Soil microbial community dynamics during ryegrass green manuring and brassica biofumigation

The use of green manure and biofumigant crops within cropping rotations is a common practice for increasing soil fertility, organic carbon levels, improving soil aggregation and preventing erosion. These practices aim to increase the resilience of agricultural soils to degradation and protect important ecosystem services. However, knowledge of how soil microbial communities respond to biofumigation and green manuring at fine time scales is lacking, and this information is required to determine when organic amendments produce their maximum effect and how long their effects last.

This study compared how a winter-grown ryegrass green manure (Lolium multiflorum, var. Tetila) and brassica biofumigant (Brassica juncea, var. Caliente 199) affected the bacterial, fungal and non-fungal eukaryote communities of an intensively cropped temperate vegetable production soil. The aim was to determine the timing, magnitude and duration of ryegrass green manuring and brassica biofumigation's effects on the microbial community dynamics of the bulk soil. We analysed the soil microbial communities with high throughput amplicon sequencing of soil DNA extracts at key times within one growing season, focussing on the period around incorporation, to determine how the communities changed in response to the treatments.

Ryegrass and biofumigant green manure crops both had significant effects on the soil microbial ecosystem, and the fungal and non-fungal eukaryote communities were more responsive than the bacteria. Community change in the biofumigant and ryegrass treated plots was greatest in the weeks immediately after incorporation and slowly receded with time. However, the fungal and non-fungal eukaryote communities of all treatments remained significantly different to each other at the end of the experiment 35 weeks post-incorporation. We also found that soil pH was variable across the site and strongly related to differences in bacterial, fungal and eukaryote community structure.