhardt et al, 2001). (Norgaard et al, 2012). The topsoil is composed of noncalcareous material and is heavily fractured and bioturbated by vertical desiccation fractures and biopores, with numerous burrows and roots.…”

“…Bulk soil samples were collected in close vicinity to the grid points at the same depth and used for determination of soil texture and OC. The same sampling strategy was applied at the Silstrup site where 65 locations were sampled (Norgaard et al, 2012).…”

“…Porosity was estimated from measured BD and an assumed particle density of 2.65 g cm . Organic carbon content was measured on setup for the leaching experiments has been previously described in Norgaard et al (2012). After attainment of steady-state flow conditions, tritium tracer was applied as a pulse input at 10 mm h −1 , replacing rainwater for 10 min.…”

“…The solid line represents a CL-to-OC ratio of n = 10. In general, the content of fines tional properties at Silstrup (Norgaard et al, 2012), Fig. 3 shows measured BTC for Faardrup grouped for different BD ranges in ascending order.…”