Processing of Legume Green Manures Slowdowns C Release, Reduces N Losses and Increases N Synchronisation Index for Two Years

The number of livestock farms decreased by 40% in Europe over the last 10-year period. Stockless organic cropping systems started to dominate in many intensive agricultural regions in Europe. Developing the sustainable management of an organic stockless agroecosystem is related to guaranteeing self-sufficiency in nitrogen (N) supply, maintaining high grain yields, and promoting carbon (C) sequestration in the soil. The aim of this study was to investigate if the processed legume green manures can be an alternative to granulated cattle manure and direct ploughing of legume biomass in order to develop the sustainability of the stockless organic cropping system. The decomposition rate and C and N release were observed for green manures made of fermented red clover and composted red clover with wheat straw. Fresh red clover biomass and granulated cattle manure were used for the comparison. Results of the 3-year field experiment showed that technologically processed legume biomass had a positive effect on the productivity of crops at least two years in rotation. Fermented red clover and red clover compost increased N use efficiency by 15% and biomass output efficiency by 16% compared with fresh red clover biomass. Processed legume green manures significantly increased the synchronisation index between crop N demand and N supply. In autumn, incorporated fresh red clover biomass lost 65.6% of its initial C and 37.6 kg ha−1 (50.1%) of its initial N under decomposition in the first non-growing season. It also increased mineral N losses deeper into the subsoil by 52.7%. Meanwhile, fermented red clover and red clover compost released 43% of its N during the first crop growing season, sustained at least one year slower C release to the soil, promoted ecosystem productivity, prevented mineral N losses to subsoil and gained high N synchrony indexes. The best N synchrony was achieved using fermented red clover, with a higher decomposition rate positively significantly correlated (r = 0.47–0.78, p < 0.05) with grain yield, total biomass, protein content and total N accumulated in the plant of spring wheat and spring barley.