Shredded straw of Miscanthus was composted in 800-L boxes with different amounts of pig slurry added as nitrogen source. The impact of the different initial C/N ratios (11, 35, 47, 50, and 54) on the composting process and the end product was evaluated by examining chemical and microbiological parameters during 12 months of composting. Low initial C/N ratios caused a fast degradation of fibers during the first three months of composting (hemicellulose: 50-80%, cellulose: 40-60%), while high initial C/N ratios resulted in 10-20% degradation of both hemicellulose and cellulose. These differences were reflected in the microbial biomass and respiration, which initially were higher in low C/N treatments than in high C/N treatments. After 12 months of composting, this situation was reversed. Composts with high initial C/N ratios had high microbial biomass (15-20 mg ATP g-1 OM) and respiration rates (200 mg CO2 h-1 g-1 OM) compared to treatments with low initial C/N ratios (less than 10 mg ATP g-1 OM and 25 mg CO2 h-1 g-1 OM). This could be explained by the microorganisms being nitrogen limited in the high C/N ratio treatments. In the low C/N ratio treatments, without nitrogen limitation, the high activity in the beginning decreased with time because of exhaustion of easily available carbon. Different nitrogen availability was also seen in the nitrification patterns, since nitrate was only measured in significant amounts in the treatments with initial C/N ratios of 11 and 35. The microbial community structure (measured as phospholipid fatty acid, PLFA, profile) was also affected by the initial C/N ratios, with lower fungal/bacterial ratios in the low compared to the high C/N treatments after 12 months of composting. However, in the low C/N treatments higher levels of PLFAs indicative of thermophilic gram-positive bacteria were found compared to the high C/N treatments. This was caused by the initial heating phase being longer in the low than in the high C/N treatments. The different fungal/bacterial ratios could also be explained by the initial heating phase, since a significant correlation between this ratio and heat generated during the initial composting phase was found.

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