Inhibition of nitrification in forest soil by monoterpenes

Nitrate production was detected in untreated soil of a Norway spruce (Picea abies L.) stand only after clear-cutting the stand. The aim of this study was to determine whether allelochemical inhibition of nitrification by mono terpenes played any role in inhibiting nitrification in the stand. Therefor e, soils from a clear-cut plot and from a forest plot were studied. In the field, monoterpenes (mostly alpha- and beta-pinenes), measured by soil micr oair diffusive samplers, were intensively produced in the forest plot, but not in the clear-cut plot. In the laboratory, soil samples taken from the f orest plot produced only small amounts of monoterpenes, indicating that mon oterpenes were mainly produced by the roots and not to great extent by the soil microbial population. The effect of a mixture of monoterpenes (seven m ajor monoterpenes detected in the field) on net nitrification, net N minera lization and denitrification activities of soil from the clear cut plot, an d on carbon mineralization of soils from both the forest and clear-cut plot s, was studied in the laboratory. In both aerobic incubation experiments an d in soil suspensions with excess NH4-N, nitrification was inhibited by exp osure to the vapours of monoterpenes at similar concentrations at which the y had been detected in forest plot. This indicates direct inhibition of nit rification by monoterpenes. Exposure to monoterpenes did not affect denitri fication. However, it increased respiration activity of both soils. This co uld also indicate indirect inhibition of nitrification by monoterpenes, due to immobilization of mineral N. Thus it seems that monoterpenes could play a role in inhibiting nitrification in the forest soil.