Miljøprojekt, 902 – The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil – The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

9 Soil treatment as a factor influencing the relation between binding and degradation

The soil treatment influences many of the chemical, physical, and biological factors in the soil and is therefore of great importance to how quickly and how the pesticides are transported through the plough layer. The binding and the degradation of the pesticides and their metabolites are also dependent on whether the soil has been treated conventionally or not as the amount of organic matter and the microbial activity have a great influence on the binding and degradation of pesticides, which in their turn will influence whether the substances are leached.

In the literature there are conflicting reports as regards degradation and leaching of pesticides in soil that had been subjected to conventional tillage (CT) and different degrees of reduced tillage (RT) or no tilllage (NT). Some publications proved that there was no difference between the leaching of pesticides when CT was compared with NT. Other publications proved that the leaching was greater from NT soil than from CT soil explaining the difference with the occurrence of a large number of macropores in the NT soil. Other publications showed that the leaching was less from NT soil than from CT soil explaining the difference with a larger binding and a more rapid degradation rate in the NT soil, which often has a higher content of organic matter and a higher biological activity than the CT soil. In Denmark the NT technique is used very infrequently. Some of the parameters that are different in soil that has been tilled differently are the amount of organic matter and the biological activity in the upper soil layers. Helling (1987) summed up the results of measurements of the amount of organic matter in the upper soil layers and found that the amount of organic matter in NT soil was between 1.5 and 2 times the amount in CT soil. Helling (1987) also quoted studies where the total number of aerobic microorganisms, the facultatively anaerobic microorganisms, and the denitrificating microorganisms were respectively 1.35, 1.57, and 7.31 times higher in NT soil than in the CT soil.

In many of the known studies where the leaching or run-off of pesticides has been compared for different tillage systems, CT has been compared with NT (Fermanich et al., 1996; Isensee et al., 1990; Levanon et al., 1993; Fermanich & Daniel, 1991; Ritter et al., 1996; Sadeghi & Isensee, 1992; Isensee & Sadeghi, 1993; Afyuni et al., 1997). In most of these cases (Fermanich et al., 1996; Isensee et al., 1990; Fermanich & Daniel, 1991; Sadeghi & Isensee, 1992; Isensee & Sadeghi, 1993; Afyuni et al., 1997) it emerged that the leaching or the run-off was significantly greater in NT than in CT for the substances atrazine and deisopropylatrazine; atrazine, deethylatrazine, alachlor, cyanazine and carbofuran; carbofuran and chlorpyrifos; atrazine; atrazine, cyanazine and alachlor; chlorimuron and nicosulfuron whereas Levanon (1993) showed that the leaching was greater in CT than in NT for the substances atrazine, carbofuran, diazinon, and metolachlor. Ritter et al. (1996) showed that there was no important difference in the leaching of atrazine, simazine, cyanazine, and alachlor in CT and NT but that rain occurrences of more than 30 mm shortly after spraying was the important factor, which determined whether the substances were found in the groundwater. Comparisons between RT and CT were made by Cox et al. (1996), who studied the leaching of metamitron in disturbed columns from RT soil and CT soil and found that 9% was leached from CT soil while 5% was leached from RT soil. The difference was explained with a higher degradation rate in RT and a greater degree of preferential flow in CT.

Gish et al. (1991) showed that the leaching of pesticides was more extensive in NT soil than in CT soil because of preferential flow, but they emphasized that it mainly applied to wettable powder and liquid formulations while encapsulated formulations reduced the significance of preferential flow.

It has often been stated that earthworm channels in the upper layers of NT soil are a very important factor when a greater degree of preferential flow is seen in NT soil than in CT soil. However, it is important to note that Stehouwer et al. (1993) showed that there was about three times as large an amount of organic matter in the linings of the earthworm channels than in the matrix soil, which might increase the binding of atrazine.

Zablotowicz et al. (2000) studied the degradation of fluometuron in CT soil and NT soil and found that the rate constant for the degradation in the upper two cm in CT was twice that in NT. 48% more organic matter was found in the upper two cm of the NT soil than in the upper two cm of the CT soil, and the microbial biomass and fluorescein diacetate hydrolysis activity was 106% and 127% higher, respectively, in the upper two cm of the NT soil compared with that of the CT soil. In this case the amount of organic matter has apparently increased the binding of the pesticide so much that it – despite the higher biological activity – is degraded more slowly in the NT soil.

