4.2 Soil and aggregate organic C fractions
Our results showed that the increase of SOC, MBC, WSOC and
KMnO4-C were associated with the application of
bio-fertilizer. Similarly, Li et al. (2017) indicated that
concentrations of SOC and WSOC were significantly higher in treatments
mixed with bio-fertilizer when compared with soil applied with chemical
fertilizer alone. In our study, the application of bio-fertilizer
significantly increased SOC in the whole soil in both years, and the
whole-soil SOC concentrations increased with the increasing
bio-fertilizer. The increases were mainly attributed to input of newly
organic matter through bio-fertilizer. What’s more, the increase of
nutrient supply by the addition of bio-fertilizer led to increase crop
residues return to soil, which increased the SOC content in turn. The
input of bio-fertilizer could provide substrate for soil microbes on one
hand and bring amounts of microorganisms on the other hand (Banger,
Toor, Biswas, Sidhu, & Sudhir, 2010). As one of the labile fractions of
C, WSOC appears to be an immediate substrate for the soil organisms.
WSOC was significantly increased after bio-fertilizer application, and
the highest WSOC was recorded in BF50. The increase of WSOC might be due
to the priming effect of supply of organic material through
bio-fertilizer application, which spurred the mineralization of organic
matter via increasing microbial activity. Similarly, the
KMnO4-C was significantly higher in treatment applied
with bio-fertilizer, while the basal respiration was generally decreased
by bio-fertilizer. In addition, the significant relationship (positive
or negative) between organic C fractions also suggested that
bio-fertilizer application was benefit for soil C sequestration.
The relationship between the organic C fractions and proportions of
macro-aggregate were positively significant except for basal respiration
in this study. This correlation confirmed that increases in soil labile
organic C content would promote the soil aggregate stability, which was
mainly dominated by the proportions of macro-aggregate (Bronick & Lal,
2005; Sui, Jiao, Liu, Zhang, & Ding, 2012). Previous studies also
indicated increased SOC or soil organic matter would cause an increase
in soil aggregate stability (Chenu, Le Bissonnais, & Arrouays, 2000;
Huang, Lan, Liu, & Gao, 2017).
Generally, the stabilization of organic C in soil aggregates is
considered one of the main mechanisms for sequestering C in soil. The
organic C in soil aggregates can be protected it from soil
microorganisms and soil enzymes, which reduced the decomposition of SOC.
The soil applied with bio-fertilizer had larger organic C concentrations
in the three aggregate fractions than the control in both years, and the
highest increase in the organic C was recorded in the BF50 treatment.
Previous studies showed that application of fungal based fertilizer or
fungus was also effective in increasing organic C of aggregates (Hu,
Cui, Dai, Wang, & Lin, 2014; Yilmaz & Sönmez, 2017). The increase in
organic C content of aggregates caused by bio-fertilizer application
might be linked to that the bio-fertilizer provided the suitable
environment with new and easily degradable C for the soil microbes. The
organic matter of bio-fertilizer used in this study accounted for 55%
of the total weight and significant increases of organic C in whole soil
was recorded with the supplement of bio-fertilizer.
In this study, the SOC was distributed among the three aggregate
fractions, and the organic C in each aggregate fraction partly linked to
concentrations of the whole soil organic C. However, the difference of
the proportion among aggregate fractions was not consistent with the
distribution of each aggregate. This might indicate that the
bio-fertilizer would first influence the physical properties and affect
soil chemical properties thereafter. Soil treated with bio-fertilizer
(organic materials) showed relative high organic C content in
micro-aggregate than the control, which was consistent with results of
Hernandez-Soriano et al. (2016). These results were very meaningful from
the perspective of soil C sequestration because it was believed that
organic C was predominantly fixed in micro-aggregates. The organic C in
micro-aggregates was more stable and remained unaltered for a relative
long time in soil (Huang et al., 2019). However, the dynamics of
macro-aggregate were also vital to C fixation for the reason that
micro-aggregate formation and the organic C fixed within the
micro-aggregates were affected by macro-aggregate (Six et al., 2000).