Fertilizer-derived nitrogen use of two varieties of single-crop paddy rice: a free-air carbon dioxide enrichment study using polymer-coated ¹⁵N-labeled urea

Figures & data

Table 1. Monthly weather conditions at the Tsukuba FACE site

Month	Year	Daily temperature			Daily solar radiation	Precipitation
		Mean (°C)	Max.(°C)	Min.(°C)	(MJ m^–2 d^–1)	(mm month^–1)
June	2015 (this study)	21.3	25.1	17.9	17.7	111
	2012–2014	21.0	24.6	18.1	17.2	200
July	2015 (this study)	25.4	29.2	22.2	17.6	156
	2012–2014	24.9	29.0	21.6	18.9	90
August	2015 (this study)	25.6	29.3	22.7	15.2	83
	2012–2014	26.5	30.8	22.7	18.5	92
September	2015 (this study)	21.6	25.4	18.5	11.9	306
	2012–2014	24.2	28.5	20.6	14.7	92

Figure 1. Design of the experimental section embedded in the Tsukuba FACE in 2015. PI, sampling plot at panicle initiation; HD, heading; and MT, maturing stages.

Figure 2. Biomass and the weight composition ratio of each part of paddy rice. Bars denote the standard deviation (n = 4 blocks). PI, HD, and MT denote the panicle initiation, heading, and maturing stages, respectively. F, elevated carbon dioxide (CO₂) plot; A, ambient CO₂ plot; Koshi, paddy rice (cv. Koshihikari); and Taka, paddy rice (cv. Takanari).

Table 2. Total nitrogen (N) concentrations and the ratio of fertilizer-derived N to total N in the rice plants at the maturing stage

		Rice (cv. Koshihikari)		Rice (cv. Takanari)		Statistical significance
		FACE^1)	Ambient^2)	FACE	Ambient	CO2	Var	CO2× Var
Total N concentration
Panicles	(% w/w)	0.99 (0.05)	1.03 (0.04)	0.98 (0.10)	1.01 (0.03)	n.s.	n.s.	n.s.
Leaves	(% w/w)	1.11 (0.13)	1.20 (0.13)	1.05 (0.19)	1.05 (0.04)	n.s.	n.s.	n.s.
Dead leaves	(% w/w)	0.51 (0.06)	0.53 (0.05)	0.45 (0.07)	0.46 (0.03)	n.s.	*	n.s.
Straw	(% w/w)	0.39 (0.05)	0.40 (0.04)	0.36 (0.06)	0.35 (0.02)	n.s.	*	n.s.
Root	(% w/w)	0.96 (0.08)	0.91 (0.04)	0.74 (0.04)	0.77 (0.03)	n.s.	***	n.s.
Ratio of fertilizer-derived N to total N
Panicles	(%)	39.9 (2.5)	42.2 (4.7)	37.4 (2.9)	40.9 (3.3)	n.s.	*	n.s.
Leaves	(%)	44.0 (2.3)	45.6 (5.4)	42.7 (3.2)	45.1 (4.2)	n.s.	n.s.	n.s.
Dead leaves	(%)	38.8 (3.3)	39.1 (3.9)	36.2 (4.3)	36.3 (1.6)	n.s.	*	n.s.
Straw	(%)	38.1 (1.9)	40.2 (4.4)	35.1 (3.0)	38.7 (3.7)	n.s.	*	n.s.
Root	(%)	30.2 (6.0)	32.3 (4.3)	27.8 (6.0)	23.8 (3.8)	n.s.	†	n.s.

1) FACE, CO₂ enriched plot; 2) Ambient, ambient air plot. Values in parentheses denote the standard deviation (n = 4 blocks). ***, P < 0.001; *, P < 0.05; †, P < 0.1; and n.s., not significant.

