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为了观察森林生态系统转变为农业生态系统后土壤有机碳的变化，选择贵州喀斯特森林区为研究对象，采集了林地和邻近的玉米地土壤剖面样品，并按土壤密度把土壤样品分级成轻质部分和重质部分。林地和玉米地土壤均为石灰土。玉米地土壤风化相对较强，淋溶严重而贫瘠，并且所返还的植物残留物数量很少。实验结果表明，林地土壤轻质部分的(13C值明显低于重质部分，而玉米地土壤轻质部分的(13C值明显高于重质部分，说明植物残留物首先进入到轻质部分。然而，由于当地的耕作习惯，输入到玉米地土壤轻质部分的玉米残留物的数量很少，只有48%，这是初级生产力低下的主要原因。
In order to observe the changes of soil organic carbon after the forest ecosystem was transformed into agro-ecosystem, the karst forest area of Guizhou Province was selected as the research object. Samples of soil profile of forest land and adjacent corn fields were collected and soil samples were classified into light fractions according to soil density And heavy parts. The soil in the woodland and corn fields are lime soil. The soil in the maize field is relatively weathered, is heavily leached and infertile, and has a small amount of plant residues returned. The results showed that the soil 13C values of the soil were significantly lower than those of the heavy parts, while those of the light parts of the soil were higher than those of the heavy parts, indicating that the plant residues first entered the light part. Due to local farming practices, the amount of corn residue that was fed to the light soil portion of the maize field was small at 48%, which is a major cause of the low primary productivity.