Changes in nitrogen and base cation concentrations in soil water due to the tree cutting in a wetland alder forest in the Kushiro Wetland, northern Japan

摘要

As part of the Kushiro Wetland Restoration Project, we evaluated the effect of the tree-cutting (February 2003) on soil water chemistry for 3 years in an alder (Alnus japonica) shrubland on the peat soil in the Kushiro Wetland, the largest wetland in Japan. The alder stand was divided into two types; low stature and high stature stands, mean heights being 1.5 and 2.6 m, respectively. The treatment plot with tree cutting and the reference plot measuring 25 m × 25 m each were established in both stands, and a soil incubation experiment was also conducted by trenching the root zone of wetland vegetation in the treatment plots in both stands in the summer of 2004. The tree cutting did not substantially increase the concentrations of ammonium ion (NH4 +) and nitrate ion (NO3 −) in the soil water, although a gradual and slight increase of NO3 − concentration was found after the tree-cutting only in the high stature stand (10 μmol/l). This increase in the NO3 − concentration in the high stature stand was probably due to the rhizosphere oxidation of wetland herbaceous vegetation, and was accompanied by the increase of Ca2+ concentration. The soil incubation experiment showed no significant change of the nitrogen concentration, suggesting that inorganic nitrogen was immobilized and NO3 − was denitrified intensively in this wetland peat soil. The results of this study suggested that this wetland ecosystem had a high potential to stabilize the soil water chemistry.