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.