Background The ecological public welfare forests in the south of China are mainly consisted of Pinus massoniana and Cunninghamia lanceolata pure forests. Because of lacking of diversity of vegetation species and simplicity of tree's structure, it is hard to harness the red soil erosion areas where the soil erosion is devastating. This research is aimed to analyze the influence on runoff and sediment and nutrient losses on slope gradient, after low-efficiency P. massoniana forest intercropping different native broad-leaved tree species, and to provide reference for red soil erosion harnessing and tree species selection of low-efficiency P. massoniana forest in similar areas.

Methods Choosing the similar low-efficiency P. massoniana forests in Changting red soil erosion area of Fujian province and intercropping Myrica rubra, Sapindus mukorossi, Camellia oleifera and Gardenia jasminoides 4 typical reconstructed stands, the undisturbed low-efficiency pure P. massoniana in the same area was used as control. By setting up typical runoff plot we investigated the runoff, sediment yield and nutrient loss characteristics and screened out broad-leaved tree species which have better runoff and sediment reduction.

Results 1) In 2018, the rainfalls in the sample plots mainly occurred in May to October, accounting for 63.7% of the total annual rainfall, and the precipitation and frequency causing runoff and sediment yield accounted for 72.8% and 95.6% in the annual year. 2) From May to October the yields of runoffs in 4 P. massoniana modes intercropping M. rubra, S. mukorossi, C. oleifera and G. jasminoides respectively decreased by 24.96%-42.19%, 13.93%-53.68%, 4.55%-26.45%, and 8.59% -32.98%, compared with the control group of pure P. massoniana forest, and sediment yield decreased by 60.19%-84.86%, 55.07%-87.14%, 20.91%-67.06%, and 27.22%-72.66% respectively. 3) In contrast to the control plots of pure P. massoniana forest, the total nitrogen, ammonium nitrogen, nitrate nitrogen loss and total carbon loss in the erosion sediment of P. massoniana forest interplanting with M. rubra and S. mukorossi were almost similar, respectively increased 48.33%-48.79%, 103.66%-139.05%, 39.75% -42.29%, and 63.14%-66.57%, however the modes of intercropping C. oleifera and G. jasminoides increased by 19.96% -22.52%, 74.89%-92.62%, 15.44%-16.42%, and 25.24%-28.14%. In general, the loss pattern of other nutrients was similar to that of runoff and sediment yield, showing that the mode of intercropping was lower than that of pure P. massoniana forest. 4) Principal component analysis showed that with the effect on rainfall and runoff, P. massoniana interplanting with M. rubra or S. mukorossi was more likely to cause the loss of soil carbon and nitrogen, while P. massoniana interplanting C. oleifera or G. jasminoides more easily caused the loss of soil phosphorus and potassium.

Conclusions Low-efficiency P. massoniana forest intercropping indigenous broad-leaved tree species which are suitable for planting, constructing a multi-layer forest structure may result in the biomass increase of litter and root system, to a certain extent it even may improve the reduction of soil erosion on the slope of the low-efficiency P. massoniana forest land. Interplanting M. rubra or S. mukorossi will be more fruitful than that of C. oleifera or G. jasminoides, in the short term.