Influence of the herbicides chlorsulfuron and glyphosate on mycorrhizal soybean inter cropped with the weeds Brassica campestris or Sorghum halepensis

The effect of the herbicides chlorsulfuron and glyphosate on arbuscular myc orrhizal (AM) colonization and plant dry matter was examined in soybean cul tivated either alone or as an intercrop with the weeds Brassica campestris (chlorsulfuron) or Sorghum halepensis (glyphosate). There were 48 treatment s, altogether, 24 with chlorsulfuron and 24 with glyphosate. Each set of 24 was designed as 2 x 3 x 4 factorial with 1) plus or minus Glomus mosseae, 2) soybean alone, weed alone or soybean plus weed combination, 3) herbicide applied at the rates 0, 0.1, 0.5 and 1 x the field recommendation dose. Th e shoot dry mass of AM soybean treated with low doses of herbicides, when g rown together with B. campestris or S. halepensis, but not when grown alone , was increased. This fact together with the absence of an increase in plan t dry mass in intercropped non-AM soybean plants, suggest that the AM fungu s mediates nutrient transfer from weeds to soybean. Neither herbicide affec ted AM colonization of plants except when glyphosate was applied at field r ecommendation dose to the weed S. halepensis grown as an intercrop. The mos t beneficial effect of G. mosseae on soybean was found when chlorsulfuron a nd glyphosate were applied at low doses, but this beneficial effect disappe ared when the herbicides were applied at high doses.