Effect of altered litter input and nitrogen addition on ecosystem aboveground primary productivity and plant functional group composition in a semiarid grassland

doi:10.17521/cjpe.2020.0126

Abstract

Abstract:

Aims Litter is the major input source of soil organic carbon and nutrients in natural ecosystems and considered as a key link between above- and belowground carbon cycles. Changes in litter input amount have been proven to exert significant impacts on plant productivity, community structure, and therefore ecosystem function. In Nei Mongol semiarid grasslands, different grassland management practices such as grazing, clipping, and fencing have caused dramatic changes in litter production and input. In addition, as a nitrogen-limited ecosystem, Nei Mongol semiarid grasslands also experienced increasing nitrogen deposition. However, how do changes in litter input and nitrogen addition impact the community productivity and composition of plant functional groups are still unclear in the semiarid grasslands. In this study, our objectives are: 1) to investigate the effects of altered litter input and nitrogen addition on community productivity; 2) to study the changes in aboveground biomass of different plant functional groups and their contribution to community productivity under different litter input and N addition treatments.
Methods We established a manipulative experiment with altered litter input and nitrogen addition treatments in a semiarid typical grassland in West Ujimqin Banner, Nei Mongol. A randomized block split-plot design was applied with five blocks. Three litter input treatments, including litter removal (C0), control (C1) and litter addition (C2), were assigned randomly in each block. Each plot (6 m × 7 m) of litter input treatment was separated into two subplots. One of the subplots was assigned as the N addition treatment (N1) and another subplot was considered as the control treatment without N addition (N0). In N addition treatment, 15 g N·m^-2·a^-1 N fertilizer (as NH₄NO₃) was applied every year since 2013. Aboveground net primary productivity (ANPP) in community and plant functional group levels of each treatment were determined during the peak season from 2013 to 2018.
Important findings Based on 6-year measurements, we found the following results. 1) Litter input increase and nitrogen addition increased community ANPP. Compared with the control, litter removal treatment significantly decreased ANPP by 8.4% and 7.6% in plots without and with N addition, respectively. Litter addition increased ANPP by 10.7% and 6.3% in plots without and with N addition, respectively. 2) Different responses of plant functional groups to altered litter input and N addition led to a change in plant functional group composition. The biomass of perennial bunch grass (PB) and perennial rhizome grass (PR) increased significantly with the increment of litter and nitrogen, which enhanced their dominant status in the community. 3) Improved soil water condition by litter input and nutrient supply by N addition are the major pathways that enhanced ANPP and changed the functional group composition. These results show that proper management, such as grazing exclusion and reducing grazing intensity, can promote productivity by increasing inputs of litter in semiarid grasslands, which leads to the maintenance of ecosystem stability. Suitable nutrients management, like nitrogen addition, is also helpful for productivity improvement and the recovery of degraded grasslands.

Key words: aboveground net primary productivity (ANPP), plant functional group, altered litter input, nitrogen addition, semiarid grassland

GAMADAERJI , YANG Ze, TAN Xing-Ru, WANG Shan-Shan, LI Wei-Jing, YOU Cui-Hai, WANG Yan-Bing, ZHANG Bing-Wei, REN Ting-Ting, CHEN Shi-Ping. Effect of altered litter input and nitrogen addition on ecosystem aboveground primary productivity and plant functional group composition in a semiarid grassland[J]. Chin J Plant Ecol, 2020, 44(8): 791-806.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2020.0126

https://www.plant-ecology.com/EN/Y2020/V44/I8/791

Figures/Tables 11

Table 1 Annual litter addition amount of litter addition treatment in 2013- 2018 (g·m-2)

处理 Treatment	2013	2014	2015	2016	2017	2018
N0	433 ± 13	199 ± 24	203 ± 26	135 ± 38	101 ± 8	129 ± 16
N1	433 ± 13	250 ± 23	291 ± 50	225 ± 47	158 ± 10	190 ± 17

Fig. 1 Changes in growing season precipitation and daily mean air temperature during the studied period (2012-2018) and the longterm mean growing season precipitation during 1955-2011.

Table 2 Effects of altered litter input (C) and nitrogen addition (N) on soil microenvironment evaluated using repeated measures analysis of variance (RANOVA)

	df	DOC含量 DOC content	DON含量 DON content	df	Ts	SWC
区组 Block	4	0.86	0.39	4	15.61^***	3.99^*
凋落物管理 C	2	0.76	0.77	2	14.44^***	10.49^***
氮添加 N	1	28.13^***	61.81^***	1	10.52^**	0.12
年 Year (Y)	2	20.14^***	15.33^***	5	780.49^***	302.17^***
C × N	2	0.06	0.36	2	0.09	0.52
C × Y	4	1.19	4.08^**	10	1.89^#	11.12^***
N × Y	2	8.41^**	5.18^*	5	2.20^#	8.42^***
C × N × Y	4	0.48	3.18^*	10	0.32	4.27^***

Fig. 2 Inter-annual variations (A, C, E, G) and the 6-year average (B, D, F, H) of soil temperature (Ts), soil water content (SWC), dissolved organic carbon (DOC) and nitrogen (DON) content in surface soil layer (0-10 cm) in different litter input and nitrogen addition treatments during 2013-2018 (mean ± SE). Significance levels were presented to show the effects of altered litter input (C) and nitrogen addition (N) treatments and their interaction (C × N) on these parameters (ns, p > 0. 1; #, p < 0.1; *, p < 0.05; **, p < 0.01; ***, p < 0.001). The letters in the bar graph indicate the results of multiple comparisons among different litter input and nitrogen addition treatments. Different lowercase letters indicate significant difference among three altered litter input treatments (p < 0.05), and uppercase letters indicate the significant difference between two nitrogen treatments (p < 0.05). C0, litter removal; C1, control; C2, litter addition; N0, without nitrogen addition; N1, nitrogen addition.

