森林の成立および皆伐が土壌の2・3の性質に及ぼす影響について : 第1報 森林の成立にともなう土壌の性質の変化

Abstract

To understand the close relationship between soil properties and forest vegetation, the study on the process of the soil development with the progress of time or with the development of vegetation is needed. In this report, the relationship between the time factor and the forest soil properties, namely, the amount of organic matter, nitrogen and mineral nutrients are considered. The studied area were the erosion control forests which widely distribut over the Setouchi district. The mother rock in this district is granite, and the mean annual temperature is about 14-15℃, while the annual precipitation is about 1000 ~ 1500mm. With the development of forest vegetation the amount of organic matter increased with the progress of time, in all parts of ecosystem, i. e. trees, Ao layer and soil. The amount of nitrogen also increased, but the results were not so definite as compared to that of the organic matter, because the vegetation changed from alder which were planted at first and has root-nodules fixing atmospheric nitrogen, to pine. On the other hand, there were no definite changes in the total amount of mineral nutrients. It seemed that this fact was due to the difference of the circulation system of each substance. The circulation system of carbon and nitrogen through forest ecosystem is open, and so forests take in carbon and nitrogen from the atmosphere and accumulate them in the form of trees, Ao layer and soil organic matter. If the rate of accumulation is more rapid than the rate of loss by way of decomposition and respiration, it will increase the amount of carbon and nitrogen in forest ecosystem with the progress of time. On the contrary, the circulation system of mineral nutrients is closed. They are absorbed by trees from the soil and are returned to the soil in the form of litter. No quantitative changes in forest ecosystem can be expected by this circulation. We may conclude that forest ecosystems can accumulate carbon or organic matter and nitrogen but cannot increase in mineral nutrients. Preciely speaking, the circulation system of mineral nutrients are also open, because rein water contains some amount of nutrients, and the weathering of rocks in the process of forest development may occur. On the other hand, forests lose mineral nutrients by the way of runoff water, surface erosion and leaching. The rate of loss of mineral nutrients may change with many site conditions, therefore, it cannot be concluded whether forest ecosystem can increase in mineral nutrients or not. The increasing process of soil organic matter with time can be expressed by the formula of geometrical progression. Soil organic matter increases rapidly in early stages, but soon the absolute value of yearly increase begin to decrease. It decreases with the progress of time, and finaly it reaches a dynamic equilibrium. The amount of soil organic matter at dynamic equilibrium (Sm) is decided by two factors: one is the amount of supply to soil (a), another is the rate of decomposition (p): Sm=a/p. The rate of decomposition (p) is the key factor that decides the amount of soil organic matter, because there are no remarkable variations in the amount of litter fall (amount of supply (p)) in the closed stands. Therefore, the slower the rate of decomposition, the richer the organic matter in the soil. However, on the status of dynamic equilibrium, the amount of supply and that of decomposition are equal, and then we find no quantitative change in the amount of soil organic matter. Nearly equal amount of mineral nutrients which are returned to the soil by litter fall may be decomposed or mineralised in the soil and the circulation through tree and soil continue to be the stable status in the dynamic equilibrium system. It take a very long time to reach the equilibrium. In this resarch, it is estimated that it may take 80 or more years to reach the equilibrium. The slower the rate of decomposition, the longer the time necesary to reach the equilibrium. The source of organic matter which is supplied to the soil is litter fall. It is accumulated on the surface of the forest soil, and the gain of nitrogen also mainly occur at the surface of the soil. Therefore the vertical distribution of organic matter and nitrogen in the soil begin to change with the progress of time. It contains a very small amount of organic matter and nitrogen at the initial stage, and there are no differences in the amount between surface (0-5cm), subsurface (5-15cm) and subsoil (15-25cm). But, with the development of forest vegetation, the accumulation of organic matter and nitrogen mainly occur at the surface of the soil and their downward transport are very slow, and then the decreasing gradient of organic matter and nitrogen content with depth began to increase with the progress of time. The vertical distribution of mineral nutrients are more complex. The amount of mineral nutrients in the soil were not so small even at the initial stage. With the development of forest vegetation, they are absorbed by trees and soon returned to the surface soil. This circulation of mineral nutrients by litter fall must have some influence on the vertical distribution of mineral nutrients. The substances that are scarce in the soil suffer a more severe influence on veitical distribution than the substances that are plentiful in the soil by this circulation above mentioned. In this resarch, phosphorus and calcium were scarse in the soil, and the total amount that returned to the soil during 60 years were about tweice as much as the amount that was contained in the soil. If the rate of downward transport of these substances is not so rapid, it seemes that they begin to be accumulated on the surface soil, and the decreasing gradient with depth begins to increase with the progress of time. As above mentioned, the forest ecosystem is able to accumulate organic matter and nitrogen in the soil, and after a long time, they reach the dynamic equilibrium in quantity. On this status, the circulation of substance through trees and soil is in stable status. The equal amount of substance that is returned to the soil by litter fall is decomposed in the soil. On the other hand, the forest ecosystem does not always have the ability to accumulate the mineral nutrients. If the loss of organic matter and nutrients occur by clear cutting or surface erosion, the amount of the loss of organic matter and nitrogen will be recovered by the development of secondary forest vegetation, but an amount equal to the loss of mineral nutrients by clear cutting may not always be recovered. If the mineral nutrients decrease in the soil, thire loss by surface erosion may increase, because they begin to accumulate on the surface soil by litter fall.