4.1 Effects of biochar amendment on change of SOC and DOC
Biochar could reduce soil carbon loss by lowering greenhouse gas emissions, enhancing productivity, and stabilizing organic matter. However, the impacts of biochar on soil carbon dynamics on longevity and magnitude varied widely from weeks to serval years (MacKenzie and Quideau, 2010). Biochar amendment has positive, neutral, or negative effects on SOC and DOC (Liu et al., 2016). SOC contents are usually stimulated by biochar amendment within a great change range, from a few percents to several folds, while DOC contents increased or reduced varied in different researches (Table 4). For instance, Smebye’s work (2016) indicated that DOC contents increased by 2775% when biochar was added into arable soils at the rate of 10%. DOC was also reduced by -5.59%~26.67% when biochar was applied into crop field (Yang et al., 2018). In contrast to results seen in arable soils, magnitude of SOC contents changes in the present work were relatively smaller compared with significant SOC increase in sugarcane, paddy field, and other agriculture fields (Table 4). SOC contents only increased within a little range, from 1.16% to 12% even biochar was added at the rate of 10%. A remarkable increase of DOC was observed, from no effect to 66.7%, and higher than that in arable soils. It indicated that driving factors affecting SOC dynamics in saline-sodic soils differed greatly those in crop soils, which might due to differences of nutrient limitation, microorganisms, and unique soil pH conditions (Zimmerman et al., 2011).
Biochar addition had fewer facilitation effects on SOC increase in saline-sodic soils than that in agricultural and coastal saline soils, and the effects varied with biochar addition. SOC contents in CK treatment had no difference with LK treatment but differed significantly with HK treatment, but only 12.8% increase was observed, which was due to serious nutrient limitation to plant growth and microorganism activities in saline-sodic soils. Besides as direct carbon source into soil carbon pool, biochar addition improved soil physicochemical properties, facilitated plant growth, and yielded more litter biomass, which introduced more carbon to SOC pools. Principal components analysis indicated that carbon and nitrogen had positive loadings on the first principal component (PC1, 38.71% of total variance), and PLFA had positive loadings on the second principal component (PC2, 17.03% of total variances) (Fig.3). This meant that nutrient limitation and microorganism activities were the predominant factors controlling SOC and DOC contents. The dependence of CPOC on SOC is presented by the neighboring location of SOC and CPOC in Fig.3. The increasing contribution of CPOC to SOC might be caused by direct biochar addition.
Proportions of MOC to SOC decreased with biochar addition, which implied that SOC decomposed mainly comes from MOC. MOC was closely related to OB in the present work. OB was as indicators of physiological or nutritional stress in bacterial communities and lower proportions meant lower stress (Bossio et al., 1998). Proportions of OB to PLFA increased to 3.64% from 2.95% with biochar addition, and this implied that bacteria face more resource stress and nutrient limitation after biochar addition, which was confirmed by negative correlations between OB and C:N and C:P (Fig.2). Bioavailable nutrient input by biochar addition, specifically total P increment, stimulated microorganism growth like FUN and AFM, this was in good agreement with previous results (Liu et al., 2018). Correlations analysis confirmed that PLFA, BAC, FUN, and AMF all significantly and positively related to TP. FUN and AMF biomass increased obviously with biochar, which would compete with bacteria for space and resources. Biochar is the solid material produced from the thermochemical conversion of biomass under oxygen limitation and is dominantly composed of condensed aromatic C (Liu et al., 2018), which is not bioavailable for bacteria. Particles less than 53 μ m contain an abundance of polysaccharides, proteins, and lipids, which composed 41.7%, 4.2%, and 11.1% of MOC, respectively (Grandy and Neff, 2008). These compounds could be as alternative carbon and nutrient sources for bacteria and consumed easily.