Composting of solids separated from anaerobically digested animal manure: Effect of different bulking agents and mixing ratios on emissions of greenhouse gases and ammonia

•Composting digested manure solids with straw or bio-char lowered N2O-N losses.•The lowest NH3-N losses occurred from straw-amended compost.•A 3:1 ratio of digested solids to bulking agent reduced GHG and NH3-N emissions.•Straw and bio-char were most effective in reducing total GHG emissions (as CO2-eq.).

We investigated the effects of bulking agents (BA) and mixing ratios on greenhouse gas (GHG) and NH3 emissions from composting digested solids (DS), separated from anaerobically digested manure and other bio-wastes, in small-scale laboratory composters. BA evaluated were plastic tube pieces (PT), woodchips (WC), bio-char (BC), barley straw (BS) and lupin residues (LR) and were included at a DS:BA of 3:1 or 6:1, resulting in nine treatments: CTDS (control, DS only), PT3:1, PT6:1, WC3:1, WC6:1, BC3:1, BC6:1, BS3:1 and LR3:1. Depending on treatment, C losses via CO2 and CH4 emissions accounted for 41.2–65.3 g C kg−1 initial total solids (TS) and 4.4–191.7 mg C kg−1 TS (8.4–16.1% and 0.001–0.05% of initial total-carbon), respectively, while N losses as N2O and NH3 emissions comprised 2.1–13.6 mg N kg−1 TS and 2.7–4.8 g N kg−1 TS (0.01–0.04% and 9.1–13.0% of initial total-nitrogen), respectively. Most of the CH4 emissions occurred during the thermophilic temperature phase, which had little or no effect on N2O emissions. BS addition to DS resulted in the lowest cumulative NH3-N and N2O-N losses. BC was as effective as BS in reducing cumulative NH3-N losses, but had non-significant effect on CH4-C emissions. Decreasing the mixing ratio from 6:1 to 3:1 reduced losses of CH4-C and N2O-N (except for BC) without any increase in NH3-N losses. BC and BS proved most effective in reducing emissions of total GHG (as CO2-equivalents). Composting of DS with C-rich BA can thus be an effective means of conserving N in DS, while also reducing GHG emissions.