Slow-Release Nitrogen from Composts: The Bulking Agent is More Than Just Fluff

Abstract

One of the goals of byproduct co-utilization is to produce products with increased value, and the amount of slow-release nitrogen (N) supplied by composts for plant growth is one component of compost value. To evaluate the effect of bulking agents on the amount of slow-release N derived from composts, we conducted a three-year field trial with a forage-type tall fescue (Festuca arundinacea Schreb. ‘A.U. Triumph’). Composts were prepared for the field trial from mixtures of food residuals (vegetables, meat, fish, dairy, and bakery by-products) with three bulking agents. Food residuals (33 g N/kg) were bulked with yard trimmings (11 g N/kg), yard trimmings + mixed waste paper (7 g N/kg), and wood chips + sawdust (1 g N/kg). After mixing, the food residual/bulking agent mixtures were composted in a turned windrow supplied with forced air for 70 days, then cured without forced air for 36 days. At the end of curing, total N concentrations in screened compost (< 11 mm) were 17 g N/kg for yard trimmings, 14 g N/kg for yard trimmings + paper, and 8 g N/kg with wood chips + sawdust bulking agent. For the field trial, 155 Mg/ha of compost was incorporated to a depth of 10 cm in a sandy loam soil. Tall fescue was seeded the day after compost application and was harvested 15 times over a three-year period to measure compost effects on grass yield and N uptake. Composts consistently increased yield and grass N uptake in the second and third year after application, demonstrating their slow-release N value. Cumulative apparent N recovery (ANR) over the three-year trial ranged from 7 to 11 % of the compost total N applied. Cumulative ANR was 282 kg N/ha for yard trimmings, 242 kg N/ha for yard trimmings + paper, and 113 kg N/ha for wood chips + sawdust bulking agent. Replacement of wood chips + sawdust bulking agent with yard trimmings more than doubled compost slow-release N value. Thus, yard trimmings are a valuable feedstock when developing compost products with slow-release N value.

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