Earthworm mucus improving decomposition of maize stover and its mechanism

Returning crop residues to soil is regarded as a beneficial agricultural management practice. The decomposition of the returned organic residues is a research concern, and the soil fauna plays an important role during the degradation process. Earthworms are generally regarded as a major and beneficial component of soil fauna. In the present study, the decomposition process was simulated by mixing the maize stover with the ignited quartz sand. The mucus of the earthworm Eiseniafetida which was an important component of drilosphere, glucose or glutamate were added to the mixture. Besides, glucose and glutamate were easily utilizable substrate for the microorganism. The total carbohydrate and crude protein contents in the mucus were analyzed. The total carbohydrate content was 1.3 mg/mL, and the crude protein content was 3.4 mg/mL. SDS-PAGE showed that the protein in the mucus was mainly comprised of two specific protein molecules with molecular weight approximately 44.6 and 21.5 kDa. The carbon and nitrogen mineralization, enzyme activity and microbial community structure were investigated in detail. In the experiment, the carbon mineralization rates of Glc and Glu groups reached a peak value at around 10th day, while that of Muc group reached a maximum value at 20th-30th day. The maximum nitrogen mineralization rates in both Glc and Glu groups were in the 20th-30th day, while those in Muc group were in the 30th-40th day. The effect of the mucus was compared with that of glucose and glutamate. During the whole experiment process, the carbon mineralization rate of Muc group was always higher than that of the control group. After comparing with the control group, it could be observed that the mucus had stimulatory effect on carbon and nitrogen mineralization. The delay effect of the mucus was observed when it was compared to glucose and glutamate. The activities of protease and dehydrogenase in Muc group were higher than those in CK group. On the 10th day, the activities of carboxymethyl cellulase and urease in Muc group were 4 times and 1.5 times of those in CK group, respectively. The average values of ACE and chao for bacterial community in Muc group were 2.6 and 3 times of those in CK group, respectively. The values of ACE and chao for fungal community in Muc group were 2.4 times and 2.3 times of those in CK group, respectively. In addition, the Shannon-wiener diversity index of bacterial and fungal communities in Muc group was 1.4 times and 4.8 times of that in CK group, respectively. The relative abundance of Fusarium in Muc group was 2.5 times of that in CK group. The activity of cellulolytic enzyme, protease, urease and dehydrogenase was increased by the addition of the mucus. The microbial abundance and diversity in the group treated with the mucus was higher than that in the control group. The predominant microbial genera in the group treated with the mucus were lingocellulolytic microorganism and some were involved in nitrogen cycle. This indicated that the mucus of drilosphere played an important role in decomposition of crop residues after being returned to soil.