Microbial assembly adapted to low-P soils in three subtropical forests by increasing the maximum rate of substrate conversion of acid phosphatases but not by decreasing the half-saturation constant

It is hypothesized that microbes (and plant roots) in tropical soils adapt to a low-phosphorus (P) environment by lowering the K-m, which represents the affinity with substrates, of the phosphatase assembly (at the community level), and thus P fertilization increases K-m. However, the hypothesis has not yet been tested, partly due to the difficulty of measuring K-m in ecosystems, where elevated organic and inorganic P through P fertilization can lead to an overestimation of the K-m values through competitive inhibition. However, a lack of response of apparent K-m (K-app(m)) to P fertilization can be interpreted as a lack of support for the hypothesis. The present study demonstrated that 10-year P fertilization in a primary forest and a secondary forest did not affect K-app(m), although K-app(m) was slightly elevated by P fertilization in a planted forest. Meanwhile P fertilization clearly reduced the V-app(max) (apparent V-max) of phosphatases in the primary and secondary forests. Our result suggested that microbes (and plants) in tropical forests adapt to the P-poor environment through increasing the phosphatase abundance, but not through reducing the K-m of the phosphatase assembly (at the community level).