During the summer of 2004, we conducted a survey of six production crops to investigate the effects of using Brassica species as fall or winter cover crops on the arbuscular mycorrhizal fungi (AMF) colonization of vegetable crops. The impetus for this survey stems from current research that indicates the secondary plant compounds found in Brassicas may reduce disease and weed pressures in subsequent production crops.  Several recent studies have demonstrated a significant increase in the release of these compounds through the pulverization of plant tissue by flail mowing and immediate irrigation, a process coined as biofumigation. Some of the reported effects of biofumigation are reduced weed competition, nematodes, bacteria, post harvest pathogenic fungi, and most notably, a reduction of some soil-borne pathogens.

Biofumigation holds promise as an important management tool for conventional farmers who are currently under great pressure to find alternatives to chemical fumigation practices, and organic farmers who seek acceptable means of reducing soil pathogens to improve yields.  There are however, large gaps in our understanding of the mechanisms and impacts of biofumigation particularly the potential negative impacts to beneficial soil organisms. Of particular concern are AMF fungi, which are thought to play an important role in nutrient acquisition and uptake in low-input organic farming systems.

Although there have been some studies that have shown reduced AMF colonization following brassica cover crops there has been little evidence that there could be subsequent impacts to crop yields or nutrient content.  To investigate the possibility that brassica cover crops reduce AMF colonization to the detriment of subsequent production crops we conducted a survey of six production crops (fresh market tomato, processing tomatoes, onions, cantaloupe, and romaine lettuce), planted after a brassica cover crop in the spring of 2004. Included in this survey was a USDA organic trial currently comparing eight cover crop treatments, which include standard planting rates of a mustard, rye and a legume/rye cover crop mix, treatments of the same cover crops at three times the standard planting rate with a fallow and a fallow without compost. The results of the survey indicate:

• There is little change in AMF colonization due to brassica cover crops planted at the standard planting rate. Differences in vegetative and fruit biomass or nutrient uptake between crops grown following the various treatments were minimal
• There is a positive correlation between Zn uptake and colonization (R2 = 0.35, P < 0.02)
• There was a significant decrease in phosphorous uptake in the onions (P < 0.002) and an increase calcium (P < 0.01) uptake in romaine following mustard.
• There may be some reduction in colonization at the higher planting rates of mustard but these reductions do not differ significantly from those of fallow or other cover crops at the standard rate