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).