Carbon sequestration and carbon emission are processes of ecosystem carbon cycling that can be affected while land area converted to grassland resulting in increased soil carbon storage and below-ground respiration. Discerning the importance of carbon cycle in grassland, we aimed to estimate carbon sequestration in photosynthesis and carbon emission in respiration from soil, root, and microbes, for four consecutive years (2007–2010) in a warm-season perennial grassland, Japan. Soil carbon emission increased with increasing growing season temperature which ranged from 438 to 1642？mg？CO2？m？2？h？1. Four years’ average soil carbon emission for growing season, nongrowing season, and annual emission was 1123, 364, and 1488？g？C？m？2, respectively. Nongrowing and snow covered season soil carbon emission contributed 23–25% and 14–17% to the annual emission. Above-ground biomass varied seasonally and variation in green biomass affected soil carbon emission with increasing temperature and precipitation. Temperature effect on root carbon emission contributed about 1/4th of the total soil carbon emission. Variation in soil and root carbon emission is affected by below-ground biomass. Long-term estimation concluded that seasonal and interannual variations in carbon sequestration and emission are very common in grassland ecosystem. 1. Introduction Carbon sequestration and emission can be affected while the ecosystem land area converted to the grassland [1–3], resulting in increased soil carbon storage [4, 5], below-ground respiration, and carbon cycling [6]. Land area converted to grassland has been reported to affect ecosystem carbon balance in short term through increased variations in ecosystem carbon cycling [7–9]. Moreover, one of the major long-term effects of such land conversion is the reduced carbon sink strength of grassland ecosystems [10]. Grasslands are able to store 10–30% of global soil organic carbon due to its large coverage (40%) of the earth’s terrestrial surface and are considered to sequester large amounts of carbon from the atmosphere and hence acquire significant sink/source capacity of the ecosystem and play a major role in the global carbon balance [11–13]. Soil as a whole is a large pool of atmospheric carbon. Globally, it accumulates 1500–2000 petagrams (Pg carbon, 1？Pg = 1015？g) in the form of soil organic carbon in the upper 1？m of soil [14, 15] or even greater [16], which is at least three times larger than atmospheric carbon. Small variation in carbon storage in grassland soil will greatly affect the concentration of atmospheric carbon and
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