Influence of vegetation on microbial degradation of atrazine and 2,4-dichlorophenoxyacetic acid in riparian soils James A. Entry and William H. Emmingham Department of Forest Science, College of Forestry, Oregon State University, Corvallis, OR 97331, U.S.A. Received 12 October 1994, accepted 28 November 1995. Entry, J. A. and Emingham, W. H. 1996. Influence of vegetation on microbial degradation of atrazine and 2,4-dichlorophe- noxyacetic acid in riparian soils. Can. J. Soil Sci. 76: 101–106. Mineralization of atrazine (2 chloro-4 [ethylamino]-6[isopropy- lamino]-s-triazine) and 2,4-D (2,4-dichlorophenoxyacetic acid) in the organic layer and the top 10 cm of mineral soil was measured with radiometric techniques seasonally in coniferous forests and deciduous forests and grassland riparian soils. Active bacterial biomass and active fungal biomass, total carbon, total nitrogen, and total phosphorus were also measured. In the organic horizon, atrazine mineralization was higher in conifer than in deciduous forests during all seasons. Mineralization of 2,4-D was higher in coniferous than deciduous forests in autumn and spring. Grassland vegetation did not form an organic horizon. In mineral soil, atrazine mineralization was higher in coniferous than deciduous forests in the spring and higher in grassland soils in all seasons of the year. In mineral soil, 2,4-D mineralization was higher in coniferous and deciduous forests than grassland soils in autumn, win- ter, and spring. 2,4-D mineralization in mineral soils did not differ between coniferous and deciduous forest soils. We found no abiotic variables or active fungal or bacterial biomass that correlated with atrazine or 2,4-D mineralization. We hypothesize that the soil microbial communities that develop under coniferous forests are capable of mineralizing greater amounts of atrazine and 2,4-D than those that develop under deciduous forests or grassland ecosystems. Key words: Forest riparian soils, forest soils, herbicides, microbial biomass Entry, J. A. et Emmingham, W. H. 1996. Influence de la végétation sur la dégradation microbienne de l’atrazine et de l’acide 2,4-dichlorophénoxyacétique dans les sols de rivage. Can. J. Soil Sci. 76: 101–106. La minéralisation de l’atrazine (chloro-2, éthylamino-4 isopropylamino-6 triazine-1,3,5) et du 2,4-D (acide 2,4-dichlorophénoxyacétique) dans la couche organique et dans les 10 cm supérieurs du sol minéral a été mesurée par radiométrie chaque saison de l’année dans des sols de rivage sous forêt de conifères, sous forêt caducifoliée et sous végétation herbacée. On mesurait également les biomasses bactérienne et fongique actives, ainsi que C, N et P totaux. Dans l’horizon organique, la minéralisation de l’atrizine était plus prononcée sous forêt de conifères que sous forêt caducifoliée durant toutes les saisons de l’année. Celle du 2,4-D était également plus prononcée sous forêt de conifères, mais seulement en automne et au printemps. La végétation prairiale ne forme pas d’horizon organique. Dans les hori- zons minéraux, la minéralisation de l’atrazine était plus forte au printemps sous couvert de conifères que sous couvert de feuillus et plus forte encore sous végétation herbacée à toutes les saisons de l’année. La minéralisation du 2,4-D était plus poussée sous couvert de conifères et de feuillus que sous végétation herbacée, en automne, en hiver et au printemps, mais il n’y avait pas de dif- férence à cet égard entre les deux formes de couverts forestiers. Aucune des variables abiotiques, ni les biomasses fongique ou bactérienne ne produisaient de corrélation avec la minéralisation de l’atrazine ou du 2,4-D. La conclusion à laquelle nous sommes arrivés est que la microfaune formée sous forêt de conifères est capable de minéraliser de plus grandes quantités d’atrazine et de 2,4-D que celles formées en écosystèmes de forêt caducifoliée ou de végétation herbacée. Mots clés: Sols forestiers de rivage, sols forestiers, herbicides, biomasse microbienne Water quality degradation results from surface water dis- particular soil will have profound effects on a soil microbial