Department of Soil Science, University of Manitoba, Winnipeg, Manitoba, Canada.
J Environ Sci Health B. 2010 Apr;45(3):204-13. doi: 10.1080/03601231003613542.
Variations in the characteristics of soil organic matter (SOM) at the field-scale are largely unknown, particularly in relation to observed variations in herbicide sorption. For the herbicide 2,4-D [2,4-dichlorophenoxyacetic acid], we found that its organic carbon-normalized sorption coefficient, Koc, varied by four-fold, from 76 to 315 L kg(-1), in the Ap-horizon along a slope transect in an undulating agricultural field in Manitoba, Canada. In order to explain the relatively large in-field variation in 2,4-D Koc values, techniques ranging from conventional chemical fractionation methods to solid state Cross Polarization and Magic-Angle Spinning (13)C-Nuclear Magnetic Resonance applied on whole soils, were used to derive SOM chemical, physical and structural parameters for correlation analyses with the measured 2,4-D Koc values on whole soils. Out of the 15 parameters considered, the 2,4-D Koc was significantly positively correlated with 1) the carbon (C) content of sodium hydroxide-extracted humic acids (r = 0.83, P < 0.01), a chemical parameter indicative of free form C in soil; 2) the molar absorptivity of humic acids at wavelength 280 nm (r = 0.81, P < 0.01), a physical parameter indicative of greater SOM aromaticity; and 3) the relatively intensity of aryl C (r = 0.92, P < 0.01) and O-aryl C (r = 0.93, P < 0.01) in whole soil, both structural parameters indicative of aromatic C. Consequently, the results suggest that in-field variations in 2,4-D Koc values are induced by variations in SOM aromaticity. Koc values are among the most sensitive parameters in herbicide fate models used in regulatory and environmental assessments. Currently, these herbicide fate models do not consider associations between SOM characteristics and Koc and hence revising model equations to include these associations may improve estimates of herbicide persistence, bioavailability and transport at the field-scale.
田间尺度上土壤有机质（SOM）特性的变化很大程度上是未知的，特别是与观察到的除草剂吸附变化有关。对于除草剂 2,4-D[2,4-二氯苯氧基乙酸]，我们发现其有机碳归一化吸附系数 Koc 在加拿大曼尼托巴省一个起伏的农业场沿坡向的 Ap 层中变化了四倍，范围从 76 到 315 L kg(-1)。为了解释 2,4-D Koc 值田间较大的变异性，我们使用了从常规化学分级方法到全土固态交叉极化和魔角旋转（13）C-核磁共振等技术，得出了 SOM 化学、物理和结构参数，以便与全土实测的 2,4-D Koc 值进行相关分析。在所考虑的 15 个参数中，2,4-D Koc 与 1）氢氧化钠提取腐殖酸中的碳（C）含量（r = 0.83，P < 0.01）呈显著正相关，该参数是土壤中游离 C 的化学指标；2）腐殖酸在 280nm 处的摩尔吸光率（r = 0.81，P < 0.01），这是一个表示更大的 SOM 芳香度的物理参数；以及 3）全土中芳基 C 的相对强度（r = 0.92，P < 0.01）和 O-芳基 C（r = 0.93，P < 0.01），这两个结构参数都表示芳基 C。因此，结果表明，2,4-D Koc 值的田间变化是由 SOM 芳香度的变化引起的。Koc 值是用于监管和环境评估的除草剂命运模型中最敏感的参数之一。目前，这些除草剂命运模型没有考虑 SOM 特性与 Koc 之间的关联，因此修订模型方程以纳入这些关联可能会改善对除草剂在田间尺度上的持久性、生物有效性和迁移性的估计。
