3.2. Arable Soils under Study

The representative 2 topsoil samples (0–30 cm) from farmlands in Ligota Piękna (near Wrocław, Poland) were randomly collected for the study (at 12 points within each location). They were selected due to the agricultural land use and texture, as sandy loam (L soil material) represents 13.3% and loam (C soil) represents 16.8% of the topsoil in south-west Poland [84]. They are the 2 most abundant groups among light- and heavy-textured soils of the region. According to the WRB classification [85], the soils were mostly Phaeozems and Umbrisols. Prior to the analyses, soil materials were air-dried, crushed gently and sieved through a sieve with a mesh size of 2 mm. The fraction of the soils <2 mm was collected and used in the study. Table 7 shows a summary of the physicochemical properties of the investigated soil materials and was derived from a previous study [10].

Basic physicochemical properties of tested arable L and C soils ¹.

¹ Previously published by Ćwieląg-Piasecka et al. [10].

In the fraction <2 µm of both soils, montmorillonite was the major clay mineral, accompanied by kaolinite and small amounts of highly dispersed quartz. In addition, both soils contained low level of carbonates at 0.6% and 1.1% in L and C soil, respectively. Ca²⁺ was the main exchangeable cation in both tested soils.

In order to correlate the soils affinity for the studied pesticides to a particular soil organic matter fraction, the extraction and chemical fractionation of the humic substances from investigated L and C soils was conducted. It was undertaken according to the method recommended by the International Humic Substances Society (IHSS), described in Swift [86], with some modification [87,88,89,90]. The first step in the procedure was sample decalcitation with H₂SO₄ [91] in order to obtain a low-molecular-weight fulvic fraction (CAC). In the next step, soil was exhaustively extracted with 0.1M NaOH until the supernatant was completely discolored. As a result, a fraction called total extractable carbon (TEC) was obtained. It consisted of humic acids (HA) and fulvic acids (FA). In the subsequent step, the alkaline extract was acidified to pH 2 to precipitate the humic acid fraction (CHA), which was then centrifuged from the CFA fraction remaining in the solution.

The carbon of humic acid (CHA) content was calculated from the difference between total extractable carbon (TEC) measured in alkali (NaOH) extract and carbon of fulvic acid (CFA) fraction after HA precipitation at a pH of less than 2. Carbon of nonhydrolyzable humic substances (also called residual carbon or humin fraction, CR) was calculated from the difference between total organic carbon (TOC) and sum of humic and fulvic acid fractions (CR = TOC − (CHA + CFA)).

Total organic carbon and concentrations of organic carbon in the subsequent fractions were measured after each extraction step on an Enviro TOC/TN analyzer (Elementar, Langenselbold, Germany).

To set a series of soil L and C at various pHs in the range of 3–7, the following procedure was implemented. An aliquot (200 g) of each soil (L or C) was placed in a beaker, soaked with 50 mL of deionized water and stirred for 12 h. Small volumes of 1 M HCl solution were added dropwise to a soil slurry and stirred to obtain the desired final pH values. The samples were equilibrated for another 12h followed by the pH measurement. The procedure was repeated until each soil series contained samples of pH adjusted to approximately: 3, 4, 5 and 7. Obtained soils were air-dried, ground and subjected to the final pH test. Each soil sample (n = 3) was equilibrated with 10 mM CaCl₂ solution in a ratio of 1:2 (used in the batch studies) to ascertain that the experimental pH values would be maintained.

The pH range was set to cover the values typical for undisturbed soils, excluding values higher than 7. This was due to the fact that investigated carbamates undergo hydrolysis in alkaline conditions [38,42] and, furthermore, in pH values exceeding 7, humic acid fraction occurs in its dissolved form. Therefore, to obtain reliable results concerning carbamate concentration in a solution after sorption and to avoid the loss of potential sorbent from the solid phase, the upper limit for the pH range investigated was set to 7.