Physicochemical Dynamics of Composting Screw Pressed Cattle Slurry Amended with Italian Ryegrass Straw or Gorse Bulking Agents

Abstract

Cattle slurry solid fraction (SF) was composted with increasing rates (0, 25, 33 and 50% v/v) of either Italian ryegrass (Lolium multiflorum L.) straw or gorse (Ulex europaes) to determine the effects of these bulking agents on the physicochemical properties during the composting process and to identify approaches to improve final compost quality. Composting temperatures increased to a maximum of 65°C after 42 days for unamended SF. In contrast, temperatures increased more rapidly in piles mixed with straw (68°C at day 7) or with gorse (74°C at day 3). Gorse or straw addition to SF, therefore, also increased the initial rates of organic matter mineralization. However, potential organic matter (OM) mineralization and compost N concentration decreased with the addition of the bulking agents. C/N ratios declined from 32-38 to a value of 13-17 towards the end of composting and followed a similar trend for all compost treatments. Low compost temperature, low C/N ratio and the small content of NH₄⁺ combined with increased concentrations of NO₃⁻ indicated SF composts were stabilized and suitable for use in agriculture. High concentrations of OM (780-840 g kg⁻¹ dry matter (DM)) and total N (28-35 g kg⁻¹ DM), and low electrical conductivity (0.7-1.2 dS m⁻¹) suggested that SF composts would be effective soil amendments with agronomic and environmental advantages. The addition of straw or gorse also enhanced compost sanitation.