Influence of Azolla incorporation and/or dual cropping on CH₄ and N₂O emission from a paddy field

Figures & data

Figure 1. Daily sunshine time (■) and average air temperature (●) during the experimental period from 10th June to 18 September 2019 in Tsuruoka, Japan. The bold and dashed lines crossing air temperature and sunshine time are the daily average values for 1981–2010, respectively. Data were derived from the Japan Meteorological Agency.

Table 1. Cumulative CH₄ and N₂O emissions during early (before 35 DAT), middle (35–70 DAT), and later (70–98 DAT) rice growth stages, and seasonal total cumulative emissions as affected by Azolla application practices

	Cumulative CH4 emission				Cumulative N2O emission
Treatments	Early	Middle	Later	Total	Early	Middle	Later	Total
	(g C m^−2)				(mg N m^−2)
NPK	4.8 ± 2.3 ab	22.6 ± 3.6 ab	7.7 ± 1.0 a	35.1 ± 6.5 ab	4.0 ± 1.2 a	3.7 ± 1.3 b	3.4 ± 0.6 a	11.1 ± 2.2 b
NPK + Cover	3.2 ± 0.8 b	19.9 ± 2.3 b	8.7 ± 1.3 a	31.7 ± 2.6 b	2.6 ± 0.7 a	27.6 ± 5.9 a	3.9 ± 1.2 a	34.0 ± 5.1 a
AGM + Cover	7.7 ± 1.3 a	24.8 ± 2.5 a	9.7 ± 0.5 a	42.2 ± 2.0 a	3.3 ± 0.4 a	33.9 ± 1.8 a	4.5 ± 1.2 a	41.6 ± 1.8 a

Values are means ± standard error (n = 4). Different letters following values within the same column indicate significant differences among treatments at P < 0.05. The tests of between-subject effects are as shown in Table S1.

Figure 2. Changes in CH₄ (a) and N₂O (b) fluxes from plots treated with chemical fertilizer (NPK), NPK plus Azolla cover (NPK + Cover), and Azolla as green manure plus Azolla cover (AGM + Cover) throughout the experimental period. Vertical bars indicate standard error (n = 4). Dashed lines show the early, middle and later rice growth stages.

Figure 3. Changes in the concentration of CO₂ (a) and CH₄ (b) dissolved in the soil solution in plots treated with chemical fertilizer (NPK), NPK plus Azolla cover (NPK + Cover), and Azolla as green manure plus Azolla cover (AGM + Cover) throughout the experimental period. Vertical bars indicate standard error (n = 4). Dashed lines show the early, middle and later rice growth stages.

Figure 4. Plant height (a), tiller number (b), and leaf color measured in SPAD values (c) of rice plants from plots treated with chemical fertilizer (NPK), NPK plus Azolla cover (NPK + Cover), and Azolla as green manure plus Azolla cover (AGM + Cover) throughout the experimental period. Vertical bars indicate standard error (n = 4). Dashed lines show the early, middle and later rice growth stages.

Table 2. Rice grain yield, harvest index, GWP (CH₄ and N₂O emissions), and yield-scaled GWP as affected by Azolla application practices

Treatments	Grain yield	Harvest index	GWP	Yield-scaled GWP
	(g m^−2)	(%)	(g CO2eq m^−2)	(g CO2eq g^−1 grain)
NPK	628.8 ± 35.7 a	51.4 ± 1.7 a	1596.0 ± 295.4 ab	2.5 ± 0.3 b
NPK + Cover	607.6 ± 18.2 a	49.0 ± 2.3 a	1455.0 ± 118.1 b	2.4 ± 0.2 b
AGM + Cover	392.9 ± 31.9 b	48.0 ± 5.2 a	1934.8 ± 89.4 a	4.9 ± 0.4 a

Values are means ± standard deviation (n = 4). Different letters following values within the same column indicate significant differences among treatments at P < 0.05. GWP of CO₂:CH₄:N₂O = 1:34:298 (IPCC 2013). The tests of between-subject effects are as shown in Table S2.

Figure 5. Relationship between daily CH₄ and night respiration (CO₂) emission from plots treated with chemical fertilizer (NPK), NPK plus Azolla cover (NPK + Cover), and Azolla as green manure plus Azolla cover (AGM + Cover) after 5 weeks rice transplanting (n = 8).

IPCC. 2013. “Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.” In Climate Change 2013 – The Physical Science Basis by Intergovernmental Panel on Climate Change, edited by T. F. Stocker, D. Qin, G. K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, 1535. Cambridge, UK and New York, NY, USA: Cambridge University Press. doi:10.1017/CBO9781107415324.

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