Effects of adjusting C/N ratio of wilted maize straw on wet storage process and subsequent methane production

In northern China, large amounts of wilted maize straw as agricultural waste are produced annually due to the cropping system, harvesting method and so on. Straw-biogas-fertilizer has become one of the promoted utilization modes in major grain-maize producing areas of northern China. Straw is anaerobically digested to produce methane while digestate (effluent of anaerobic digestion process) is utilized as organic fertilizer. Wet storage is always reported as a preferred strategy to preserve energy crops for methane production. It can control microbial activity by a combination of an anaerobic environment and a natural fermentation of sugars by lactic acid bacteria on the crop to achieve lower dry matter loss compared to hay or open-air storage. However, the C/N ratio of wilted maize straw is generally 100-60:1, not only affects the microbial activity during storage, but also is not conducive to the anaerobic digestion. In this study, the effects of adjusting the C/N ratio of wilted maize straw by adding sucrose and soybean meal on the storage quality and subsequent methane production were studied. Wilted maize straw has been stored for 60 days with five treatments, including CK group (without additives), S group (only sucrose and water were added where the water-soluble carbon content and moisture content were adjusted to 3.5% and 65%, respectively), S-SM1 group (sucrose, soybean meal and water were added where, the water-soluble carbon content, C/N ratio and moisture content were adjusted to 3.5%, 30:1 and 65%, respectively), S-SM2 group (sucrose, soybean meal and water was added where the water-soluble carbon content, C/N ratio and moisture content were adjusted to 3.5%, 25:1 and 65%, respectively), S-SM3 group (sucrose, soybean meal and water were added where the water-soluble carbon content, C/N ratio and moisture content were adjusted to 3.5%, 20:1 and 65%, respectively). The results of 60-day wet storage experiment showed that, when the C/N ratio of wilted maize straw was adjusted to 30:1, 25:1 and 20:1, there was no significant effect (P>0.05) on pH value and dry matter loss but decreased the hemicellulose content compared to S group. At the same time, the lactic acid content was increased by 19.0%, 22.2% and 31.7%, respectively. The acetic acid content was increased by 20.0%, 30.0% and 30.0%, respectively. The butyric acid content was decreased by 55.0%, 65.0% and 65.0%, respectively. The relative abundance of lactic acid bacteria (Lactobacillus, Weissella, Enterococcus, Pediococcus) was increased by 13%, 45% and 63%, respectively. The relative abundance of Clostridium was reduced to 0%. The results of LEfse analysis showed that Lactobacillus and Cellulosimicrobium become biomarker when the C/N ratio of wilted corn straw was 20:1. The results of biomethane potential test showed that, the specific methane yield of CK group, S group, S-SM1 group, S-SM2 group and S-SM3 group was increased by 5.2%, 3.7%, 3.9%, 6.1% and 10.8%, respectively, compared with treatment groups before wet storage. Simulating the anaerobic digestion performance by the modifying Gompertz equation shows that adjusting the C/N ratio of straw can reduce the lag period. In summary, by adding water-soluble carbon and protein nitrogen to adjust the C/N ratio of wilted maize straw, it can effectively improve the storage quality, biodegradability and specific methane yield of wilted corn straw. In practical engineering applications, in order to reduce the cost of the project, it is appropriate to supplement other agricultural wastes with high nitrogen content for mixed storage.