29 Degradation of Herbicides in the Forest Floor Review of the Literature

PLEASE DO NOT REMOVE FROM Reprinted from Tree Growth and Forest Soils FILES Edited by C. T. Youngberg and C. B. Davey © OREGON STATE UNIVERSITY PRESS, 1970 29 Degradation of Herbicides in the Forest Floor Logan A. Norris HERBICIDES ARE USED extensively to accomplish forest and range management goals. However, the continued availability of such chemical tools depends on strong proof that they can be used with minimum impact on the quality of the environment. When economic chemicals are applied in the forest, only part of the material reaches the intended target (Figure 1). The forest floor is a major receptor of aerially applied chemicals (19). Herbicide adsorption, surface runoff, leaching, or volatilization from the forest floor result only in the temporary storage or redistribu- tion of chemicals. Chemical or biological degradation is the only means by which the load of environmental pollutants can be reduced. For this reason, special attention is given to herbicide degradation, particularly in the forest floor, the initial point of chemical entry into the soil. The research reported or summarized herein is part of a con- tinuing study of the behavior of chemicals in the forest environment. Goals of these investigations are to determine 1) the relative persistence of different herbicides in forest floors; 2) the kinetics of degradation; and 3) the impact of various economic chemicals on the persistence of associated herbicides. Review of the Literature Under various conditions reported in the literature, most herbicides undergo some degree of chemical or biological degradation in soil. Herbicide degradation under laboratory conditions or in agri- L. A. Norris is Research Chemist, United States Department of Agriculture Forest Service, Forestry Sciences Laboratory, Corvallis, Oregon. This paper is a contribution from USDA Forest Service and the Depart- ment of Agricultural Chemistry, Oregon State Universit y. (Technical Paper No. 2525, Oregon Agricultural Experiment Station.) 397 I • 398 TREE GROWTH AND FOREST SOILS AERIALLY APPLIED CHEMICALS DRIFT AND VOLATILIZATION VEGETATION SURFACE WATER FOREST FLOOR Figure 1. Distribution of aerially applied chemicals in the forest. cultural soils is reviewed by Audus (4), Woodford and Sagar (26), and Sheets and Harris (24). Burschel (6) considered the behavior in soil of herbicides important in forestry. A report prepared for the De- partment of Defense contains a valuable collection of references to the persistence characteristics of herbicides intended for noncropland uses (12). The early work of DeRose (9) and Dries (14) established that 2,4-D ( 2.4-dichlorophenoxyacetic acid) is degraded in the soil, prob- -LOGAN A. NORRIS 399 ably by microbial action. In field studies, 2,4-D remained active for 6 weeks and 2,4,5 T 2,4,5 trichlorophenox■aLctic acid) lasted for 19 weeks (17). However, the kinetics of degradation of 2,4-D and 2,4,5-T are similar (2, 3). Amitrole (3-amino-1,2,4-triazole) has been reported to lose phyto- toxicity in soil with time. Freed and Furtick (10) found no amitrole residues two to six months after field application to several soil types in Oregon. Amitrole degradation has a temperature optimum of 20 to 30 C, with higher rates of inactivation occurring with high moisture availability (8). Picloram (4-amino-3,5,6 trichloropicolinic acid) is of more recent origin than the phenoxy or amitrole herbicides, so the principles of its persistence are less well developed. Youngson et al. (27) found that increasing organic matter and temperature favored picloram degradation as did increasing soil moisture up to 55 percent of capacity. Hamaker, Youngson, and Goring (11) indicate that the rate of picloram degradation follows half-order kinetics. Most studies of the behavior of herbicides in soil have been con- ducted with fairly pure chemicals applied alone. However, the grow- ing intensity of forest management and the widespread use of pesticides indicate that multichemical residues will become more common. Nearly all pesticide residue monitoring programs now report finding several biologically active chemicals in many samples. Brownbridgel found that soil microbes could degrade 2,4-D faster if they were first adapted to either 2,4-D or 2,4-DP [2 (2,4- dichlorophenoxy)propionic acid]. Kaufman (13) reports that the degradation of dalapon (2,2 dichloropropionic acid) was retarded in the presence of amitrole. Dalapon, on the other hand, had little effect on amitrole degradation. Nash (16) reports that two organic phosphate insecticides interact with a urea herbicide to alter patterns of phyto- toxicity. These reports indicate that one pesticide may influence the action or degradation of another. Earlier Studies of Herbicide Degradation in Forest Floor Material Degradation of radioactive 2,4-D and 2,4,5-T Norris (18) treated forest floor material from a red alder (Alnus rubra Bong.) stand with 2.4-D-1-4 C or 2,4,5-T-1-4C at a rate of 2.24 N. Brownbridge, 1956. Ph.D. thesis, London University. Data in Audus (4). 