Nitrogen Addition Affects Ecosystem Carbon Exchange by Regulating Plant Community Assembly and Altering Soil Properties in an Alpine Meadow on the Qinghai-Tibetan Plateau

Ling Han; Hasbagan Ganjurjav; Guozheng Hu; Jianshuang Wu; Yulong Yan; Luobu Danjiu; Shicheng He; Wendong Xie; Jun Yan; Qingzhu Gao

doi:10.3389/fpls.2022.900722

Nitrogen Addition Affects Ecosystem Carbon Exchange by Regulating Plant Community Assembly and Altering Soil Properties in an Alpine Meadow on the Qinghai-Tibetan Plateau

Front Plant Sci. 2022 Jun 13:13:900722. doi: 10.3389/fpls.2022.900722. eCollection 2022.

Authors

Ling Han¹, Hasbagan Ganjurjav^{1

2}, Guozheng Hu^{1

2}, Jianshuang Wu¹, Yulong Yan³, Luobu Danjiu⁴, Shicheng He⁴, Wendong Xie⁴, Jun Yan⁴, Qingzhu Gao^{1

2}

Affiliations

¹ Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing, China.
² National Agricultural Experimental Station for Agricultural Environment, Nagqu, China.
³ China New Era Group Corporation, Beijing, China.
⁴ Nagqu Grassland Station, Nagqu, China.

Abstract

Nitrogen (N) deposition can affect the global ecosystem carbon balance. However, how plant community assembly regulates the ecosystem carbon exchange in response to the N deposition remains largely unclear, especially in alpine meadows. In this study, we conducted a manipulative experiment to examine the impacts of N (ammonium nitrate) addition on ecosystem carbon dioxide (CO₂) exchange by changing the plant community assembly and soil properties at an alpine meadow site on the Qinghai-Tibetan Plateau from 2014 to 2018. The N-addition treatments were N0, N7, N20, and N40 (0, 7, 20, and 40 kg N ha^-1year^-1) during the plant growing season. The net ecosystem CO₂ exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) were measured by a static chamber method. Our results showed that the growing-season NEE, ER and GEP increased gradually over time with increasing N-addition rates. On average, the NEE increased significantly by 55.6 and 65.2% in N20 and N40, respectively (p < 0.05). Nitrogen addition also increased forage grass biomass (GB, including sedge and Gramineae) by 74.3 and 122.9% and forb biomass (FB) by 73.4 and 51.4% in N20 and N40, respectively (p < 0.05). There were positive correlations between CO₂ fluxes (NEE and GEP) and GB (p < 0.01), and the ER was positively correlated with functional group biomass (GB and FB) and soil available N content (NO₃ ^--N and NH₄ ⁺-N) (p < 0.01). The N-induced shift in the plant community assembly was primarily responsible for the increase in NEE. The increase in GB mainly contributed to the N stimulation of NEE, and FB and the soil available N content had positive effects on ER in response to N addition. Our results highlight that the plant community assembly is critical in regulating the ecosystem carbon exchange response to the N deposition in alpine ecosystems.

Keywords: aboveground biomass; alpine meadow; net ecosystem carbon exchange; nitrogen deposition; plant community; soil available nitrogen.