Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters.

To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice ( L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Herbicide sorption coefficients in relation to soil properties and terrain attributes on a cultivated prairie.

The sorption of 2,4-D and glyphosate herbicides in soil was quantified for 287 surface soils (0-15 cm) collected in a 10 x 10 m grid across a heavily eroded, undulating, calcareous prairie landscape. Other variables that were determined included soil carbonate content, soil pH, soil organic carbon content (SOC), soil texture, soil loss or gain by tillage and water erosion, and selected terrain attributes and landform segments. The 2,4-D sorption coefficient (Kd) was significantly associated with soil carbonate content (-0.66; P < 0.001), soil pH (-0.63; P < 0.001), and SOC (0.47; P < 0.001). Upper slopes were strongly eroded and thus had a significantly greater soil carbonate content and less SOC compared with lower slopes that were in soil accumulation zones. The 2,4-D Kd was almost twice as small in upper slopes than in lower slopes. The 2,4-D Kd was also significantly associated with nine terrain attributes, particularly with compounded topographic index (0.59; P < 0.001), gradient (-0.48; P < 0.001), mean curvature (-0.43; P < 0.001), and plan curvature (-0.42 P < 0.001). Regression equations were generated to estimate herbicide sorption in soils. The predicted power of these equations increased for 2,4-D when selected terrain attributes were combined with soil properties. In contrast, the variation of glyphosate sorption across the field was much less dependent on our measured soil properties and calculated terrain attributes. We conclude that the integration of terrain attributes or landform segments in pesticide fate modeling is more advantageous for herbicides such as 2,4-D, whose sorption to soil is weak and influenced by subtle changes in soil properties, than for herbicides such as glyphosate that are strongly bound to soil regardless of soil properties.

In-field variation in 2,4-D mineralization in relation to sorption and soil microbial communities.

This study was undertaken to assess 2,4-D mineralization in an undulating cultivated field, along a sloping transect (458 m to 442 m above sea level), as a function of soil type, soil microbial communities and the sorption of 2,4-D to soil. The 2,4-D soil sorption coefficient (Kd) ranged from 1.81 to 4.28 L kg(-1), the 2,4-D first-order mineralization rate constant (k) ranged from 0.04 to 0.13 day(-1) and the total amount of 2,4-D mineralized at 130 days (M(130)) ranged from 24 to 39%. Both k and M(130) were significantly negatively associated (or correlated) with soil organic carbon content (SOC) and Kd. Both k and M(130) were significantly associated with two fatty-acid methyl esters (FAME), i17:1 and a18, but not with twenty-two other individual FAME. Imperfectly drained soils (Gleyed Dark Grey Chernozems) in lower-slopes showed significantly lesser 2,4-D mineralization relative to well-drained soils (Orthic Dark Grey Chernozems) in mid- and upper-slopes. Well-drained soils had a greater potential for 2,4-D mineralization because of greater abundance and diversity of the microbial community in these soils. However, the reduced 2,4-D mineralization in imperfectly drained soils was predominantly because of their greater SOC and increased 2,4-D sorption, limiting the bioavailability of 2,4-D for degradation. The wide range of 2,4-D sorption and mineralization in this undulating cultivated field is comparable in magnitude and extent to the variability of 2,4-D sorption and mineralization observed at a regional scale in Manitoba. As such, in-field variations in SOC and the abundance and diversity of microbial communities are determining factors that require greater attention in assessing the risk of movement of 2,4-D by runoff, eroded soil and leaching.

Effect of manure on glyphosate and trifluralin mineralization in soil.

Manure additions to soil may alter soil chemical, physical, and biological characteristics, and thereby change pesticide fate processes in soil. This is the first study to examine the impact of liquid hog manure amendments on glyphosate and trifluralin mineralization in soil. Experiments were conducted in soil microcosms in the laboratory for a total of 332 (glyphosate) and 430 (trifluralin) days. The rate and amount of mineralization of both glyphosate and trifluralin were significantly influenced by the additions of fresh manure to soil in the laboratory and by the history of manure applications in the field. However, the maximum difference in herbicide mineralization between soils that were free of manure application and those amended with manure in the field or in the laboratory was only 6.1% and 7.3% of that initially applied, for trifluralin and glyphosate, respectively. Therefore, we conclude that liquid hog manure application to soil will have no significant effect on the mineralization of glyphosate and trifluralin under field conditions.

Sorption of herbicides in relation to soil variability and landscape position.

Using the soil-water sorption partitioning coefficient (Kd), this study quantified the spatial variation of 2,4-D sorption by soil in an undulating-to-hummocky terrain landscape near Minnedosa, MB, Canada. Herbicide sorption was most strongly related to soil organic matter content and slope position, with greatest sorption occurring in lower landscape positions with greater soil organic matter content. The relation between sorption and slope position was more pronounced under conventional tillage (CT) than under long-term zero-tillage (ZT). Using multivariate regression and three independent variables (soil organic matter content, soil clay content and soil pH), the prediction of herbicide sorption by soil was very good for CT (R2 = 0.89) and adequately for ZT (R2 = 0.53).

Sorption of atrazine and metolachlor by burrow linings developed in soils with different crop residues at the surface.

Atrazine and metolachlor sorption by earthworm (Lumbricus terrestris L.) burrows was measured by introducing herbicides into the burrows and collecting the effluent between 0 to 3, 3 to 6, and 6 to 9 min of simulated burrow flow. On average, sorption by burrow linings reduced the herbicide concentration to 78% (atrazine) and 74% (metolachlor) of the applied herbicide solution concentration. For both herbicides, the amount sorbed was dependent on the food source available to the earthworm, as well as the duration of burrow flow. On average, soybean-fed- and corn-fed-earthworm-burrows significantly retained more herbicides relative to the Control Treatment (unfed-earthworms). More herbicides were transported through the burrows with time because the lateral flow movement from the burrow wall into the soil matrix decreased. It is also likely that herbicides retained on burrow linings during the first 3 min of flow saturated the adsorption sites on the burrow wall, which decreased the subsequent retention potential of herbicides in flow between 3 to 9 min. Based on these results, we conclude that herbicide transport through earthworm burrows in the field will be related to crop and crop residue management practices.

Sorption of atrazine and metolachlor by earthworm surface castings and soil.

Atrazine and metolachlor were more strongly retained on earthworm (Lumbricus terrestris L.) castings than on soil, suggesting that earthworm castings at the surface or at depth can reduce herbicide movement in soil. Herbicide sorption by castings was related to the food source available to the earthworms. Both atrazine and metolachlor sorption increased with increasing organic carbon (C) content in castings, and Freundlich constants (Kf values) generally decreased in the order: soybean-fed > corn-fed > not-fed-earthworm-castings. The amount of atrazine or metolachlor sorbed per unit organic carbon (Koc values) was significantly greater for corn-castings compared with other castings, or soil, suggesting that the composition of organic matter in castings is also an important factor in determining the retention of herbicides in soils. Herbicide desorption was dependent on both the initial herbicide concentration, and the type of absorbent. At small equilibrium herbicide concentrations, atrazine desorption was significantly greater from soil than from any of the three casting treatments. At large equilibrium herbicide concentrations, however, the greater organic C content in castings had no significant effect on atrazine desorption, relative to soil. For metolachlor, regardless of the equilibrium herbicide concentration, desorption from soybean- and corn-castings treatments was always less than desorption from soil and not-fed earthworm castings treatments. The results of this study indicate that, under field conditions, the extent of herbicide retention on earthworm castings will tend to be related to crop and crop residue management practices.