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.