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Title: SOIL MICROBIAL, FUNGAL AND NEMATODE RESPONSES TO SOIL FUMIGATION AND COVER CROPS UNDER POTATO PRODUCTION

Author
item Collins, Harold
item Alva, Ashok
item Boydston, Rick
item Cochran, Rebecca
item HAMM, P - OREGON STATE UNIV
item RIGA, E - WASHINGTON STATE UNIV

Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2005
Publication Date: 6/1/2006
Citation: Collins, H.P., Alva, A.K., Boydston, R.A., Cochran, R.L., Hamm, P., Riga, E. 2006. Soil microbial, fungal and nematode responses to soil fumigation and cover crops under potato production. Biology and Fertility of Soils. 42:247-257.

Interpretive Summary: The Columbia Basin of Eastern Washington provides ideal conditions for high potato yields of up to 90 Mg ha-1. In the Basin, commercial soil fumigants such as metam-sodium are very effective for the control of soil borne pathogens, weeds, and nematodes that reduce crop yield and quality. Soil fumigation has been assumed to have minor impacts on the general soil microbial community. However, few published data are available that adequately describe changes in microbial populations. The use of cover crops may serve as an alternative to fumigation as well as mitigating degradation to soil and environmental quality. Soil erosion continues to be a threat to agricultural productivity worldwide, with soil losses in the United States alone exceeding 3 billion tons annually. Wind and water erosion cause dramatic declines in soil productivity. Cover crops play a vital role in filling open 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 micro-flora, as well as enhancing microbial populations antagonistic to pathogenic organisms. Several studies have shown that certain cover crops (Sorghum sudanense, Brassica spp.) are also effective in reducing the incidence of pathogenic fungi, plant parasitic nematodes, and weeds through the production and transformation of secondary plant compounds. A study was conducted on five soil types within the Columbia Basin to determine the effects of cover crops and fumigation on soil microbial activities in a potato production system under center pivot irrigation. Results show fumigation significantly reduced soil fungi, but had minor effects on bacterial populations, soil respiration, or N-mineralization potentials. The percentage of bacteria and fungi surviving fumigation were 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 an adequate reduction in soil-borne pathogens.

Technical Abstract: Metam-sodium (sodium N-methyldithiocarbamate) and 1, 3 dichloropropene are widely used in potato production, for the control of soil borne pathogens, weeds, and 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, soilborne pathogens (Verticillium dahliae, Pythium spp., and Fusarium spp.), free-living and plant parasitic 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-d 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 (range 4-22%) than cover cropped treatments. Soil fungal populations and free-living nematodes levels were significantly lowered in fumigated 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 cover crop treatment. The percentage of bacteria and fungi surviving fumigation were 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 an adequate reduction in soil-borne pathogens.