Bacterial growth and decomposition are regulated by soil pore network characteristics, while fungi are independent: insights from computed tomography

The characteristics of the soil pore network condition the flow of nutrients, liquids, gases, and temperature through this medium. We hypothesized that soil structure properties will also control soil microbial processes. To test this, we selected soil samples where land use generated differences in soil structure. Unaltered soil samples from three adjacent plots were analysed: a potato field soil, ploughed after harvesting, two weeks before sampling (Control); another ploughed potato plot, in which bioaugmentators (plant growth promoting bacteria) had been applied to increase soil biodiversity (Bio); and a third soil, dedicated to melon cultivation, without bioaugmentation, followed by six months of fallow (Melon). In each soil, three different depths were analysed (between 0/-3.33 cm; -3.33/-6.67 cm; and -6.67/-10 cm), and, at each depth, we separated aggregates into three size categories: >0.8 cm, 0.8-0.2 cm, and <0.2 cm. The structure of each core was analysed by computed tomography, while the leucine incorporation method (bacterial growth) and the acetate incorporation into ergosterol method (fungal growth) were used to estimate rates of microbial growth, and respiration was measured to estimate soil decomposer functioning.

The land uses affected soil structural variables. Bio pores had a significantly higher number of branches than Control and Melon. Regarding the aggregate fraction, most of the parameters considered (physical and biological) showed significant differences: the matrix fraction pores (aggregates < 0.2 cm) had a higher connectivity, were more tortuous, but presented a lower number of branches and junctions. On the other hand, there were also lower rates of bacterial growth, fungal growth and respiration in larger aggregates. No significant differences were found considering depth.

We detected links between rates of microbial growth, decomposer functioning and the porous network characteristic differences between samples. The fractal dimension was generally correlated with bacterial growth (r = 0.43, p-value = 0.04), also within Control (r = 0.86, p-value = 0.03) and Melon (r = 0.88, p-value = 0.02) land uses but not for Bio (r = 0.51, p-value = 0.30). Bacterial growth also increased in higher pore tortuosity (r = 0.49, p-value = 0.02), but was inversely correlated with the proportion of pores that end in the matrix (r = -0.41, p-value = 0.05). In the Bio treatment, microbial growth and decomposer were more independent of the pore architecture, and less correlated than in Control and Melon treatments. There, bacterial growth was favoured by higher connectivity (r > 0.86) and optical density (r > 0.69), while respiration increased with the number of pores (r > 0.75) and pore length (r > 0.71). The respiration rate within small aggregates (0.2 to 0.8 cm) increased with the length and number of pores (r > 0.84).

In conclusion, aggregation seems to have a greater effect on the physical and biological properties of the soil than differences between land uses studied and the depths considered. On the other hand, characteristics such as connectivity, tortuosity, and the length and number of pores seem to regulate both bacterial growth and respiration, while fungal growth appears independent.