The potential importance of grazing to the fluxes of carbon dioxide and methane in an alpine wetland on the Qinghai-Tibetan Plateau

doi:10.1016/j.atmosenv.2005.05.036

Atmospheric Environment

Volume 39, Issue 29, September 2005, Pages 5255-5259

https://doi.org/10.1016/j.atmosenv.2005.05.036 Get rights and content

Abstract

To assess the impact of livestock grazing on the emission of greenhouse gases from grazed wetlands, we examined biomass growth of plants, CO₂ and CH₄ fluxes under grazing and non-grazing conditions on the Qinghai-Tibetan Plateau wetland. After the grazing treatment for a period of about 3 months, net ecosystem CO₂ uptake and aboveground biomass were significantly smaller, but ecosystem CH₄ emissions were remarkably greater, under grazing conditions than under non-grazing conditions. Examination of the gas-transport system showed that the increased CH₄ emissions resulted from mainly the increase of conductance in the gas-transport system of the grazed plants. The sum of global warming potential, which was estimated from the measured CO₂ and CH₄ fluxes, was 5.6- to 11.3-fold higher under grazing conditions than under non-grazing conditions. The results suggest that livestock grazing may increase the global warming potential of the alpine wetlands.

Introduction

Although wetlands cover about 5% of the global land surface, the ecosystems contribute significantly to the global greenhouse gases (e.g., CO₂ and CH₄) budget (e.g., Matthews and Fung, 1987). Many wetlands are used for livestock grazing, which may alter greenhouse gas fluxes in these ecosystems (e.g., Jensen, 1985; Robertson, 1997; Morris and Jensen, 1998). Despite the potential importance, little evidence is available to assess the effects of livestock grazing on the greenhouse gases budget in wetland ecosystems.

The Qinghai-Tibetan Plateau is the highest (average 4000 m a.s.l.) plateau in the world, and it has a total wetland area of 50,000 km² (Zhao, 1999). These alpine wetlands contain a large amount of soil organic carbon, which is estimated to compose about 0.2% of the global pool of soil carbon (Wang et al., 2002). The large carbon pool in the wetland ecosystems suggests that the wetlands on the plateau could become a significant source of CH₄. On the Qinghai-Tibetan Plateau, almost all wetlands are now being managed for livestock grazing. However, we have little knowledge of the effects of livestock grazing on greenhouse gases dynamics in the plateau's wetlands.

The direct effects of livestock grazing on wetlands can be slowing down photosynthetic CO₂ uptake by plants due to the reduction of assimilatory organs. Reducing the aboveground biomass can also change gas transport through plant conduit between soil and the atmosphere. The decease of transporting conductance may increase CH₄ emission from soil, but may also increase the entry of oxygen into soil, which will favor CH₄-oxidizing bacteria and suppress methanogenesis (Epp and Chanton, 1993).

Methane has a much greater global warming potential (GWP) than CO₂. Therefore, it is critical to assess both CH₄ and CO₂ emission if we are to clarify the contribution of a wetland to global warming.

To demonstrate the potential importance of grazing to global warming gases in the plateau wetlands, we measured plant biomass and CO₂ and CH₄ fluxes under experimental grazing and non-grazing conditions. To understand the underlying mechanism, we examined the gas-transport systems of major aquatic plants. We further assessed the grazing impact on the wetland's contribution to the radiative forcing calculated from the measured CO₂ and CH₄ fluxes and the GWP.

Section snippets

Study site and experiment design

The study site was located in the Luanhaizi wetland at the northeast edge of the Qinghai-Tibetan Plateau (37°35′N, 101°20′E, 3250 m a.s.l.). The catchment was flooded at an average water depth of 30 cm over the growing season, 2002. The annual mean temperature is −2 °C, and the annual precipitation is 500 mm (Klein et al., 2001). Vegetation of the wetland was composed of four major species dominating in different zones along a gradient of water depth. There were three emergent-plant zones,

Plant biomass and gas fluxes

In the non-grazing plot, aboveground biomass in the different vegetation zones increased from the deeper water to the drier edges of the wetland. The biomass for ZPot, ZHip, ZSci, and ZCar was 174, 230, 419, and 489 g dry weight m⁻², respectively. In the grazing plot, the aboveground biomass in the shallower zones ZCar and ZSci decreased (by 85.9% and 87.2%, respectively) significantly compared with that in the non-grazing plot (paired t-test; $P < 0.001$ ). The deeper zones, ZPot and ZHip, however,

Discussion

From the short-term measurement, we estimated that about 85% of aboveground biomass was reduced by grazing. Decreased aboveground biomass can lead to a decrease in the assimilatory capacity of emergent plants. Morris and Jensen (1998) reported that grazing decreases the aboveground net primary productivity (ANPP) and affects the carbon cycle in grassland ecosystems. Data compiled from the world's grasslands show that grazing consumes 75% of ANPP (Frank et al., 1998). However, the grazing effect

Acknowledgments

This work was part of a joint research project of the National Institute for Environmental Studies, Japan, and the Northwest Plateau Institute of Biology, Chinese Academy of Science (Grant no. 13575035 and S1 (B13)), and was supported by Asahi Breweries Scientific Foundation.

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Atmospheric Environment

Abstract

Introduction

Section snippets

Study site and experiment design

Plant biomass and gas fluxes

Discussion

Acknowledgments

References (19)

Gas exchange through dead culms of reed, Phragmites australis (Cav.) ex Steudel

Aquatic Botany

Methane emissions from different vegetation zones in a Qinghai–Tibetan plateau wetland

Soil Biology and Biochemistry

Carbon exchange and species composition of grazed pastures and exclosures in the shortgrass steppe of Colorado

Agriculture Ecosystems & Environment

Role of plant aeration in zonation of Zizania latifolia and Phragmites australis

Aquatic Botany

The effects of grazing: confounding of ecosystem, community, and organism scales

American Naturalist

Internal pressurization and convective gas flow in some emergent freshwater macrophytes

Limnology and Oceanography

Rhizospheric methane oxidation determined via the methyl fluoride inhibition technique

Journal of Geophysical Research

The ecology of the Earth's grazing systems

BioScience

Cited by (58)

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Methane emissions during different freezing-thawing periods from a fen on the Qinghai-Tibetan Plateau: Four years of measurements

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Clonal integration in Phagmites australis mitigates effects of oil pollution on greenhouse gas emissions in a coastal wetland