Table 1.

Description of independent variables related to the soil samples.

Variable	Description	Class	Number of samples	Percentage (%)
DIST	Roadside distance	0 m: Road edge	32	33.3
		10 m: 10 m distance from road edge	32	33.3
		50 m: 50 m distance from road edge	32	33.3
SOIL	Roadside soil type	Meadow: Pure meadow soil	69	71.9
		Mea-ls: Meadow soil with low content of sand	9	9.3
		Mea-hs: Meadow soil with high content of sand	18	18.8
PRE	Air Pressure measured at sampling sites	560 Pa–570 Pa	3	3.1
		570 Pa–580 Pa	9	9.4
		580 Pa–590 Pa	12	12.5
		590 Pa–600 Pa	9	9.4
		600 Pa–610 Pa	6	6.3
		610 Pa–620 Pa	57	59.4
ALT	Altitude of sampling sites	4,100 m–4,200 m	51	53.1
		4,200 m–4,300 m	3	3.1
		4,300 m–4,400 m	3	3.1
		4,400 m–4,500 m	6	6.3
		4,500 m–4,600 m	9	9.4
		4,600 m–4,700 m	6	6.3
		4,700 m–4,800 m	12	12.5
		4,800 m–4,900 m	6	6.3
ROAD	The road pavement surface type	Asphalt	63	65.6
		Gravel	33	34.4
VOLUME	The level of road segment traffic volume	Low	60	62.5
		Middle	18	18.8
		High	18	18.8
REGION	The sampling region in terms of road segment	ZE: connected to Zhaling Lake and Eling Lake	51	53.1
		MQ: nearby Zhaling Lake and Eling Lake	18	18.8
		NMS: connected to Nam Co Lake	9	9.4
		NL: nearby Nam Co Lake	18	18.8