Background In the north of China, large area of bare land accompanied with leaf dropping of broad leaved plant in winter, easily resulting the dust pollution and air pollution. It is important reason for the gradually worse of haze pollution in the northern cities of China. Thus, accurately grasping the distribution of bare land in winter and establishing a reasonable management plan is of significance for environmental protection and ecological construction of northern cities of China.

Methods Beijing, as one of the most representative cities in the north of China, was taken as the survey object in this study. The bare lands of gardens and greenbelts within the 5th Ring of Beijing were investigated using survey method defined in the National Soil Survey Technical Regulation. Total 100 survey samples were set up with board type location method. And 250 000 m², accounting for 3.73% of the total area of 5th Ring of Beijing, were investigated. The study was carried out by 3 types of bare lawn, bare shrubs, and bare tree-pit land.

Results Area of greenbelts was 87 539.84 m², accounting for 35.02% of total area of this survey, suggesting that the majority within 5th Ring of Beijing occupied by hardening land such as buildings and roads. Among the surveyed greenbelts, the area of lawn green, shrubs green, and tree-pit green was 66 647.30 m², 16 329.64 m², and 4 562.91 m², indicating the lawn green was the main proportion of greenbelts within the 5th Ring of Beijing. The results for bare land demonstrated that the contribution rate of bare lawn to total bare land was 78.11%, i.e., it was the major type of bare land within the 5th Ring of Beijing and corresponding measures should be firstly taken to improve bare lawn in future. Among 100 surveyed samples, 43.42% lawn green and 24.25% shrubs green became bare land in winter; moreover, the proportion of tree-pit green becoming bare land was up to 90.91%, indicating that there was no coverage for most tree-pits in winter. The regional distribution of bare land showed that 13 of top 20 surveyed plots of bare land were distributed in the area between the South 4th Ring and the South 5th Ring, accounting for 51.66% of the total surveyed bare land. This indicated that the area between South 4th Ring and the South 5th Ring exposed to severe bare land in winter.

Conclusions Bare land not only caused soil erosion and soil nutrient loss with the 5th Ring of Beijing, but also increased haze pollution degree and deteriorated urban air quality. From the results that lawn green occupied a large proportion of gardens and greenbelts within the 5th Ring of Beijing, we suggest that we may strictly control the area of bare land in winter and ensure the protection of the ecological environment in Beijing in winter by planting cold-resistant turf grass varieties within the 5th Ring of Beijing to replace easy wilt turf grass, and using cut branches and bushes from maintaining gardens and greenbelts as cover materials to efficiently cover the bare land.