Fig. 3 Inter-annual variation (A) and the 6-year average (B) of aboveground net primary productivity (ANPP) in different litter input and nitrogen addition treatments during 2013-2018. The data in the figure are mean ± SE. Significance levels were presented to show the effects of altered litter input (C) and nitrogen addition (N) treatments and their interaction (C × N) on ANPP (ns, p > 0.1; #, p < 0.1; *, p < 0.05; **, p < 0.01; ***, p < 0.001). Different uppercase letters in the bar graph indicate significant difference results of multiple comparisons between two nitrogen treatments (p < 0.05). C0, litter removal; C1, control; C2, litter addition; N0, without nitrogen addition; N1, nitrogen addition.

Table 3 Effects of altered litter input (C) and nitrogen addition (N) on community above-ground net primary productivity (ANPP) and absolute and relative biomass of each functional group evaluated using repeated measures analysis of variance (RANOVA)

	df	地上净初级生产力 ANPP	绝对生物量 Absolute biomass					相对生物量 Relative biomass
	df	地上净初级生产力 ANPP	PB	PR	PF	SS	AB	PB%	PR%	PF%	SS%	AB%
区组Block	4	1.72	1.60	0.66	1.17	1.39	1.16	0.87	0.31	1.11	1.94	0.42
C	2	3.20^#	0.77	3.30^#	0.01	0.66	3.13^#	0.18	1.65	0.36	0.70	3.70^*
N	1	62.87^***	31.22^***	7.60^*	1.45	0.17	42.97^***	9.04^**	0.24	12.50^**	2.81	37.54^***
年Year (Y)	5	57.28^***	15.66^***	22.04^***	17.54^***	1.51	4.32^**	4.87^***	8.73^***	6.41^***	1.50	4.32^**
C × N	2	0.02	0.02	0.02	0.08	1.33	2.65^#	0.08	0.18	0.23	1.21	2.96^#
C × Y	10	0.59	1.31	6.36^***	0.76	0.68	1.85^#	2.78^**	5.03^***	0.86	0.44	1.78^#
N × Y	5	7.64^***	8.05^***	2.63^*	5.27^***	2.46^*	3.91^**	8.59^***	0.30	10.38^***	2.72^*	3.67^**
C × N × Y	10	0.84	0.95	1.63	1.22	0.65	1.81^#	0.39	2.26^*	1.46	0.84	1.70^#

Fig. 4 Inter-annual variations (A, C, E, G, I) and 6-year average (B, D, F, H, J) of aboveground net primary productivity (ANPP) of five functional groups in different litter input and nitrogen addition treatments during 2013-2018. The data in the figure are mean ± SE. Significance levels were presented to show the effects of altered litter input (C) and nitrogen addition (N) treatments and their interaction (C × N) on ANPP of each functional group (ns, p > 0.1; #, p < 0.1; *, p < 0.05; **, p < 0.01; ***, p < 0.001). C0, litter removal; C1, control; C2, litter addition; N0, without nitrogen addition; N1, nitrogen addition. Different uppercase letters in the bar graph indicate significant difference of multiple comparisons between two nitrogen addition treatments. AB, annual and biannual plants; PB, perennial bunch grasses; PF, perennial forbs; PR, perennial rhizome grasses; SS, shrubs and semi-shrubs.

Fig. 5 Variations in the relative biomass of the five functional groups in different litter input and nitrogen addition treatments. The data in the figure are mean ± SE. Asterisk indicated significant difference between different nitrogen treatments (**, p < 0.01; ***, p < 0.001). C0, litter removal; C1, control; and C2, litter addition; N0, nitrogen-free control; N1, nitrogen addition. AB, annual and biannual plants; PB, perennial bunch grasses; PF, perennial forbs; PR, perennial rhizome grasses; SS, shrubs, and semi-shrubs.

Fig. 6 Correlation analysis of aboveground net primary productivity (ANPP), soil temperature (Ts), soil water content (SWC) and its variation (CVSWC) with litter biomass added in the previous year under each nitrogen treatment. The correlation coefficient (R2) and significance levels are presented in the figure (*, p < 0.05; **, p < 0.01; ***, p < 0.001). N0, nitrogen-free control; N1, nitrogen addition.

Fig. 7 Correlation analysis between aboveground net primary productivity (ANPP) and precipitation (A), soil temperature (Ts)(B), soil water content (SWC)(C), and seasonal variation of soil water content (CVSWC)(D) during the growing season. The correlation coefficient (R2) and the significance levels are presented in the figure (ns, p > 0.05; *, p < 0.05; **, p < 0.01; ***, p < 0.001). C0, litter removal; C1, control; C2, litter addition; N0, without nitrogen addition; N1, nitrogen addition.

Fig. 8 Structural equation model (SEM) on how litter manipulation and nitrogen addition affect the community functional group composition and aboveground net primary productivity (ANPP) through soil environment. CVSWC, variance of soil water content; DOM, soil dissolved organic matter (the first principal component of DOC and DON, which explained 0.88 of them); PB, perennial bunch grasses; PF, perennial forbs; PR, perennial rhizome grasses; SWC, soil water content; Ts, soil temperature. AIC, Akaike Information Criterion; BIC, Bayesian Information Criterion; Fisher’s Chi, result of Fisher exact test.

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