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Alfalfa-clover grass – biogas digestate – cooperation

The farm investigated in the project supplies around 65 t of alfalfa-clover grass (dry matter), from a total of 8.4 ha of donor area, to the biogas plant. About 70 % of the nitrogen contained in the alfalfa-clover grass is returned from the biogas plant to the farm in the form of liquid and solid digestate.

An analysis of the causes of the losses showed that about 8 % of the nitrogen is lost during storage in silo windrows, and 1 % during the uncovered storage of liquid digestate. Most of the C contained in the clover grass is utilised as CH4 for the production of electricity and heat for about 1,000 households. Around 17 % of the C supplied in the alfalfa-clover grass is returned to the farm as organic fertiliser. This portion mainly consists of compounds that remain relatively stable in the soil after fertiliser application. A site-adapted humus analysis of the farm showed a positive humus balance of +233 kg C/ha over the 7-year crop rotation, benefitting mainly from the two-year cultivation of lucerne-clover grass. Machinery use for 1,000 kg of C applied on the receiving field in the course of the cooperation is comparatively high at 461 kg CO2e, but the CO2 consumption for 100 kg N is very low at 202.13 kg CO2e. With a cost of € 299 for 100 kg N, the biogas digestate cooperation caused less than half the cost incurred for the corresponding amount of N in the form of commercial organic fertilisers that meet organic standards.