Fig. 4
From: Suppressed N fixation and diazotrophs after four decades of fertilization
a A structural equation model describing the effects of relative abundance of main ecological clusters on the nitrogen fixation rates. Soil properties include soil pH, total phosphorus (TP), total carbon (TC), and total nitrogen (TN). Numbers labeling the arrow lines are indicative of the correlations. R2 denotes the proportion of variance explained. Significance levels of each predictor are *P < 0.05 and **P < 0.01. b Standardized total effects (STE) from the SEM. This is the sum of direct and indirect effects from each variable on nitrogen fixation rate. Fertilization treatments: control, non-fertilization; NPK fertilization, NPK (urea, superphosphate, and potassium chloride); NPK + WS, NPK with wheat straw; NPK + PM, NPK with pig manure; NPK + CM, NPK with cow manure (NPK + CM)