charge from agricultural lands which carries dissolved and community inhabiting that soil. Forest ecosystems deposit a sediment-adsorbed herbicides into adjacent water systems. substantial amount of coarse woody debris on the ground Dissolved herbicides may also percolate into groundwater (Harmon et al. 1986) which select for soil microorganisms and be discharged via subsurface flow. Recent studies have that can more effectively degrade atrazine and 2,4-D (Entry shown that forest vegetation in riparian areas can increase et al. 1994b). When designing ecosystems to act as filter- the degradation rate of herbicides and mitigate their input to belts to mitigate nutrient and herbicide input to lakes and lakes and streams (Entry et al. 1994a,b). streams, the influence of vegetation on the soil microbial Microbial degradation of herbicides in soils is a function community and its actions on nonpoint source pollutants of three key variables: the ability of the microorganisms to must be taken into account. The objective of this study is to degrade the pesticides, the quantity of these microorganisms determine the influence of coniferous, deciduous, and grass- in the soil, and the activity of the soil microbial enzyme sys- land vegetation on soil microbial mineralization of atrazine tems (Anderson 1984). The type of vegetation growing on a and 2,4-D. 101 102 CANADIAN JOURNAL OF SOIL SCIENCE MATERIALS AND METHODS munitum, Clintonia uniflora (Schult.) Kunth., and Viola The experiment was arranged in a randomized block design sempervirens Greene. (Kirk 1982) of soils sampled from each of three riparian The deciduous stand has an overstory of 60- to 80-yr-old areas (blocks) supporting forest ecosystem vegetation of A. rubra and A. macrophyllum. Shrubs are H. discolor, three types (treatments): coniferous, deciduous, and grass- Vaccinium membranaceum, Amelancher anifolia Nutt., G. land. The sampling sites were on Oak Creek, Jackson Creek, shallon, and R. gymnocarpa; forbs are P. munitum, C. uni- and Soap Creek in the Oregon Coast Range. We sampled the flora, and A. caudatum Lindl; forbs are P. munitum, litter layer and the top 10 cm of mineral soil in each vegeta- Smilacena stellata, and C. unifloria. tion type (coniferous forest, deciduous forest and grass) at The grassland is dominated by F. arundinacea, T. each of three locations, three times in each four seasons pratense, L. perenne, and F. occidentalis. within the year. SOAP CREEK. The Soap Creek site is on a 20–30% slope on Site Descriptions the Dunn State Forest near Corvallis, Oregon (lat. 44°38′, OAK CREEK. The Oak Creek site is on a 20–30% slope in long. 123°21′). The soil is a Xeric Haplohumult clay mixed McDonald Forest near Oregon State University, Corvallis mesic in the Jory series (Knezevich 1975). Annual precipi- (Lat. 44°33′ Long. 123°15′). Annual precipitation is 150 cm tation ranges from 100 to 150 cm yr–1 with annual tempera- yr–1, 4% or less occurring as snowfall. Mean annual air tem- tures of 10–12°C and a frost-free season of 165–210 d. The perature ranges from 10 to 12°C. The soil is a Dystric site is classified as a Tsuga heterophylla/Acer circina- Xerochrept mixed mesic in the Price-Ritner series tum/Gaultheria shallon community type (Hubbard 1991). (Knezevich 1975); pH is 5.7. Permeability is slow and avail- The conifer stand has an overstory of 50- to 80-yr-old Douglas-fir, with a midstory of T. heterophylla and T. brev- able water capacity is from 15 to 25 cm. The site is classi- ifolia. Shrub species are H. discolor, R. ursinus, R. parvi- fied as a Tsuga heterophylla/Acer circinatum/Gaultheria florus, S. mollis, and R. gymnocarpa; the forbs are C. shallon community type (Hubbard 1991). laciniata, V. sempervirens, C. uniflora, and A. caudatum. The conifer stand has an overstory of 60- to 80-yr-old and Ferns are Blechnum spicant (L.) Roth, P. munitum, and older Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], Dryopteris austriaca (Jacq.) Woynar. with Tsuga heterophylla (Raf.) Sarg. and Taxus brevifolia The deciduous stand is dominated by an overstory of 50- Nutt. as midstory