八島 未和

Canada bluejoint grass [Calamagrostis canadensis (Michx.) Beauv., referred to as bluejoint below] is a competitive understory species widely distributed in the boreal region in North America and builds up a thick litter layer that alters the soil surface microclimate in heavily infested sites. This study examined the effects of understory removal, N fertilization, and litter layer removal on litter decomposition, soil microbial biomass N (MBN), and net N mineralization and nitrification rates in LFH (the sum of organic horizons of litter, partially decomposed litter and humus on the soil surface) and mineral soil (0-10 cm) in a 13-year-old white spruce [Picea glauca (Moench.) Voss] plantation infested with bluejoint in Alberta, Canada. Removal of the understory vegetation and the litter layer together significantly increased soil temperature at 10 cm below the mineral soil surface by 1.7 and 1.3 degrees C in summer 2003 and 2004, respectively, resulting in increased net N mineralization (by 1.09 and 0.14 mg N kg(-1)day(-1) in LFH and mineral soil, respectively, in 2004) and net nitrification rates (by 0.10 and 0.20 mg N kg(-1) day(-1) in LFH and mineral soil, respectively, in 2004). When the understory vegetation was intact, nitrification might have been limited by NH4+ availability due to competition for N from bluejoint and other understory species. Litter layer removal increased litter decomposition rate (percentage mass loss per month) from 2.6 to 3.0% after 15 months of incubation. Nitrogen fertilization did not show consistent effects on soil MBN, but increased net N mineralization and nitrification rates as well as available N concentrations in the soil. Clearly, understory removal combined with N fertilization was most effective in increasing rates of litter decomposition, net N mineralization and nitrification, and soil N availability. The management of understory vegetation dominated by bluejoint in the boreal region should consider the strong effects of understory competition and the accumulated litter layer on soil N cycling and the implications for forest management.

Canada bluejoint grass (Calamagrostis canadensis (Michx.) Beauv.) is a very competitive understory species and can create a thick litter layer that may decrease tree growth in white spruce (Picea glauca (Moench.) Voss) stands in the boreal region. We examined the effects of Canada bluejoint grass dominated understory competition, N fertilization, and litter layer removal on the nutrition and growth of white spruce, in relation to soil N availability in a 13-year-old plantation near Whitecourt, Alberta, Canada, in 2003 and 2004. We hypothesized that: (1) understory competition reduces white spruce growth as a result of competition for available N; (2) litter layer removal increases white spruce growth by increasing soil temperature and N availability; (3) N fertilization improves white spruce growth by increasing N availability. Soil gravimetric moisture content in LFH was reduced by understory competition and litter layer removal in 2003, which had a very dry summer. Understory removal increased white spruce diameter growth, 100-needle biomass, and needle N concentration and content. In 2003, N fertilization consistently increased 100-needle weight and needle N content when litter layer was removed, but decreased or did not affect those values without litter layer removal. Nitrogen fertilization increased needle N concentration in 2004. The inconsistent effects of N fertilization may be due to the short-term nature of fertilization effects and immobilization of N by organic matter. Rates of white spruce diameter growth and soil mineral N supply in 2004 were related (R-2 = 0.65, P < 0.001). Vector analysis showed that N fertilization or litter layer removal with intact understory vegetation either increased or did not affect needle N concentration but decreased tree biomass. We conclude that understory control will benefit tree growth in young white spruce stands in the boreal region that have passed the free-to-grow stage. However, N fertilization or litter layer removal alone in bluejoint infested sites may not benefit the trees. (c) 2006 Elsevier B.V. All rights reserved.

The analysis of humic acid and microbiological characteristics of soils in Atagawa Subtropical Farm, Chiba University, was performed in 2000 by the five laboratories of Chiba University, Nihon University, Tokyo University of Agriculture and Technology and Meiji University. Soil samples were taken from the Site 2 and Site 4 of the Farm. Site 2 under evergreen broad leaf forest was located on the gentle slope, from a small ridge to valley, and soil materials were moved from the upper part of the site. The volcanic products at the Site 2 were affected by the upper horizons. The Site 4 consisted of volcanic products in the whole profile and utilized for the orchard (orange). Based on the humic acid analysis, such as the absorption curve of humic acid, Δlog K, RF and PQ, the humic acid from A horizon of the Site 2 and A1 horizon of the Site 4 were classified as B_+ type (B type humic acid showing specific peaks of P type humic acid) and A type, respectively. Total number of microorganisms, biomass nitrogen content, respiration amount and qCO_2 of the Site 4 sample were determined. Fungi in A horizon were highest, and they were lower in the deeper layers. Bacteria in Bw1 and BC horizon were higher than in shallower layers. Biomass C and N showed the highest values in A1 horizon, and lowered but there was no tendency in deeper layers. The qCO_2 in A2 horizon seemed relatively to be low. The factors influencing the activities of microorganisms in Site 4 were considered as following; 1) Site 4 was a part of slope. 2) Site 4 remained influence caused by the land-creation of this farm. 3) There was little water permeability in Bw horizon that was sticky. Both nitrate and ammonium were detected in deeper layers, not in A1 and A2, indicating that ammonium fixation by clay minerals takes places in subsurface layers.