Relationships between geochemical properties and microbial nutrient acquisition in tropical forest and cropland soils

Land-use conversion can profoundly modify geochemical and microbial properties that drive organic carbon (C) dynamics in tropical soils. However, it is unclear how microbes adjust nutrient acquisition strategies to changing geochemical properties across deeply weathered soils developed from geochemically contrasting parent material. Here we show that along a geochemical gradient in forest and cropland, a proxy variable, the chemical index of alteration (CIA), is sufficiently sensitive to reflect geochemical controls on microbial nutrient acquisition in tropical soils. We found that the CIA negatively correlates with rock-derived nutrient concentrations and pH, but positively with clay content, iron oxides, as well as total iron, aluminum, and manganese concentrations. Thus, using the CIA, which integrates effects of soil fertility and C stabilization by minerals, reduced the complexity of relating microbial nutrient acquisition to geochemical soil properties. Effects of the CIA on microbial C and phosphorus (P) acquisition were stronger in cropland than in forest soil. Microbial nutrient acquisition strategy shifted with increasing CIA from predominating C demand to P. Changes in soil properties at higher CIA (less rich in rock-derived nutrients) were favorable to fungi, which pursue a conservative nutrient allocation strategy to cope with acidic and nutrient-depleted soil conditions, reducing C loss through respiration. In low CIA soils (more rock-derived nutrients), bacteria-dominated communities increasingly invested in C acquisition at the expense of the community biomass, with subsequent greater C loss through respiration. We conclude that microbial communities adapt nutrient acquisition strategies to changing geochemical soil properties in a way that might affect C input versus C storage and release in tropical soils. Here, high soil fertility may favor plants to build up more C in biomass, thus increasing C input, whereas, it may also favor the establishment of microbial communities whose nutrient acquisition and allocation strategies limit long-term soil C storage.