trees. Acer circinatum Pursh. is scattered to 70-yr-old A. rubra and A. macrophyllum. Shrubs are H. throughout the stand. Regenerating understory trees are discolor, R. parviflorus, and R. gymnocarpa; the forbs are P. Abies grandis (Dougl.) Lindl., and Thuja plicata Donn. munitum and D. austriaca. Shrub species are Rubus parviflorus Nutt., Rubus ursinus Grassland vegetation consists of F. arundinacea, Cham. & Schlect., Berberis nervosa Pursh, and Holodiscus T. pratense, and Festuca occidentalis. discolor (Pursh) Maxim.; the forbs are Coptis laciniata Gray, Trientalis latifolia Hook., Viola sempervirens Greene, Soil Measurements and Polystichum munitum (Kaulf.) Presl. On each site, we randomly sampled each soil type three The deciduous stand is dominated by 50- to 70-yr-old times around each of three sampling centers on 12 January, Alnus rubra, Bong and Acer macrophyllum Pursh. 13 April, 29 July, and 19 November 1991 (n = 648; three Understory trees are a few small T. heterophylla. Shrubs are vegetation types × two soil layers × three riparian areas × H. discolor, R. ursinus, R. parviflorus, Symphocarpos mol- three sampling centers × three samples × four seasons). lis Nutt., C. laciniata, T. latifolia, and Rosa gymnocarpa Moisture and temperature of the different soil types at each Nutt. The understory has a 20% covering of P. munitum. site were taken at each sampling (Table 1). The grassland vegetation consists of Festuca arundinacea The litter layer and the top 10 cm of each mineral soil Schreb., Trifolium pratense L., and Lolinum perenne L. were subjected to procedures for estimating microbial bio- mass and atrazine and 2,4-D degradation. Soil moisture was JACKSON CREEK. The Jackson Creek site is located on a determined gravimetrically; soil temperatures were taken at 40–60% slope in the Siuslaw National Forest near Corvallis, the time of sampling; pH measurements were made on a 1- Oregon (Lat. 44°33′ Long. 123°14′). The soil is classified as to-1 paste mixture with a model 901 ion analyzer (Orion a Typic Haplumbrept, loamy skeletal mixed mesic in the Instruments). All soil samples were held in the laboratory at Klickitat series (Knezevich 1975). Annual precipitation 4°C with moisture conditions similar to those in the field averages from 130 to 190 cm yr–1. Average annual temper- and were prepared for herbicide mineralization experiments ature ranges from 10 to 12°C, with a frost-free season of and microbial testing within 24 h. This method does not sig- 165–210 d. The site is classified as a Tsuga nificantly alter microbial activity (West et al. 1986). heterophylla/Gaultheria shallon community type (Hubbard Concentrations of total N, NH4, NO3, and mineralized 1991). NH4 in soils were determined by standard techniques; total The overstory of the conifer stand is 60- to 80-yr-old and N with methods described by Bremmner and Mulvaney older Douglas-fir; the midstory is T. heterophylla and T. (1982); extractable concentrations of NH4 and NO3 by a brevifolia. Shrubs are A. circinatum, H. discolor, micro-diffusion method (Keeney and Nelson 1982); miner- Symphocarpus albus Lindl., Rosa woodsi Lindl., V. parvi- alizable NH4 with methods of Giest (1977); and total P with folium Smith., G. shallon, and R. gymnocarpa; forbs are P. methods described in Olsen and Sommers (1982). Total C ENTRY AND EMMINGHAM — MICROBIAL DEGRADATION OF ATRAZINE AND 2,4-D IN RIPARIAN SOILS 103 Table 1. Mean temperature (°C) and soil moisture (kg water/kg dry weight soil) measured at sampling sites at the time of soil collection (n=9) Jackson Creek site Oak Creek site Soap Creek site Season Soil type Temperature Moisture Temperature Moisture Temperature Moisture (°) (°) (°) Winter Conifer litter 4±1 2.10±0.37 4±1 2.21±0.40 4±1 2.06±0.38 Conifer mineral soil 5±1 0.65±0.21 5±1 0.67±0.21 5±1 0.65±0.19 Deciduous litter 5±1 1.88±0.30 5±1 1.97±0.33 4±1 2.31±0.43 Deciduous mineral soil 5±1 0.63±0.22 5±1 0.65±0.26 5±1 0.62±0.14 Grassland soil 5±1 0.58±0.19 6±1 0.72±0.21 6±1 0.67±0.14 Spring Conifer litter 14±2 1.72±0.35 16±2 1.89±0.35 15±2 1.76±0.33 Conifer mineral soil 15±2 0.47±.20 16±2 