Potential response of soil processes to diterpenes, triterpenes and tannins: Nitrification, growth of microorganisms and precipitation of proteins

The aim of this study was to determine the influence of higher terpenes (diterpenes – abietic acid and colophony, and a triterpene – beta-sitosterol), belonging to plant secondary compounds, on certain microbial populations and processes related to N cycling in boreal soil. We studied how they affect nitrification process and bacterial and fungal growth in forest soil. We also investigated whether they have any ability for protein precipitation and in this way possibly make N less available in forest soil. For comparison, in nitrification studies also tannins (tannic acid and condensed tannins), belonging also to phenolic plant secondary compounds, were included. Terpenes decreased nitrification potential in a soil suspension experiment with excess NH4-N; tannins showed a less clear effect, but a tendency to decrease nitrification. Studies based on incorporation of 3H-thymidine in macromolecules and 14C-acetate-in-ergosterol showed that addition of terpenes to forest soil increased bacterial growth but inhibited fungal growth. Terpenes showed some ability to precipitate a protein (BSA – bovine serum albumin), but mainly at low pH. In conclusion, all these separate studies suggest that these plant secondary compounds may regulate soil microbial populations and processes in several ways.

► Tannins and higher terpenes can decrease directly nitrification potential.
► Higher terpenes promote bacterial growth but decrease fungal growth.
► Terpenes showed some ability to precipitate BSA in low pH.