Soil microbial, fungal, and nematode responses to soil fumigation and ...

Biol Fertil Soils (2006) 42: 247–257 DOI 10.1007/s00374-005-0022-0

ORIGINA L PA PER

H. P. Collins . A. Alva . R. A. Boydston . R. L. Cochran . P. B. Hamm . A. McGuire . E. Riga

Soil microbial, fungal, and nematode responses to soil fumigation and cover crops under potato production Received: 29 June 2004 / Revised: 17 May 2005 / Accepted: 17 May 2005 / Published online: 9 August 2005 # Springer-Verlag 2005

Abstract Sodium N-methyldithiocarbamate (metam sodium) and 1,3 dichloropropene are widely used in potato production for the control of soil-borne pathogens, weeds, and plant parasitic nematodes that reduce crop yield and quality. Soil fumigation with metam sodium has been shown in microcosm studies to significantly reduce soil microbial populations and important soil processes such as C and N mineralization. However, few published data report the impact of metam sodium on microbial populations and activities in potato production systems under field conditions. Fall-planted white mustard (Brassica hirta) and sudangrass (Sorghum sudanense) cover crops may serve as an alternative to soil fumigation. The effect of metam sodium and cover crops was determined on soil microbial populations, soil-borne pathogens (Verticillium dahliae, Pythium spp., and Fusarium spp.), free-living and plantparasitic nematodes, and C and N mineralization potentials under potato production on five soil types in the Columbia Basin of Eastern Washington. Microbial biomass C was 8– 23% greater in cover crop treatments compared to those

fumigated with metam sodium among the soil types tested. Replacing fumigation with cover crops did not significantly affect C or N mineralization potentials. Cumulative N mineralized over a 49-day laboratory incubation averaged 18 mg NO3-N kg−1 soil across all soil types and treatments. There was a general trend for N mineralized from fumigated treatments to be lower than cover-cropped treatments. Soil fungal populations and free-living nematode levels were significantly lowered in fumigated field trials compared to cover-cropped treatments. Fumigation among the five soil types significantly reduced Pythium spp. by 97%, Fusarium spp. by 84%, and V. dahliae by 56% compared to the mustard cover crop treatment. The percentage of bacteria and fungi surviving fumigation was greater for fine- than coarse-textured soils, suggesting physical protection of organisms within the soil matrix or a reduced penetration and distribution of the fumigants. This suggests the potential need for a higher rate of fumigant to be used in fine-textured soils to obtain comparable reductions in soil-borne pathogens.

H. P. Collins (*) . A. Alva . R. A. Boydston . R. L. Cochran USDA-ARS, Vegetable and Forage Research Unit, 24106 North Bunn Rd., Prosser, WA, 99350, USA e-mail: [email protected] Tel.: +1-509-7869250 Fax: +1-509-7869277

Keywords Soil fumigation . Cover crops . Microbial populations . Pathogens

P. B. Hamm Hermiston Agricultural Research and Extension Center, Oregon State University, P.O. Box 105 Hermiston, OR, 97838, USA A. McGuire Center for Sustaining Agri. and Nat. Resources, Washington State University, P.O. Box 37, Courthouse, 1st and C Street, Ephrata, WA, 98823, USA E. Riga Irrigated Agricultural Research and Extension Center, Washington State University, 24106 North Bunn Rd., Prosser, WA, 99350, USA

Introduction The Pacific Northwest (PNW) of the United States is an important area for potato (Solanum tuberosum) production, accounting for about 128 million tons of potato production annually (55% of the total US production) on about 253,000 ha (45% of the total acreage under potato in the USA). The Columbia Basin of Eastern Washington and Oregon provides ideal conditions for high potato yields of up to 90 Mg ha−1 (USDA-NAS, 2002). Fumigation with sodium N-methyldithiocarbamate (metam sodium) and the nematicide 1,3 dichloropropene is a commonly used practice and has been shown to be an effective method for the control of soil-borne pathogens, weeds, and plant-parasitic nematodes that reduce crop yield and quality (Smelt and Leistra 1974; Ben-Yephet and Frank 1984; Magyarosy et

