Effects of long-term grazing and nitrogen addition on the growth of Stipa bungeana population in typical steppe of Loess Plateau

doi:10.17521/cjpe.2023.0086

Abstract

Abstract:

Aims Populations are the basis for the formation and development of the structure and function of grassland ecosystems. However, long-term grazing and global climate change like nitrogen addition profoundly impact the growth and reproduction of populations, such as Stipa bungeana, a dominant species in typical steppe of the Loess Plateau that has a high ecological and economic value. This study investigated how grazing and nitrogen addition affect the growth of S. bungeana.

Methods The study was based on a long-term rotational sheep grazing experiment in the typical steppe of the Loess Plateau. A completely randomized split-plot experimental design was employed, with stocking rate (0, 2.7, 5.3, 8.7 sheep·hm^-2) as the main factor and nitrogen addition levels (0, 5, 10, 20 g·m^-2) as the secondary factor. Morphological traits, aboveground biomass, the proportion of population biomass to total community aboveground biomass (PPB) and the relationship between them in S. bungeana were examined to investigate the effects of stocking rate, nitrogen addition and their interaction.

Important findings As the stocking rate increased, the plant height, canopy diameter, tiller density, seedlings, aboveground biomass, and PPB all followed a “single peak” curve trend, while the population density decreased. Nitrogen addition increased the plant height, canopy diameter, reproductive branch density, tiller density, aboveground biomass and PPB, while density of seedlings initially increased and then decreased as the nitrogen addition levels rose. The total effect of grazing on population aboveground biomass and the PPB was small compared with that of nitrogen addition. Specifically, grazing had a direct negative effect on aboveground biomass and affected PPB by regulating tiller density, population density and aboveground biomass. Nitrogen addition had a positive effect on aboveground biomass, both directly and indirectly through increasing plant height, reproductive branch density. It also impacted the PPB through regulating population density, canopy diameter, tiller density, and reproductive branch density. Overall, nitrogen addition increased canopy diameter and reproductive branch density and grazing increased density of seedings. The interaction of grazing and nitrogen addition significantly affected reproductive branch density. Stipa bungeana had maximum aboveground biomass or community status at a stocking rate of 4.10 or 5.29 sheep·hm^-2. These results indicated that grazing and nitrogen addition regulated the aboveground biomass and community status of S. bungeana through affecting its morphological characteristics, providing a basis for the scientific management and sustainable development of grassland populations.

Key words: grazing, nitrogen addition, population, morphological traits, aboveground biomass, community status

HUANG Ling, WANG Zhen, MA Ze, YANG Fa-Lin, LI Lan, SEREKPAYEV Nurlan, NOGAYEV Adilbek, HOU Fu-Jiang. Effects of long-term grazing and nitrogen addition on the growth of Stipa bungeana population in typical steppe of Loess Plateau[J]. Chin J Plant Ecol, 2024, 48(3): 317-330.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2023.0086

https://www.plant-ecology.com/EN/Y2024/V48/I3/317

Figures/Tables 8

Fig. 1 Climatic characteristics of the study area. A, Year of 2000-2021. B, Year of 2021. CV, coefficient of variation.

Fig. 2 Effects of stocking rate and nitrogen addition on growth traits of Stipa bungeana (mean ± SE). A, Plant height. B, Canopy diameter. C, Plant height/canopy diameter. Different uppercase letters indicate significant differences among different stocking rates (p < 0.05), different lowercase letters indicate significant differences among different nitrogen addition rates (p < 0.05). N, nitrogen addition rate; SR, stocking rate. ns, p > 0.05.

Fig. 3 Contributions of stocking rate (A), nitrogen addition rate (B) and their interaction (C) to the growth index of Stipa bungeana. *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Fig. 4 Effects of stocking rate and nitrogen addition on reproductive traits of Stipa bungeana (mean ± SE). A, Reproductive branch density. B, Tiller density. C, Productive branch density/tiller density. Different uppercase letters indicate significant differences among different stocking rates (p < 0.05), different lowercase letters indicate significant differences amang different nitrogen addition rates (p < 0.05). N, nitrogen addition rate; SR, stocking rate. **, p < 0.01; ns, p > 0.05.

Fig. 5 Effects of stocking rate and nitrogen addition on reproductive traits of Stipa bungeana (mean ± SE). A, Population density. B, Seedlings. C, Seedlings/population density. Different uppercase letters indicate significant differences among different stocking rates (p < 0.05), different lowercase letters indicate significant differences among different nitrogen addition rates (p < 0.05). N, nitrogen addition rate; SR, stocking rate. ns, p > 0.05.

Fig. 6 Effects of stocking rate and nitrogen addition on aboveground biomass and community status of Stipa bungeana (mean ± SE). A, Aboveground biomass. B, Proportion of population biomass to total community aboveground biomass. Different uppercase letters indicate significant differences among different stocking rates (p < 0.05), different lowercase letters indicate significant differences among different nitrogen addition rates (p < 0.05). N, nitrogen addition rate; SR, stocking rate. ns, p > 0.05.

Fig. 7 Correlation analysis between the growth indicators of Stipa bungeana. *, p < 0.05; **, p < 0.01.

Fig. 8 Effects of stocking rate and nitrogen (N) addition on Stipa bungeana. A, Effects on the growth of Stipa bungeana. B, Direct and indirect effects on aboveground biomass and the proportion of population biomass to total community aboveground biomass. In A, the black line indicates a significant path (p < 0.05), the gray line indicates that the path of action is not significant (p > 0.05), the thickness of the line indicates the effect size, the solid line indicates a positive effect, and the dashed line indicates a negative effect, numbers at arrows are standardized path coefficients, R2 indicates variation that can be explained. *, p < 0.05; **, p < 0.01; ***, p < 0.001. RMSEA, root mean square error of approximation.

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