Movement of Nutrients in a Small Mountainous and Forested Watershed
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概要
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The movement of nutrients was investigated in a small mountainous and forested watershed as an example of a forest ecosystem unit. This experimental watershed is located in the southeast of Lake Biwa, Kiryu Kamitanakami, Shiga Prefecture, on the granite area and a plantation of pine (Pinus densiflora) and Hinoki (Chamaecyparis obtusa SIEB. et ZUCC.) for erosion control. At this experimental watershed, weekly samples of precipitation, throughfall, stem flow and stream water and a monthly sample of litter fall were collected for chemical analysis from May 1968 to April 1969. Quantities of nutrients contained in the throughfall, stem flow and litter fall, which moved from tree crown layer to forest floor, and nutrient inputs in the precipitation to the forest ecosystem and outputs in the stream water from the ecosystem and nutrients content of AO layer and mineral soil were measured. The results obtained from these investigations were as follows: 1) The annual outputs of sodium, potassium, calcium and magnesium in the discharge water exceeded the annual inputs in the precipitation, while the outputs of phosphorus, nitrate- and ammonia-nitrogen and dissolved organic matter were less than the inputs. 2) Of the total nutrients contained in the annual throughfall, stem flow and litter fall, 55% calcium, 71% phosphorus and 88% nitrogen reached the forest floor in the litter fall, while 900 sodium, 76% potassium, 63% magnesium in the throughfall and stem flow reached the forest floor. Thus movements of phosphorus and nitrogen from the tree crown layer to the forest floor are mainly by the litter fall and those of sodium, potassium and magnesium are mainly by the rainwater passed through the tree crown layer. 3) The outputs of phosphorus and nitrogen from the ecosystem were only 13% and 4% respectively of the total amount of each nutrient contained in the throughfall, stem flow and litter fall reaching the forest floor, while the outputs of sodium, potassium, calcium and magnesium were 511%, 29%, 38% and 45% respectively reaching the forest floor. Phosphorus and nitrogen in the throughfall, stem flow and litter fall seem to be retained in the soil and to recycle within the ecosystem in higher proportion than the other mineral nutrients. 4) The annual net losses of sodium, potassium, calcium and magnesium from the ecosystem were equivalent to 118%, 5%, 6% and 2% respectively of the available nutrients in the soil and 852%, 22%, 25% and 26% respectively of the annual amounts of nutrient return by the litter fall and the leachate from the canopy (nutrients contained in the throughfall and stem flow minus nutrients contained in the precipitation). The net gains of phosphorus and nitrogen were 0.7% and 65-130% respectively of the available nutrients and 4% and 12% respectively of the nutrients return. It may be noted that the annual net losses of the nutrients from the forest ecosystem represent considerable proportions to the annual nutrients return within the ecosystem.
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