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

Abstract: 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 was conducted in situ for 3 years to assess the comprehensive effects of cultivation of temperate steppe on soil organic carbon (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 10–20cm in the spring wheat field were found to have decreased by 38.3 and 17.4% respectively from 29.5 and 21.9gkg−1 to 18.2 and 18.1gkg−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 237.4gCm−2 respectively in the spring wheat field, which were slightly lower than the values of 342.2, 412.0 and 312.1gCm−2 in the S. baicalensis steppe, while ecosystem respiration of 690.9, 991.2 and 569.6gCm−2 respectively in the spring wheat field were markedly higher than those of 447.0, 470.9 and 429.7gCm−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 0–10cm and soil temperatures at 5 and 10cm were the main factors affecting the variations in ecosystem respiration and soil respiration rates in droughty years of 2002 and 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 growth stages, which should be fully considered when assessing and predicting the effects of cultivation on the net CO2 balance of grassland ecosystems. [Copyright &y& Elsevier]