Divergent mineralization of hydrophilic and hydrophobic organic substrates and their priming effect in soils depending on their preferential utilization by bacteria and fungi

Hydrophilic and hydrophobic organic compounds extracted from 13C-labelled maize residues were incubated with soils to evaluate their mineralization and priming effect (PE) caused by their utilization by microbial groups. Two soils with contrasting soil properties were collected from well-drained upland and water-logged paddy. Mineralization of the 13C-labelled fractions and their PE were quantified by monitoring the CO2 efflux and 13C enrichment during a 40-day incubation. The composition of main microbial groups (bacteria and fungi) involved in the utilization of 13C-labelled fractions was determined based on phospholipid fatty acids (PLFAs) analysis. At the initial stage (6–24 h), hydrophilic fraction had faster mineralization rate (3.6–70 times) and induced 1.5–10 times stronger PE (positive in upland soil and negative in paddy soil) than those of hydrophobic fraction. The 13C-PLFAs data showed that the incorporation of hydrophilic fraction into bacteria was 11.4–16.4 times greater than that into fungi, whereas the hydrophobic fraction incorporated into fungi was 1.0–1.5 times larger than that into bacteria at day 2. This indicated greater contributions of r-strategists (fast-growing bacteria) for the uptake of hydrophilic fraction versus K-strategists (slow-growing fungi) for hydrophobic fraction. Compared with K-strategists, the r-strategists possessed a much faster metabolism and thus triggered stronger apparent PE by accelerating microbial biomass turnover, resulting in higher mineralization and stronger PE for the hydrophilic than hydrophobic fraction. The slower and less mineralization of both fractions in paddy than in upland soils is due to the suppression of microbial activity and substrate utilization under flooding. At the end of 40-day incubation, the cumulative mineralization of hydrophilic and hydrophobic fractions was similar. Consequnently, microbial mechanisms underlying the utilization of organic compounds with contrasting solubility (hydrophilic or hydrophobic) are crucial for evaluating the stabilization and destabilization (e.g., priming) processes of soil organic matter.