Effects of Nitrogen Addition on Ecosystem CO2 Exchange in a Meadow Steppe, Inner Mongolia

doi:10.11983/CBB17041

Abstract

Abstract: Increasing nitrogen deposition influences carbon sequestration in grassland ecosystems, but we have no consistent results on how it impacts the exchange of CO₂ at the ecosystem level. As well, the impact of different types of N fertilizers and rates of N increase are not clear yet. This study aimed to evaluate the effect of N addition on ecosystem CO₂ exchange and was performed in a meadow steppe in Erguna, Inner Mongolia. A field experiment compared two types of N fertilizers (urea and slow-release urea) with five N addition rates (0, 5.0, 10.0, 20.0 and 50.0 g N·m^-2·a^-1). At the start and middle stage of the growing season, N addition had weak and inhibitive effects on ecosystem CO₂ exchange when precipitation was low, and significantly increased ecosystem CO₂ exchange in the late growing season when precipitation was high. Both net ecosystem CO₂ exchange and gross ecosystem photosynthesis increased significantly with N addition rate but showed a tendency of saturation when the N addition rate reached 10 g N·m^-2·a^-1. The two types of N fertilizers resulted in slightly different responses of ecosystem CO₂ exchange: slow-release urea had a stronger positive effect at 5 g N·m^-2·a^-1 with no significant difference at other addition rates. Our study suggests that increasing N deposition has significant effects on carbon assimilation in this semi-arid grassland, but the direction and magnitude of effects are strongly affected by seasonal pattern and amount of precipitation. The effects of different types of N fertilizers (i.e., urea and slow-release urea) on ecosystem CO₂ exchange may differ.

Key words: nitrogen addition, net ecosystem CO₂ exchange, ecosystem respiration, gross ecosystem photosynthesis, meadow steppe

Muqier Hasi, Xueyao Zhang, Guoxiang Niu, Yinliu Wang, Jianhui Huang. Effects of Nitrogen Addition on Ecosystem CO₂Exchange in a Meadow Steppe, Inner Mongolia[J]. Chinese Bulletin of Botany, 2018, 53(1): 27-41.

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URL: https://www.chinbullbotany.com/EN/10.11983/CBB17041

https://www.chinbullbotany.com/EN/Y2018/V53/I1/27

Figures/Tables 13

Figure 1 Daily precipitation (bars) and daily mean air temperature (line) of the research plots in 2016

Table 1 Means of growing season soil temperature, soil moisture, net ecosystem CO2 exchange, ecosystem respiration and gross ecosystem photosynthesis under different nitrogen addition rates

	N addition rates (g N·m^-2·a^-1)	θν	Ts	NEE	ER	GEP
Urea	0	11.48^a±0.74	17.94^a±0.66	-2.98^a±0.38	3.85^ab±0.12	6.81^ab±0.48
	5	12.79^a±0.97	17.69^a±0.76	-3.13^a±0.49	3.35^a±0.22	6.48^a±0.31
	10	12.29^a±0.61	17.66^a±0.32	-3.77^a±0.32	4.10^ab±0.42	7.88^b±0.29
	20	13.79^a±0.47	17.71^a±0.49	-3.82^a±0.27	4.23^ab±0.23	8.05^b±0.19
	50	13.41^a±0.71	17.22^a±0.22	-2.88^a±0.44	4.59^b±0.33	7.48^ab±0.61
Slow-release urea	0	11.48^a±0.74	17.94^a±0.66	-2.98^b±0.38	3.85^a±0.12	6.81^a±0.48
	5	12.01^a±0.74	18.07^a±0.35	-3.77^ab±0.32	3.81^a±0.25	7.58^a±0.19
	10	12.51^a±0.55	17.72^a±0.35	-4.80^a±0.39	4.29^a±0.32	9.10^b±0.39
	20	14.49^a±1.28	17.06^a±0.27	-3.15^b±0.46	4.19^a±0.37	7.34^a±0.23
	50	14.26^a±0.82	17.44^a±0.42	-2.93^b±0.54	4.55^a±0.27	7.47^a±0.40

Table 2 Results (P-values) of repeated-measures ANOVA on the effects of sampling time, N addition (urea and slow-release urea), and their interactions on soil temperature, soil moisture, net ecosystem CO2 exchange, ecosystem respiration and gross ecosystem photosynthesis during the growing season

		θν	Ts	NEE	ER	GEP
Urea	Time	<0.001	<0.001	<0.001	<0.001	<0.001
	N	0.22	0.91	0.29	0.08	0.07
	Time×N	0.66	0.56	0.01	0.01	0.03
Slow-release urea	Time	<0.001	<0.001	<0.001	<0.001	<0.001
	SN	0.38	0.49	0.04	0.34	0.01
	Time×SN	0.63	0.74	0.003	0.15	0.002

Figure 2 Seasonal dynamics of soil NH4+-N and NO3--N at 0-10 cm depth(A), (B) Addition with urea; (C), (D) Addition with slow-release urea. In-set figures showed mean soil inorganic N contents with addition of N (means±SE, n=4). Different lowercase letters among columns indicate significant differences among treatments (P<0.05).

