4.1 Soil water aggregate distribution and stability
Generally, the proportion of water stable macro-aggregate is an early
indicator of improved aggregation, which usually boosts MWD. The
addition of bio-fertilizer affected soil aggregation and improved
aggregate stability in this study. The orders of effectiveness of the
bio-fertilizer on the content and stability of macro-aggregate were as
follows: BF50 > BF30 = BF70 > BF0. This might
be that the mycorrhizal development and activity have increased after
application of bio-fertilizer, which enhanced aggregate stability in the
end. Studies have shown that preparations of fungal origin are vital for
the soil aggregate stability (Bethlenfalvay & Barea, 1994; Yilmaz &
Sönmez, 2017). The bio-fertilizer supplement half of chemical fertilizer
was found to be more effective in ensuing stability of macro-aggregate.
The supplement of bio-fertilizer provided soil organic addition since it
was produced from wheat straw. This organic matter brought from
bio-fertilizer would act as a core for soil aggregation which
contributed to formation of macro-aggregate according to the aggregate
formation model (TIisdall & Oades, 1982). What’s more, the organic
matter would also provide nutrients to soil microbes, and promote soil
microbial activity and enzyme activity. Soil aggregate stability
increased with microbial development, and that hypha development outside
the rhizosphere, particularly by mycorrhizal fungus, provided nutrients
to microorganisms, which enhanced the development of microorganisms and
increased the soil aggregate stability in turn (Zhang, Liu, Chen, &
Tang, 2016). Consequently, the application of organic materials would
favor the aggregation of clay and micro-aggregate, which caused a
decrease in clay and micro-aggregate fractions and an increase in
macro-aggregate.
However, the effects of bio-fertilizer supplemented with 30% or 70%
chemical fertilizer were not much effective in both years. The result
obtained may have been due to the threshold effect of bio-fertilizer
application, the inadequate dose of bio-fertilizer application would not
be benefit for the soil aggregation. Promotion of bacterial and fungal
activity increased with the amount of bio-fertilizer when it was less
than 50%, then the promotion would generally diminish when the amount
of bio-fertilizer was higher than 50%. The higher proportion of
macro-aggregate and MWD values in treatments with bio-fertilizer accord
with previous study and proved our hypotheses that bio-fertilizer are
able to improve soil aggregation and the promotion of bio-fertilizer may
be related to its dose in the given area. In our study, the
bio-fertilizer applied alone showed no positive effect on soil
macro-aggregate content and water stable aggregate stability. The
inappropriate application of bio-fertilizer would not achieve expected
result.
Different from the effect on macro-aggregate, bio-fertilizer generally
had a negative effect on the micro-aggregate. The orders of
micro-aggregate were as follows: BF0 = BF30 > BF70 = BF100
> BF50. The maximum negative on the content of
micro-aggregate was achieved with BF50. Our results showed that the
micro-aggregate was more stable than the macro-aggregate since the
variation of micro-aggregate was 4.5 g 100 g-1 while
it reached to 23.2 g 100 g-1. This might be linked to
other dominant factor such as the sand and silt content of soil (high
sand content of soil which was above 70% in this study soil) that
played a vital role in the soil aggregation. Generally, combined with
bio-fertilizer application would lead to increased soil aggregate
stability than applied chemical fertilizer or bio-fertilizer alone.
Previous study showed that fungal based bio-fertilizer contributed more
to macro-aggregate, while the bacterial based bio-fertilizer linked more
to micro-aggregate (Hernandez-Soriano, Kerré, Kopittke, Horemans, &
Smolders, 2016; Yilmaz & Sönmez, 2017).