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).