Effects of application of microbial fertilizer on aggregation and aggregate-associated carbon in saline soils

A pot experiment was conducted to elucidate the process of aggregate formation and the accumulation of soil organic C as affected by application of microbial fertilizer in coastal saline soils east of Shandong Province, China. The study was designed with five levels of electrical conductivity (EC): 0.33, 0.62, 1.13, 1.45 and 2.04 ds m−1. For each EC level, a treatment group received microbial fertilizer (MF) and a control (CK) did not. The mass and organic C concentration of aggregates (>2000 μm large macroaggregate, 250–2000 μm small macroaggregate and 53–250 μm microaggregate, <53 μm silt + clay fraction) were measured. Treatments and controls were denoted as MF1–MF5 and CK1–CK5 from lowest to highest EC values. The soil organic C concentrations of MF1–MF3, but not MF4 and MF5, were significantly higher than that of their controls. For MF1–MF3, application of microbial fertilizer significantly increased the proportion of macroaggregates. MF1–MF3 treatments exhibited significantly increased organic C concentration in the large macro-aggregates and free silt + clay fractions, but the differences were not significant for EC values of 1.45 and 2.04 ds m−1. The mass proportion of large and small macroaggregates was significantly related with organic C concentration in the microaggregates. For EC values 1.45 and 2.04 ds m−1, the silt C was too low to form the microaggregates, and the aggregates were not significantly different. Significant linear relationships existed between the organic C concentration in the silt + clay fraction and mass ratio of the macro- to micro-aggregates. We suggested that microbial fertilizer reduced the silt + clay fraction and increased the microaggregate mass by increasing soil organic C in the silt + clay fraction, which promoted the formation of macroaggregates. The mass and organic C of microaggregates played an important role in aggregation and C accumulation.