4.2 The predominant factor affecting SOC and DOC
SOC and DOC contents were governed by multiple environmental factors,
and the multiple linear regression analyses could identify the most
important one. ACT biomass and N:P ratios were the common factors that
govern DOC contents and the proportions of DOC to SOC (RC)
(Table 5). ACT species, as
pioneers on nitrogen-poor sites, have a predilection for barren soils
and can tolerate environmental stress such as drought, high salinity,
and extreme pH. Biochar addition introduced soil pores to increase (low
BW), drought (low SWC), and high pH, which were suitable for ACT growth
but meanwhile restricted other microorganisms. Actinomycetes are
effective in decomposing C compounds with poor nutrient and they are
usually booming when N is limiting in soil (MacKenzie and Quideau,
2010). DOC is expected to be derived from ACT excretion or products of
refractory SOC degraded by ACT and other microorganisms, and this was
supported by the positive Beta coefficients of ACT in regression models,
which was 0.635.
N:P ratios were the common negative factor predominant DOC and RC in
soils, which suggested the high nitrogen limitation presence in
saline-sodic soils. Soil C:P and N:P ratios decreased while N:P ratios
increased with biochar addition, and this confirmed rising N limitation
to microorganisms after biochar addition. In the regression models, the
Beta coefficients of N:P ratios to DOC and RC were both negative values,
which matched well with the diagonal location of DOC, PLFA, and TN and
N:P (Fig.3). Globally, there is a Redfield-like atomic C:N:P ratio,
60:7:1, for the soil microbial community (Lehmann et al., 2011).
Nitrogen limitation to microorganisms in saline-sodic soils was
aggravated after biochar addition, which the well-constrained N:P ratios
reduced to 5.3 in HK, 7.1 in LK from 8.3 in CK treatment on average.
Only the CPOC was remained in the regression model for predicting SOC
contents, and this matched well with positive loading of CPOC on SOC in
Fig.3. CPOC could explain 73% of SOC change as the Beta coefficient
showed (Table 5). Considering there was no aggregate that larger than
2000 μ m was separated and biochar was applied as fine powders
into soils, it was guessed that CPOC increase was due to biochar powder
addition directly. Biochar interacts with minerals in soils and forms an
organic-inorganic complex, which resulted in the protection of the
enclosed biochar carbon against further decomposition in soils
(Brodowski et al., 2005). Besides, SWC and BW decrease implied soil
pores increase, which accelerates Fe3+ and
Al3+ deposition in biochar surface. This reduces the
microbial accessibility to biochar and protects biochar-derived C from
decomposition (Sollins et al., 1996). However, this hypothesis needs
more evidence or parameters, such as black carbon biomarker, to confirm
links between carbon in CPOC aggregates and biochar carbon.