248

al. 1988; Becker et al. 1990; ICI 1992; Ingham et al. 2000; Hamm et al. 2003). The use of cover crops may serve as an alternative to fumigation as well as mitigating degradation to soil and environmental quality. Cover crops play a vital role in filling gaps in crop rotations where soil is left bare and provide protective mulches in conservation tillage systems. Benefits of cover crops include sequestering excess soil nitrogen, replenishing soil organic matter reserves, increasing the size and activity of the soil microflora, as well as enhancing microbial populations antagonistic to pathogenic organisms (Doran and Smith 1991; Lal et al. 1991; Shennan 1992; Weinert et al. 1995; Mendes et al. 1999). Several studies have shown that certain green manure cover crops (Sorghum sudanense and Brassica sp.) or their seed meals are also effective in reducing the incidence of pathogenic fungi (Lewis and Papavizas 1974; Hansen et al. 1990; Smolinska et al. 1997; Brown and Morra 1997), plantparasitic nematodes (Mojtahedi et al. 1993a,b; Widmer and Abawi 2002), and weeds (Brown and Morra 1995; Boydston and Hang 1995; Buhler et al. 2001; Boydston and Vaughn 2002) through the production of secondary plant compounds, e.g., cyanogenic and isothiocyanate compounds. Due to the high cost of using soil fumigants, potato growers are increasingly interested in growing cover crops as green manures that serve as a biofumigation method that might replace soil fumigation prior to planting. Acreage planted to cover crops of white mustard (Brassica hirta) and Indian mustard (Brassica juncea) preceding potatoes in 2004 will exceed 8,000 ha in Washington State, representing more than 10% of the projected potato acreage. Planted in mid to late August, white mustard emerges quickly and produces from 5,000 to 7,000 kg ha−1 dry weight plant residues before succumbing to freezing temperatures during the winter months. Soil fumigation in combination with minimal residue inputs has been implicated in reducing soil microbial populations to levels that could disrupt nutrient cycling. Soil fumigation with metam sodium has been shown in microcosm studies to significantly reduce soil microbial

populations and important soil processes such as C and N mineralization (Sinha et al. 1979; Toyota et al. 1999; Macalady et al. 1998; Ibekwe et al. 2001). The objectives of the study were to determine the effect of metam sodium and green manure cover crops, mustard (B. hirta) or sudangrass (S. sudanense) on soil microbial populations, selected soil fungi (Verticillium dahliae, Pythium spp., and Fusarium spp.), free-living and plantparasitic nematodes, and C and N mineralization potentials from five soil types under potato production.

Materials and methods Field plots In 2000 and 2001, a series of studies were conducted on a Quincy sand located on the USDA-ARS research farm in Benton County, WA, and on commercial fields representing four additional soil types (Table 1) found in the Columbia Basin of Eastern Washington. The field study at the USDA-ARS research farm (Paterson, WA) was a 4year crop rotation of winter wheat/sweet corn/sweet corn/ potato (Triticum aestivum/Zea mays/Z. mays/S. tuberosum), incorporating both cover crops and soil fumigation as treatments. The experimental design was a randomized complete block with four replications. Plots were prepared in early September for planting of cover crops and fumigation by incorporating the previous season’s sweet corn residues using a rolling cultivator and packer. A mustard cover crop (12 kg seed ha−1) was planted in September using a Vermillion planter and fertilized with 56 kg N ha−1 as UN32 through the irrigation system. Fumigated plots were treated with 1,3 dichloropropene (16 November 2000 and 24 October 2001) by subsurface injection using 45 cm shanks, 50 cm apart, at a rate of 140 l ha−1 followed by a packer to seal the soil surface (Table 1). The following week (23 November 2000 and 31 October 2001) 280 l ha−1 metam sodium (anhydrous, 42%) was applied to the soil surface with a field sprayer and incorporated immediately by roto-tilling and irriga-

Table 1 Soil classification, type of cover crop, date of fumigation, and chemical characteristics of sample sites Site soil series/texture

Cover crop

Date of final fumigation

Total soil −1

C (g kg Quincy sand, mixed, mesic Xeric Torripsamments Quincy loamy fine sand mixed, mesic Xeric Torripsamments Timmerman sandy loam, sandy, mixed mesic, Xerollic Camborthids Shano silt loam, coarse–silty, mixed, mesic Xerollic Camborthids Warden silt loam, coarse–silty, mixed, mesic Xerollic Camborthids

Mustard

C:N soil)

−1

N (g kg

pH

soil)

4.3 (0.4)

0.51 (0.05)

8.4

7.0

Sudangrass

23 Nov 2000 and 31 Oct 2001 20 Oct 2001

7.3 (0.9)

0.79 (0.16)

9.2

7.0

Mustard

28 Mar 2001

7.4 (0.7)

0.62 (0.08)

11.9

6.5

Mustard

7 Mar 2002

7.2 (0.8)

0.57 (0.02)

12.7

6.6

Mustard

20 Oct 2001

8.9 (1.1)

0.50 (0.07)

17.8

6.7

Values in parentheses are confidence intervals at p=0.05 Mustard, var. Martigena; Sudangrass, var. Sorden 79

