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Influence of vegetation on microbial degradation of atrazine and 2

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					      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.
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