Experiment design

We chose the Pinus massoniana forest with 47a in the study area as the research object. In the typical Pinus massoniana forest, the separate layers of litter (semi-decomposed and non-decomposed layers) were collected from several 1 m × 1 m quadrat and placed in grid bags. The litter of the semi-decomposed layer have no complete outline, and the color was brown. As the litter leaves of the completely decomposed layer are powdery and are combined with the soil layer, this layer is difficult to collect. Before testing, it was necessary to clean the soil off the pine needles and then allow the litter to dry naturally. The characteristics of the semi-decomposed and non-decomposed litter layers are shown in Table ​Table1.1. The soil samples need to be dried and screened by 10 mm. When filling the soil trough, every 0.1 m of soil thickness was one layer, for a total of four layers (0.4 m). The characteristics by soil particle sizes are different (Fig. 2). The soil samples were dried naturally, crushed, and then sieved. The soil trough (2 m long, 0.5 m wide and 0.5 m deep) was filled to have a bulk density of 1.53 g·m⁻³. In this process, an appropriate amount of water was sprinkled on the surface of each soil layer to achieve a soil moisture content consistent with the surrounding, undisturbed, or natural, state. The simulation experiment was conducted in the Jiufeng rainfall laboratory at Beijing Forestry University, China. We used a rainfall simulation system (QYJY-503T, Qingyuan Measurement Technology, Xi’an, China) used a rotary downward spray nozzle. The system is able to simulate a wide range of rainfall intensities (10 to 300 mm h⁻¹) using various water pressure and nozzle sizes controlled by a computer system.

Characteristics of the non-decomposed and semi-decomposed layers of Pinus massoniana litter.

Litter characteristics were determined by Win RHIZO Pro 2.0 image analysis software, and litter density measured by a volumetric drainage method.

Soil particle composition of study area soil layers.

According to the results of the field forest investigation, the litter was covered with the experimental treatments shown in Table ​Table2.2. The treatments mass coverage of non-decomposed litter layer was named as follows: N1 denoted litter mass coverage 0 g·m⁻², N2 was ‘the non-decomposed litter mass coverage 100 g·m⁻²’, N3 was ‘the non-decomposed litter mass coverage 200 g·m⁻²’, and N4 was ‘the non-decomposed litter mass coverage 400 g·m⁻²’, N5 was ‘the semi-decomposed litter mass coverage 100 g·m⁻²’, N6 was ‘the non-decomposed litter mass coverage 100 g·m⁻² and the semi-decomposed litter mass coverage 100 g·m⁻²’, N7 was ‘the non-decomposed litter mass coverage 200 g·m⁻² and the semi-decomposed litter mass coverage 100 g·m⁻²’. N2, N3 and N4 were the undissolved state of litter layer, and N4 (non-decomposed state, ND), N7 (initial stage of litter decomposition, ID), N6 (middle stage of litter decomposition, MD) and N5 (final stage of litter decomposition, FD) respectively represent different stages of litter decomposition.

The experimental design of this study.

In this study, biomasses of the non-decomposed and semi-decomposed layers of litter were combined, and then divided into seven treatments (N1–N7).

ND is non-decomposed state, ID is initial stage of litter decomposition, MD is middle stage of litter decomposition, FD is final stage of litter decomposition.

According to the rainfall in the Taizishan area of Hubei Province, erosive rainfall and extreme rainstorms were selected as the research conditions. Summer rainfall events occur mainly in the summer in this area, and a rainfall intensity of 60 mm·h⁻¹ was the most common erosive rainfall intensity. Under extreme weather conditions, the rainfall intensity can reach up to 120 mm·h⁻¹. Our experiments were conducted with 60 and 120 mm·h⁻¹ rain intensities with a rainfall that lasted 1 h. According to the field investigation data of forest land, this area is a low mountain and hilly area with a slope mostly between 5° and 10°. Therefore, 5° and 10° were selected for the slope treatments in this study. The combination of slope and rainfall intensity was named as follows: T1 denoted ‘Slope 5° and rainfall intensity 60 mm·h⁻¹’, T2 was ‘Slope 10° and rainfall intensity 60 mm·h⁻¹’, T3 was ‘Slope 5° and rainfall intensity 120 mm·h⁻¹’, and T4 was ‘Slope 10° and rainfall intensity 120 mm·h⁻¹’. With two rainfall intensities, two slopes, seven litter coverage gradient and two repetitions combined, this study had a total of 56 rainfall events.