3.1 Effect of fertilization strategies on microbial communities along the paddy soil profile
The absolute abundance of total and specific microbial groups (G−, G+ bacteria, actinomycetes, and fungi) in all treatments significantly decreased with soil depth (Table 1, P <0.05). The abundance of total and specific PLFAs were the lowest and highest in the control and NPK + OM treatments, respectively, at the 0−40 cm soil depth. Compared with that in the NPK treatments, NPK + ST showed significantly higher bacterial (G−, G+, and actinomycetes) and fungal abundance at the 10−20 cm soil depth (P <0.05). The fungi to bacteria, G+ bacteria to actinomycetes, and fungi to G− bacteria ratios gradually decreased with soil depth in all treatments between 0 and 30 cm. The G+ to G−bacteria ratio increased with soil depth for all treatments (Figure 1).
Both mineral and organic fertilizers significantly increased the G+ to G− bacteria and actinomycetes ratios compared with those in the control group at 0–30 cm soil depths (Figures 1b and c,P <0.05). NPK + ST showed the lowest fungi to bacteria and G− bacteria ratios at 0–10 cm compared with those in the other treatments (Figures 1a and d, P <0.05). At 10−40 cm, the fertilized treatments showed higher fungi to bacteria and G− bacteria ratios compared to those of the control.
There was no uniform pattern throughout the soil profile regarding the influence of organic fertilizers on microbial functional PLFA ratios. With equal amounts of fertilizer input, compared with NPK, NPK + OM further stimulated more G+ bacteria than G− bacteria and actinomycetes at all depths (Figures 1b and c). Soil respiration rates gradually decreased with soil depth and the lowest values for each soil depth were observed in the control (Figure 2a, P <0.05). NPK + ST significantly increased the soil respiration rate at 0−20 cm (P <0.05). NPK + OM showed a constant soil qCO2 between the 0 and 40 cm soil depths, while both the NPK treatments and the control significantly increased the qCO2 values with soil depth. NPK + ST treatment showed the lowest qCO2 at 10–20 cm compared to that at other depths (Figure 2b, P <0.05).