Brassicaceae seedmeals, by-products of
mustard and canola oil production, average 6% nitrogen (N) by weight and have
the potential for use as soil amendments to improve nutrient availability. Brassicaceae
seedmeals contain different types and concentrations
of glucosinolates (GLSs)
that produce biologically active and often toxic compounds. Field and
laboratory studies were initiated to investigate the influence of Brassica juncea, Brassica napus, and
Sinapis alba seedmeals on N mineralization. Apparent N mineralization
(ANM) during the growing season was 2 to 5 times greater in seedmeal-amended
(2 t ha-1) soils than in the untreated controls in field studies
conducted in 2005 and 2006. For both years, seasonal ANM in the B. juncea and B. napus
treatments was equivalent to ~70% of the total N applied in the seedmeal. High-GLS B. juncea
and S. alba seedmeals
resulted in significantly lower microbial biomass N than in the low-GLS B.
napus treatment at 4 and 8 d after seedmeal application. In the laboratory study, 15N-labelled
seedmeals were amended to soil at a 2 % (wt. meal:wt. soil) rate and incubated aerobically at constant
temperature and moisture content for 45 d. Microbial respiration was lower and
delayed in the high-GLS B. juncea and S.
alba seedmeals compared to the low-GLS B.
napus seedmeal.
Mineralization of seedmeal N was initially rapid with
40 to 46% of seedmeal-15N recovered in the soil inorganic N pool by
day 15. Nitrification appeared to be inhibited in the S. alba treatment in the laboratory study. Brassicaceae
seedmeals substantially increased plant-available
inorganic N in soil and may serve as an effective organic source of mineralizable N. Although it was of short duration,
evidence of the biological activity of GLS-degradation products was observed in
decreased microbial biomass nitrogen, soil respiration, and nitrification in
high-GLS seedmeals.