249

tion. Treatments sampled at Paterson, WA, were the (1) fallow, (2) fallow-fumigated, with 1,3 dichloropropene/ metam sodium, and (3) the white mustard cover crop (B. hirta) plots, that preceded potato. Soil sampling Soil samples at all sites were collected prior to fumigation, approximately 1 month following fumigation and at preplant of potato at the USDA-ARS field site (Quincy sand) and just at preplant for the remaining soil series (Table 1). Twenty soil cores (∼1 kg) were collected with a 2.5 cm diameter probe from the surface 30 cm of each treatment replicate. In the spring of 2002, soil samples were collected from four commercial on-site field trials that had been fumigated with 140 l ha−1 of 1,3 dichloropropene and 346 l ha−1 metam sodium and treatments planted to white mustard, representing four different soil series (Table 1). Five replicated field plots sampled at these sites consisted of (1) mustard (B. hirta) or sudangrass (S. sudanense) cover crop planted in the fall of 2001 and (2) either in fall or spring fumigated with 1,3 dichloropropene/metam sodium. Samples from all five soil series were subsampled within 48 h for microbial, selected soil fungi, and nematode analyses. Microbial enumeration assays Populations of culturable aerobic soil bacteria were enumerated on 0.3% tryptic soy agar, Pseudomonas on King’s B, and fungi on potato dextrose agar. Populations of several major soil-borne pathogens (V. dahliae, Pythium spp., and Fusarium spp.) were determined using standard enumeration techniques for taxonomic and phenotypic identification. Fusarium spp. and Pythium spp. were assayed by the methods reported by Hansen et al. (1990). Soil was screened (0.64 cm mesh), mixed, weighed, diluted in sterile water agar (1%), and plated onto selective medium, containing either a modified Komada’s or modified clarified V8 agar, for isolating Fusarium spp. and Pythium spp., respectively. The number of colonies of each genus formed after 3 (in the dark) and 7 days (under continuous light) were counted for Pythium sp. and Fusaria, respectively. No attempt was made to separate pathogenic isolates and/ or to identify species colonies belonging to either genus. V. dahliae numbers was determined using the method of Johnson et al. (1988). A portion of each sifted sample was scooped into a 118-ml polypropylene cup (about 1/2 full), any clumps fractured with a pestle, and left to air dry for 12–14 days at room temperature. Approximately 0.1 g of soil was then blown through an Anderson Air Sampler (Anderson Air Samplers and Consulting Service, Provo UT 84601) onto five petri plates containing Sorenson-NP. A control soil known to have V. dahliae was used to check procedures, colony morphology, and length of time re-

quired for microsclerotia to form. Petri plates were incubated in plastic bags in the light at room temperature for at least 14 days. After incubation, the plates were handwashed in tap water to remove soil from the surface of the plates and then examined under a dissecting microscope. The number of colonies that formed distinctive microsclerotia consistent with the known soil was recorded as V. dahliae. Soil population levels of Fusarium spp., Pythium spp., and V. dahliae were calculated as colony forming units (CFU) per gram of dry soil. Nematodes analyses Soil subsamples (250 g) were processed via an elutriator and sugar flotation technique to extract both free-living and plant-parasitic nematodes from each soil sample in only the 2002 crop year (CY). Nematode species were identified under the microscope, and nematode abundance was determined (Fortuner 1991). Soil microbial biomass and C and N mineralization potentials Total soil C and N was determined by dry combustion on duplicate subsamples from each treatment using a LECO CNS analyzer, Model 2000. Microbial biomass C and N was determined for soil samples using the chloroform fumigation–incubation method described by Jenkinson and Powlson (1976). Soil respiration was determined by incubating duplicate100 g moist-sieved samples from the 0–30 cm depth increment of each field replicate. Samples were adjusted to 60% of field capacity and incubated in 0.95-l bottles in the dark at 25°C for 100 days. Headspace CO2 was measured using the NaOH base trap method (Zibilske 1994), initially at 4-, 7-, and 10-day then at 7-day intervals through day 100. Following each interval of CO2 analysis, the headspace was returned to ambient CO2 by degassing with compressed air. Soil N mineralization potentials were determined by incubating seven sets of 10 g soil samples from each treatment replicate in 60-ml bottles over 7 weeks at 25°C, adjusted to 60% of field capacity. Bottles were opened weekly to reduce the potential for generating anaerobic conditions. Incubated soil was extracted at weekly intervals with 50 ml of 1 M KCl, shaken on a rotary shaker for 1 h, then filtered through a Type A/E glass fiber filter (Gelman Sciences Inc., Ann Arbor, MI). The NO3-N and NH4-N in the extracts were determined on an FIA Series auto-analyzer. Data analysis Data were analyzed using analysis of variance for multiple treatment comparisons and then treatment means separated by Fisher’s least significance difference test where ANOVA was significant.

250

Results and discussion Microbial biomass

difference in microbial biomass C was attributed to the death of organisms resulting from fumigation stimulating surviving populations. The change in microbial biomass N between fumigated and cover-cropped treatments among the soil series was similar to that of microbial biomass C, increasing from coarse- to fine-textured soils (Fig. 2b). Microbial N increased from 2.2% in the Quincy sand to 5.8% in the silt loams of total soil N, respectively. The microbial biomass C–N ratio ranged from 7.7 to 14.3 for the cover-cropped treatments, with the lowest ratios found in the Warden and Shano silt loams. Fumigation lowered the ratio with an average of 0.7 units for the Quincy sand and Timmerman sandy loam and 1.5 units for both the Warden and Shano silt loams. The increase in microbial N most likely resulted from the uptake of N by the microbial biomass released from populations of organisms killed during the fumigation.

