Microbial Carbonisation and its potential for on-farm composting : exploring reductive composting as an approach for regenerative agriculture

Sammanfattning: The search for agronomic solutions to the decline of soil health, open-ended nutrient cycles and greenhouse gas (GHG) emissions from agriculture, against the background of impending climate change, were motivations for this thesis. A composting trial on a Swedish farm was designed to find answers to the overarching research question of how feasible and climate-friendly on-farm composting using Microbial Carbonisation (MC) is, compared to conventional windrow composting (CC). MC can be understood as the biological transformation of biomass under mesophilic and anoxic conditions, in contrast to CC, which is an aerobic and partly thermophilic decomposition process. The investigation of the MC method was approached using natural and social science methodologies. Field trials were carried out, accompanied by substrate, soil, emission and pore-gas measurements, as well as records of machinery use. In addition, interviews were conducted with farmers already using MC in Germany, to gain a better insight into its practical application and farmers’ needs. The results suggest that MC substrates can be richer in nitrogen (N) and carbon (C) after composting than CC. The machinery requirement of MC was only one tenth of the more labour-intensive CC process, which is associated with lower fossil emissions. A novelty of the present research was that nitrous oxide (N2O) emissions were measured for the first time on a MC compost. Overall, on a weight basis, MC showed 30 – 40 % lower GHG emissions during composting, compared to CC. This advantage, however, was offset by 28 – 40 % higher emissions in the field on an area basis. In addition, GHG balances are highly dependent on the appropriateness of the measurementmethodology, the period under consideration and the reference unit in which the emissions are expressed. As CC showed higher N-losses during the composting process, MC overall emitted 5 – 29 % less GHG per kg N applied to the field. It was therefore not entirely clear whether MC or CC performed better in terms of GHG emissions. As MC can provide N- and C-rich substrates in a cost-efficient way, it appears promising for the use in regenerative agriculture. The farmers’ interviews supported the results of MC being cost-efficient and practicable for on-farm composting. Nevertheless, the field application of compost can substantially increase the GHG balance of what at first sight appears to be a climate friendly composting process. Future studies need to further address this issue, as well as the impact of MC substrates on soil health.