Table 3 Significant regression coefficients of trend-surface analysis and coefficients of determination (r²) for soil MBC, MBN concentrations and MBC:MBN under three N fertilization treatments (i.e. NN, LN and HN) in SG and GG cropland soils in a three-year long fertilization experimental site at the Tennessee State University (TSU) Agricultural Research Center in Nashville, TN, USA.

Crop	Variable	Fertilization	Plot	β ₀	β₁x	β₂y	β₃xy	β₄x²	β₅y²	r²
SG	MBC	NN	P1	0.161***	—	—	—	—	—	0.407
		NN	P2	0.131***	—	—	—	—	—	0.130
		LN	P1	0.229**	—	—	—	—	—	0.296
		LN	P2	0.136**	—	—	—	—	—	0.253
		HN	P1	0.188**	—	—	—	—	—	0.290
		HN	P2	0.200***	—	—	0.022*	—	—	0.289
	MBN	NN	P1	0.028***	−0.018*	—	—	0.006*	—	0.408
		NN	P2	—	—	—	—	—	—	0.134
		LN	P1	0.0280*	—	—	—	—	—	0.152
		LN	P2	0.0143*	—	—	—	—	—	0.545
		HN	P1	—	—	—	—	—	—	0.140
		HN	P2	—	—	—	—	—	—	0.287
	MBC:MBN	NN	P1	—	—	—	—	—	—	0.092
		NN	P2	11.443*	—	—	—	—	—	0.078
		LN	P1	—	—	—	—	—	—	0.105
		LN	P2	11.157***	—	—	—	2.519*	0.544*	0.538
		HN	P1	16.528**	—	—	—	—	—	0.416
		HN	P2	—	—	—	—	—	-	0.303
GG	MBC	NN	P1	—	—	—	—	—	—	0.184
		NN	P2	0.226***	—	—	—	—	—	0.150
		LN	P1	0.162**	—	—	—	—	—	0.074
		LN	P2	0.157**	—	—	—	—	—	0.097
		HN	P1	0.227***	—	—	—	—	—	0.153
		HN	P2	0.138***	—	—	−0.010*	—	—	0.379
	MBN	NN	P1	—	—	—	—	—	—	0.176
		NN	P2	—	—	—	—	—	−0.00278	0.346
		LN	P1	0.028**	—	—	—	—	—	0.210
		LN	P2	0.041***	—	−0.0152***	—	—	0.0019**	0.582
		HN	P1	—	—	—	—	−0.0030*		0.346
		HN	P2	0.015*	—	—	—	—	—	0.142
	MBC:MBN	NN	P1	10.379*	—	—	—	—	—	0.090
		NN	P2	20.195***	−14.846*	—	1.591**	—	—	0.430
		LN	P1	—	—	—	—	—	—	0.304
		LN	P2	—	—	1.537*	—		—	0.332
		HN	P1	13.969**	—	—	1.541*	—	—	0.414
		HN	P2	8.503***	—	—	−0.720*	1.752*	—	0.485

*** and ***represent significance at P < 0.05, 0.01 and 0.001, respectively. The abbreviations are referred to Table 1.

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