Pedogenesis of volcanic ash soils determines the soil microbial pool and its contribution to nutrient availability

Pedogenesis is a key factor driving nutrient immobilization and mineralization, particularly relevant in the fertility management of volcanic soils. These processes have a strong effect on plant production, but also on the functionality of a soil microbial pool. This research aimed to determine the effect of substrate addition on soil microbial pool, substrate use and nitrogen availability by performing a double labeling study (¹³C and ¹⁵N isotope labeling) in five different volcanic ash soils under permanent pastures. Disturbed soil samples were collected in five volcanic ash soils between the Andes and Coastal Mountains of Chile to evaluate contrasting pedogenesis. Soils were incubated for 16 days at 20 °C and a 60 % water holding capacity. Destructive sampling was carried out on days 1, 2, 3, 6, 9 and 16. Labeling was tracked through the consumption by the microbial biomass, functional genes, stoichiometric imbalances between microbial C/N, enzymatic C/N and the release of CO₂ during the incubation period. Ultisol and well-developed Andisol (“Older soils”) with higher organic matter content (∼14 %) showed higher available ¹⁵N contents, higher ¹³C-CO₂ respiration, and immobilization of substrate into microbial biomass (i.e., ¹³C-MBC). This study found that depending on the pedogenesis of the soil, N availability from mineral N and N-acquiring enzyme activities changed as a function of a microbial pool inherent in each soil. Thus, the effect of the initial activity of the soil microbial pool on N availability will allow adjusting fertilization strategies in soils with high levels of organic matter such as volcanic ash soils.