Effects of mire disturbance by Sika deer on nitrogen fixation and denitrification in Ozegahara Mire, Japan

Abstract

ABSTRACT Ozegahara Mire is the largest alpine mire in Japan, and it has been preserved as the precious peatland ecosystem under oligotrophic condition. However, after the late 20 century, the mire has been exposed to ecosystem disturbance by invaded Sika deer. Browsing and digging of the mire vegetation by Sika deer have given considerable impacts on not only vegetation but also biochemical property of the peat soil, resulting in the changes in microbiological activities and nutrient dynamics. However, little information has been provided about the effect of deer disturbance on nitrogen (N) dynamics in the mire. This study aims to provide information about the effects of deer disturbance on nitrogen dynamic including N2 fixation and denitrification in Ozegahara Mire. Soil samples were collected at 6 plots on 2 sites with and without mire disturbance by Sika deer at eastern Naka-Tashiro of Ozegahara mire in July 2018. Mineral N contents and other physicochemical properties of the soil samples were measured. N2 fixation and denitrification activities of the soil samples were also measured at different incubation temperatures (15, 20 or 25°C) under either aerobic or anaerobic conditions by acetylene reduction activity (ARA) and inhibition methods, respectively. As results, the ARA in the disturbed sites were lower than those in the undisturbed sites and also lower under aerobic condition than anaerobic condition. Negative correlation was found between ARA and ammonium N in the soil (R 2 = 0.353, p< 0.05), suggesting that mechanical disturbance of the peat soils by deer enhanced mineralization of organic N to suppress N2 fixation. ARA was increased by elevated incubation temperature in the undisturbed sites, while no change in the disturbed sites. Denitrification activities was not affected by the disturbance by Sika deer, while higher under aerobic condition than in anaerobic condition, and in higher temperature than lower temperature. Multi-regression analysis of ARA affecting factors revealed that ARA was significantly influenced by temperature, ammonium N and disturbance. These results indicate the marked influences of the mire disturbance by Sika deer on N2 fixation and denitrification activities of the peat soils and N dynamics in Ozegahara Mire.