4.3 How biochar amendment changes the soil carbon pool?
Change of soil carbon pool are usually results of multiple combined
factor blocks including soil physical condition, nutrient availability,
microorganism activity, and aggregation processes. Attempting to
apportion the impacts of their interactive effects on SOC is vital to
evaluate the benefit of biochar application on soil properties and
fertility. The partial least squares path model (plspm) could provide
visual structural equation modeling that studying complex multivariate
relationships among observed and latent variables. In the present work,
five blocks were established aiming to reveal which blocks had great
impacts on carbon change in saline-sodic soils before and after biochar
addition.
The Phy block contained SWC, BW and pH variables, the Nut block
contained TN, TP, C:N, C:P and N:P variables, the agg block contained
CPOC, FPOC, and MOC variables, the carbon block contained DOC and SOC
variables, and the mic block contained PLFA, BAC, ACT, FUN, AMF, PRO and
OB variables. The Phy, nut, agg, carbon, and mic blocks represent
information of soil basic physical-chemical properties, nutrient
availability, aggregation process, carbon dynamics, and microorganism
communities, respectively.
The plspm models indicated that the biochar amendment had greatly
changed interactions among these five blocks. In CK treatment, Phy, nut,
mic, and agg all had varied but positive effects on carbon, of which nut
had the largest effects on carbon while mic had the smallest. After
biochar was added, agg was the only block that had positive effects
while other blocks had negative effects on carbon, of which nut still
had the largest values in absolute (Fig.4).
The effects of microorganisms on carbon dynamic changed to weakly
negative in LK+HK (-0.0768) from weakly positive in CK treatment
(0.0043). It was concluded that biochar addition triggered the negative
priming effects of microorganisms on the soil carbon pool, which
coincided with the previous report (Zimmerman et al., 2011). However, no
significant effects on carbon from mic were observed as expected in
other work (Prayogo et al., 2014). It confirmed that the roles of
microorganisms in regulating carbon cycles in saline-sodic soils greatly
depended on nutrient limitation, especially nitrogen bioavailability.
Biochar initially promoted microorganism biomass via nutrient input by
biochar, which was proved by rising CO2 production
greatly over the short term in arable soils (Prommer et al., 2014).
However, microorganisms would utilize SOC previously associated with
clay minerals as nitrogen or carbon sources when extra nitrogen from
biochar was exhausted. This was confirmed by the great change of effects
of nut on carbon, which changed to -0.6284 in LK+HK from 0.9684 in CK
treatment. Nutrients were the primary driver affect organic carbon pool
in saline-sodic soils,
Meanwhile, the effects of agg on carbon changed from 0.0820 in CK, the
weakly positive, to 0.3478 in LK+HK, which implied the only positive
effects. Soil organic carbon pool would benefit from aggregation change
caused by biochar addition, and this was in good agreement with the
results of PCA and the regression analysis. However, as mentioned above,
SOC increase after biochar addition might come from biochar materials
but not the native soil organic matter, and effects of biochar on native
SOC preserve should be studied over the long-term timescale (Liu et al.,
2018).