Greenhouse gas emissions during composting of two-phase olive mill wastes with different agroindustrial by-products

The evolution of CO2, CH4 and N2O were monitored in five composting mixtures prepared from two-phase olive mill waste (TPOMW) and different agroindustrial by-products in order to assess the effect of the initial composition and the N source on greenhouse gas emission. Surface gas fluxes were measured using a closed static chamber and compared to the changes in different organic matter fractions (organic and watersoluble C) and N forms (NH4+ and NO3-). CH4 emissions depended on the organic matter mineralisation dynamics and the incorporation of manure in the starting mixture. The highest CH4 fluxes were registered during the intense degradation at early stages of the process (up to 100 g C m−2 d−1). The emission of N2O (0–0.9 g N m−2 d−1) occurred from 6th to 10th wk of composting (bioxidative phase), coinciding with an intense nitrification in the pile. The use of urea enhanced the N2O emission up to 3.7 g N m−2 d−1, due to an increase in available mineral N in the pile. Even though well managed TPOMW composting piles only represent a minor source of CH4 and N2O emissions, the addition of urea and easily available C fractions to the starting mixtures can significantly increase the environmental impact of TPOMW composting as far as greenhouse gas emissions are concerned.