Stale seedbed technique for weed control negatively impacts the indigenous AM fungus population

Authors: Douds, David; Lee, Joseph - Joe; McKeever, Lindsay; DUFFIELD, ROSS - Rodale Institute; ZIEGLER-ULSH, CHRISTINE - Rodale Institute; MOYER, JEFF - Rodale Institute

Submitted to: Biological Agriculture and Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2018
Publication Date: 2/5/2018
Citation: Douds, D.D., Lee, J., McKeever, L., Duffield, R., Ziegler-Ulsh, C., Moyer, J. 2018. Stale seedbed technique for weed control negatively impacts the indigenous AM fungus population. Biological Agriculture and Horticulture. 34:199-210. https://doi.org/10.1080/01448765.2018.1426493.
DOI: https://doi.org/10.1080/01448765.2018.1426493

Interpretive Summary: One method organic farmers commonly used to control weeds is the “stale seedbed technique.” This is when a plot of ground is allowed to undergo several cycles of weed seed germination followed by cultivating the soil to uproot and kill the weed seedlings prior to planting the crop. This procedure was hypothesized to have a negative impact upon the native population of arbuscular mycorrhizal [AM] fungi. These are soil fungi that form a beneficial symbiosis with the roots of most crop plants, assisting the plant in nutrient uptake from the soil and disease and drought resistance. Experiments were conducted at The Rodale Institute over two years, using leek seedlings as the crop plant. Each year, the stale seedbed technique significantly reduced weed growth for one month after planting the leeks. Further, plots receiving the stale seedbed treatments showed reduced populations of AM fungi in the soil relative to control plots. In addition, colonization of the roots of leek seedlings by AM fungi was slower in those plots than plots which did not receive the stale seedbed treatment. Farmers who utilize the stale seedbed technique for weed control may want to consider use of AM fungus inoculum to ensure their vegetable seedlings can take full advantage of the symbiosis due to the harmful effects of the repetitive cultivations upon the native AM fungus population.

Technical Abstract: Among the weed control tools available to organic farmers are a variety of stale seedbed techniques. Stale seedbed methods create a surface layer of soil depleted in viable weed seeds through repetitive cycles of seed germination followed by cultivation to uproot and kill those seedlings. Experiments were conducted in 2011 and 2014 to examine the impact of two stale seedbed techniques, roto-tilling and spading (each at two frequencies), upon the native AM fungus communities in organically farmed high P soil. Weed biomass, propagule densities of the indigenous AM fungus population, development of mycorrhizas in roots of outplanted leek (Allium porrum L.) seedlings, and impact of AM fungus inoculation upon growth and yield were measured for repetitively cultivated vs. control planting beds. Stale seedbed treatments had significant negative impact upon total weed biomass through the first month after planting the leek seedlings. Control beds, cultivated only just before planting, had significantly greater propagule density than soils roto-tilled (Pf>F=0.0004) or spaded (Pr>F=0.0008) at 2-4 week intervals. Early season development of mycorrhizas in outplanted seedlings was greater in the control beds than in repetitively cultivated beds, eg. 6% vs 1% of root length colonized (Pr>F=0.0001) two weeks after outplanting in 2014. Even though the stale seedbed techniques had negative impacts upon the indigenous AM fungus populations, there was no measurable impact upon crop yield in the high P soil used here. Nevertheless, farmers may want to consider use of AM fungus inoculation of vegetable seedlings outplanted into stale seedbed-treated fields low in plant available P.