Jul 9, 2013
Insight: grazing alters carbon fluxes in a desert steppe
Compared with a seven-year fenced desert steppe, a near-mid-grazed steppe can contribute more to atmospheric carbon uptake in a year with average precipitation, but not in a subsequent dry year. In a new study, we identified the major reasons behind this greater gross ecosystem production (GEP), which is accompanied by increased ecosystem respiration and enhanced carbon uptake in grazed areas.
We found that the consistently greater growing-season GEP occurring at a grazed site is mainly caused by greater photosynthetic capacity thanks to suitable environmental conditions and longer growing times per day and over the growing period. Growing-season ecosystem respiration also appears to be greater at a grazed site than a fenced one, especially in a non-severe drought season when the grass is active.
We believe that increased heterotrophic respiration may be among the primary mechanisms behind this phenomenon. Lack of increased carbon storage in un-grazed systems indicates that eliminating grazing to increase carbon sequestration is not likely to be a viable option in natural grassland systems. Higher carbon-dioxide uptake is possible in a semi-arid desert steppe if the land in this area and soil/vegetation is suitably managed.
In our study, we used so-called paired eddy-covariance systems to measure the net ecosystem exchange (NEE) and microclimate of adjacent pastures of grazed and ungrazed steppes to determine the role of grazing on carbon-dioxide fluxes in a desert steppe. Although the grazed site takes up more carbon in a normally rainy year in contrast to a dry year, the daily NEE variation for both years is lower in the fenced site, suggesting that the latter is more resistant to a changing climate. It also means that future models should take into account year-to-year differences in carbon balance because relationships between fluxes and climatic regulators change annually in different land-use-change scenarios.
About the author
Changliang Shao, PhD, is a research assistant at the Key Laboratory of Vegetation and Environmental Change at the Institute of Botany, Chinese Academy of Sciences.