Contribution of rock fragments on formation of forest soil macropores in the stoney mountains of the Loess Plateau, China

The abundance of rock fragments plays a very important role in the formation of macropores in stony mountain soils of the Loess Plateau in China. This paper reports on the effect of the volumetric content of rock fragments on macropores based on an investigation of 13 plots. The results indicated that the increase of rock fragment content leads to increased radii and volume of macropores, rock fragments caused increases in the density of macropores with radii >1.4 mm. The cause of the formation of macropores was quantitatively analyzed by dividing the quantity of macropores formed into the contribution by rock fragments or by biological factors. The results show that the contribution of biological factors to soil macroporosity is important and dominant for the topsoil (0-10 cm), but it decreases with increasing soil depth, while the contribution of rock fragments increases quickly with increasing soil depth. According to the contribution of biological factors to the formation of macropores, the soil profile can be divided into 3 depth ranges: 0-40,40-80 and >80 cm. The impact of biological factors on macroporosity is strong and dominant at the depth of 0-40 cm, with a contribution of 50.73%, the impact is markedly decreased and less strong at the depth of 40-80 cm with a contribution of 25.62% and it is very low and weak at depths greater than 80 cm with a contribution of 1.81%. The enhancement of soil macroporosity from biological factors is mainly limited within the upper soil layer, within the range of 0-40 cm. In general, broadleaf forests enhance the soil macroporosity better than coniferous forests, and the effect of shrubs on macroporosity is similar to that of coniferous forests.