Soil fumigation and the incorporation of white mustard green manure in the 2-year study on the Quincy sand at the Paterson, WA, field site had variable effects on soil microbial biomass C compared to the fallow control treatment in both years (Fig. 1a,b). Although not significantly different, microbial biomass C was consistently greater, over all sample dates, for the mustard green manure, averaging 160 mg C kg−1 soil compared to 130 and 118 mg C kg−1 soil, for the fallow and fallow/fumigated treatments, respectively. Fumigation decreased microbial biomass 22% in the 2000–2001 CY and 10% in the 2001–2002 CY compared to the fallow control, whereas the mustard green manure increased biomass C 13 and 17% in the 2000–2001 and 2001–2002 crop years, respectively, compared to the fallow control. Microbial biomass C was 3.7, 3.0, and 2.7% of the soil organic C (SOC) for the cover cropped, fallow, and fumigated treatments, respectively. Microbial biomass C was not significantly different between fumigated and cover-cropped (white mustard or sudangrass green manure) treatments within any of the soil series sampled (Fig. 2a). There was a general trend for microbial biomass C in fumigated treatments to be lower (range 8–23%) than cover-cropped treatments, except for the Warden silt loam which showed a slight increase. Microbial biomass C was 157 mg C kg−1 soil in the Quincy sand and increased to 260 mg C kg−1 soil in the Shano silt loam. The size of the microbial biomass was a function of the total SOC, which increased from 4.3 g C kg−1 soil in the Quincy sand to 8.9 g C kg−1 soil for the Warden silt loam (Table 1). Although total SOC was greater in the silt loam soils, the proportion of microbial biomass C to total SOC was similar among all soil series. Microbial biomass C averaged 3.2% of the SOC for the cover cropped and 2.8% for the fumigated treatments across all soil types. The 0.4%

The amount of C mineralized from the Quincy sand did not change significantly over the sampling dates within any of the treatments for either the 2000–2001 and 2001– 2002 crop years (data not shown). Mineralized C averaged 185, 155, and 160 mg C kg−1 soil for the mustard green manure, fallow, and fallow/fumigated treatments, respectively. Cumulative C mineralized over the 49-day laboratory incubation accounted for 4.3, 3.6, and 3.7% of the SOC for the green manure, fallow, and fumigated treatments, respectively. Carbon and N mineralized over the 49-day laboratory incubation were not significantly different between fumigated and cover-cropped treatments for any of the soil series sampled in the spring of 2002 (Fig. 3), although there was a general trend for C mineralized from fumigated treatments to be lower (range 0–20%) than cover-cropped treatments. Cumulative C mineralized was 170 mg CO2-C

A

B

2000-2001 Crop year, Quincy sand series

Mineralizable C and N

250 White Mustard Fallow Fumigated

200

150

100

50

0

Microbial Biomass C (mg kg-1 soil)

250

soil) -1

Microbial Biomass C (mg kg

2001-2002 Crop year, Quincy sand series White Mustard Fallow Fumigated

200

150

100

50

0 12 Oct 00 Pre-fumigate

5 Dec 00 Post-fumigate

Sample date

1 Mar 01 Pre-plant

9 Sept 01 Pre-fumigate

26 Nov 01 Post-fumigate

2 Apr 02 Pre-plant

Sample date

Fig. 1 Microbial biomass C of the Quincy sand soil for the white mustard cover crop, fumigated (metam sodium/1,3 dichloropropene) and fallow treatments during the 2000–2001(a) and 2001–2002 CY (b). Error bars at p=0.05

251

A

B 50

Microbial Biomass N (mg kg-1 soil)

Cover crop Fumigated

300

Microbial Biomass C (mg kg

-1

soil)

350

250 200 150 100 50


40

30

20

10

0

0 Quincy sand

Quincy Timmerman loamy fine sandy loam sand

Warden silt loam

Shano silt loam

Quincy sand


Soil Series

Bacterial and fungal populations Aerobic bacterial populations (Fig. 4a,b) of the mustard green manure and fumigated treatments in the 2-year study on the Quincy sand, prior to fumigation, were not significantly different compared to the fallow control both years, averaging 4.5×106 CFU g−1 soil in 2000 and 3.2×106 g−1 soil in 2001. Following fumigation, bacterial populations

had a cover crop of sudangrass var. Sorden 79, the remaining sites were mustard var. Martigena. Error bars at p=0.05

C mineralization (mg CO2-C kg-1 soil)

A 500 450 400


350 300 250 200 150 100 50 0 Quincy sand


Warden silt loam

Shano silt loam

Warden silt loam

Shano silt loam

soil)

B -1

kg−1 soil for the Quincy sand and increased to 385 mg CO2-C kg−1 soil for the Shano silt loam (Fig. 3a). The amount of C mineralized was a function of the total SOC, which increased from 4.3 g C kg−1 soil in the Quincy sand to 8.9 g C kg−1 soil for the Warden silt loam (Table 1) and was also correlated with the size of the microbial biomass (data not shown). Although total SOC was greater in the silt loam soils, the proportion of C mineralized to total SOC was similar. Cumulative C mineralized accounted for 4.2 and 3.8% of the SOC for the cover crop and fumigated treatments among all series, respectively. Among the five-soil series, soil fumigation and the incorporation of cover crops had variable effects on N mineralization (Fig. 3b). There was no pattern of the N mineralized changing as a function of soil texture or total soil N, as observed for SOC, microbial biomass C and N, or mineralized C. However, there was a general trend for N mineralized from fumigated treatments to be lower (range 4–22%) than cover-cropped treatments, except for the Timmerman sandy loam which showed a 16% increase in NO3-N for the fumigated treatment. The lower N mineralized would suggest an effect of fumigation on the activity of soil nitrifiers. Cumulative N mineralized averaged 3.2 and 2.9% among all series of the total N for the cover crop and fumigated treatments, respectively.