Table 3. Total nitrogen (N) content per unit area in the rice plants at the maturing stage

		Rice (cv. Koshihikari)		Rice (cv. Takanari)		Statistical significance
		FACE^1)	Ambient^2)	FACE	Ambient	CO2	Var	CO2× Var
Total N in rice (whole plant)	(kg N ha^–1)	140.7 (11.7)	126.4 (6.8)	144.6 (21.4)	132.2 (6.9)	n.s.	n.s.	n.s.
Panicles	(kg N ha^–1)	80.9 (2.1)	71.7 (1.1)	94.3 (11.2)	85.2 (6.0)	*	**	n.s.
Other aboveground part	(kg N ha^–1)	48.1 (7.9)	46.5 (6.2)	37.1 (7.8)	35.1 (3.2)	n.s.	***	n.s.
Root	(kg N ha^–1)	11.6 (2.5)	8.3 (1.8)	13.3 (2.6)	12.0 (1.4)	n.s.	*	n.s.

1) FACE, CO₂ enriched plot; 2) Ambient, ambient air plot. Values in parentheses denote the standard deviation (n = 4 blocks). ***, P < 0.001; **, P < 0.01; *, P < 0.05; and n.s., not significant.

Figure 3. Total nitrogen (N) content and the weight composition ratio of each part of paddy rice. Bars denote the standard deviation (n = 4 blocks). PI, HD, and MT denote the panicle initiation, heading, and maturing stages, respectively. F, elevated carbon dioxide (CO₂) plot; A, ambient CO₂ plot; Koshi, paddy rice (cv. Koshihikari); and Taka, paddy rice (cv. Takanari).

Figure 4. Fertilizer-derived nitrogen (N) content and the weight composition ratio of each part of paddy rice. Bars denote the standard deviation (n = 4 blocks). PI, HD, and MT denote the panicle initiation, heading, and maturing stages, respectively. F, elevated carbon dioxide (CO₂) plot; A, ambient CO₂ plot; Koshi, paddy rice (cv. Koshihikari); and Taka, paddy rice (cv. Takanari).

Table 4. Fertilizer-derived nitrogen (N) content per unit area in the rice plants and soils, and the use efficiencies and fate of fertilizer-derived N at the maturing stage

		Rice (cv. Koshihikari)		Rice (cv. Takanari)		Statistical significance
		FACE^1)	Ambient^2)	FACE	Ambient	CO2	Var	CO2× Var
Applied fertilizer	(kg N ha^–1)	80.0 (0.0)	80.0 (0.0)	80.0 (0.0)	80.0 (0.0)	–	–	–
Fertilizer N in rice (whole plant)	(kg N ha^–1)	55.0 (4.0)	52.3 (4.1)	52.5 (4.2)	51.9 (3.8)	n.s.	*	*
Panicles	(kg N ha^–1)	32.3 (1.8)	30.2 (3.0)	35.1 (2.4)	34.8 (2.3)	n.s.	**	n.s.
Other aboveground part	(kg N ha^–1)	19.2 (2.9)	19.4 (1.2)	13.9 (2.2)	14.3 (2.3)	n.s.	***	n.s.
Root	(kg N ha^–1)	3.5 (1.1)	2.7 (0.7)	3.6 (0.5)	2.8 (0.1)	†	n.s.	n.s.
Fertilizer N remained in soil	(kg N ha^–1)	19.6 (7.1)	13.6 (1.6)	14.4 (3.2)	11.4 (5.2)	†	n.s.	n.s.
Fertilizer N loss as the balance	(kg N ha^–1)	5.4 (7.9)	14.1 (4.9)	13.1 (3.9)	16.7 (1.9)	n.s.	n.s.	n.s.
REag ^3) (dilution method)	(%)	64.3 (4.3)	62.1 (4.2)	61.2 (5.5)	61.4 (4.8)	n.s.	*	†
REwp ^4) (dilution method)	(%)	68.7 (5.0)	65.4 (5.1)	65.7 (5.2)	64.9 (4.8)	n.s.	*	*
AE ^5) (dilution method)	(%)	40.3 (2.3)	37.8 (3.8)	43.8 (3.1)	43.5 (2.9)	n.s.	**	n.s.
RSR ^6) (dilution method)	(%)	24.5 (8.9)	17.0 (2.0)	18.0 (4.0)	14.2 (6.5)	†	n.s.	n.s.
ELR ^7) (dilution method)	(%)	6.7 (9.8)	17.6 (6.2)	16.3 (4.9)	20.9 (2.4)	n.s.	n.s.	n.s.