400 TREE GROWTH AND FOREST SOILS kg/ha (2 lb/acre).2 The evolution of radioactive CO2 was measured as a function of time (Figure 2). Both 2,4-D and 2,4,5-T were degraded, but the rate of CO2 evolution was not the same for both herbicides. The evolution of "CO2 from 2,4,5-T followed zero-order kinetics up to 29 days after treat- ment, but evolution from 2,4-D was clearly of mixed order. The re- lease of radioactive CO 2 cannot be quantitatively equated with herbicide degradation, however, since there is no assurance that all of the radioactive carbon removed from the herbicide molecule was released as CO2. Thus, the amount of degradation could be greater but not less than that indicated by these data. NOUNS •FTER TRE•TMENT Figure 2. Liberation of "CO, from red alder forest floor material treated with 2,4-D-1 "C or 2,4.5-T-1"C. Degradation of nonradioactive 2,4-D Norris and Greiner (20) reported experiments in which they chemically determined the level of 2,4-D in forest floor material to obtain a direct measure of herbicide degradation. Except where other- wise indicated, 2,4-D was applied at 3.36 kg/ha (3 lb/acre) to red alder litter. These experiments are summarized below. 2 All rates of application of 2,4-D, 2,4,3-T, and picloram are expressed in terms of acid equivalent. LOGAN A. NORRIS 401 Influence of litter type. The rate of 2,4-D degradation was only slightly different in forest floor material collected beneath red alder, ceannthus i Ccanothus iclutinur vat- larz•igatus How.), vine maple (Acer circinatum Pursh), bigleaf maple (Acer macrophyllum Pursh), and Douglas-fir (Pseudotsuga menzicsii (Mirb.) Franco) (Figure 3). Thus, variation in the degradation rate of 2,4-D in various litter types may occur in the field, but it is more likely to be a function of microsite environment than of litter type. The general shape of the recovery curves suggests that the rate of degradation follows a mixed order of kinetics similar to that found in the first test (Figure 2). 100 90 z U.1 80 ALDER w> 7 0- CEANOTHUS 0 A VINE MAPLE cc b K BIGLEAF MAPLE 60_ DOUGLAS FIR 3 6 10 15 TIME (DAYS) Recoveries at 15 days having a letter in common are not significantly different at the 5 percent level. Figure 3. Recovery of 2,4-D from various types of forest floor material. Influence of formulation. A 2,4-D acid and a triethanolamine salt formulation contained only components of high purity and were most rapidly degraded. Commercial formulations of 2,4-D isooctyl ester and solubilized acid contained the usual mixture of impurities, emulsifiers, and solvents and were less readily degraded (Figure 4). The most striking difference was between the pure acid and the solubilized acid in which the 2.4-D is in exactly the same chemical form. This retarded rate of degradation is attributed to constituents of formulation, indi- cating the influence that one chemical may have on the degradation 402 TREE GROWTH AND FOREST SOILS 100 90 41114111 .1 —- 80 • z Q ^ 2,4-D TR1ETHANOL AMINE SALT CI. — 70 2,4-0 ACID 0 • cc 0 2,4-D SOUJENLIZED ACID (COM. FORM • 0 2,4-D ISOOCTYL ESTER (COM FORM ) ,t160 A 50 • 0 6 10 15 TIME (DAYS) Recoveries at 15 days having a letter in common are not significantly different at the 5 percent level. Figure 4. Recovery of various formulations of 2,4-D from red alder forest floor material. 100 90 ":7Lz 80 cc • >. 70 cc A 2,4-D ISOOCTYL ESTER (CONTROL) 0 0 2,4-D ISOOCTYL ESTER + DIESEL OIL cc 60 ^ 2,4-0 ISOOCTYL ESTER + DDT 0 0 6 10 15 TIME (DAYS) Recoveries at 15 days having a letter in common are not significantly different at the 5 percent level. Figure 5. Recovery of 2,4-D from red alder forest floor material treated with DDT or diesel oil. LOGAN A. NORRIS 403 of another. This point assumes large proportions when we realize that the 125 herbicides produced commercially in 1964 were offered in 8,000 different formulations /2). Influence of other chemicals. The insecticide DDT at 1.12 kg/ha (1 lb/acre) significantly stimulated the rate of degradation of 2,4-D isooctyl ester) when they were applied at the same time. Diesel oil at 37.4 liters/ha (4 gal/acre), commonly used as a carrier, had no effect on 2,4-D degradation (Figure 5). The persistence of the herbicide thus will not likely be adversely affected by either chemical in field applications.

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