N₂ fixation of nodules and N absorption by soybean roots associated with ridge tillage on poorly drained upland fields converted from rice paddy fields

Figures & data

Table 1 Soil classification, texture, pH, soil N, amount of mineralized N and height of water table in the two fields

	Site A	Site B
Classification of cultivated soils in Japan^†	Mottled Gley Lowland soils	Strong Gley Lowland soils
Soil Taxonomy^‡	Typic Hydraquents	Typic Hydraquents
Clay content (%)	27	45
Silt content (%)	30	41
Sand content (%)	44	19
pH (H2O)	5.5	5.0
Soil N (%)	0.15	0.28
Mineralized N (mg N kg^−1)	50.3	69.3
Height of water table (m)^§	0.7	0.4
^† Cultivated Soil Classification Committee (1995);
^‡ Soil Survey Staff (1994).
^§Measurement was conducted in October 2003.

Figure 1  Difference in the volumetric water content between two cultivation treatments (ridge tillage [RT] and conventional tillage [CT]) at 5 cm depth from the soil surface at sites A and B in 2003.

Figure 2  Average temperature, solar radiation and precipitation for each 5-day period during the cropping season of soybean at sites A and B. Arrows indicate the rainy season in a normal year.

Table 2 Yield and related properties of soybean at harvest

			Grain yield (g m^−2)	Dry matter (g m^−2)	Pod number (m^−2)	Seed number (m^−2)	Seed weight (g)
2002	Site A	CT	353	539	755	1,260	0.280
		RT	378	577	786	1,270	0.296
	Site B	CT	348	515	734	1,210	0.286
		RT	375	586	800	1,310	0.287
2003	Site A	CT	317	506	668	1,090	0.289
		RT	398	601	742	1,250	0.320
	Site B	CT	292	435	519	941	0.311
		RT	331	481	537	943	0.351
RT, ridge tillage; CT, conventional tillage.

Table 3 Sources of variation, degrees of freedom and mean squares for soybean properties at harvest in an analysis across years, sites and methods of cultivation

Source	df	Mean squares
		Grain yield	Dry weight	Pod number	Seed weight
Year (Y)	1	5,687**	16,114**	159,198****	63.075****
Site (S)	1	3,278*	14,197**	42,101***	7.253**
Y×S	1	3,060*	13,144**	51,482***	13.872***
Cultivation (C)	1	11,618***	25,980***	16,032*	27.195****
Y×C	1	1,992	468	11	12.826***
S×C	1	493	11	38	0.373
Y×S×C	1	886	2,918	3,504	2.751
Error	20	936	2,025	4,813	1.333
*P < 0.1;
**P < 0.05;
***P < 0.01;
****P < 0.001.

Figure 3  Comparison of the time-course of the ratio of N derived from N2 fixation of nodules per plant N (ratio of fixed N) with the differences in years, cultivation treatments and sites. Letters above the x-axis denote the growth stage of soybeans. RT, ridge tillage; CT, conventional tillage.

Table 4 Estimated amount of N derived from N2 fixation of nodules (fixed N) and of N absorbed by roots (absorbed N)

	Site A						Site B
	Total		Fixed N		Absorbed N		Total		Fixed N		Absorbed N
	CT	RT	CT	RT	CT	RT	CT	RT	CT	RT	CT	RT
2002
V5	1.3	1.3	0.25	0.26	1.1	1.0	0.41	0.83	0.11	0.19	0.30	0.64
R1	4.6	5.7	3.2	4.2	1.4	1.5	4.5	5.0	3.3	3.6	1.1	1.4
R3	7.3	7.6	5.6	6.0	1.8	1.6	9.6	11	8.0	9.3	1.6	2.1
R5	16	14	14	13	2.6	1.9	19	21	16	17	2.7	3.2
R7	18	23	14	18	3.1	5.2	26	26	20	20	5.6	6.0
2003
V4	0.29	0.29	0.18	0.11	0.11	0.18	0.21	0.25	0.07	0.13	0.14	0.12
V5	1.2	1.7	0.31	0.42	0.9	1.3	0.65	0.76	0.15	0.18	0.50	0.59
R1	4.8	5.9	3.4	4.1	1.3	1.8	2.8	2.9	2.0	2.0	0.84	0.90
R3	11	11	8.5	8.5	2.6	2.8	8.2	9.7	6.0	7.5	2.2	2.2
R5	14	19	11	15	3.4	4.5	12	15	8.8	11	3.1	3.8
R7	19	21	12	15	6.0	5.6	18	26	12	16	5.7	9.8
Values are g N m^−2.

Table 5 Sources of variation, degrees of freedom and mean squares for soybean N fixation and absorption of R1 and R7 stages in an analysis across years, sites and methods of cultivation

Source	df	Mean Squares
		R1			R7
		Total	Fixed N	Absorbed N	Total	Fixed N	Absorbed N
Year (Y)	1	0.061*	2.04***	0.061**	25.2	74.61****	13.09*
Site (S)	1	2.052****	3.94***	0.824****	54.1**	14.46****	12.69*
Y×S	1	0.001	2.45***	0.273****	12.8	15.31**	0.111
Cultivation (C)	1	0.273***	0.980**	0.17***	75.9**	31.22***	9.78
Y×C	1	0.015	0.102	0.019	4.87	2.63	0.334
S×C	1	0	0.456	0.015	0.098	1.23	2
Y×S×C	1	0.166	0.009	0.101**	30.2	5.7	9.63
Error	8	0.114	0.165	0.009	8.27	1.98	3.08
*P < 0.1;
**P < 0.05;
***P < 0.01;
****P < 0.001.

Figure 4  Ratio of exchangeable Rb and K in vertical soil profile at sites A and B. The ratio is expressed on a weight basis.

Table 6 Ratio of Rb and K in soybean at R1 and R7 stages

	Site A		Site B
	CT	RT	CT	RT
2002
R1	14	8.8	11	12
R7	13	13	19	25
2003
R1	15	13	16	13
R7	20	17	24	29
Values are mole ratio ×100,000.

Table 7 Sources of variation, degrees of freedom and mean squares for Rb/K at R1 and R7 growth stages in an analysis across years, sites and methods of cultivation

Source	df	R1	R7
Year (Y)	1	36.6****	81.9***
Site (S)	1	0.123	274****
Y×S	1	0.122	0.004
Cultivation (C)	1	19.8****	15.6
Y×C	1	0.203	0.666
S×C	1	8.12	8.45
Y×S×C	1	15.6	0.193
Error	8	0.125	5.72
***P < 0.01;
****P < 0.001.

Cultivated Soil Classification Committee . 1995 . Classification of cultivated soils in Japan 3rd edn . MiscPublNatl InstAgro-EnvironSci , 17 : 1 – 79 .

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Soil Survey Staff . 1994 . Keys to Soil Taxonomy , 6th edn , Washington : USDA .

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