Impacts of Brassica Biofumigation on Non-Target Belowground Organisms, Aboveground Herbivores, and Soil Health Indicators

Abstract

Biofumigation using Brassicaceae cover crops and their byproducts is gaining increasing attention as a natural alternative to synthetic fumigants in both conventional and organic production systems. While its efficacy against soilborne pests and pathogens is well documented, the effects of biofumigation on non-target beneficial organisms in the soil and the potential for these belowground changes to cascade above ground and influence plant-herbivore interactions remain poorly understood. This dissertation evaluated the impacts of soil biofumigation with brown mustard (Brassica juncea (L.)) plant and its seed meal (BMSM) on non-target belowground organisms, soil health indicators, and subsequent impact on aboveground herbivores. The first study evaluated the impact of brown mustard seed meal (BMSM) on the springtail Folsomia candida Willem (Entomobryomorpha: Isotomidae) and the earthworm Eisenia fetida Bouche (Opisthopora: Lumbricidae) under greenhouse condition. BMSM caused an 85% reduction in springtail populations within one hour of application; however, populations recovered by day 19 and surpassed the springtail populations in control treatments by day 26. Earthworms avoided BMSM-incorporated media immediately after incorporation but exhibited significantly greater body weight, cocoon production, and hatch rate when reared in biofumigated media compared to untreated controls. The second study examined the impact of soil biofumigation with brown mustard (Brassica juncea L.) plant, its seed meal (BMSM), and canola (Brassica napus L.) plant on bird cherry-oat aphid (Rhopalosiphum padi (L.), Hemiptera, Aphididae) preference and reproduction on wheat host, in the presence or absence of springtail (F. candida) under greenhouse conditions. The impact of these treatments on wheat biomass and root morphology was also assessed. In preference bioassays, aphids were more frequently found on untreated control plants. The bird cherry-oat aphid reproduction was reduced on wheat plants grown in biofumigated media. The presence of springtails modified these effects only in the canola treatment, where aphids preferred plants grown in canola-incorporated media containing springtails over untreated control. The biomass and root architecture of plants were negatively impacted in treatment involving brown mustard or BMSM containing springtails. The third study evaluated the impact of soil-incorporated brown mustard green manure and seed meal (BMSM), as well as canola green manure on soil arthropods, microbial communities, plant tissue decomposition rate, soil nutrients, and yield in a wheat (Triticum aestivum L.) cropping system field experiment. Arthropod abundance was significantly lower on BMSM and brown mustard-incorporated plots, one month after biofumigation. Soil microbial biomass was significantly greater in BMSM-incorporated plots within 24 hours of incorporation, driven by an increase in fungal biomass, however, by 2- and 10-months microbial biomass was similar among treatments. Plant tissue decomposition rate was greatest in BMSM-incorporated plots 4 months after biofumigation. In BMSM-incorporated plots, soil nitrate was elevated at 2 months, sulfate at both 24 hours and 2 months, and manganese at 24 hours post-biofumigation. Wheat yield did not differ significantly among treatments. These findings fill a critical knowledge gap by demonstrating that biofumigation effects on non-target organisms and soil health indicators are transient and that biofumigation impacts are not limited to subterranean organisms; it can impact aboveground herbivores likely through host-mediated effects.