Nitrogen and phosphorus addition enhances deterministic processes in community assembly of alpine grasslands in Tianshan Mountains

doi:10.17521/cjpe.2025.0347

Abstract

Abstract:

Aims Understanding how nutrient addition influences community assembly is essential for predicting the responses of alpine grassland ecosystems to eutrophication. However, the relative importance of deterministic versus stochastic processes following nutrient addition remains debated.

Methods We conducted a nutrient-addition experiment in the Bayanbulak alpine grassland on the southern slope of Tianshan Mountains, with four treatments: control, nitrogen (N) addition, phosphorus (P) addition, and combined N and P addition. We measured species relative abundance (SRA), aboveground biomass, and plant functional traits. Using trait-abundance relationships, the β Raup-Crick index, functional diversity metrics, niche overlap, and the responses of soil physicochemical properties to fertilization, we evaluated how nutrient addition alters the balance between deterministic and stochastic assembly, and further assessed the relative importance of environmental filtering versus limiting similarity.

Important findings Following N addition, SRA exhibited a significant positive linear relationship with plant height, while under N and NP additions SRA showed a significant negative linear relationship with seed size, indicating that taller species and species with smaller seeds became dominant via advantages in light competition and rapid colonization, respectively—consistent with trait-based niche assembly. Null-model analyses (β Raup-Crick) further revealed that between-community dissimilarity under nutrient addition was significantly lower than expected by randomization, indicating a shift from stochastic to deterministic assembly. Nitrogen-containing treatments significantly increased aboveground biomass, while significantly decreasing functional richness, functional dispersion, and Rao’s Q, and increasing niche overlap. These results suggest that nutrient addition enhanced environmental filtering, promoting functional trait convergence and species compositional homogenization. However, short-term nutrient addition did not significantly reduce species richness, likely because the regional species pool contains a relatively low proportion of rare species. Overall, nitrogen is the primary limiting nutrient for aboveground biomass in our study area, and nutrient enrichment promotes deterministic assembly by strengthening environmental filtering. These results reveal a trend toward functional convergence under nutrient enrichment and provide theoretical support for biodiversity conservation in alpine grasslands.

Key words: nutrient addition, community assembly, species relative abundance, functional trait, niche overlap, alpine grassland

HONG Ling-Gui, AN Qi, ZHANG Fo-Jun, REN Yu-Fei, REN Zheng-Wei, ZHOU Xiao-Long. Nitrogen and phosphorus addition enhances deterministic processes in community assembly of alpine grasslands in Tianshan Mountains[J]. Chin J Plant Ecol, 2026, 50(4): 883-894.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2025.0347

https://www.plant-ecology.com/EN/Y2026/V50/I4/883

Figures/Tables 6

Fig. 1 Conceptual framework illustrating the effects of nutrient addition on grassland community assembly processes.

Fig. 2 Linear regression relationships between species relative abundance and functional traits. CK, control; N, nitrogen addition; NP, combined nitrogen and phosphorus addition; P, phosphorus addition. R2 and p values were estimated using standardized major axis (SMA) regression. Significant relationships are indicated in the top-right corner.

Fig. 3 Effects of nutrient addition on the Raup-Crick dissimilarity index of alpine grassland communities in Tianshan Mountains (mean ± SE). CK, control; N, nitrogen addition; NP, combined nitrogen and phosphorus addition; P, phosphorus addition. Different lowercase letters indicate significant differences (p < 0.05).

Fig. 4 Effects of nutrient addition on functional diversity of alpine grassland communities in Tianshan Mountains (mean ± SE). CK, control; N, nitrogen addition; NP, combined nitrogen and phosphorus addition; P, phosphorus addition. Different lowercase letters indicate significant differences (p < 0.05).

Table 1 Comparison of observed niche overlap and null model-simulated niche overlap under different nutrient addition treatments

处理 Treatment	观测指数 Observed index	模拟指数 Mean of stimulated index	95%置信区间 95% CI	p
CK	0.944	0.288	0.234-0.388	<0.001
N	0.963	0.302	0.248-0.401	<0.001
P	0.966	0.291	0.227-0.401	<0.001
NP	0.960	0.303	0.239-0.423	<0.001

Fig. 5 Conceptual model illustrating the processes of community assembly under nutrient addition. The borders represent the size of the functional space occupied by species in different communities, and the different shapes denote species with distinct functional traits.

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