0.32±0.11 15±2 0.48±0.17 Deciduous litter 15±2 1.57±0.32 15±2 1.63±0.32 14±2 1.82±0.36 Deciduous mineral soil 15±2 0.50±0.16 15±2 0.46±0.17 15±2 0.54±0.20 Grassland soil 14±2 0.47±0.13 16±3 0.33±0.13 16±3 0.40±0.16 Summer Conifer litter 20±3 0.38±0.09 21±3 0.35±0.15 20±2 0.37±0.17 Conifer mineral soil 19±3 0.31±0.11 20±2 0.32±0.14 19±2 0.35±0.12 Deciduous litter 20±3 0.20±0.06 21±2 0.17±0.01 20±2 0.32±0.11 Deciduous mineral soil 19±3 0.23±0.12 20±2 0.73±0.23 19±2 0.20±0.07 Grassland soil 23±3 0.18±0.10 23±3 0.18±0.06 22±3 0.18±0.05 Autumn Conifer litter 12±2 1.93±0.27 13±2 1.77±0.34 12±2 1.86±0.29 Conifer mineral soil 12±2 0.47±0.13 13±2 0.56±0.21 12±2 0.54±0.16 Deciduous litter 12±2 2.07±0.36 13±2 1.86±0.33 12±2 1.97±0.35 Deciduous mineral soil 13±2 0.53±0.15 13±2 0.50±0.19 12±2 0.50±0.14 Grassland soil 12±2 0.40±0.13 13±2 0.48±0.17 11±2 0.46±0.16 was estimated by dry ashing (Nelson and Sommers 1982). cence microscopy for INT-stained (active) bacteria at The C:N ratio was calculated by dividing total C by total N approximately × 1000 magnification. Microbial observa- in each sample. tions were made with a Leitz-Dialux phase-contrast micro- scope. Phase objectives were adapted for epifluorescence Microbial Biomass Measurements with a mercury light source, an H2 filter module containing We estimated total and active bacterial and fungal biomass- a wide-band exciter filter at 390–470 nm, a dichromatic es using methods described by Ingham and Klein (1984). beam splitter passing 510 nm reflected light, and a barrier 1.0-mL aliquot of soil diluted to 10–4 was further diluted in filter restricting light range to 515 nm. We used an edge fil- 4 mL of 60 mM of phosphate buffer. One milliliter of this ter to narrow the exitation range to 455–490 nm in order to solution was incubated with 1 mL of filter-sterilized (0.22- reduce autofluorescence interference. Total bacteria report- mm pore size) 20 mg liter–1 fluorescein diacetate (FDA) ed here are stained (active) and nonstained bacteria. solution for 3 min at 20°C. The final solution was passed Minimum and maximum hyphal diameters were mea- through a polycarbonate filter (25-mm diameter, 0.22-mm sured in one field per slide, and the mean diameter was used pore size). Fungal hyphae were removed from the filters by for calculating fungal biovolume. We computed bacterial shaking the filters for 1 min with 1 mL of sterile buffer in biovolume from the number of soil bacteria per gram of soil 25- × 55-mL screwcap vials. Filters were removed and 1 mL with the assumption that bacterial spheres were 1 mm in of 3% malt agar was added to the soil suspension and mixed; diameter (Jenkinson and Ladd 1981). A biovolume-to-bio- then 0.1 mL of mixture was transferred to a microscope mass conversion factor of 130 mg C mm-3 was used for both slide containing a cavity of known volume (Ingham and bacteria and fungi, assuming 1.1 g cm-3 wet density, 0.25 Klein 1984). Two slides were prepared for each sample and dry matter content, and 0.37 C content in the bacterium or placed inside a humidifier to prevent agar dehydration. fungus (Jenkinson and Ladd 1981). Slides were examined for FDA-stained hyphal length by epifluoroscent microscopy immediately after preparation Herbicide Degradation because most fluorescence is lost after 4.5 h of storage in a Ring-labeled 14C atrazine (purity > 99.5%) was donated by humidifier at room temperature (Stamatiadis et al. 1990). Ciba-Geigy Corp., Greensboro, NC, and 2,4-D (purity > Three fields per slide were examined with phase contrast 98%) was purchased from Sigma Chemical Co., St. Louis, microscopy for total hyphal length, and three transects were MO. We dissolved 1.0 mM of unlabeled atrazine plus 1995 examined for FDA-stained (active) hyphal length at × 100 Bq of ring-labeled 14C atrazine in 10 mL of 95% ethanol. In total magnification. a separate container, we dissolved 1.0 mM of unlabeled 2,4- We used iodonitrotetrazolium (INT) stain to count total D plus 2557 Bq of ring-labeled 2,4-D in 10 mL of 95% and live bacteria with a method described by Stamatiadis et ethanol. Each mixture was brought to 