4 Discussion
Despite some fertilizer inputs to the cropland in this study,
significant losses of SOC and STN occurred in cropland compared with
grassland, which suggested that the nutrient pools may be vulnerable to
grassland conversion over time. This was consistent with previous
studies that demonstrated that the conversion of grassland to cropland
usually induces SOC and STN loss
(Don et al. 2011; Wang et al.
2011; Ding et al. 2013). It is mainly because the removal of plant
biomass reduces nutrient cumulate in soil, which in turn decreases the
microbial biomass because less energy is available from soil organic
matter decomposition (Schnitzer et al. 2011). Moreover, the breakdown by
tillage results in large aggregates transforming into fine aggregates
(Figure S2), which makes them more susceptible to erosion by wind and
water that will lead to soil nutrient loss
(Six et al. 2000). However, soil
C:N ratios were significantly different between grassland and cropland
sites. This indicates that the decrease in nitrogen could keep up with
the pace of soil carbon loss after grassland cultivation.
In this study, grassland conversion consistently reduced the total soil
microbial biomass, Gram+, Gram-, Fungi, AMF and Act. The greater
microbial biomass under grassland soil has been previously reported
(Jangid et al. 2011) as there are more favorable soil environmental
conditions for microbes under grassland resulting in larger microbial
biomass. The decreasing soil microbial biomass induced by the grassland
conversion probably resulted from the declines in the concentrations of
SOC and STN, which provide energy sources for microbe turnover. Crop
growth during growing season (August is the mid-growing season in this
ecotone ecosystem) significantly influences soil microbial biomass
because crops compete with microorganisms for substrates (Zhang et al.
2012), and the higher vegetation biomass in cropland soils than in
grassland soils during the growing season suggests that crops require
more available nutrients, which might lead to the rapid depletion of
labile substrates without inputs from plant residues and rhizodeposition
(Bever et al. 2010; Zhang et al. 2012). In addition, the soil
microclimate is cooler and moister under grassland soils compared to the
drier and warmer in cropland soils. Loss of soil nutrients and water has
been linked to increased susceptibility to other stresses (soil pH and
soil aggregates) (Bever et al. 2010).
We found that neither F:B ratio nor relative abundance of bacteria and
fungi changed with grassland conversion. This indicated that grassland
conversion not only reduced the soil microbial activity but also altered
the soil microbial community structure. One possible interpretation
might be that grassland conversion affect microbial composition
primarily by altering the soil nutrient (soil organic carbon, total
nitrogen), as indicated by the correlation analysis. Soil microbial
community may acclimate grassland environments and sustained a
relatively stable structure. A higher F:B ratio in topsoil of grassland
is associated with higher decomposition efficiency and greater carbon
storage potential in soil (Ding et
al. 2013). Moreover, this indicated soil fungi is likely to more
sensitive and easily degradation to grassland reclaim
(Poeplau et al. 2011), implying
that the turnover rate may increase in whole microbial communities (Six
et al. 2000). Previous studies have suggested that soil fungi often
dominates the decomposition of soil organic matter because the lower
nutrient demands and metabolic activities than bacteria in a low
nutrient content (Jangid et al. 2011;
Ma et al. 2015; Moon et al. 2016).
Furthermore, the translocation of nutrients can be promoted by the
hyphae of soil fungi (Klein et al. 2004), which would decrease
significantly in response to physical disturbances (Helgason et al. 2008;
Drenovsky et al. 2010), such as plowing under cultivation, so grassland
soils are more favorable for the formation of fungal hyphal networks
that play an significant role in the cycling of soil carbon and nitrogen
(Hu et al. 2014).
The site (S), land use (LU) and soil depth (SD) have significantly effect
on soil microbial biomass (Table 3). However, there was no statistically
significant interactions between site (S) and the others factors (LU and
SD). It indicated that decrease in soil microbial biomass after
grassland cultivation is a prevailing phenomenon among different site.
Moreover, soil depth has been observed to be a determinant of SOC and
STN concentrations and soil microbial community composition in grassland
soils. Soil depth provided varied environmental conditions as reflected
by changes in soil characteristics and PLFA biomass, and the decreases
in soil nutrient with increasing soil depth may be a major reason for
the pronounced decrease in soil microbial biomass in lower grassland
soil layers (Guo et al. 2002; Ding et al. 2013;
Moon et al. 2016). In contrast,
the decreases in soil microbial biomass, SOC and STN were less
pronounced in cropland because tillage practices homogenize the soil
substrates and resources across the plow layer
(Drenovsky et al. 2010), thus
leading to higher soil microbial biomass, SOC and STN in 0-30 cm soil
layer in cropland.
Our results revealed that grassland cultivation affected microbial
biomass mainly through enhanced soil nutrient resources rather than and
soil pH, moisture and aggregation. This finding is consistent with recent
studies reporting that resource availability controlled the responses of
the plant soil system to land use change
(Lange et al. 2015).These findings
suggest that soil microbes are highly vulnerable to grassland
cultivation and that this vulnerability is determined by the disruption
of feedback processes between soil nutrient properties and soil microbes
due to grassland conversion. It is essential to effectively evaluate
soil properties before grasslands are converted to cropland (Jangid et
al. 2011), especially in agro-pastoral ecosystems that are particularly
sensitive to environmental changes and are difficult to restore.