Influence of initial C/N ratio on chemical and microbial composition during long term composting of straw

Shredded straw of Miscanthus was composted in 800-L boxes with different am ounts of pig slurry added as nitrogen source. The impact of the different i nitial CIN ratios (11, 35, 47, 50, and 54) on the composting process and th e end product was evaluated by examining chemical and microbiological param eters during 12 months of composting. Low initial C/N ratios caused a fast degradation of fibers during the first three months of composting (hemicell ulose: 50-80%, cellulose: 40-60%), while high initial C/N ratios resulted i n 10-20% degradation of both hemicellulose and cellulose. These differences were reflected in the microbial biomass and respiration, which initially w ere higher in low C/N treatments than in high C/N treatments. After 12 mont hs of composting, this situation was reversed. Composts with high initial C IN ratios had high microbial biomass (15-20 mug ATP g(-1) OM) and respirati on rates (200 mug CO, h(-1) g(-1) OM) compared to treatments with low initi al C/N ratios (less than 10 mug ATP g(-1) OM and 25 mug CO2 h(-1) g(-1) OM) . This could be explained by the microorganisms being nitrogen limited in t he high C/N ratio treatments. In the low C/N ratio treatments, without nitr ogen limitation, the high activity in the beginning decreased with time bec ause of exhaustion of easily available carbon. Different nitrogen availabil ity was also seen in the nitrification patterns, since nitrate was only mea sured in significant amounts in the treatments with initial C/N ratios of 1 1 and 35. The microbial community structure (measured as phospholipid fatty acid, PLFA, profile) was also affected by the initial C/N ratios, with low er fungal/bacterial ratios in the low compared to the high C/N treatments a fter 12 months of composting. However, in the low C/N treatments higher lev els of PLFAs indicative of thermophilic gram-positive bacteria were found c ompared 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 differen t fungal/bacterial ratios could also be explained by the initial heating ph ase, since a significant correlation between this ratio and heat generated during the initial composting phase was found.