Effects of dicyandiamide and Mg/P on the global warming potential of swine slurry and sawdust cocomposting.

Composting is an emerging strategy for swine slurry treatment; nonetheless, significant greenhouse gases (GHG) emissions may occur during this process. We carried out two separate assays with increasing doses of dicyandiamide (DCD; up to 1.1% w/w) as a nitrification inhibitor and solutions of MgCl2 and H3PO4 (Mg/P; up to 0.09/0.06 mol kg-1) to promote struvite crystallization in order to assess their efficiencies as additives to decrease GHG emission during swine slurry cocomposting with sawdust (1:1v/v). We monitored the nitrous oxide (N2O-N), methane (CH4-C), and carbon dioxide (CO2-C) emissions and the ammonia (NH4+-N) and nitrate/nitrite (NOx-N) concentrations in compost reactors (35 L) during the first 4-5 weeks of composting. DCD had no effect on CH4-C and CO2-C emissions but decreased N2O-N losses by up to 56% compared with control. However, DCD inactivation was favored by thermophilic conditions and N2O-N emissions increased to same levels of control after 13 days. Mg/P was effective to decrease N2O-N losses only at the highest dose, which also sustained higher [NH4+-N] in the compost by the end of the assessment. Nonetheless, the use of 0.09/0.06 mol kg-1 of Mg/P also decreased CH4-C and CO2-C emissions compared with lower doses of Mg/P and unamended treatments. Overall, DCD and Mg/P amendments decreased the global warming potential (GWP) of swine slurry composting by up to 46 and 28%, respectively. The Mg/P application may be also interesting to increase the compost quality by increasing its NH4+-N availability. Graphical abstract.