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RESEARCH PRODUCT

Effect of the conversion of grassland to spring wheat field on the CO2 emission characteristics in Inner Mongolia, China

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Chinese grasslands have undergone great changes in land use in recent decades. Approximately 18.2% of the present arable land in China originated from the cultivation of grassland, but its impact on the carbon cycle has not been fully understood. This study wasconducted insitu for3yearstoassessthecomprehensive effects ofcultivationof temperatesteppe onsoilorganiccarbon(SOC) and soil respiration rates as well as ecosystem respiration. As compared with those in the Stipa baicalensis steppe, the SOC concentrations at depths of 0‐10 and 10‐20 cm in the spring wheat field were found to have decreased by 38.3 and 17.4% respectively from 29.5 and 21.9 g kg � 1 to 18.2 and 18.1 g kg � 1 after a cultivation period of 30 years. Accordingly, the total amounts of soil respiration through the growing season (from April to September) in 2002, 2003 and 2004 were 265.2, 282.2 and 237.4 g C m � 2 respectivelyinthespringwheatfield,whichwereslightly lowerthanthevaluesof342.2,412.0and312.1 g C m � 2 in the S. baicalensis steppe, while ecosystem respiration of 690.9, 991.2 and 569.6 g C m � 2 respectively in the spring wheat field were markedly higher than those of 447.0, 470.9 and 429.7 g C m � 2 in the steppe plot. Similar seasonal variations of ecosystem respiration and soil respiration existed in both sample sites. Respiration rates were higher and greater differences existed in both ecosystem respiration and soil respiration during the exuberant growth stage of plants (from mid-June to mid-August). However, in the slower-growth period of the growing season (before late May and after late August), the CO2 effluxes of the two sample sites were similar and remained at a relatively low level. The results also showed that ecosystem respiration and soil respiration were under similar environmental controls in both sample sites. Soil water content at a depth of 0‐10 cm and soil temperatures at 5 and 10 cmwere themain factors affectingthevariations in ecosystemrespiration and soil respirationrates indroughty yearsof 2002and 2004 and in the rainy 2003, respectively. This study suggests that the conversion of the grassland to the spring wheat field has increased the carbon loss of the whole ecosystem due to the change of vegetation cover type and significantly reduced the carbon storage of surface soil. In addition, the tillage of grassland had different effects on ecosystem respiration and soil respiration. The effects were also dissimilar in different growthstages, which should be fully considered when assessing and predicting the effects of cultivation on the net CO2 balance of grassland ecosystems. # 2006 Elsevier B.V. All rights reserved.