Soil phosphorus fractions, phosphatase activity, and the abundance of phoC and phoD genes vary with planting density in subtropical Chinese fir plantations

Abstract Plants and microorganisms have developed mechanisms to acquire soil phosphorus (P). The present study occurred in Chinese fir plantations with five planting densities (1667, 3333, 5000, 6667, and 10,000 stems∙ha−1) at age 5 and 35 in acidic soils. We aim to (1) assess the inorganic (Pi) and organic (Po) forms of P and the labile P (including resin-Pi (AER-Pi), NaHCO3-Pi, and NaHCO3-Po), moderately labile P (including NaOH-Pi, NaOH-Po, and HCl-Pi), and residual P in soils; (2) to evaluate phosphatase activity and the abundance of phoC and phoD genes; and (3) to identify the correlations among P fractions, phosphatase activity, and the abundance of phoC and phoD genes. The results showed that the total Pi, AER-Pi, and NaHCO3-Pi were higher in low densities (≦ 3333 stems∙ha−1) than high densities (≧ 5000 stems∙ha−1) at age both 5 and 35, but the NaOH-Po and NaHCO3-Po were the opposite at age 35 only. Moreover, the NaOH-Pi content was higher in low densities than high densities at age 35 only. The phosphatase activity and the abundance of phoC and phoD genes were higher in high densities than low densities at age 5, whereas the opposite was true for age 35. Therefore, the decrease in planting density was expected to increase soil P bioavailability in acidic soils with the development of plantations from 5 to 35 years. The result of unary linear regression showed that phosphatase activity was positively correlated with the abundance of phoC and phoD genes in 35-year-old plantations and with the abundance of phoC gene in 5-year-old plantations. In 35-year-old plantations, the phosphatase activity and the abundance of phoC and phoD genes were positively correlated with NaHCO3-Pi and NaOH-Pi, but significantly negatively correlated with NaHCO3-Po and NaOH-Po. These findings together demonstrate that P bioavailability and fractions, phosphatase activity, and the abundance of phoC and phoD genes varied with planting density in acidic soils, with potential implications for subtropical and tropical biogeochemical processes and for understanding how microbial metabolism regulates P biotransformation.