## 1 Introduction  
Ruminants contribute significantly to global methane (CH\({}_{4}\)) emissions. CH\({}_{4}\), a greenhouse gas (GHG), has a major role in climate warming [1]. Agriculture is responsible for approximately 40-50% of global CH\({}_{4}\) emissions, with the enteric fermentation CH\({}_{4}\) from domestic livestock producing 76-92 Tg/year, accounting for 25% of anthropogenic methane sources [2; 3]. CH\({}_{4}\) emissions from domesticated livestock (cattle, sheep, and pigs) in China were 13.21 Tg [4]; however, this data did not include CH\({}_{4}\) emissions from yaks. Yaks, as the main domesticated animals on the Qinghai-Tibet Plateau (QTP) in China, are an important source of income for local herdsmen. There are approximately 15 million yak in Chinese territories; this population continues to grow due to the nutritional value of their by-products [5].  
Enteric CH\({}_{4}\), which results from the fermentation of feed, is primarily emitted from ruminants by eructation. Methanogens in the rumen mainly use H\({}_{2}\) to reduce CO\({}_{2}\) to CH\({}_{4}\). Studies have shown that H\({}_{2}\) production management is the most important consideration in reducing CH\({}_{4}\) emissions in ruminants [6; 7]. There are two main mechanisms to effectively abate CH\({}_{4}\) formation this way [8]: the first is to maintain a low H\({}_{2}\) pressure by inhibiting H\({}_{2}\) liberating reactions. Monensin is an ionophore that can effectively inhibit hydrogen-producing microorganisms (Gram-positive bacteria and protozoa), resulting in more propionate, which means less acetate, formate, butyrate and hydrogen are produced in the rumen [9; 10]. Monensin has been shown to decrease the methane emissions of rumen [11; 12].  
A second option is to promote alternative H\({}_{2}\)-using reactions to compete for the hydrogen used to produce methane. This can be achieved by using propionate enhancers, and other electron acceptors, to decrease H\({}_{2}\) availability for CH\({}_{4}\) production. Propionate precursors (i.e., fumarate, malate, or acrylate) consume hydrogen generated in the rumen through the succinate fermentation pathway [13]. Fumarate is one of propionate precursors, and fumarate-reducing bacteria use H\({}_{2}\) to promote the production of rumen propionate [14]. [PERSON] et al. (2002) noted that fumarate had the most consistent effects, with 50% of added acids being fermented to propionate, resulting in a 14% decrease in methane production [15].  
The effects of monensin sodium salt (MSS) or disodium fumarate (DF) on in vitro fermentation and methanogenesis have been comprehensively reported [16; 17; 18]. However, few studies have explored the combination effects of these two additives on rumen ecosystem function. MSS and DF have different mechanisms of action for reducing rumen methane emissions; therefore, it was hypothesized that, in combination, rumen methane production would be further decreased, as compared to the use of a single compound.  
Here, the effect of supplementation with MSS and DF (alone and in combination) on rumen fermentation, methanogenesis, and the inventory of \"microbial types\" is reported in vitro. The objective of this work was to study: (a) the effect of MSS and/or DF on rumen fermentation and CH\({}_{4}\) production of two different substrates; (b) the relationships between microbial community, ruminal fermentation parameters and CH\({}_{4}\) production.