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Soil Biology and Biochemistry
Vol. 95, 2016, Pages: 233–242

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

Yuping Chen, Guangshui Chen, David Robinson, Zhijie Yang, Jianfen Guo, Jinsheng Xie, Shenglei Fu, Lixia Zhou, Yusheng Yang

School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China.

Abstract

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.

Keywords: Evergreen broad-leaved forest; Chinese fir plantation; Soil organic carbon; Microbial growth strategy; Soil easily decomposable carbon; Soil carbon stability.

 
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