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[AGE30-P07] Analysis of the permeability distribution characteristics of forest soils based on soil-pore connection properties
Keywords:Continuous Outflow Evaporation Method(COFEM), soil-pores conductance, soil hydraulic properties, Soil water retention model
Introduction
Soil hydraulic properties are associated with the soil pore systems. The soil pore system is divided into two parts. One part is the basic primary pore system. The other part associated with forest development 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, only a few studies have characterized soil hydraulic properties from forests in temperate climates according to secondary soil pore systems. The purpose of this study is to derive various pore-related properties of unsaturated forest soil in temperate forests by evaluating the results of hydraulic property measurements at different depths and to reveal the variation pattern of soil-pore properties among different soil depths.
Method
Soil samples were taken using soil core samplers (height 5 cm, volume 100 cm3) from the Kamigamo Experimental Forest Station located in the southern part of Kyoto Prefecture, central Japan. The parent material is sedimentary rock. The soil type is Brown Forest soil. The main forest type is a natural forest with a mixture of Japanese Cypress, Japanese Cedar, and red pine.
Two types of forest were selected for sampling. The first one is the Longleaf pine (Pinus palustris) forest. One soil profile was excavated, and four samples were taken at every 10 cm depth from 15cm to 55cm. The second forest is the Japanese cedar (Cryptomeria japonica) forest. Two soil profiles were excavated, and two samples were taken at every 10 cm depth from 15cm to 55cm.
We obtained hydraulic properties using the Continuous Outflow Evaporation Method (COFEM) developed by Masaoka and Kosugi (2021). Soil pore connection properties C(r) were determined according to the equation proposed by Kosugi (1992). The water retention model (Kosugi, 1996) was fitted to obtain the soil-pore-distribution-related parameters.
Results and Discussion
In the forest system, the soil hydraulic properties show a certain variation pattern at different depths of the soil.
In the top soil (depth of 15cm and 25cm), effective porosity (θs-θr) and saturated hydraulic conductivity (Ks) are large. Also, the median soil pore radius (rm) of top soil was larger than those of other depths. This indicates that the amount and proportion of large soil-pore increased and soil water movement ability increased as the crumb structure develops in forest soils. 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)
Soil pore connection properties C(r) in the top soil are slightly larger than that in the subsurface soil in the large pore size range. The opposite tendency is found in the small pore size range. This indicates that the top soil has better connections of large pores and worse connections of small pores.
The top soil had a larger variation in water retention and hydraulic conductivity from wet to dry range, compared with other depths. There is an increase in soil water movement ability at the top soil. The performance of connection degree is related to both soil pore system and water movement ability. One possible reason is that the structure of small pores becomes complicated when fine soil particle is consolidated during the process of soil agglomeration, resulting in a decrease in pore connection degree. To reveal the possible reason, it requires an optical investigation of how small pores are connected.
Soil hydraulic properties are associated with the soil pore systems. The soil pore system is divided into two parts. One part is the basic primary pore system. The other part associated with forest development 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, only a few studies have characterized soil hydraulic properties from forests in temperate climates according to secondary soil pore systems. The purpose of this study is to derive various pore-related properties of unsaturated forest soil in temperate forests by evaluating the results of hydraulic property measurements at different depths and to reveal the variation pattern of soil-pore properties among different soil depths.
Method
Soil samples were taken using soil core samplers (height 5 cm, volume 100 cm3) from the Kamigamo Experimental Forest Station located in the southern part of Kyoto Prefecture, central Japan. The parent material is sedimentary rock. The soil type is Brown Forest soil. The main forest type is a natural forest with a mixture of Japanese Cypress, Japanese Cedar, and red pine.
Two types of forest were selected for sampling. The first one is the Longleaf pine (Pinus palustris) forest. One soil profile was excavated, and four samples were taken at every 10 cm depth from 15cm to 55cm. The second forest is the Japanese cedar (Cryptomeria japonica) forest. Two soil profiles were excavated, and two samples were taken at every 10 cm depth from 15cm to 55cm.
We obtained hydraulic properties using the Continuous Outflow Evaporation Method (COFEM) developed by Masaoka and Kosugi (2021). Soil pore connection properties C(r) were determined according to the equation proposed by Kosugi (1992). The water retention model (Kosugi, 1996) was fitted to obtain the soil-pore-distribution-related parameters.
Results and Discussion
In the forest system, the soil hydraulic properties show a certain variation pattern at different depths of the soil.
In the top soil (depth of 15cm and 25cm), effective porosity (θs-θr) and saturated hydraulic conductivity (Ks) are large. Also, the median soil pore radius (rm) of top soil was larger than those of other depths. This indicates that the amount and proportion of large soil-pore increased and soil water movement ability increased as the crumb structure develops in forest soils. 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)
Soil pore connection properties C(r) in the top soil are slightly larger than that in the subsurface soil in the large pore size range. The opposite tendency is found in the small pore size range. This indicates that the top soil has better connections of large pores and worse connections of small pores.
The top soil had a larger variation in water retention and hydraulic conductivity from wet to dry range, compared with other depths. There is an increase in soil water movement ability at the top soil. The performance of connection degree is related to both soil pore system and water movement ability. One possible reason is that the structure of small pores becomes complicated when fine soil particle is consolidated during the process of soil agglomeration, resulting in a decrease in pore connection degree. To reveal the possible reason, it requires an optical investigation of how small pores are connected.