Soil incorporation of brassica materials and seed treatment with Trichoderma harzianum: Effects on melon growth and soil microbial activity

•Brassica meal and liquid formulation (DSM&LF) showed fertilizing properties in pots.•DSM&LF increased shoot dry weight on melon, soil microbial size and enzyme activity.•Trichoderma harzianum seed treatment plus DSM&LF increased also root length.•DSM&LF effects are linked to ameliorated nutrient cycles due to soil microorganisms.

Brassica nigra defatted seed meal (DSM) and liquid formulation were added to three different soils in a pot experiment to investigate its effect on the growth of melon, on soil microbial size and on six enzyme activities. DSM was added as a pre-sowing treatment; the liquid formulation, based on B. carinata oil and B. nigra DSM, was then repeatedly applied as fertilizer with allelopathic activity. Brassica materials were also evaluated in combination with the melon seed treatment with an isolate of Trichoderma harzianum. This beneficial fungus was previously selected for tolerance to toxic compounds released by brassica DSM. At the flowering stage, brassica treatments determined a doubling in shoot dry weight in all soils; this was paralleled by an increase of both soil microbial biomass (+46%) and enzyme activities, up to +90%. The combination of the applications of brassica materials and seed treatment with T. harzianum also increased root length by 20%. These effects were most probably due to enhanced microbial activity and enhanced biogeochemical cycles of nutrients caused by the treatments with brassica materials and the stimulation of root activity by T. harzianum.