Brassica crops stimulate soil mineral N accumulation

The impact of Brassica crops and their tissues on accumulation of mineral N in soil was examined in the field and in laboratory incubation experiments. Mineral N accumulation over the summer fallow increased by an additional 39-49 kg/ha in the top 0.10 m of soil following brassicas compared with wheat at 2 sites. At a third site there was no increase in the top 0.10 m, but this was possibly due to leaching, as a 21-39 kg/ha increase was detected over the 1.50 m profile. The accumulation of mineral N in soil collected after harvest of canola crops and incubated in the laboratory was double that of soil collected after non-Brassica crops. This outcome was not evident in soil collected when crops were flowering, only occurred in the top 0.05 m of soil, and did not persist beyond week 3 of the incubation. In further laboratory incubations using tissues from wheat and a range of brassicas matched for C: N ratio but differing in glucosinolate concentration, Brassica root tissues initially immobilised, and later released, mineral N at a greater rate than wheat root tissues. These results suggest that enhanced accumulation of mineral N following Brassica crops compared with cereal crops is unlikely to be due to biofumigation of the soil microbial community. Shifts in the composition of the soil microbial community and differences in the chemical constituents of root tissues and in above-ground crop residue inputs may instead be responsible.