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Journal Articles

Wintertime grassland dynamics may influence belowground biomass under climate change; A Model analysis

Katata, Genki*; Grote, R.*; Mauder, M.*; Zeeman, M. J.*; Ota, Masakazu

Biogeosciences, 17(4), p.1071 - 1085, 2020/02

 Times Cited Count:8 Percentile:54.32(Ecology)

Mountain grassland productivity is limited by cold and long winters; thus, rising temperatures and changes in snow cover expected in the future may have large impacts on the grassland yields. To investigate this, we enhanced land surface model (SOLVEG) to account for snow, freeze-thaw events, and grass growth, and the model was applied to the managed grasslands affected by extremely warm winter. The model reproduced temporal variability of observed heat fluxes, soil temperatures and snow depth throughout the 3-year simulation period. High physiological activity during the extremely warm winter led to a CO$$_{2}$$ uptake of 100 g-C m$$^{-2}$$, which was, surprisingly, mainly allocated into the below-ground biomass and rarely used for plant growth during spring. This process, which is so far largely unaccounted for in global terrestrial biosphere models, may lead to carbon accumulation in the soil and/or heterotrophic respiration as a response to global warming.

Journal Articles

Root and dissolved organic carbon controls on subsurface soil carbon dynamics; A Model approach

Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun

Journal of Geophysical Research; Biogeosciences, 118(4), p.1646 - 1659, 2013/12

 Times Cited Count:36 Percentile:74.36(Environmental Sciences)

We investigated the role of root litter input and dissolved organic carbon (DOC) transport in controlling subsurface SOC dynamics by a soil C model. The model involves C turnover by decomposition, interaction between SOC and DOC, and DOC transport along water flows, for three C pools (characterized by turnover times of years, decades and millennia). Model simulations with a range of rooting profiles demonstrated a large proportion (36-78%) of SOC is apportioned to the subsurface (below the first 30 cm) soils and a significant part (39-73%) of the subsurface SOC turns over decadally. DOC transport appeared to be dominant for distributing the C to the deeper horizons. Our results suggest soil C studies focusing on the surface alone significantly underestimate the stock of decadally cycling C and underpredict the responses of soil C to global changes.

Journal Articles

Cesium, iodine and tritium in NW Pacific waters; A Comparison of the Fukushima impact with global fallout

Povinec, P. P.*; Aoyama, Michio*; Biddulph, D.*; Breier, R.*; Buesseler, K. O.*; Chang, C. C.*; Golser, R.*; Hou, X. L.*; Je$v{s}$kovsk$'y$, M.*; Jull, A. J. T.*; et al.

Biogeosciences, 10(8), p.5481 - 5496, 2013/08

 Times Cited Count:99 Percentile:94.76(Ecology)

Radionuclide impact of the Fukushima Dai-ichi Nuclear Power Plant accident on the distribution of radionuclides in seawater of the NW Pacific Ocean is compared with global fallout from atmospheric tests of nuclear weapons. Surface and water column seawater samples collected during the international expedition in June 2011 were analyzed for $$^{134}$$Cs, $$^{137}$$Cs, $$^{129}$$I and $$^{3}$$H. The $$^{137}$$Cs, $$^{129}$$I and $$^{3}$$H levels in surface seawater offshore Fukushima varied between 0.002-3.5 Bq/L, 0.01-0.8 $$mu$$Bq/L, and 0.05-0.15 Bq/L, respectively. At the sampling site about 40 km from the coast, where all three radionuclides were analyzed, the Fukushima impact on the levels of these three radionuclides represent an increase above the global fallout background by factors of about 1000, 30 and 3, respectively. The water column data indicate that the transport of Fukushima-derived radionuclides downward to the depth of 300 m has already occurred. The observed $$^{137}$$Cs levels in surface waters and in the water column are in reasonable agreement with predictions obtained from the Ocean General Circulation Model, which indicates that the radionuclides have been transported from the Fukushima coast eastward. Due to a suitable residence time in the ocean, Fukushima-derived radionuclides will provide useful tracers for isotope oceanography studies on the transport of water masses in the NW Pacific Ocean.

Journal Articles

Iodine-129 concentration in seawater near Fukushima before and after the accident at the Fukushima Daiichi Nuclear Power Plant

Suzuki, Takashi; Otosaka, Shigeyoshi; Kuwabara, Jun; Kawamura, Hideyuki; Kobayashi, Takuya

Biogeosciences, 10(6), p.3839 - 3847, 2013/06

 Times Cited Count:23 Percentile:57.25(Ecology)

no abstracts in English

Journal Articles

Dynamics of decadally cycling carbon in subsurface soils

Koarashi, Jun; Hockaday, W.*; Masiello, C.*; Trumbore, S.*

Journal of Geophysical Research; Biogeosciences, 117, p.G03033_1 - G03033_13, 2012/09

 Times Cited Count:45 Percentile:77.83(Environmental Sciences)

Subsurface horizons contain more than half of the global soil carbon (C), yet the dynamics of this C remains poorly understood. We estimated the amount of decadally-cycling subsurface C from the incorporation of "bomb' radiocarbon ($$^{14}$$C) using samples taken over 50 years from grassland and forest soils in the Sierra Nevada Mountains, California. The $$^{14}$$C content of all organic matter fractions increased from the pre- to post-bomb samples, indicating about half of the subsoil C consists of C fixed since 1963. The $$^{14}$$C content of mineral-associated C increased rapidly from 1992-2009, indicating a significant time lag ($$>$$20 years) for the arrival of "bomb" $$^{14}$$C to this fraction. Our results demonstrate that a lagged response of the large subsurface C stores to changes in environmental conditions is possible.

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