## 4 Discussion
Many studies have explained that inhalation of fragrances has a significant effect on brain function and various psychophysiological parameters such as blood pressure, muscle tension, pupil dilation, skin temperature, pulse rate, and brain activity, because fragrance compounds can cross the blood-brain barrier and interact with receptors in the central nervous system [10; 36; 37; 38]. Therefore, these results suggest that the olfactory stimulation caused by soil-derived VOCs also affected the central nervous system and cerebral cortex during the soil mixing activity with the soil containing _S. rimosus_, and it is thought that it would induce a pleasant and clear arousal state of the brain. It is also thought that such a positive psychophysiological response had a positive effect on the subjective emotional states of the participants. A previous study reported that serum metabolites can be affected by physical activity [14]. Serum metabolomics showed that the _S. rimosus_ treatment group (_SS_) displayed relatively higher levels of fatty acids, lipids, and other molecules compared to the control group (S) (Figure 7C). However, most of the organic acids and amino acids were relatively higher in the control group. In particular, cystine and serotonin levels in the _SS_ group were significantly higher than in the control group (Figure 7C). Generally, serotonin is known as an important neurotransmitter involved in the regulation of numerous biological and behavioral functions in the body, playing a crucial role in psychological processes in the central nervous system, including cognition and pain sensitivity [25; 29]. [PERSON] et al. [40] reported that serotonin has been correlated with emotional and motivational aspects of human behavior, such as anxiety disorders and depression. A previous study reported that psychological disorders such as depression have been correlated with serotonin contents, which showed decreasing patterns [25]. Cystine, which is the major form of cysteine under physiological conditions, is more stable in blood plasma than the free amino acid cysteine. In the cells, cystine is promptly converted to cysteine, which is an essential substrate for the synthesis of biomolecules such as lipids, proteins, glutathione (SGH), and Coenzyme A [41; 42]. Moreover, cysteine supports various processes, including detoxification and steroid metabolism [43]. In particular, several 5-HT (serotonin) receptors are expressed highly by most excitatory neurons in the occipital cortex [44; 45]. Therefore, this positive correlation may have appeared as an association between the activity of the occipital cortex and the activity of the serotonin system. In a previous study, it was reported that BDNF regulates aging and brain activity in the hippocampus and surrounding areas [46; 47]. Also, BDNF has been found to be associated with brain and psychological disorders such as memory dysfunction, depression, and anxiety. As a result of our serum analysis, the soil mixing activity in the presence or absence of _S. rimosus_ did not change the amount of BDNF. On the other hand, the CRP level decreased after the soil mixing activity containing _S. rimosus_. CRP is a pentagonal protein produced in the liver, and CRP levels in the serum increase due to acute and chronic inflammatory reactions [29]. For this reason, CRP is a major inflammatory factor used as an inflammatory biomarker [48]. It has also been reported that high CRP levels are associated with psychological disorders such as depression and bipolar mood disorders [30; 31]. Accordingly, the current results imply that soil mixing activities including _S. rimosus_ can reduce inflammation and alleviate elevated serotonin levels in a short period of time; they may consequently cause psychophysiological changes, including being comfortable or relieved from stress. With similar results, contact with soil microorganism _M. vaccine_ has been shown to have positive effects on human metabolic and autonomic reactions. A soil mixing activity with _M.