Vegetation coverage in terrestrial ecosystem affects distribution of organic matter in soils and soil density fractions,thus is an important factor controlling storage of soil organic matter. An experiment was conducted to determine soil organic carbon and nitrogen concentration,by using density fractionation,of soil samples collected from the fields of three long-term experiments and evaluate the impact of land use on soil and density fractions. Results show that(ⅰ)soils different in land use differed significantly(p<0.05)in total organic carbon(TOC)or total nitrogen(TN),and TOC had a significant positive correlation with TN(p<0.01). Natural ecosystems,subjected to human disturbance or transferred into cropland,suffered chrono TOC and TN losses,however,application of chemical fertilizer plus organic manure increased significantly TOC and TN contents in cropland. Organic carbon storage of black soil under grassland has been increasing in the past 20 years of vegetation rehabilitation;(ⅱ)OC or N contents in free-LF(free light fraction)and HF(heavy fraction)were significantly related to TOC or TN in the whole soil at p<0.05 or p<0.01,respectively,indicating that accumulation of free-LF and HF is very important to net increase in TOC or TN. Occluded-LF,randomly distributed in soils,was less important to accumulation of TOC or TN;(ⅲ)C/N ratio of free-LF was higher than that of the whole soil and other fractions,which suggested that free-LF is a major part of the labile carbon pool on account of its susceptibility to microbial decomposition;(ⅳ)OC contents of humic acid of loosely combined humus significantly correlated with TOC,indicating that increase in humic acid leads to improvement of soil fertility. Application of organic manure to cropland not only led to accumulation of organic matter and improved soil quality,but also increased the capacity of agroecosystem to sequester atmospheric CO₂.