Figure 3 Seasonal means of pH at 0-10 cm soil (means±SE, n=4)(A) Addition with urea; (B) Addition with slow-release urea. Different lowercase letters among columns indicate significant differences among treatments (P<0.05).

Table 3 Aboveground net primary productivity, grasses aboveground biomass, forb aboveground biomass and total belowground biomass under different nitrogen addition rates (means±SE, n=4)

	N addition rates (g N·m^-2·a^-1)	ANPP	GB	FB	BGB
Urea	0	142.52^a±16.85	96.86^a±19.68	42.37^a±13.05	1718.44^a±172.36
	5	157.74^a±5.51	129.37^a±11.10	24.33^a±8.48	1575.31^a±64.48
	10	216.29^a±23.24	190.59^a±19.71	24.07^a±5.09	1941.26^ab±226.12
	20	167.54^a±21.06	125.55^a±38.81	37.01^a±16.07	2320.85^b±99.68
	50	191.38^a±40.72	182.04^a±42.34	10.26^a±3.16	1915.27^ab±74.55
Slow-release urea	0	142.52^a±16.85	96.86^a±19.68	42.37^a±13.05	1718.44^a±172.36
	5	166.24^ab±11.86	129.16^ab±10.48	28.49^a±3.39	1632.48^a±67.81
	10	238.34^bc±35.71	188.39^bc±45.45	50.16^a±25.57	2127.48^b±111.51
	20	261.06^c±26.01	220.86^c±19.61	40.48^a±7.82	1942.56^ab±123.40
	50	254.41^c±24.63	227.08^c±16.94	31.36^a±16.22	1750.71^ab±105.22

Figure 4 Seasonal variations in net ecosystem CO2 exchange, ecosystem respiration and gross ecosystem photosynthesis under different nitrogen addition rates (means±SE)(A)-(C) Addition with urea; (D)-(F) Addition with slow-release urea. NEE, ER and GEP see Table 1.

Figure 5 Seasonal patterns of net ecosystem CO2 exchange, ecosystem respiration and gross ecosystem photosynthesis at different nitrogen addition rates with urea in each month (means±SE, n=4)NEE, ER and GEP see Table 1. Different lowercase letters among columns indicate significant differences among treatments (P<0.05).

Figure 6 Seasonal patterns of net ecosystem CO2 exchange, ecosystem respiration and gross ecosystem photosynthesis at different nitrogen addition rates with slow-release urea in each month (means±SE, n=4) NEE, ER and GEP see Table 1. Different lowercase letters among columns indicate significant differences among treatments (P<0.05).

Figure 7 Relationships of net ecosystem CO2 exchange, ecosystem respiration or gross ecosystem photosynthesis with 0-10 cm soil temperature(A)-(C) Addition with urea; (D)-(F) Addition with slow-release urea. NEE, ER and GEP see Table 1.

Figure 8 Relationships of net ecosystem CO2 exchange, ecosystem respiration or gross ecosystem photosynthesis with 0-10 cm soil moisture(A)-(C) Addition with urea; (D)-(F) Addition with slow-release urea. NEE, ER and GEP see Table 1.

Figure 9 Relationships of net ecosystem CO2 exchange, ecosystem respiration or gross ecosystem photosynthesis with belowground biomass(A)-(C) Addition with urea; (D)-(F) Addition with slow-release urea. NEE, ER and GEP see Table 1.

Table 4 Sensitivity of ecosystem respiration (ER) and gross ecosystem photosynthesis (GEP) to soil moisture under different nitrogen addition rates (means±SE, n=4)

	N addition rates (g N·m^-2·a^-1)	ER	GEP
Urea	0	0.72^a±0.09	1.86^a±0.64
	5	0.95^a±0.16	3.23^ab±0.62
	10	0.59^a±0.15	3.72^ab±0.71
	20	0.76^a±0.19	3.67^ab±1.35
	50	0.74^a±0.09	4.54^b±0.34
Slow-release urea	0	0.72^a±0.09	1.86^a±0.64
	5	0.89^a±0.14	4.21^b±0.16
	10	1.43^b±0.27	5.86^c±0.61
	20	0.69^a±0.12	3.46^b±0.62
	50	0.92^a±0.12	4.99^bc±0.36

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Effects of Nitrogen Addition on Ecosystem CO₂Exchange in a Meadow Steppe, Inner Mongolia

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