Effects of the two carvone enantiomers on soil enzymes involved in the C, P, and N cycles.

Background: Essential oils exert stimulatory or inhibitory effects on the size and activity of the soil microbial communities. Given that microbial biomass is the main source of soil enzymes, in this study, we examined how R-(-)- and S-(+)-carvone affect the activity of dehydrogenase, urease, and alkaline phospho-monoesterase, and the overall microbial activity, as expressed by soil respiration. Enzymatic and microbial activities were recorded every week, for a period of four weeks, during which the two carvone enantiomers were added twice, with a two-week interval, into soil samples. For all dependent variables, we analysed the deviations of the experimental from control values.

Results: Treatment per se had a significant effect only on urease. Its activity was inhibited in the S-carvone samples, while it was enhanced or inhibited, depending on the time of incubation, in the R-carvone ones. The activity of alkaline phospho-monoesterase was not affected by S-carvone, but it increased with R-carvone. Soil respiration markedly increased in presence of the two carvones with highest values being recorded in the R-carvone samples. None of the temporal patterns of the three enzymes' activity followed the pattern of soil respiration.

Conclusions: The significant treatment by time interactions for the activities of all three enzymes indicates that responses are not consistent over time; this suggests differently functioning or structured microbial communities. Given their differing effects on soil enzymes, these compounds and the aromatic plants bearing them could find use in sustainable agriculture for the control of soil enzymes and, hence, the soil processes that they are associated with.