Background Soil moisture is one of the main factors that restrict vegetation growth, vegetation restoration and stand stability in the Loess Plateau of China. It also shows obvious space-time characteristics under different land use, topography and geomorphology and vegetation condition due to the effects of rainfall factor, surface run off, plant transpiration, root absorption and so on. Soil moisture variation research at steep slope is the important premise of vegetation construction reasonable allocation. In the Loess Plateau of China, soil erosion has led to severe soil quality degeneration and it is important to assess the soil quality of micro-topography. Thus, the study of soil moisture in vertical profiles has important significance for understanding the utilization of soil moisture in artificial forests. The aim of this study was to investigate the distribution and the hierarchical character of 0-180 cm layers soil moisture in vertical profiles.

Methods The study investigated not only micro-topography soil moisture content of steep slope, but also the effect the micro-topography soil moisture spatial and temporal distribution characteristic which is influenced by different precipitation years. The results showed that the different precipitation years have a great deal of effect on the micro-topography soil moisture.

Results 1) Soil moisture content grow in step with precipitation in high flow years, and the annual soil moisture changing curve of gully is very closed to the "V" shape among high flow years; however, soil moisture content grow in delay two months with precipitation in low flow years; the annual soil moisture changing curve is very closed to the "M" shape among high flow years, of which the peak value was presented in March and August; 2)In the perpendicular direction of the steep slope, soil moisture content of the gully, original slope and scarp first decrease and then increase with depths increasing, on the contrary, soil moisture content of the platform, collapse and gentle slope first increase and then decrease with depths increasing, in addition, soil moisture content of scarp is very well, which moisture better than not only platform, gully and scarp, but also original slope and gentle slope; 3) Fast changing layer and active layer of soil moisture was displayed in turn in low flow years, such as collapse > gully > original slope > shallow gully > gentle slope > platform > scarp, of which soil moisture of the collapse is very well. Fast changing layer and active layer of soil moisture gradually disappear in high flow years, on contrary, the fast changing layer and active layer of soil moisture gradually move down with depths increasing in low flow years。

Conclusions  In order to increase the survival rate of afforestation, promote the quick restoration of forests and grass for soil and water conservation, it was suggested that the collapse can carry trees; and gentle slope and platform of steep slope can carry shrub and original slope and shallow gully of steep slope can carry herbaceous plant. Besides, It is difficult for plants to grow in scarp of steep slope, In particular, collapse of steep slope can carry the tall arbor tree.