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.