## 4 Discussion  
Various effects in psychological and physiological aspects obtained through interaction with nature have been reported [14], but there have been few studies on the role of soil and soil microorganisms. In this study, we investigated the effects of contact with _S. rimosus_ using metabolic approaches and various psychophysiological parameters, focusing on VOCs derived by _Streptomyces_ soil microorganisms. The result of this study showed that horticultural activity using soil containing _S. rimosus_ causes psychophysiological changes in human brain activity, metabolomics response, and serum BDNF and CRP. First, in order to select a strain to be used for the experiment, candidate strains based on previous studies were selected (Control; SA: _S.aversinitiis_ KCTC 9063; SG1: _S.griseus_ KACC 20084; SG2: _S.griseus_ KACC 20731; SR: _S. rimosus_ KACC 20082) and analyzed via SPME. Among them, geosmin and 2-MIB, the target VOCs, were detected in the _S. rimosus_ strain and showed relatively high contents (_p_ < 0.05). The selected strain, _S. rimosus_ 20082, was inoculated into the soil (_SS_), and the effects of VOC profiling and a soil mixing activity with various soil sample samples (S, SM) were investigated. As a result of VOC analysis of various soil samples, most of the VOCs were found to be higher in _S. rimosus_ samples than in other soil samples. Some VOCs of terpenoid and benzoid series and the target VOCs geosmin and 2-MIB showed relatively high contents in the _SS_ soil group. It has been reported that these VOCs from soil play an important role in relieving inflammation and stress, improving sleep disorders, and regulating the immune system when humans are exposed to them [32; 33]. In addition, a study of soil  
Figure 8: Results of (**A**) BDNF and (**B**) CRP levels in serum after the soil mixing activity with or without _S. rimosus_ (n = 16 per group). Data were analyzed via paired _t_-test. *_p_ < 0.05 compared to the sterilized soil group. Values are the mean \(\pm\) SD. microorganism _M. vucae_, which was found to be effective in improving respiratory diseases and cognitive function, found that the immune system tends to activate and increase serotonin, which can affect emotional stability and behavioral responses [33]. In addition, to understand the effect of the soil mixing activity, EEG, serum metabolite analysis, and subjective emotional evaluation were performed between the experimental group (soil with _S. rimosus_) and the control group (soil without _S. rimosus_). As a result, the effect of the soil mixing activity on human metabolic and autonomic responses was found to differ according to the presence or absence of _S. rimosus_ microorganisms in the soil. As a result of the EEG, ASEF50 of the right occipital lobe was significantly higher in the treatment group (soil with _S. rimosus_) than in the control group (soil without _S. rimosus_) (\(p<0.01\)). ASEF50 is the frequency of the point occupying 50% of the 8-13 Hz range, which is the alpha wave band, in the power spectrum graph. Increased cortical alpha activity indicates brain comfort and is associated with a clear wakefulness state in the brain just prior to concentration. Also, inhalation of 2-MIB (a major odor molecule in soil), which was placed as a target VOC, significantly increases the rapid alpha band among human brain waves. In the olfactory process, fragrant molecules attach to the olfactory receptor cilia in the olfactory epithelium located in the nasal cavity, activate guanine nucleotide binding protein coupled receptors, and generate electrical signals [34]. Thereafter, electrical signals are transmitted to the brain by olfactory sensory neurons through the olfactory bulb and higher olfactory cortex to modulate brain functions, including memory, thoughts, and emotions [35]; furthermore, they act on the neuroendocrine system, neurotransmitters, and neuromodulators, influencing psychological behavior and body function [6].