Objective Planting green manure is proved to be an important practice to increase soil organic matter and fertility, however, the root exudates of green manure are clearly unknown.

Method A pot experiment was conducted by planting white clover, ryegrass and soybean, versus control with no plants, to explore the secretion capacity and exudates compounds of green manure root, as well as their effects on soil microbial function.

Result The results showed that the biomass of soybean was the highest (average 9.11 g), while its secretion organic carbon (OC 8.01 mg) was the lowest which was significantly lower than that of white clover (10.37 mg) and ryegrass (14.78 mg). The root secretion OC based on unit biomass of dry plant and root weight was the highest for white clover (0.408% and 1.249%) and the lowest for soybean (0.088% and 0.312%), indicating that white clover had a strong root secretion capacity. The exudate components were identified by ethyl acetate extraction and GC-MS analysis, there were respective 29, 21 and 28 types of compounds were detected for white clover, ryegrass and soybean. The major compounds were hydrocarbon, ester and alcohol. Ethyl propionate, propyl acetate, tributyl phosphate and heterocyclic pyridine are the dominant components for all green manures, but they vary in proportion. The Biolog method was used to evaluate the characteristics of soil microbial function of three green manure. It was found that soil microorganisms had the strongest ability to metabolize organic carbon for soybean, and the average well color development (AWCD) was in the order of soybean > white clover > ryegrass > control. To the utilization capacity of six types of carbon sources including carbohydrates, amino acids, carboxylic acids, phenolic acids, polymers and amines, the first three types of carbon sources, phenolic acids and polymers, and amines were used the strongest respectively by soybean, white clover and ryegrass.

Conclusion In conclusion, Legume green manure has a better positive effect on soil microbial function than grass ryegrass and was recommended for newly reclaimed land when the major purpose is improving soil microbial function.