Association of aqueous hydrogen concentration with methane production in continuous cultures modulated to vary pH and solids passage rate

The objective of this study was to evaluate the effects of altering pH and solids passage rate (kp) on concentration of aqueous H2 [H2(aq)], CH4 production, volatile fatty acids (VFA) production, and fiber digestibility in a continuous culture fermentation system. The present study was conducted as a 2 × 2 factorial treatment arrangement in a Latin square design using continuous culture fermentors (n = 4). Our continuous culture system was converted to a closed system to measure CH4 and H2 emission while measuring H2(aq) concentration and VFA production for complete stoichiometric assessment of fermentation pattern. Treatments were control pH (CpH; ranging from 6.3 to 6.9) or low pH (LpH; 5.8 to 6.4) factorialized with solids kp that was adjusted to be either low (Lkp; 2.5%/h) or high (Hkp; 5.0%/h); liquid dilution was maintained at 7.0%/h. Fermentors were fed once daily (40 g of dry matter; 50:50 concentrate:forage diet). Four periods lasted 10 d each, with 3 d of sample collection. The main effect of LpH increased nonammonia nitrogen flow, and both LpH and Hkp increased nonammonia nonbacterial N flow. We observed a tendency for Hkp to increase bacterial N flow per unit of nonstructural carbohydrates and neutral detergent fiber degraded. The main effect of LpH decreased H2(aq) by 4.33 µM compared with CpH. The main effect of LpH decreased CH4 production rate from 5 to 9 h postfeeding, and Hkp decreased CH4 production rate from 3 to 9 h postfeeding. We found no effect of LpH on daily CH4 production or CH4 produced per gram of neutral detergent fiber degraded, but Hkp decreased daily CH4 production by 33.2%. Both the main effects of LpH and Hkp decreased acetate molar percentage compared with CpH and Lkp, respectively. The main effect of both LpH and Hkp increased propionate molar percentage, decreasing acetate-to-propionate ratio from 2.62 to 2.34. We noted no treatment effects on butyrate molar percentage or total VFA production. The results indicate increasing kp and decreasing pH decreased acetate-to-propionate ratio, but only increasing kp decreased CH4 production; lack of differences for LpH might be a result of compensatory methanogenesis during the second half of the day postfeeding.