| Natural grassland and the adjacent six farmlands of different reclamation years (5, 10, 15, 20, 35 and 50 years) in Taipusi County of the Inner Mongolia Autonomous Region were studied. The study investigated the long time effect of a conversation from grasslands to croplands on soil physical-chemical properties, using direct field samplings. The natural grassland, five years cropland, ten years cropland and fifty years cropland was selected to study the effects of reclamation years(5,10, 50 years)on soil CH4 absorption and N2O emission. The seasonal dynamics changes of soil CH4 uptake and N2O emissions in four land plots were observed in field, using the static black-box method. On the basis of field observation experiment, under the same conditions of water and temperature, the effects of soil physical-chemical properties of the Different-reclamation-years cropland (5, 10, 15, 20, 35 and 50 years) converted from typical steppe in Taipusi County on CH4 absorption and N2O emission were studied using a laboratory incubation method. This study was designed to provide a scientific basis to use the land rationally, to assess comprehensivly the grassland conditions such as soil quality and fertility, to develop the mitigation measures of soil greenhouse gas emissions, and so on. The major results are as follows:1. Soil physical-chemical properties were influenced significantly after natural grassland was changed into cropland. Reclamation induced a reduction of soil organic carbon (SOC) content, total nitrogen (TN) content, soil available kalium content, dissolved organic carbon content (DOC), pH levels and clay content, and increased soil bulk density and sand content in the 0-30cm soil layer, with the use length of the farmland.The results showed that soil desertification, soil acidification and loss of soil nutrient were more serious, with the increase of reclamation years. 2. The capacity of CH4 uptake was enhanced significantly after grassland was changed into cropland, soil CH4 absorption decreased with cultivation years. With natural grassland as control, N2O emission was reduced in five years cropland, but N2O emission was increased in ten years cropland and fifty years cropland.3. Field observation experiment showed that: moisture and temperature in soil surface were the main factors of CH4 absorption, and soil surface moisture content and NO3--N content were the main factors of soil N2O emissions. The number of soil bacteria influenced significantly N2O emission of grassland. Soil microbial biomass nitrogen (BN) affected significantly N2O emission of ten years cropland and fifty years cropland.4. Incubation experiment showed that: under the same conditions of water and temperature, the CH4 uptake and N2O emission were reduced significantly after grassland was changed into cropland, soil CH4 absorption and N2O emission decreased with cultivation years. The Correlation analysis indicated that: soil organic carbons (SOC), total nitrogen (TN), soil available kalium content, pH levels, sand content and dissolved organic carbon content (DOC) were the main influencing factors of CH4 absorption ; It also founded that there was clear dependency of soil properties (soil organic carbons content; total nitrogen content) on soil N2O emission.
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