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 mu m large macroaggregate, 25-02000 mu m small macroaggregate and 53-250 mu m microaggregate, <53 mu m 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. (C) 2016 Elsevier B.V. All rights reserved.