100 mL volume with al. (1990). A 1-mL sample of initial soil suspension was fur- deionized water. We placed 15 g equivalent dry weight fresh ther diluted to 0.2 mg-soil in 4 mL buffer. The soil suspen- soil in a 100-mL container, added approximately 1.0 mL sion was incubated in the dark with 4 mL of filtered INT solution containing 1.0 mM of unlabeled atrazine plus 1995 buffer for 60 min at room temperature. We examined two Bq of ring-labeled 14C atrazine or 1.0 mL solution contain- slides per sample and 10 fields per slide with epifluores- ing 1.0 mM of unlabeled 2,4-D plus 2557 Bq ring-labeled 104 CANADIAN JOURNAL OF SOIL SCIENCE Table 2. Total nitrogen (TN), carbon (TC), phosphorus (TP), and carbon:nitrogen (C:N) ratios of riparian soils growing conifer forest, deciduous forest, and grassland vegetation Organic horizon Mineral soil TN TC TP C:N TN TC TP C:N Season Vegetation ( %) (% ) Autumn Conifer forest 1.316a 43.5a 0.176a 37a 0.414a 7.3a 0.104bc 19bc Deciduous forest 1.25a 30.4b 0.127b 18b 0.393a 6.1a 0.092bc 16c Grassland — — — — 0.332a 9.0a 0.216a 28a Winter Conifer forest 1.315a 43.5a 0.177a 38a 0.415a 7.2a 0.112b 17c Deciduous forest 1.625a 30.4b 0.137b 18b 0.399a 6.0a 0.088c 15c Grassland — — — — 0.363a 8.9a 0.211a 28a Spring Conifer forest 0.981a 34.7b 0.111c 36a 0.293a 7.9a 0.108bc 20b Deciduous forest 1.015a 21.7c 0.115c 31a 0.327a 6.9a 0.119b 20b Grassland — — — — 0.306a 5.3a 0.115b 28a Summer Conifer forest 0.908a 42.2a 0.116c 37a 0.323a 6.9a 0.0806c 22b Deciduous forest 1.357a 32.2b 0.107c 31a 0.364a 7.1a 0.0947c 18bc Grassland — — — — 0.325a 6.0a 0.0728c 28a a–cIn each column, values followed by the same letter are not significantly different as determined by Fisher’s protected least significant difference (LSD) test (P<0.05) n=27. 14C-2,4-D to each soil sample and thoroughly mixed the soil Table 3. Active fungal and bacterial biomass in riparian soils growing and herbicides. Each container was then placed in a 0.89-L conifer forest, deciduous forest and grassland vegetation container with a vial containing 10 mL of 1 M NaOH and a Organic layer Mineral soil vial containing 10 mL of deionized water (to maintain Fungal Bacteria Fungal Bacteria humidity) and incubated for 30 d at temperatures similar to Season Vegetation (µg C g-1 soil) those measured in the field: winter 5°C, spring 15°C, sum- Autumn Conifer forest 9.4a 98a 62a 55a mer 20°C, and fall 12°C. Previous studies have shown that Deciduous forest 4.3b 32b 45b 29b soil microbes in 20-g equivalent dry weight of soil in this Grassland — — 49b 11c system do not alter O2 content inside the container relative Winter Conifer forest 4.7b 17c 7d 6d Deciduous forest 3.6b 15c 5d 5d to O2 content outside the container (Entry et al. 1987). We Grassland — — 7d 4d ran one control and one blank for each set of 27 samples in Spring Conifer forest 5.4b 107a 89a 60a order to establish background counts. Control soil samples, Deciduous forest 4.9b 37b 32c 28b autoclaved for 1 h (252°C, 1.4 kPa) before being run, con- Grassland — — 39bc 13c Summer Conifer forest 2.5c 15c 12d 6d sisted of 15 g equivalent dry weight of soil with 14C labeled Deciduous forest 2.4c 15c 13d 6d herbicide added. Blanks consisted of a run of the procedure Grassland — — 8d 5d without soil placed in the container. a–dIn each column, values followed by the same letter are not significant- After a 30-d incubation, the containers were opened and ly different as determined by Fisher’s protected least significant difference the NaOH vials removed. One-half milliliter of the NaOH test (P≤0.05), n=27. was removed from the vial and mixed with 1.0-mL deion- ized H2O and 17-mL scintillation cocktail (Bio-Safe II, sites, only differences among soil types and seasons are Research Products International Corp., Mount Prospect, IL). reported here (Kirk 1982). When the solutions cleared, the samples were counted for 10 min with a Beckman LS 7500 autoscintillation counter. The RESULTS amount of 14C counts from control and blank samples was Soil temperatures did not consistently vary among the sites not significantly different from background