Table 3 Stepwise regression analysis of soil influencing factors (only factors with significant correlations are listed, 0 < VIF < 10 denotes there is no collinearity, where VIF is Variance Inflation Factor).

Soil layer (cm)	Soil properties	Affecting factor	Coefficient estimate	SD	P	VIF	R ²
0–10	TK	SOC	−0.589	0.143	0.001	1.668	0.805
		CS	0.402	0.139	0.011	1.590
		W	0.312	0.114	0.015	1.070
	TP	Stand age	1.512	0.243	<0.001	3.850	0.769
		SOC	−1.245	0.223	<0.001	3.230
		TN	−0.582	0.216	0.017	3.031
		TK	−0.472	0.203	0.035	2.675
	TN	Stand age	1.250	0.271	<0.001	5.636	0.805
		SOC	−0.620	0.265	0.033	5.393
		TP	−0.517	0.188	0.015	2.717
		CS	−0.408	0.149	0.015	1.715
	W	SOC	1.133	0.328	0.004	3.230	0.500
		TK	0.893	0.299	0.009	2.675
		Stand age	−0.800	0.358	0.041	3.850
10–20	TP	Stand age	0.985	0.122	<0.001	1.537	0.845
		SOC	−0.706	0.123	<0.001	1.565
		CS	−0.395	0.103	0.001	1.090
	CS	TP	−0.929	0.182	<0.001	1.594	0.667
		W	−0.804	0.184	<0.001	1.628
		SOC	−0.409	0.147	0.013	1.042
	TN	Stand age	0.636	0.237	0.016	1.537	0.417
20–40	TK	Stand age	−1.187	0.125	<0.001	1.927	0.878
		W	−0.534	0.136	0.001	2.275
		SOC	0.237	0.108	0.045	1.436
	BD	TP	−0.733	0.142	<0.001	1.233	0.740
		TN	−0.588	0.132	<0.001	1.063
		TK	−0.561	0.146	0.001	1.302
40–60	CS	W	−0.384	0.189	0.046	1.521	0.623
60–80	TN	TK	0.811	0.220	0.002	1.606	0.550
		TP	0.696	0.245	0.012	1.998
		SOC	−0.393	0.182	0.048	1.109
80–100	CS	TK	0.539	0.139	0.001	1.214	0.746
		Stand age	0.467	0.158	0.009	1.575
		W	−0.404	0.151	0.017	1.443
	TP	pH	0.570	0.194	0.009	1.000	0.325

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