The influence of organic matter on sorption and fate of glyphosate in soil – Comparing different soils and humic substances
Introduction
Glyphosate, the active ingredient in Round Up®, is one of the most commonly used herbicides in the world. Its physical/chemical properties are very different from most other herbicides, however, e.g. having a negative log Kow-value of −3.4 (Tomlin, 1997). From this value, soil organic matter (SOM) would not be expected to sorb glyphosate, and indeed it is often stated, that SOM is not important for the sorption of glyphosate. Madhun et al. (1986) showed, however, that glyphosate could be sorbed by a purified soil humic acid (HA), and one group (Piccolo and co-workers) have published several studies showing a large potential of both pure HAs and an HA–iron complex for sorbing glyphosate (Miano et al., 1992, Piccolo and Celano, 1994, Piccolo et al., 1995, Piccolo et al., 1996). The mechanism of the sorption seemed, from studies using infrared spectroscopy, to be hydrogen bonding between the –O− and OH of the phosphono-group in glyphosate and unspecified functional groups of the HAs (Miano et al., 1992, Piccolo and Celano, 1994). The binding seemed to be stronger at pH = 4 than at pH = 6, which was ascribed to deprotonation of the phosphono-group (pK3 = 5.46, see Fig. 1) and the resulting elimination of hydrogen bonding to e.g. COO−-groups in HA. The strong indication of the importance of organic matter in the sorption of glyphosate in these studies is not easily transferred to whole soils, however. Actually, correlations between SOM content and sorption of glyphosate in soils are rarely seen (Vereecken, 2005, Borggaard and Gimsing, 2008), and it has even been suggested that SOM competes with glyphosate for sorption sites in clay minerals and Fe/Al-oxides, which are known to sorb glyphosate strongly (Gerritse et al., 1996, Day et al., 1997). Several studies have shown positive correlations between content of amorphous Fe/Al-oxides in soil and sorption of glyphosate (e.g. Piccolo et al., 1994, Gerritse et al., 1996, Vereecken, 2005), and the strong sorption by these minerals is probably a main reason for the difficulty in quantifying the importance of sorption by SOM when whole soils are investigated. Yu and Zhou (2005) showed a 46–75% decrease in glyphosate sorption by two soils when SOM was removed by ignition at 700–800 °C but failed to discuss the possible importance of the change in soil structure that such a treatment will cause. An infrared spectroscopy study by de Santana et al. (2006) concluded that bonds between glyphosate and metal constituents of soil were decreased or lacking after ignition of the soil (15 min at 650 °C).
There are thus indications that SOM in general and humic substances (HS) in particular are important for the sorption and hence the fate of glyphosate in soil and the purpose of this study is to increase the knowledge on the possible interactions between organic matter and glyphosate in soil.
Section snippets
Soils and humic substances
Six different soils were used in this study, one A- and one B-horizon from a clay soil in Sjælland, DK (SlA and SlB), a dark clayey wet zone soil (WZ) from Sjælland, DK and an A-, B-, and C-horizon from a sandy soil in the western part of Jylland, DK (FlA, FlB, FlC). SOM was determined as loss on ignition at 550 °C for 2 h. Textural characteristics were determined by standard sieving. pH was determined in a 2:1 water/soil-slurry. The humic and fulvic acids were purified as described elsewhere (
Sorption of glyphosate to humic substances
The HS-fractions used in this study have been previously described (Albers et al., 2008a), but the main parameters are summarized in Table 1.
Despite the great hydrophilicity of glyphosate (log Kow ≈ −3.4) the purified HS-fractions all showed a potential for sorption of glyphosate (Table 2). The KF-values were a little larger for the HA-fractions than for the FA-fractions, and for both HS-fractions, KF increased with a factor of 2–5 when pH was raised from 2–3 to ∼7.
The HS-fractions from the WZ
Conclusions
We have shown that glyphosate can be sorbed to purified HS-fractions and that a possible binding mechanism could be hydrogen bonds to phenolic groups of HS. The sorption experiments with whole soils indicate that soil organic matter, including HS, might be responsible for part of the sorption, especially in sandy soils. This was further substantiated by the fate experiment which showed that after 80 days, ∼40% of the added glyphosate was associated with HA or FA in the sandy A- and B-horizons.
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