4.2 Changes in soil MBC, MBN and MBC/MBN
Our results demonstrated that MBC and MBN had a significant increase in amended treatments over the incubation period. In general, the variability in MBC and MBN after amendment addition was first affected by the characteristics of biochar. Biochar, with its extensive surface area and a porous structure, can better coordinate soil water, fertilizer, air and heat, providing an excellent environment for the growth and reproduction of microorganisms(Clough et al ., 2013); also, the surface of biochar has a high density of negative charge that adsorbs substances toxic to microorganisms, thereby promoting their growth and reproduction, and increasing microbial biomass (Ok et al ., 2015). This also explains why higher contents of M contributed to further increase in MBC and MBN. However, our study showed that MBC and MBN increased up to day 45, and then decreased during the remaining incubation time; this indicated that microorganisms started growing in the presence of easily-available organic substrates (de Mora et al ., 2005), which were rapidly depleted or stabilized after 45 days. This suggested that biochar did not have the capacity to provide additional substrates for microbial growth, and microorganisms could not achieve continuous increase in this mine soil; in turn, we also confirmed that the addition of M was essential for soil microbial growth (also shown by beta diversity of fungi). In addition, soil microbial growth was not only affected by carbon sources, but also regulated by N fertilizers. In our study, MBC and MBN contents were higher in N2 level, in which C-M2-N2 supported highest value, indicating that biochar+N at 1.2 g N kg-1 soil+ microbial agents at 0.8 g kg-1 (corresponding to C/N ratio of 25:1) could satisfy microbial nitrogen demand and contribute to the increase of soil microbial biomass. However, low N and abundant C (C/N ratio of 35:1) may reduce soil microbial biomass.
The level of soil MBC/MBN ratio can reflect the supply capacity of soil nitrogen. A small value of MBC/MBN with high bioavailability of nitrogen can improve the utilization rate of soil N (Liang et al ., 2006). In our study, the average value of MBC/MBN ratio in all amended treatments was significantly lower than that of the CK treatment, indicating that the combination of amendments could effectively improve the utilization rate of nitrogen. Furthermore, MBC/MBN ratio decreased significantly in N2 levels, in which C+M2+N2 with the lowest MBC/MBN ratio. This may be due to the biological activity of N increasing in the combination of biochar and N fertilizer at this level; as a result, more nitrogen can be assimilated by the microorganisms, which increases the contents of MBN, resulting in a decrease in the MBC/MBN ratio. It is also possible that the combination of biochar and N2 level is more conducive to the growth and reproduction of bacteria, which increases the proportion of bacteria in soil microbial community, and causes a decrease in MBC/MBN ratio due to smaller MBC/MBN ratio in bacteria than fungi (Tao et al. , 2016). Overall, our results confirmed proper C/N ratio of 25:1 (corresponding to biochar+N fertilizer at 1.2 g N kg-1soil+microbial agent at 0.8 g kg-1) could contribute to the increase of microbial biomass and effectively improve the utilization rate of soil N in short-term.