Effects and pathways of long-term nitrogen addition on plant diversity and primary productivity in a typical steppe

doi:10.17521/cjpe.2019.0260

Abstract

Abstract:

Aims Nitrogen (N) deposition has made great impacts on the structure and function of terrestrial ecosystems in recent decades, and N is known as the main limiting element of plant growth and primary productivity in the northern grassland of China. Species diversity and functional diversity have been widely recognized as key indicators of the mechanisms of biodiversity maintenance for ecosystem functions. However, the effect of long-term N addition on the relationship between species diversity and functional diversity, and the corresponding impacts on primary productivity have rarely been studied; particularly the underpinning mechanisms remain unclear. Our objective is to examine the effects and pathways of long-term N addition on species diversity, functional diversity and primary productivity in a typical steppe.
Methods A manipulative N addition experiment located in a typical steppe of the Nei Mongol grassland has been conducted for 18 years. The experimental design included seven levels of N addition rate (i.e., control, 0, 1.75, 5.25, 10.50, 17.50, 28.00 g·m^-2·a^-1) with nine replicates for each treatment. The plant functional traits of dominant species, species richness and composition, and aboveground net primary productivity (ANPP) were determined. In addition, species diversity, functional attribute diversity and community-weighted mean traits were calculated.
Important findings 1) N addition significantly reduced species richness and Shannon-Wiener index, but had little effect on functional diversity, including functional attribute diversity and community-weighted mean traits. 2) Structural equation model analyses showed that functional diversity was mainly affected by species richness, whereas the decrease in species diversity didn’t lead to the decrease in functional diversity. The community- weighted mean traits did not change under N addition, which was mainly due to the shift in functional group composition, that is, the proportion of perennial rhizome grass in plant community increased greatly under N addition. 3) The effect of N addition on ANPP was through direct pathways of species richness and shift in functional group composition, and further via an indirect pathway of community-weighted mean traits. Particularly, the community-weighted mean traits were the most important factors and explained 48% of total variation in ANPP, implying that the primary productivity is mainly determined by the biomass and functional traits of dominant species in the plant community. Our results well supported the mass-ratio hypothesis.

Key words: plant functional trait, species diversity, functional diversity, community-weighted mean traits, functional group composition, biodiversity, aboveground net primary productivity

WANG Yu-Bing,SUN Yi-Han,DING Wei,ZHANG En-Tao,LI Wen-Huai,CHI Yong-Gang,ZHENG Shu-Xia. Effects and pathways of long-term nitrogen addition on plant diversity and primary productivity in a typical steppe[J]. Chin J Plant Ecol, 2020, 44(1): 22-32.

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Figures/Tables 6

Fig. 1 Effects of N addition on species richness (A) and Shannon-Wiener index (B) in a typical steppe (mean + SE). Control, absolute control, no fertilizer added. Different lowercase letters indicate significant difference among treatments (p < 0.05).

Fig. 2 Effect of N addition on functional attribute diversity (FAD) in a typical steppe (mean + SE). A, Plant trait functional diversity, calculated by plant height (PH), individual biomass (PB) and stem:leaf biomass ratio (SLR). B, Leaf trait functional diversity, calculated by specific leaf area (SLA), leaf dry mass content (LDMC), and leaf N content (LNC). C, Whole-plant trait functional diversity, calculated by six indices of plant and leaf traits. Control, absolute control, no fertilizer added. No significant difference among treatments (p > 0.05).

Fig. 3 Effects of N addition on community-weighted mean traits in a typical steppe (mean + SE). Community-weighted mean traits included community-weighted plant height (PHCWM), individual biomass (PBCWM), stem:leaf biomass ratio (SLRCWM), specific leaf area (SLACWM), leaf dry mass content (LDMCCWM), and leaf N content (LNCCWM). Control, absolute control, no fertilizer added. No significant difference among treatments (p > 0.05).

Fig. 4 Effects of N addition on the relative aboveground biomass (RAB) of six dominant species in a typical steppe (mean ± SE). PR, perennial rhizome grasses; PB, perennial bunch grasses; PF, perennial forbs. Control, absolute control, no fertilizer added.

Fig. 5 Effect of N addition on aboveground net primary productivity (ANPP) in a typical steppe (mean + SE). Control, absolute control, no fertilizer added. No significant difference among treatments (p > 0.05).

Fig. 6 Structural equation model (SEM) analyses of pathways of N addition on aboveground net primary productivity (ANPP). Results of SEM fitting: c2 = 3.611, p = 0.89, df = 8, n = 13, RMSEA < 0.001, AIC = 29.61, CFI = 1.000. ANPP, aboveground net primary productivity; CWM, community-weighted mean traits; FAD, functional attribute diversity; PR/PB, biomass ratio of perennial rhizome grass to perennial bunchgrass, indicating a shift in functional group composition. Solid and dashed arrows indicate significant (p < 0.05) and non-significant (p > 0.05) effects, respectively. Values associated with solid arrows represent standardized path coefficients (β); *, p < 0.05; **, p < 0.01; ***, p < 0.001. r2 values associated with response variables indicate the proportion of variation explained by relationships with other variables.

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