Shano silt loam

Soil Series

N mineralization (mg NO3-N kg

Fig. 2 Effect of soil fumigation (metam sodium/1,3 dichloropropene) and cover crops on microbial biomass-C (a) and microbial biomass-N (b) of five soil series. The Quincy loamy fine sand soil

Warden silt loam

30


25 20 15 10 5 0 Quincy sand


Soil Series

Fig. 3 Effect of soil fumigation (metam sodium/1,3 dichloropropene) and cover crops on cumulative 49-day C mineralization (a) and N mineralization (b) in five soil series. The Quincy loamy fine sand soil had a cover crop of sudangrass var. Sorden 79, the remaining sites were mustard var. Martigena. Error bars at p=0.05

252

A


C

25

30

White Mustard Fallow Fumigated

CFU's Bacteria x 106 g-1 soil

CFU Bacteria x 106 g-1 soil

30

All Soil Series

20 15 10 5

25 20 15 10 5 0



28 Mar 01 Pre-plant

Quincy sand

Sample date

B


Shano silt loam

Warden silt loam

Shano silt loam

Warden silt loam

Shano silt loam

D CFU's Pseudomonads x 104 g-1 soil

CFU Bacteria x 106 g-1 soil

Warden silt loam

30

30 25



20 15 10 5 0

25

Mustard Fallow

20

15

10

5

0 9 Sept 01 Pre-fumigate


2 Apr 02 Pre-plant

Quincy sand

Sample date


CFU's Pseudomonads x 103 g-1 soil

E 30 25

Mustard Fallow

20 15 10 5 0 Quincy sand

Fig. 4 Culturable bacterial populations during the 2000–2001 (a) and 2001–2002 (b) CY of the Quincy sand for the white mustard cover crop, fumigated (metam sodium/1,3 dichloropropene), and fallow treatments. Culturable bacteria (c), Pseudomonas (d), and


fluorescent Pseudomonas (e) populations for the soil series. The Quincy loamy fine sand soil had a cover crop of sudangrass var. Sorden 79, the remaining sites were mustard var. Martigena. Error bars at p=0.05

253

A

2000-2001 Crop year, Quincy sand series 90

CFU Fungi x 102 g-1 soil

80 White Mustard Fallow Fumigated

70 60 50 40 30 20 10

*


*


28 Mar 01 Pre-plant

Sample date

B

2001-2002 Crop year, Quincy sand series 90


CFU Fungi x 102 g-1 soil

80 70 60 50 40 30 20 10 0 9 Sept 01 Pre-fumigate


2 Apr 02 Pre-plant

Sample date

C

All Soil Series 70

CFU's Fungi x 103 g-1 soil

increased significantly, remaining greater (20.2×106 CFU g−1 soil) prior to planting in the spring of 2001, compared to the fallow or mustard green manure, but were similar (5.0×106 CFU g−1 soil) in 2002. Among the five-soil series, soil fumigation and the incorporation of cover crops had variable effects on soil bacterial populations (Fig. 4c). There was no pattern of total bacterial numbers changing as a function of soil texture, as observed for SOC and microbial C and N. Bacterial populations in the fumigated soils averaged 5.5×106 CFU g−1 soil for all series, except for the Timmerman sandy loam which was 11.6×106 CFU g−1 soil. Bacterial populations were significantly lower in the fumigated Quincy loamy fine sand and Warden silt loam series, significantly greater in the Timmerman sandy loam, and were not different in the Quincy sand or Shano silt loam series compared to the cover crop treatment. Fumigated treatments had significantly greater total Pseudomonas (Fig. 4d) and fluorescent Pseudomonas (Fig. 4e) populations than the cover crop treatments among all soils, except for the Quincy sand series. Pseudomonas represented, on average, 1.0 and 2.3% of the total culturable bacterial populations for the cover crop and fumigated treatments, respectively. Fumigated treatments supported 2.5 times (14.5×104 CFU g−1 soil) the populations of Pseudomonas compared to the cover-cropped treatment (6.5×104 CFU g−1 soil), except for the Quincy sand series which was not significantly different. Fluorescent Pseudomonas represented, on average, 9 and 11% of the Pseudomonas populations for the cover crop and fumigated treatments, respectively. Fumigated treatments supported twice (12.7×103 CFU g−1 soil) the populations of fluorescent Pseudomonas compared to the cover-cropped treatment (6.8×103 CFU g−1 soil). Elliott and Des Jardin (2001) reported variable results in bacterial numbers following metam sodium fumigation. They followed the change in selected bacterial populations following fumigation of sandy soils. They found that after 50 days, Pseudomonas populations in treated soils were either not significantly different or had increased significantly from prefumigation population levels, including control treatments. In contrast, several microcosm studies have shown significant reductions in soil bacterial populations as well as their diversity that did not recover to prefumigation levels even after 100 days of incubation (Toyota et al. 1999). It seems likely that in situ studies result in a greater survival and variability in the response of bacterial populations than confined soil systems, due to the heterogeneous nature of both environmental and physical conditions of natural soils. Fungal populations (Fig. 5a,b) in the mustard green manure and fumigated treatments in the 2-year study on the Quincy sand were not significantly different prior to fumigation, compared to the fallow control in both years, averaging 33×102 CFU g−1 soil in the fall 2000 and 50×102 CFU g−1 soil in the fall 2001. Fungal populations in the mustard green manure were nearly double (61×102 CFU g−1 soil) compared to that of the fallow or fumigated treat-

