Decomposition characteristics of soil humic acid under different vegetation types

Humic acid (HA) from the 0-10 cm depth from an arable soil, as well as four forest soils (Chinese pine, black locust, Sea-buckthorn and mixed forest) was extracted from a typical area in land conversion projects. The soils were the sole energy constituents in the HA medium. Several species of microorganisms, from soil of these vegetation types and capable of utilizing HA as a carbon and nitrogen sources, were trained and isolated in the medium. Results showed that the HA extracted from the four forest soils had a higher carbon content, and lower nitrogen content, than those of the arable land. Four bacteria strains and one strain actinomycetes were isolated by the HA substrate. The species were Bacillus licheniformis (99.65%), Rhizobium nepotum (99.78%), Microbacterium resistens (98.71%), Stenotrophomonas maltophilia (99.23%) and Streptomyces azureus (99.78%) based on the 16s rRNA sequence analysis. B. licheniformis was the dominant species in HA substrate of arable land, Chinese pine, black locust and mixed forest, and S. azureus was the dominant species in Sea-buckthorn HA medium. R. nepotum was not found in the black locust HA substrate, and S. maltophilia was not found in Sea-buckthorn or mixed forest soils. The colony forming unit (CFU) of decomposing bacteria in HA medium was in the decreasing order Sea-buckthorn>black locus>arable land>Chinese pine>mixed forest. CFU in mixed forest was only 58.87% that of Sea-buckthorn. Soil HA total degradation rate (TDA) was comprised of two parts: respiration carbon (RC) and microbial biomass carbon (MBC). The Microbial RC of mixed forest, Chinese pine, Sea-buckthorn and black locust was higher by 25.35%, 11.28%, 10.36% and 7.11% respectively than that of arable land. The difference between the mixed forest and arable land was significant (P<0.05). MBC of decomposing bacteria in Sea-buckthorn, black locust, arable land, and Chinese pine was higher by 131.91%, 68.48%, 64.98% and 54.47% respectively over that of the mixed forest. The MBC of black locust, arable land and Chinese pine showed no significant differences. However, the difference among the Sea-buckthorn and Mixed forest was significant (P<0.05). TDR of Sea-buckthorn soil HA was significantly higher than that in arable land and mixed forest (P<0.05). Correlation analysis showed that CFU was correlated positively with MBC and negatively correlated with RC negatively (P<0.01). This indicated the change of vegetation type had some influence on the community of HA decomposition bacteria and their decomposition characteristics.