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CHANGES OF NUTRIENT LOADING CAUSED BY CLEAR-CUTTING OF A DECIDUOUS BROADLEAF FOREST AND PLANTING OF JAPANESE CEDAR

ABSTRACT Two adjoining small forested watersheds having weirs were prepared to evaluate changing in the loading rate of nutrients caused by clear-cutting. One of the experimental watersheds (Kutsuki-L, 1.09 ha) was clear-cut from December 1996 to May 1997, and young Japanese cedars (Cryptomeria Japonica D. DON.) were planted in April 1998. The other (KutsukiR, 1.92 ha) has been conserved. Before clear-cutting, the annual average concentrations of total nitrogen (TN), total phosphorus (TP) and potassium ion (K) of the stream water of Kutsuki-L were 0.145, 0.010 and 0.16 mg l, respectively. After the clear-cutting practice, abrupt and short-term increasing of TN, TP, and TOC concentrations was occurred. It seemed to result from disturbance of surface soils by the practice. TN increased to around 0.5 mg l after about one year from the event, and began decreasing gradually in 2000. However in 2002, it maintained yet slightly higher level than that of conserved Kutsuki-R. The high concentration was mainly due to increasing discharge of nitrate. TP and TOC did not show such delayed and prolonged effect. K decreased concentration slightly for two years and then recovered the level before cutting. The averages of output fluxes of TN, TP and K were 2.18, 0.15 and 2.67 kg ha y, respectively, before clear-cutting. After that, TN increased to the maximum in the third year, which was 5.6 times of the above value, and TP to 1.3 times in the first year. These results indicated that clear-cutting of the large area might give serious effect on eutrophication of lakes and bays in the lower reaches, and that it is necessary to develop logging techniques to limit the draining of nitrate.