In regard to soil mechanical composition, compared to luxuriant growth, the sand content in well growth, moderate growth, and poor growth is lower, while the silt content is higher. This discrepancy can be attributed to the specific soil texture requirements of tea trees. Tea trees typically thrive in loose and well-aerated soil[17], which facilitates root development and extension, enhancing the absorption of oxygen and nutrients. Conversely, overly sticky soil can impede root respiration, leading to root anoxia and adversely affecting tea tree growth. Moreover, tea trees in luxuriant growth areas typically have longer growth periods compared to other regions, a phenomenon supported by research[18]. Over time, the soil particle composition and texture types tend to optimize alongside the extended growth years of tea trees.
In this study, it was observed that in the soil water characteristic curve, during the suction stage before 3.5 kPa, the water holding capacity of the luxuriant growth was the lowest. This can be attributed to factors such as high root density, the lowest soil bulk density (1.37 g/cm3), the highest content of organic matter (12.45 g/kg), and the highest total soil porosity (35.12%) within the 20 cm soil layer. This finding aligns with previous research by Mohammadi[19]. Furthermore, according to the study of Burger[20], there exists a significant negative correlation between porosity and the parameter θs of the V-G model model of the soil water characteristic curve (Table 4).
In the low suction section, soil water is discharged primarily in the form of gravity water, resulting in poor water holding capacity of the luxuriant growth before 3.5 kPa. Additionally, Fooladmand's research[21] indicates that soil water holding capacity is positively correlated with clay and silt content, and negatively correlated with sand content and bulk density. In this study, it was found that the water holding capacity of the poor growth is relatively better, followed by the moderate growth. This difference can be attributed to the significantly higher porosity of the luxuriant growth (35.12%) compared to the poor growth (26.14%), consistent with the negative correlation observed by Burger[20].
Soil particle size composition profoundly affects soil ventilation, permeability, and fertility, thus influencing soil water holding capacity. In this study, the V-G model utilized in the soil moisture characteristic curve was obtained by fitting soil particle size data using the RETC method. Combined with Fig. 2, which illustrates the soil textures of the four types of growth regions as silty loam, it was noted that except for the significantly higher silt and clay content in the poor growth compared to other growth regions, there were no significant differences in the content of sand and silt between well growth and moderate growth (P > 0.05) (Table 1). Consequently, there were few differences in water holding capacity among these regions. Additionally, the V-G parameter 'n' exhibited minimal variation among the four growth regions, ranging from 5.1631 to 5.6253, resulting in consistent alterations in soil moisture characteristic curves across these regions.
When the specific water capacity drops below 10− 2, the soil's capacity to release water significantly diminishes, posing challenges for vegetation to access water[22]. Figure 4 illustrates that the specific water capacity of the four growth regions is 10− 3, indicating that vegetation growth in these tea-producing areas is constrained by water availability. To facilitate optimal tea growth, it may be necessary to augment soil moisture levels appropriately.
In this study, a positive correlation between unsaturated hydraulic conductivity and porosity was observed, which aligns with findings from Su's research[22]. In luxuriant growth areas, the accumulation of litter and high root density within the 20 cm soil layer contributes to soil loosening. Consequently, the increased number of pores leads to higher porosity[17], thereby resulting in the highest unsaturated hydraulic conductivity observed in luxuriant growth regions.
In this study, it was observed that D(θ) increases with the rise in θ, which corresponds to the empirical formula D(θ) = aebθ[23]. At lower soil moisture content levels, soil moisture primarily moves as water vapor, resulting in gradual changes in soil water movement. Conversely, as soil moisture increases, the rate of change in soil water movement decreases, and soil resistance to water diminishes. Consequently, soil water diffusivity escalates sharply with increasing soil water content, consistent with findings by Yao[24].
According to the study of Cornelius[25], soil type, mechanical composition, bulk density, porosity, and organic matter content influence soil water diffusivity, with soil porosity exhibiting a positive correlation with soil water diffusivity. In this study, under identical water content conditions, soil water diffusivity is ranked as follows: luxuriant growth, well growth, moderate growth, and poor growth, from highest to lowest. This ranking can be attributed to the lowest viscous weight and porosity of the soil structure in poor growth areas, while the porosity in luxuriant areas is the highest.