Ten years of green manuring and biofumigation alters soil characteristics and microbiota

Green manure and biofumigant crops are common within cropping rotations. These practices aim to maintain or increase the yield of agricultural soils by preventing degradation and protecting important ecosystem services, and their benefits to organic carbon levels, soil aggregation, erosion prevention and overall soil fertility are well documented. However, most studies have only examined the short term (one to three year) impacts of green manures and biofumigants, and information on their long term effects is lacking. This study examines the effects of 10 to 13 years of continuous ryegrass (Lolium multiflorum, var. Tetila) green manuring and brassica biofumigation (Brassica juncea, var. Caliente 199) on the soil's chemical, physical characteristics, microbial communities and resulting yields of a fertile, clay rich soil (Red Ferrosol) within an intensive vegetable cropping rotation. The aim was to assess the treatments' efficacy for improving or altering the forementioned parameters to improve sustainable yields, and to examine how the treatments affected the soil's microbial ecology. Soil aggregate stability was assessed at cash crop maturity, and soil chemistry, soil microbiota, and vegetable yields were sampled after the cash crop harvest for three consecutive years. To determine how the soil microbial communities had changed in response to the treatments soil DNA extracts were analysed by high throughput amplicon sequencing.Ryegrass green manuring and brassica biofumigation both altered the soil microbial communities and appeared to favour copiotrophic, organic matter degrading bacteria and fungi. Both treatments increased the relative abundance of arbuscular mycorrhizal fungi (AMF) by 800 % to 1400 % relative to the fallow in two of the three years. Soil aggregate stability was consistently higher in the ryegrass and biofumigant plots and a 20.4 % increase in soil carbon was observed in the treated plots on one side of the trial site. A 20 % increase in cash crop yield (carrots) was recorded in one of the three years.