counts. All her- and were lower in the winter and higher in summer than in bicide mineralization values are, therefore, reported as val- spring or autumn (Table 1). Soil moisture was higher in the ues above control values. organic than in mineral soil. Soil moisture was higher in Statistical Analysis winter, autumn, and spring than summer. Soil nitrogen in All dependent variables were tested for normal distribution. organic or mineral soil did not vary among conifer, decidu- Data were then analyzed by means of ANOVA procedures ous forests, and grassland ecosystems (Table 2). Conifer for a randomized block design with Statistical Analysis forests had higher amounts of carbon in the organic layer Systems (SAS Institute, Inc. 1982). Residuals were equally than the deciduous forests. The amount of carbon in the distributed with constant variances. All digits reported are mineral soil did not vary among coniferous or deciduous the sample values minus control values. Differences report- forests and grassland vegetation types in all seasons of the ed throughout are significant at P ≤ 0.05, as determined by year. Total phosphorus was higher in the organic layer of the Fisher’s Protected Least significant difference test. Because coniferous than the deciduous forests in winter and autumn, analysis of variance for soil chemicals, active and total fun- but not in spring and summer. In mineral soil, total phos- gal and bacterial biomasses, and atrazine and 2,4-D miner- phorus was higher in the grassland than in coniferous or alization did not indicate significant differences among deciduous forests. Total phosphorus in mineral soils did not ENTRY AND EMMINGHAM — MICROBIAL DEGRADATION OF ATRAZINE AND 2,4-D IN RIPARIAN SOILS 105 Table 4. Atrazine and 2,4-D mineralization in riparian soils growing ization occurred in coniferous and deciduous forest soils conifer forest, deciduous forest and grassland vegetation than grassland mineral soils in all seasons. Atrazine 2,4-D organic mineral organic mineral DISCUSSION layer soil layer soil Temperature and moisture are the main abiotic variables Season Vegetation (% 14CO2 recovered as CO2) that affect mineralization of 2,4-D and atrazine (Parker and Autumn Conifer forest 2.00b 1.78ab 12.23c 12.40c Doxtader 1983; Wolf and Martin 1975). Soil moisture and Deciduous forest 1.76c 0.64c 13.09c 13.00c temperature varied less than 2°C among vegetation type Grassland — 0.71c — 3.50e within each season. High amounts of additional nitrogen has Winter Conifer forest 2.50b 2.16a 15.56c 6.49d Deciduous forest 1.93c 0.98b 13.78c 7.54d been shown to decrease atrazine and 2,4-D mineralization in Grassland — 1.01b — 3.61e pasture soils (Entry et al. 1993). Total nitrogen did not vary Spring Conifer forest 4.31a 2.16a 64.75a 56.22a among vegetation type in organic or mineral soil. Soil car- Deciduous forest 1.60c 2.47a 34.61b 51.62a bon was greater in the organic layer of coniferous forests in Grassland — 1.20b — 40.76b autumn and winter but not spring or summer and did not Summer Conifer forest 2.70b 1.24b 1.26d 3.19e Deciduous forest 1.72c 0.85bc 1.01d 3.33e vary among vegetation type in mineral soil. We found no Grassland — 0.60c — 1.33f differences in soil abiotic variables that could explain the a–fIn each column, values followed by the same letter are not significantly increased atrazine mineralization in coniferous forest soils different as determined by Fisher’s protected least significant difference or increased 2,4-D mineralization in forest mineral soils test (P≤0.05), n=27. compared to grassland soils. Although microbial degradation of herbicides in soil is vary among vegetation types in spring and summer. The believed to be a function of microbial activity (Anderson C:N ratio in the organic layer was higher in conifer than the 1984), we found no correlation of active fungal or bacterial deciduous forest in autumn and winter but not spring or biomass with atrazine or 2,4-D mineralization. The activity summer. The C:N ratio in mineral soil was higher in the and production of the various enzymes involved in herbicide grassland