Abstract:Instability of high solid-state anaerobic digestion often happened during the longterm operation of reactor. To investigate underlying methanism of this problem, the characteristics of semicontinuous anaerobic digestion for rice straw were studied with three organic loading rates (OLR) in a horizontal reactor, when being co-inoculated with ruminal contents and anaerobic sludge. The results indicated that the highest volume biogas production reached 1.04L/(L·d) in the reactor. The average CH4 productivity reached 280.90mL/g VS with 54.39% CH4 volume content at the OLR of 2.26g/(L·d), which was up to 80.29% of the theoretical value of rice straw. The highest digestion rates of hemicellulose and cellulose were 49.71% and 31.25%, respectively. And the cellulase activities were increased obviously in the horizontal reactor, which was indispensable to the degradation of cellulose. At the OLR of 2.47g/(L·d), the average concentration of ammonium nitrogen reached 1082.63mg/L. Especially the highest concentration reached 1196.37mg/L after 192d of operation. It was indicated that the methanogens suffered different influences from ammonium nitrogen using quantitative realtime PCR(Q-PCR). Specifically, the hydrogenotrophic Methanobacteriales was decreased to 1.04×106 copies/g from 1.70×109copies/g dominated in the solid residues and CH4 productivity was decreased to 256.54mL/g for being inhibited significantly. In contrast, the acetotrophic Methanosarcinales was increased to 9.44×106 copies/g from 7.89×106 copies/g. At the same time, the average propionate concentration reached 253.32mg/L in the anaerobic reactor. Thus the study revealed the adaptive shifts of underlying methanogenic communities by ammonium nitrogen inhibition during the high solidstate digestion of rice straw.