Reese et al. (1999) found no significant difference in the degradation rates of isoproturon and diflufenican when comparing CT and RT. Giupponi et al. (1996), on the other hand, found that soil tillage influences the degradation rate of isoproturon. In a prolonged study in Italy between 1993 and 1995, the results showed that the K_d-values were higher in RT soil than in CT soil. The degradation was faster in a ploughless ridge-cultivated soil than in both CT and NT soils. Modelling in VARLEACH, where the measured values for degradation rates and sorption were applied, showed that ploughless ridge cultivation was the form of tillage that had the least influence on the quality of the groundwater as regards leaching of pesticides.

Fomsgaard and Kristensen (2002) carried out a number of studies of the degradation of isoproturon in three types of soil that had been tilled differently: a) newly ploughed: Soil from a field that for 20 years previous to the study had only been harrowed but was ploughed shortly before samples were taken (RT-CT), b) harrowed: Soil that for 20 years had not been ploughed but only harrowed (RT), and c) ploughed: Soil that had been ploughed for a number of years before sampling (CT). The studies were carried out in different concentrations (0.1 µg/g and 0.01 µg/g) in both plough layers and deeper soil layers. ¹⁴C-labelled pesticide was used so that the total mineralisation to ¹⁴CO₂ could be followed. At the time samples were continuously interrupted during the incubation of some of the experiments so that the remaining amount of isoproturon and formed metabolites could be followed over time. Some of the samples were analysed by two extraction methods, an aqueous and an organic one, by means of which it was possible to a certain extent to distinguish between desorbable pesticide and non-desorbable pesticide.

The mineralisation rate for isoproturon in plough layers at the concentration of 0.1 µg/g was highest in RT soil. The biological activity, measured as the mineralisation rate of Na-acetate, was also highest in this soil. Figure 23 shows the results of the measurements of the mineralisation where the rise in the first part of the curve is the measure of the rate that is calculated by the mathematical modelling. The mineralisation rate for isoproturon was higher in soil that normally had been ploughed (CT) than in soil that had been ploughed for the first time in many years (RT-CT). A similar difference between the mineralisation rates in the different types of soil was seen in the concentration of 0.01 µg/g. The recent ploughing of the soil that formerly had only been harrowed has also reduced the mineralisation rate considerably. The microcosmos in which the bacteria live has been decisive for their activity. And the activity has been so high that it has been able to cause a more rapid mineralisation even though the binding, measured by K_d-values, was not significantly different in the harrowed and the newly ploughed soil (Fomsgaard & Kristensen, 2002).

The influence of the amount of organic matter on sorption and degradation, respectively, has been discussed in many publications. Generally, it will be expected that a larger amount of organic matter causes an increased binding which then is expected to reduce the degradation rate because the substance is less available to the microorganisms in the aqueous phase. However, the opposite – that a greater amount of organic matter is linked to a greater biological activity – is often seen to be the case, which then increases the degradation rate for the pesticides, as seen here in Fomsgaard & Kristensen (2002) where the binding was not influenced when the K_d-values for newly ploughed and harrowed soil are compared. A more detailed knowledge of the type of organic matter would in this case have been interesting. What the connection between the mineralisation presented in Figure 23 and the disappearance of the parent substance (which for one series of experiments is shown in Figure 17) is like has not been fully analysed yet. Only with these results can it be determined whether the lower mineralisation rates result in residues of extractable substance in the soil.

Figure 23. Mineralisation rates for isoproturon at 15°C and 5°C and mineralisation rates for Na-acetate. The trials/experiments have been carried out in CT soil (soil that has been ploughed normally for a number of years), RT (soil that has only been harrowed for 20-30 years), and RT-CT (soil that has only been harrowed in the previous 20-30 years but was ploughed for the first time shortly before sampling) (Fomsgaard & Kristensen, 2002).
Figur 23. Mineraliseringshastigheder for isoproturon ved 15°C og 5°C samt mineraliserings-hastigheder for Na-acetat. Forsøgene er udført i TJ (jord der har været pløjet normalt i en årrække), RJ (jord, der kun har været harvet i 20-30 år) og RJ-TJ (jord, der kun har været harvet i de foregående 20-30 år, men som kort før prøveudtagningen blev pløjet for første gang) (Fomsgaard & Kirstensen, 2002).