Effect of artificial nitrogen load on soil respiration in cool-temperate forests in Japan

To investigate the effect of artificial nitrogen load on soil respiration, the experiment was carried out in a cooltemperate forest in Japan. Three experimental sites, which consists of a coniferous forest, a broad-leaved deciduous forest with Sasa community and a broad-leaved deciduous forests without Sasa community, were established in the cooltemperate forest. We loaded 0kg N haE-1 yrE-1, 20kg N haE-1 yrE-1 and 40kg N haE-1 yrE-1 as NO3NH4 in each experimental site. The treatment of nitrogen load was started from 1999 to 2001, and soil respiration rates were measured in 2000 and 2001 using the closed chamber method. Soil respiration rates showed clear seasonal changes in both years of 2000 and 2001, irrespective of the amount of nitrogen load. The soil respiration rates in 20kg N load treatment were higher than those in 0kg N load in 2000 in all experimental sites. On the contrary, in 2001, the soil respiration rates in 0kg N load were higher than those in 20kg N load in both of the broad-leaved experimental sites, although the soil respiration in the coniferous site showed the same result as in 2000. Moreover, developing a statistical model based on the relationships between soil respiration and soil temperature, we compared temperature sensitivities obtained from each experimental site. Temperature sensitivities obtained from 20kg N load site had a largest sensitivities in all experimental forests, whereas that from 0kg N load site had a smallest sensitivities. These facts suggests that temperature sensitivities of soil respiration might become stronger by the nitrogen load treatment. In addition, considering global environmental issues which are both of global warming and increase in nitrogen deposition, we compared the differences in soil respiration among the experimental sites. The amount of soil respiration increased in 33%-47% when soil temperature was raised by 3 deg C than the present condition. The effects of the global environmental issues on soil respiration were different among vegetation types and nitrogen load treatments.