Mustard Fumigated

60 50 40 30 20 10 0

Quincy sand


Warden silt loam

Shano silt loam

Fig. 5 Fungal populations for the 2000–2001 (a) and 2001–2002 (b) CY of the Quincy sand for the white mustard cover crop, fumigated (metam sodium/1,3 dichloropropene), and fallow treatments. Fungal populations for the five soil series (c). The Quincy loamy fine sand soil had a cover crop of sudangrass var. Sorden 79, the remaining sites were mustard var. Martigena

254 Table 2 Colony forming units (CFU) of selected soil-borne plant pathogens for the Quincy sand soil for the 2000–2001 crop year Sample date

Treatment Pythium spp. Fusarium spp. V. dahliae (CFU/g soil) (CFU/g soil) (CFU/g soil)

March Fumigated 0a 2001 Fallow 106b (preplant) Mustard 130b

1,200a 5,479b 4,320b

0a 11b 6b

Fumigated metam sodium/1,3 dichloropropene in fall, 2000. Mustard, var. Martigena. Treatments followed by the same letter, within a soil series, are not significantly different at p=0.05

ments (36×102 CFU g−1 soil) in the fall of 2001. Preplant samplings showed that the fumigated treatment supported a significantly smaller fungal population, averaging 10% of the fallow control in the spring of 2001, but were similar in spring of 2002 (10×102 CFU g−1 soil). Reductions in the general fungal populations on the Quincy sand were mirrored by reductions in populations of plant pathogenic fungi belonging to Pythium spp., Fusarium spp., and V. dahliae (Tables 2, 3). Fumigation significantly reduced Pythium spp. by 88% and Fusarium spp. by 78% compared to the fallow or mustard green manure in the spring of 2001. In 2002, these fungi were reduced 46 and 58% in the fallow/fumigated treatment, respectively. The number of Pythium spp. and Fusarium spp. in the mustard green manure treatment was not significantly different from the fallow treatment in the spring of 2001 (Table 2), but was significantly greater only in Pythium numbers compared to the fallow in 2002 (Table 3). V. dahliae populations in the fumigated treatment were only significantly different from the fallow and mustard prior to planting in 2001. Counts of V. dahliae in the mustard Table 3 Colonies forming units (CFU) of selected soil-borne plant pathogens in fumigated and cover crop treatments of five soil series sampled prior to spring planting 2002 Soil series

Treatment

Quincy sand Fumigateda Fallow Mustard Quincy Fumigateda loamy fine Sudangrass sand Timmerman Fumigated sandy loam Mustard Warden silt Fumigateda loam Mustard Shano silt Fumigated Mustard loam

Pythium Fusarium spp. (CFU spp. (CFU g−1 soil) g−1 soil)

V. dahliae (CFU g−1 soil)

151ab 136a 279b 6a 123b

1,756a 3,633b 4,170b 1,199a 6,019b

0.3NS 0.5 0.5 3a 8b

0a 130b 2a 67b 266NS 269

170a 1,549b 753a 4,784b 3,320NS 3314

10a 22b 57a 117b 15NS 17

Mustard, var. Martigena; Sudangrass, var. Sorden 79 Fumigated with metam sodium/1,3 dichloropropene in fall, 2001, remaining soils fumigated in the spring, 2002 b Treatments followed by the same letter, within a soil series, are not significantly different at p=0.05. Only one field replicate NS Not significantly different a