than the conifer or deciduous forest in all seasons degradation are not fully known. This experiment measured of the year. The C:N ratio in mineral soil did not vary atrazine and 2,4-D mineralization and microbial biomass as between conifer and deciduous vegetation types. opposed to measuring disappearance of the compounds in Active bacterial biomass in the organic layer was greater the soil. Herbicide degradation may be able to be predicted in the coniferous and deciduous forest soils in spring and in the future by improved indices of microbial activity, how- autumn than in winter or summer (Table 3). Active fungal ever, other factors may need to be considered. biomass in the organic layer was greater in the winter, Forest riparian ecosystems develop a soil microbial com- spring, and autumn than in the summer. In autumn and munity that results in higher rates of atrazine and 2,4-D min- spring active fungal and bacterial biomass in mineral soil eralization than grassland soils. Coniferous forest soils was higher in coniferous forest soil than deciduous forest or develop microbial communities that result in higher grassland soil. Fungal biomass in mineral soil did not vary amounts of atrazine and 2,4-D mineralization than decidu- between deciduous forest and grassland vegetation types. ous forest or grassland soils. Although more research is nec- Active fungal and bacterial biomass in mineral soils did not essary, land managers designing forest filterstrips to vary among the three vegetation types in the summer or mitigate the input of agricultural herbicides to lakes and winter months. Active bacterial biomass in mineral soils streams should consider establishing coniferous forest over was higher in coniferous forest than deciduous forest or deciduous forest or grassland ecosystems. grassland vegetation types in autumn and in spring, but not summer or winter. Active bacterial biomass was higher in Anderson, J. P. E. 1984. Herbicide degradation in soil: Influence mineral soils in deciduous forests than in grasslands in of microbial biomass. Soil Biol. Biochem. 16: 483–489. Bremmner, H. M. and Mulvaney, C. S. 1982. Nitrogen—total. spring and autumn but not winter or summer. Pages 595–622 in P. A. Page, R. H. Miller, and D. R. Keeney, eds. In the organic horizon, atrazine mineralization was high- Methods of soil analysis. Part 2. Chemical and microbiological er in conifer than deciduous forest soils in all seasons of the properties. Agronomy no. 9. American Society of Agronomy, year and was higher in spring than all other seasons (Table Madison, WI. 4). In mineral soil, atrazine mineralization was higher in Entry J. A., Donnelly, P. K. and Emmingham, W. H. 1994a. conifer forest than grassland vegetation types in all seasons. Microbial mineralization of atrazine and 2,4-D in pasture and for- Atrazine mineralization in mineral soils was higher in decid- est riparian soils. Biol. Fertil. Soils 18: 89–94. uous forest than grassland soils in spring, but not autumn, Entry, J. A., Donnelly P. K. and Emmingham, W. H. 1994b. winter, and summer. In the organic horizon, mineralization Atrazine and 2,4-D mineralization in riparian soils of young, sec- of 2,4-D was higher in the coniferous forests than in decid- ond, and old-growth forests. Appl. Soil Ecol. (in press). Entry, J. A., Mattson, K. G. and Emmingham, W.H. 1993. The uous forests in the spring. In the organic horizon 2,4-D min- influence of nitrogen on mineralization of atrazine and 2,4- eralization was higher in the spring than the winter or dichlorophenoxyacetic acid in pasture soils. Biol. Fertil. Soils 16: autumn and was lowest in the winter. In mineral soil, min- 179–182. eralization of 2,4-D did not differ between coniferous and Entry, J. A., Stark, N. M. and Loewenstein, H. 1987. Timber deciduous forest soils. Greater amounts of 2,4-D mineral- harvesting: Effects on degradation of cellulose and lignin. For. 106 CANADIAN JOURNAL OF SOIL SCIENCE Ecol. Manage. 22: 79–88. Nelson, D. W. and Sommers, L. E. 1982. Total carbon, organic Giest, J. M. 1977. 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