Large amounts of easily decomposable carbon stored in subtropical forest subsoil are associated with r-strategy-dominated soil microbes

• Density of easily decomposable C in subsoil significantly higher than in topsoil under both plantation and native forests.
• r-strategy microbes with inefficient C use dominated subsoil under both forests.
• r-strategy soil microbes dominated under Chinese fir plantations.
• Undisturbed forest subsoils can accumulate significant amounts of easily decomposable C.

Subsoils store over 50% of the total soil organic carbon (SOC) in terrestrial ecosystems, but the stability of this fraction of SOC and the contributions of the associated soil microbes to C dynamics remain unclear. A natural evergreen broad-leaved forest and a Chinese fir plantation converted from natural forest in Fujian Province, China, were used to test if differences in soil C decomposability and soil microbial characteristics between topsoil (0–10 cm) and subsoil (40–60 cm) were associated with the prevalence of microbes expressing a characteristic growth strategy (r-versus K-strategies). A combined approach, including modified Michaelis-Menten kinetics, substrate-induced respiration, soil C decomposition, soil basal respiration measurements, and phospholipid fatty acid (PLFA) analysis was used. Compared with topsoil, the subsoil had 3.6 and 1.6 times higher concentrations of readily decomposable C substrate (as glucose equivalents) estimated in terms of Michaelis-Menten kinetics, and 2.7 and 2.8 times faster mineralization per unit SOC, respectively, under the natural and plantation forests. Soil microbes in the subsoil tended to be r-strategist-dominated in both forests, characterized by higher maximum rate of soil respiration and half-saturation constant, higher maximal specific growth rate, higher relative abundance of Gram-negative bacteria, and higher metabolic quotient, the latter indicating smaller C use efficiency. In contrast, soil microbes in the topsoil tended to be K-strategist-dominated. Soil microbial communities shifted from K-strategy to r-strategy in the topsoil of Chinese fir plantation, reflecting lower microbial C use efficiencies, compared with natural forest. It is concluded that a substantial pool of easily decomposable C accumulated in subsoils of these two subtropical forests, a product partly of r-strategists replacing K-strategy microbes. These findings improve our understanding of the mechanisms regulating C dynamics between topsoil and subsoil and have implications for the effects of forest conversion on soil C storage.