treatments were not significantly different either year compared to the fallow treatment. Among all the soil series, fumigated treatments had significantly smaller fungal populations than the cover crop treatments prior to planting, with the greatest differences found for the Quincy sand, Quincy loamy fine sand, and the Timmerman sandy loam (Fig. 5c). Fungi surviving fumigation were 33, 18, and 23% of the cover crop treatments, respectively, for these three soil series. Fungal populations surviving fumigation in the two silt loam soils (Warden, Shano) averaged 70% of the cover crop treatment. Reductions in the general fungal populations again reflected reductions in populations of Pythium spp., Fusarium spp., and V. dahliae across soil series (Table 3). Fumigation, on average, significantly reduced Pythium spp. by 97%, Fusarium spp. by 84%, and V. dahliae by 56%, compared to the cover crop treatment in the Quincy loamy fine sand, Timmerman sandy loam, and Warden silt loam soils. Soil-borne numbers of Pythium and Fusarium, but not V. dahliae, were reduced by fumigation compared to the mustard treatment. Populations of Pythium spp., Fusarium spp., and V. dahliae in the Shano silt loam where not significantly different between fumigated and covercropped treatments. The reduction of these fungal genera (Pythium spp., Fusarium, and V. dahliae) due to fumigation with metam sodium and 1,3 dichloropropene is not surprising. A recent study details the use of these fumigants and particularly shows the efficacious use of metam sodium when growing potatoes (Hamm et al. 2003). Their work compared effects on control of these soil-borne pathogens on a single soil type. Results reported here suggest that different soil types apparently cause differing responses to fumigation. The numbers of fungi surviving fumigation were greater for the fine- than coarse-textured soils, suggesting poor penetration of the fumigant, physical protection of fungal structures by the heavier textured soil matrices, biodegradation of bioactive compounds (Warton et al. 2001), or sorption and inactivation of the fumigant by soil constituents (Brown and Morra 1997). Sorption of bioactive compounds isothiocyanates (ITCs) from Brassica spp. or cyanogens from sudangrass to soil constituents is an important mechanism that decreases their effectiveness. Disappearance and reduced efficacies in soil have been correlated with greater organic carbon and nitrogen contents (Borek et al. 1995). This may in part explain the reduced efficacy in the silt loam soils. The failure of the mustard/sudangrass cover crop treatments to reduce numbers of soil-borne pathogens compared to fumigated soils may be due to several issues. Factors such as the amount of cover crop biomass produced and the subsequent biofumigation products, timing of incorporation, how residues are incorporated, soil type, organic matter content, or depth of incorporation may have all contributed to the level of control. The amount of cover crop biomass returned varied up to almost threefold among the sites sampled, ranging from 2,360 kg ha−1 on the Shano silt loam soil to 6,400 kg ha−1 on the Warden silt loam (Table 4). The variability of cover crop biomass

255 Table 4 Planting and incorporation dates and cover crop biomass yields for the five soil types

Mustard, var. Martigena; Sudangrass, var. Sorden 79 CY Crop year

Soil type

Cover crop type

Date planted

Date of incorporation

Biomass yield dry weight (kg/ha)

Quincy sand (2001 CY) (2002 CY) Quincy loamy fine sand Timmerman sandy loam Warden silt loam Shano silt loam

Mustard Mustard Sudangrass Mustard Mustard Mustard

15 Aug 2000 15 Sep 2001 20 Jul 2001 9 Aug 2001 17 Aug 2001 8 Aug 2001

23 Mar 2001 25 Mar 2002 6 Oct 2001 24 Oct 2001 12 Oct 2001 10 Oct 2001

4,928 4,368 5,135 5,346 6,397 2,360

produced was dependent upon the time of planting, fertilization, and period of growth before winter kill or incorporation. The efficacy of the biofumigation effect is dependent upon the breakdown of the cover crop residue and release of the active compounds (e.g., cyanogenic and isothiocyanates) into the soil. The release of ITCs from mustard residues results from the enzymatic breakdown of glucosinolates contained in plant tissues. Generally, the concentration of glucosinolates peaks just before flowering. However, biomass continues to increase until the plants begin to dry or are frost killed. The time of incorporation for obtaining the maximum biofumigation effect is yet to be established (McGuire, 2002). Second, to produce ITCs, the glucosinolates must be exposed to specific enzymes (myrosinases), which are normally separated from the glucosinolates in plants tissues (Brown and Morra 1997). This is also true for producing HCN from dhurrin in sudangrass (Conn 1981). The current practice is to chop the green manure cover crop before incorporating to insure that effective enzyme mixing occurs. It has been suggested that fall incorporation is best for maximal release of the biofumigant compounds and inhibition of soil-borne pathogens, as well as reducing degradation of the bioactive compounds by the soil microflora (McGuire 2002, 2003). Reduced control of soil-borne pathogens by the cover crop treatments within the root zone of potato does not necessarily mean a reduced impact on yield. Of the three fungal genera isolated during the study, only one was enumerated as a known plant pathogen, V. dahliae, the most important causal agent of potato early dying. Potato yields were only significantly different among the fumigated and cover crop treatments (85.6 and 60.5 Mg ha−1, respectively) for the Quincy fine sandy loam soil that had a cover crop of sudangrass (Table 5). A higher incidence of potato early dying was observed in the sudangrass cover Table 5 Potato yields for fumigated and cover crop field trials for four soil types

Soil type

The Warden silt loam was not planted to potatoes. Mustard, var. Martigena; Sudangrass, var. Sorden 79 a Values within a soil type row followed by the same letter are not significantly different at the 1% level

