Chemical properties of agro-waste compost affect greenhouse gas emission from soils through changed C and N mineralisation

The effect of different composts and their chemical composition on soil C and N mineralisation and greenhouse gas (GHG) emissions was explored by determining NH₄⁺ and NO₃⁻ dynamics and monitoring CO₂, N₂O and CH₄ fluxes from amended soils under laboratory conditions. Eight different composted agro-industrial wastes were incubated with a clay soil for 55 days at 20 °C and 60% water-holding capacity (WHC). Mineralisation of the C added to the soils was directly correlated to C/N, total phenolic/N, gallic acid/N and NH₄⁺ and negatively correlated with total organic N (TON) and lignin content of the composts. Soils amended with straw compost presented the highest C mineralised with significantly higher CO₂ emissions throughout the incubation period. N mineralisation was only negatively correlated with C/N ratio and straw compost exhibited a significant and strong immobilisation when compared with the rest of treatments. The study demonstrates that the C/N ratio by itself is not suitable for predicting N mineralisation. The results revealed that composts with high initial soluble N contents and narrow C/N ratio, as in the case of coffee grounds, produce higher N₂O emissions. With the exception of mimosa (Acacia dealbata) and coffee grounds that behave as sources of methane, soils amended with composts displayed an overall sink effect for methane. These findings reveal that the effect of compost incorporation on GHG emissions depends essentially on their chemical composition.