Fig. 3

Structural equation modeling (SEM) illustrating the direct and indirect effects of depth, sediment properties (water content and salinity), and molecular attributes (diazotrophic community richness and structure) on NFR. Continuous and dashed arrows represent the significant and non-significant relationships, respectively. Arrows indicate the hypothesized direction of causation. Blue and red arrows indicate positive and negative relationships, respectively. The numbers adjacent to arrows are standardized path coefficients proportional to thickness of the lines, with p values in the brackets. Significance levels are denoted with **p < 0.01; ***p < 0.001. Standardized total effects (direct plus indirect effects) calculated by the SEM are displayed beside the SEM. The hypothetical model fits our data well as suggested by the goodness-of-fit statistics: chi-square = 1.374, degrees of freedom = 4, probability level = 0.849, GFI = 0.951, CFI = 1.000, and RMSEA = 0.000