Variations of nitrogen and phosphorus limitation along the environmental gradient in alpine grasslands on the Northern Xizang Plateau

doi:10.17521/cjpe.2020.0135

Abstract

Abstract:

Aims Understanding the mode of nutrient limitation on ecosystem net primary production is an important issue of modern ecology. Nutrient availability is a key determinant of ecosystem dynamics, but the relationship between soil resource availability and ecosystem nutrient limitation is still unclear.
Methods A series of nitrogen and phosphorus nutrient addition experiments were set up in four types of alpine grasslands (alpine meadow, alpine meadow-steppe, alpine steppe and alpine desert-steppe) along the precipitation gradient on the Northern Xizang, to systematically study the effects of nitrogen and phosphorus addition on different types of alpine grasslands, and to explore the nitrogen and phosphorus limitation models of different alpine grasslands.
Important findings The results showed that: (1) The effects of nitrogen and phosphorus addition on different alpine grasslands varied. Nitrogen addition significantly increased the aboveground biomass of alpine meadows and alpine meadow grasslands, but had no effect on alpine meadows and alpine desert grasslands. The addition of phosphorus alone had no significant effect on the four alpine grasslands, while the addition of nitrogen and phosphorus had a promoting effect on the aboveground biomass of the four alpine grasslands. (2) With the decrease of precipitation, the nitrogen limitation index of the alpine grasslands gradually decreased from 1.18 to 0.52-0.64, and the nutrient limitation mode transitioned from nitrogen limitation to co-limitation by nitrogen and phosphorus; the phosphorus limitation index was negative in the alpine meadow-steppe and alpine steppe, indicating that phosphorus addition alone has side effects on these two grassland types. These results suggest that alpine meadow is mainly limited by nitrogen availability, and phosphorus addition alone has side effects; the alpine meadow-steppe is between the nitrogen limit and the joint nitrogen and phosphorus limitation, and phosphorus addition alone also has side effects; the alpine steppe is limited by both nitrogen and phosphorus availability, and the addition of phosphorus has side effects; the alpine desert-steppe is jointly limited by nitrogen and phosphorus availability. These results show that nutrient limitation mode transits from nitrogen limitation to nitrogen and phosphorus co-limitation with the decrease of precipitation. This study implies that the impacts of increasing nitrogen deposition under future climate change on different types of alpine grasslands may be different. Additionally, the differences in nitrogen and phosphorus limitation mode should also be taken into consideration when nutrient addition is used to restore different types of degraded alpine grasslands.

Key words: control experiment, nitrogen and phosphorus limitation, grassland type, plant functional group

ZONG Ning, SHI Pei-Li, ZHAO Guang-Shuai, ZHENG Li-Li, NIU Ben, ZHOU Tian-Cai, HOU Ge. Variations of nitrogen and phosphorus limitation along the environmental gradient in alpine grasslands on the Northern Xizang Plateau[J]. Chin J Plant Ecol, 2021, 45(5): 444-455.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2020.0135

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Figures/Tables 7

Fig. 1 Biomass response patterns of the four main types of nitrogen and phosphorus co-limitation. Assuming that nutritional additions have no negative effects. CK, control (no resource addition); +A, adding resource A; +B, adding resource B; +AB, adding A and B together.

Table 1 Description of study sites in alpine grasslands on the Northern Xizang Plateau

	研究站点 Study site
	高寒草甸 Alpine meadow	高寒草甸草原 Alpine meadow-steppe	高寒草原 Alpine steppe	高寒荒漠草原 Alpine desert-steppe
经纬度 Latitude and longitude	31.57° N, 92.57° E	31.38° N, 90.23° E	31.78° N, 87.23° E	32.37° N, 82.27° E
海拔 Altitude (m)	4 570	4 590	4 580	4 520
年平均气温 Mean annual air temperature (℃)	-0.9	-1.0	-1.4	-1.4
年降水量 Mean annual precipitation (mm)	444.9	335.4	327.4	175.2
群落盖度 Coverage (%)	70-80	40-50	20-30	15-25
优势种群 Dominant species	高山嵩草 Kobresia pygmaea	紫花针茅、窄叶薹草 Stipa purpurea, Carex montis-everesti	紫花针茅 S. purpurea	紫花针茅、小叶棘豆 S. purpurea, Oxytropis microphylla

Fig. 2 Inter-annual variations of precipitation in different types of alpine grasslands and the relationship between soil nitrogen (N) and phosphorus (P) content and precipitation on the Northern Xizang Plateau. ADS, alpine desert-steppe; AM, alpine meadow; AMS, alpine meadow-steppe; AS, alpine steppe.

Table 2 Using year as the repeated factor, Repeated Measure ANOVA analysis of the effects of nitrogen and phosphorus addition on the aboveground biomass of different types of alpine grasslands

功能群 Functional group	年份 Year		施肥 Fertilization		草地类型 Grassland		年份×施肥 Year × Fertilization		年份×草地 Year × Grassland		草地×施肥 Grassland × Fertilization		年份×草地×施肥 Year × Grassland × Fertilization
	df = 2		df = 3		df = 3		df = 6		df = 6		df = 9		df = 18
		F	p	F	p	F	p	F	p	F	p	F	p	F	p
群落 Community	14.81	<0.001	24.70	<0.001	336.91	<0.001	0.83	0.55	2.92	0.014	2.78	0.016	0.77	0.73
禾草 Grasses	16.98	<0.001	33.60	<0.001	30.68	<0.001	5.64	<0.001	12.59	<0.001	6.92	<0.001	4.19	<0.001
莎草 Sedges	2.63	0.080	3.82	0.019	403.75	<0.001	0.53	0.79	2.07	0.069	3.04	0.01	0.56	0.91
豆科 Legumes	11.57	<0.001	3.64	0.023	24.66	<0.001	0.92	0.48	11.34	<0.001	6.81	<0.001	1.29	0.48
杂类草 Forbs	6.49	0.003	7.38	0.001	52.69	<0.001	2.15	0.060	7.23	0.003	1.44	0.21	3.27	<0.001

Fig. 3 Effects of nitrogen (N) and phosphorus (P) addition on the aboveground biomass of different plant functional groups of alpine grasslands. Different uppercase letters in the same year represent significant differences among fertilization treatments (p < 0.05). F and P represent the differences in different functional groups among fertilization treatments. A-C, Alpine meadow. D-F, Alpine meadow-steppe. G-I, Alpine steppe. J-L, Alpine desert-steppe.

Fig. 4 Relative co-limitation index of nitrogen and phosphorus for different types of alpine grasslands. ADS, alpine desert-steppe; AM, alpine meadow; AMS, alpine meadow-steppe; AS, alpine steppe. RCIN, nitrogen limitation index; RCIP, phosphorus limitation index. Different uppercase letters in the same year represent significant differences among grassland types (p < 0.05).

Fig. 5 Relationships between relative co-limitation index and precipitation as well as soil nutrient content in alpine grasslands on the Northern Xizang Plateau. RCIN, nitrogen limitation index; RCIP, phosphorus limitation index.

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