Biogas in stockless organic Farming: Effects of Digestion of Clover/grass, Cover Crops and Crop Residues on Nitrogen Cycles and Crop Rotation Productivity

Summary

A common practice in stockless organic farming systems is to leave the biomass from clover/grass-ley and crop residues in the field for their residual fertility effect. No farmyard manures for transfer of nutrients within the system are available. Clover/grass-ley biomass and crop residues represents an unexploited energy potential that could be harnessed by the digestion in biogas plants for production of methane, thus replacing ruminants by the biogas digester. In field trials by implementing a whole crop rotation comprehending six crops were carried out in 2002-2005 to evaluate whether the use of N could be improved by processing biomass described above in a biogas digester and using the effluents as a fertilizer, compared to general practice.
Results indicate that digestion of crop residues resulted in more efficient manuring systems, not only by the implementation of an additional “product” (power energy), but also by getting more efficient cropping systems with higher DM and N yields of most of the non-legume crops, combined with a reduction of N losses due to denitrification and a reduction of the nitrate leaching risk. The causes were a better and more evenly allocation of nutrients within the whole crop rotation, a higher N input via N2 fixation, lower N losses and probably a higher N availability of digested in comparison to the same amounts of nutrients in undigested organic manures.