Nitrogen mineralization of lime-stabilized fishwaste in acid soils and its effect on plant growth

Fishwaste is often disposed of in landfills or the ocean, adversely affecting water quality. With its high protein content, fishwaste could instead be used as a N source for crop production. The viability of a lime-stabilized fishwaste (LSF) product as a N source was tested by determining its effects on net N mineralization, growth of bacteria, and the growth of a variety of plant species in two acidic soils. Net N mineralization and viable bacteria number were significantly increased (P < 0.001) by the addition of LSF. At the highest rate of 30 g LSF kg -1 soil, high NH 4 -N accumulation was extended to several weeks and a longer lag period before bacteria growth occurred. At such a high rate, nitrification, but not the activities of proteolytic enzymes, was severely impaired. The considerably high inactivation rate for the bacteria following the exhaustion of the available energy in the protein in the Sultan soil [half life = 4.5 ± 0.5 d (SD)] may explain why the Sultan soil had a comparatively higher net N mineralization rate than the Puyallup soil [half life = 14.7 ± 3.5 d (SD)]. At 6 g LSF kg -1 soil in a greenhouse trial, the N availability to a variety of crops was generally increased. Unless a soil is extremely acidic, using high rates of LSF can lead to undesirable soil conditions (high pH and NH 4 -N accumulation) that interfere with plant growth.