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).