Quincy sand (2001 CY) (2002 CY) Quincy loamy fine sand Timmerman sandy loam Shano silt loam

crop than the fumigated treatment (A. McGuire, personal communication). Best suppression of potato early dying disease is thought to result from the prompt incorporation of the sudangrass cover crop and the subsequent release of cyanogenic compounds from decomposing residues. Incorporation of the sudangrass residue was delayed at the Quincy fine sandy loam site for 3 days due to a mechanical breakdown which may have reduced the efficacy of the cyanogenic compounds released from the sudangrass cover crop. Nematodes Fumigation significantly reduced nonplant parasitic, freeliving nematodes in all soil types and had variable effects on lesion and stunt nematodes (Table 6). Stunt (Tylenchorhynchus spp.) and Lesion (Pratylenchus spp.) nematodes were not found in all soil series with a significant difference between treatments detected only in the Shano silt loam soil. Among all soil series, fumigated treatments had significantly smaller nematode populations than the cover crop treatments, with the greatest differences found for the Quincy loamy fine sand, Shano silt loam, and Quincy sand. Free-living nematode populations following fumigation were 16, 33, and 65% less of the cover crop treatments for these three soil series, respectively. The incorporation of Brassica spp. and sudangrass as green manures to suppress plant-parasitic nematode populations has been widely studied (Mojtahedi et al. 1993a; Brown and Morra 1997; Potter et al. 1998; Abawi and Widmer 2000). Under field conditions, sudangrass incorporated as green manure has been shown to reduce the rootknot nematode, Meloidogyne chitwoodi (Mojtahedi et al. 1993b). Recently, Widmer and Abawi (2002) showed that

Potato variety

Cover crop type

Potato yield Fumigated (Mg ha−1)

Ranger Russett Ranger Russett Russett Burbank Norkotah Russet Burbank

Mustard Mustard Sudangrass Mustard Mustard

70.6aa 72.4a 85.6a 77.5a 76.2a

Cover crop (Mg ha−1)

71.2a 73.7a 60.5b 76.4a 76.2a

256 Table 6 Nematode counts in fumigated and cover crop treatments of four soil series sampled prior to planting Mustard, var. Martigena; Sudangrass, var. Sorden 79 a Fumigated with metam sodium/1,3 dichloropropene in fall of 2001. Remaining soil types fumigated, spring 2002 b Treatments significantly different within a soil series at *p=0.05, **p=0.1 NS Not significantly different, nf not found

Soil series

Treatment

Nonplant parasitic

Pratylenchus spp. (lesion, number/250 g soil)

Tylenchorhynchus spp. (stunt, number/250 g soil)

Quincy sand

Fumigateda Fallow Mustard Fumigateda Sudangrass Fumigateda Mustard Fumigated Mustard

90ab 74a 138b** 29a 178b* 36NS 89 132a 404b*

8NS 1 30 nf nf 11NS 37 151a 238b**

3NS 8 11 nf nf nf nf nf nf

Quincy loamy fine sand Warden silt loam Shano silt loam

there is a direct relationship between cyanide concentration in sudangrass hybrids used as green manure and suppression of nematodes. Sudangrass as green manure suppressed both root-knot nematodes in Idaho and V. dahliae, the causal agent of potato early dying complex (Davis et al. 1994). Brassica spp. and sudangrass used as green manures might effectively reduce plant-parasitic nematodes directly and indirectly by encouraging either competitive exclusion or organisms antagonistic to nematodes to increase in the soil (Riga et al. 2003).

Conclusion Under field conditions, the response of soil microbial populations and selected activities were investigated among five soil series that were planted to cover crops and fumigated with metam sodium and 1,3 dichloropropene. Broad-scale properties such as microbial biomass C and N, total culturable bacterial, and fungal numbers and C and N mineralization potentials were generally not different between these treatments. However, fumigated treatments generally had smaller microbial biomass and overall lower activity. Fumigation resulted in greater populations of Pseudomonas than cover-cropped treatments. The influence of fumigation on total fungal numbers, selected soil fungi (Pythium spp., Fusarium spp., and V. dahliae ), and nematodes were more pronounced, supporting significantly smaller populations compared to the cover crop treatments among soil series. The numbers of bacteria and fungi surviving fumigation were greater for the fine- than coarse-textured soils, suggesting physical protection of organisms within the soil matrix or potentially a reduced penetration and distribution of the fumigant in fine-textured soils. This suggests the potential need for a higher rate of fumigant to be used in fine-textured soils to obtain a comparable reduction in soil-borne pathogens. The use of white mustard and sudangrass cover crops in lieu of fumigation resulted in higher bacterial, fungal, and nematode numbers than fumigated treatments. Although, the numbers of potential soil-borne pathogens were greater in the cover cropped compared to fumigated treatments, final yields of potatoes were not different between the two.

Apparently, the reduction of soil-borne populations of potential pathogenic fungi is not the sole factor that relates to reduced disease and subsequent yield. More work is needed to determine the factors in the soil that are being impacted by fumigation and/or cover cropping and how these factors contribute to plant health. At this time, it appears by this limited study that cover cropping is as effective at maintaining yield during potato production as is soil fumigation. Acknowledgements This research was supported, in part, by a grant from the Washington State Potato Commission. The authors wish to thank M. Seymour for field assistance; W. Boge and D. Moy (USDA-ARS, Prosser, WA), J. Jaeger (Oregon State University, Hermiston, OR), and M. Lauer and J. Wilson (Washington State University) for sample processing and laboratory analyses. We also thank the commercial potato growers, R. Calloway, H. Friehe, and D. Gies, for access to their fields.

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