Delayed application of N-rich substrate affects GHG emissions during composting

Dissertação de Mestrado em Engenharia AgronómicaA significant amount of nitrogen (N) is lost during composting and this loss reduces the fertilizing value of the final product. Greenhouse gas emissions (GHG) – mainly methane (CH4) and nitrous oxide (N2O) – also occur during composting. High temperatures (> 45°C) increase ammonia volatilisation. The addition of an N-rich substrate after the thermophilic phase (late addition) is therefore proposed as a way of reducing nitrogen losses and GHG emissions from composting thereby improving the agronomic value of the final product. Vermicomposting – composting by earthworms – has also been suggested for reducing N losses from composting. However, the effect of these practices on the GHG emissions is unclear. The objective of this thesis was, therefore, to investigate the effect of delayed addition of N-rich substrate on greenhouse gas emissions during composting and link this effect to the different composting method. The results showed that the treatments with early addition increased CO2 production (P = 0.03), irrespective of the composting method. Apart of the mixing ratios of the substrate, vermicomposting increased the CO2 emissions by 22 - 47% compared with traditional composting (P < 0.01). Vermicomposting also increased N2O emissions (P = 0.01) by 160 – 230%, but decreased CH4 emissions (P = 0.01) from 97-253%. The timing of substrate addition on GHG emission showed the same pattern regardless the composting methods. Delayed addition increased N2O emissions, but reduced CH4 production. With vermicomposting, the NO3- concentrations increased from 90%-140% and the NH4+ concentrations decreased by 23 – 44% except in the 3rd treatment where it increased 98%. The pH dropped ( 0,5 – 3%) in the presence of earthworms. In conclusion, the timing of addition of N-rich substrate influences the total GHG emissions and chemical properties of compost and hence further studies are suggested to understand the mechanisms