1) FACE, CO₂ enriched plot; 2) Ambient, ambient air plot; 3) RE_ag, recovery efficiency of aboveground part; 4) RE_wp, recovery efficiency of whole plant; 5) AE, agronomic efficiency; 6) RSR, ratio of fertilizer-derived N remained in soil to the applied amount; and 7) ELR, ratio of fertilizer-derived N lost to the environment to the applied amount. Values in parentheses denote the standard deviation (n = 4 blocks). ***, P < 0.001; **, P < 0.01; *, P < 0.05; †, P < 0.1; and n.s., not significant.

Table 5. Comparison between the dilution and difference methods for recovery efficiency (aboveground part) and agronomic efficiency

			Method^1)			Rice (cv. Koshihikari)				Rice (cv. Takanari)
						FACE^2)		Ambient^3)		FACE		Ambient
REag ^4)		(%)	Dilution			64.3 (4.3)		62.1 (4.2)		61.2 (5.5)		61.4 (4.8)
REag(diff) ^5)		(%)	Difference			86.1 (18.1)		66.3 (4.3)		71.5 (15.3)		64.5 (14.6)
AE ^6)		(%)	Dilution			40.3 (2.3)		37.8 (3.8)		43.8 (3.1)		43.5 (2.9)
AE(diff) ^7)		(%)	Difference			53.6 (6.9)		35.8 (6.0)		47.6 (7.2)		41.9 (10.5)
	Statistical significance
	Method			CO2	Var		Method×CO2		Method×Var	CO2× Var	Method×CO2× Var
RE	**			n.s.	n.s.		†		n.s.	n.s.	n.s.
AE	†			n.s.	n.s.		**		n.s.	n.s.	n.s.

1) Method, ‘Dilution’ denotes that the fertilizer-derived N was determined using ¹⁵N-labeled fertilizer (dilution method) and ‘Difference’ the fertilizer-derived N was estimated as the difference in N content between the fertilized and non-fertilized plots (difference method); 2) FACE, CO₂ enriched plot; 3) Ambient, ambient air plot; 4) RE_ag, recovery efficiency of aboveground part (dilution method); 5) RE_ag(diff), recovery efficiency of aboveground part for reference (difference method); 6) AE, agronomic efficiency (dilution method); and 7) AE_(diff), agronomic efficiency for reference (difference method). Values in parentheses denote the standard deviation (n = 4 blocks). **, P < 0.01; †, P < 0.1; and n.s., not significant.

Figure 5. Total nitrogen (N) and the remaining fertilizer-derived N content in the soil (0–15 cm depth) at the maturing stage.

Figure 6. Ammonium concentrations in the soil solution. Bars denote the standard deviation (n = 4 blocks). F, elevated carbon dioxide (CO₂) plot; A, ambient CO₂ plot; Koshi, paddy rice (cv. Koshihikari); and Taka, paddy rice (cv. Takanari).

Table 6. Total nitrogen (N) content in the rice plants at the maturing stage separating its N sources

		Koshihikari		Takanari		Statistical significance
		FACE^1)	Ambient^2)	FACE	Ambient	CO2	Var	CO2× Var
Total N in whole plant	(kg N ha^–1)	140.7 (11.7)	126.4 (6.8)	144.6 (21.4)	132.2 (6.9)	n.s.	n.s.	n.s.
Fertilizer N	(kg N ha^–1)	55.0 (4.0)	52.3 (4.1)	52.5 (4.2)	51.9 (3.8)	n.s.	*	*
N of other sources^3)	(kg N ha^–1)	85.7 (9.5)	74.1 (9.0)	92.1 (17.6)	80.3 (7.3)	*	†	n.s.

1) FACE, CO₂ enriched plot; 2) Ambient, ambient air plot; 3) N of other sources, N from sources other than fertilizer = Total N – Fertilizer N. Values in parentheses denote the standard deviation (n = 4 blocks). *, P < 0.05; †, P < 0.1; and n.s., not significant.