10:45 AM - 12:15 PM
[AGE27-P06] Hydraulic properties distribution of forest soil considering structural developmen
Keywords:Continuous Outflow Evaporation Method(COFEM), soil pores properties, soil hydraulic properties
Introduction
Soil hydraulic properties are associated with the soil pore systems. Understanding how the soil pore structure development influence is critical for forest soil hydraulic properties study. The forest soil pore system is divided into two parts. One part is the basic primary pore system. The other part is the secondary pore system, formed by the influence of biological activities, has a considerable impact on the soil-pore structure of forest soils.
However, studies of soil hydraulic properties from forests in temperate climates according to secondary soil pore systems are not adequate. The purpose of this study is to derive various pore-related properties of unsaturated forest soil by evaluating the results of hydraulic properties measurements and to reveal the variation pattern of soil-pore properties in forest soil.
Method
Soil samples were taken using soil core samplers (height 5.1 cm, volume 100 cm3) from the Kamigamo Experimental Forest Station located in the southern Kyoto, Japan. The parent material is sedimentary rock. The soil type is Brown Forest soil.
The Longleaf pine (Pinus palustris) forest named B4 was selected for sampling. Undisturbed forest soil samples were collected from one soil profile, and four samples were taken at every 10 cm depth from 15cm to 55cm. After measurement, we prepared disturbed samples by crushed the oven-dried soils carefully to break up aggregate structures without breaking soil grains.
We obtained hydraulic properties using the falling-head method and Continuous Outflow Evaporation Method (COFEM) developed by Masaoka and Kosugi (2021). The water retention model (Kosugi, 1996) and hydraulic conductivity model (Kosugi, 1999) were fitted to obtain the pore-related parameters.
Results and Discussion
In the forest system, the soil hydraulic properties showed a certain variation pattern at different depths of the soil. After removing the influence of soil aggregates, the soil pore system changed significantly.
In the undisturbed top soil (depth of 15cm and 25cm), effective porosity (θs-θr), saturated hydraulic conductivity (Ks) and median soil pore radius (rm) were large compared with other depths. This may indicate the proportion of large soil pore increased and soil water movement capacity increased. According to previous studies, there is a high correlation between the development of large pores and the development of biological pore systems in forest soils (Hayashi et al, 2006). However, Soil pore tortuosity parameters α and βσ showed no certain variation pattern between different depths.
In the disturbed soil, top soil showed similar trend with undisturbed soil that they had larger values of effective porosity, saturated hydraulic conductivity and median soil pore radius compared with other depths. Meanwhile, Soil pore tortuosity parameters α and βσ showed no certain variation pattern between different depths as same as undisturbed soil.
Compared with undisturbed soil, disturbed soil showed lower values of width of soil-pore size distribution (σ) and saturated hydraulic conductivity in both depths, but it had similar values of effective porosity. The values of median soil pore radius showed a trend that they were close to middle soil of undisturbed soil. As for soil pore tortuosity, disturbed soil showed lower α value, and no significant difference showed in βσ values. Considering α increased means pore tortuosity increased, it indicates that the pores connected with each other more tortuously.
Soil hydraulic properties are associated with the soil pore systems. Understanding how the soil pore structure development influence is critical for forest soil hydraulic properties study. The forest soil pore system is divided into two parts. One part is the basic primary pore system. The other part is the secondary pore system, formed by the influence of biological activities, has a considerable impact on the soil-pore structure of forest soils.
However, studies of soil hydraulic properties from forests in temperate climates according to secondary soil pore systems are not adequate. The purpose of this study is to derive various pore-related properties of unsaturated forest soil by evaluating the results of hydraulic properties measurements and to reveal the variation pattern of soil-pore properties in forest soil.
Method
Soil samples were taken using soil core samplers (height 5.1 cm, volume 100 cm3) from the Kamigamo Experimental Forest Station located in the southern Kyoto, Japan. The parent material is sedimentary rock. The soil type is Brown Forest soil.
The Longleaf pine (Pinus palustris) forest named B4 was selected for sampling. Undisturbed forest soil samples were collected from one soil profile, and four samples were taken at every 10 cm depth from 15cm to 55cm. After measurement, we prepared disturbed samples by crushed the oven-dried soils carefully to break up aggregate structures without breaking soil grains.
We obtained hydraulic properties using the falling-head method and Continuous Outflow Evaporation Method (COFEM) developed by Masaoka and Kosugi (2021). The water retention model (Kosugi, 1996) and hydraulic conductivity model (Kosugi, 1999) were fitted to obtain the pore-related parameters.
Results and Discussion
In the forest system, the soil hydraulic properties showed a certain variation pattern at different depths of the soil. After removing the influence of soil aggregates, the soil pore system changed significantly.
In the undisturbed top soil (depth of 15cm and 25cm), effective porosity (θs-θr), saturated hydraulic conductivity (Ks) and median soil pore radius (rm) were large compared with other depths. This may indicate the proportion of large soil pore increased and soil water movement capacity increased. According to previous studies, there is a high correlation between the development of large pores and the development of biological pore systems in forest soils (Hayashi et al, 2006). However, Soil pore tortuosity parameters α and βσ showed no certain variation pattern between different depths.
In the disturbed soil, top soil showed similar trend with undisturbed soil that they had larger values of effective porosity, saturated hydraulic conductivity and median soil pore radius compared with other depths. Meanwhile, Soil pore tortuosity parameters α and βσ showed no certain variation pattern between different depths as same as undisturbed soil.
Compared with undisturbed soil, disturbed soil showed lower values of width of soil-pore size distribution (σ) and saturated hydraulic conductivity in both depths, but it had similar values of effective porosity. The values of median soil pore radius showed a trend that they were close to middle soil of undisturbed soil. As for soil pore tortuosity, disturbed soil showed lower α value, and no significant difference showed in βσ values. Considering α increased means pore tortuosity increased, it indicates that the pores connected with each other more tortuously.