The ecosystem carbon budget was estimated in a Japanese Zoysia japonica grassland. The green biomass started to grow in May and peaked from mid-July to September. Seasonal variations in soil CO(2) flux and root respiration were mediated by changes in soil temperature. Annual soil CO(2) flux was 1,121.4 and 1,213.6 g C m(-2) and root respiration was 471.0 and 544.3 g C m(-2) in 2007 and 2008, respectively. The root respiration contribution to soil CO(2) flux ranged from 33% to 71%. During the growing season, net primary production (NPP) was 747.5 and 770.1 g C m(-2) in 2007 and 2008, respectively. The biomass removed by livestock grazing (GL) was 122.1 and 102.7 g C m(-2), and the livestock returned 28.2 and 25.6 g C m(-2) as fecal input (FI) in 2007 and 2008, respectively. The decomposition of FI (DL, the dry weight loss due to decomposition) was very low, 1.5 and 1.4 g C m(-2), in 2007 and 2008. Based on the values of annual NPP, soil CO(2) flux, root respiration, GL, FI, and DL, the estimated carbon budget of the grassland was 1.7 and 22.3 g C m(-2) in 2007 and 2008, respectively. Thus, the carbon budget of this Z. japonica grassland ecosystem remained in equilibrium with the atmosphere under current grazing conditions over the 2 years of the study.

Coarse woody debris (CWD) is an important component of the forest carbon cycle, acting as a carbon pool and a source of CO2 in temperate forest ecosystems. We used a soda‐lime closed‐chamber method to measure CO2 efflux from downed CWD (diameter ≥5 cm) and to examine CWD respiration (RCWD) under field conditions over 1 year in a temperate secondary pioneer forest in Takayama forest. We also investigated tree mortality (input to the CWD pool) from the data obtained from the annual tree census, which commenced in 2000. We developed an exponential function of temperature to predict RCWD in each decay class (R2 = 0.81–0.97). The sensitivity of RCWD to changing temperature, expressed as Q10, ranged from 2.12 to 2.92 and was relatively high in decay class III. Annual C flux from CWD (FCWD) was extrapolated using continuous air temperature measurements and CWD necromass pools in the three decay classes. FCWD was 3.0 (class I), 17.8 (class II), and 13.7 g C m−2 year−1 (class III) and totaled 34 g C m−2 year−1 in 2009. Annual input to CWD averaged 77 g C m−2 year−1 from 2000 to 2009. The budget of the CWD pool in the Takayama forest, including tree mortality inputs and respiratory outputs, was 0.43 Mg C ha−1 year−1 (net C sink) owing to high tree mortality in the mature pioneer forest. The potential CWD sink is important for the carbon cycle in temperate successional forests.

Secondary mixed forests are one of the dominant forest cover types in human-dominated temperate regions. However, our understanding of how secondary succession affects carbon cycling and carbon sequestration in these ecosystems is limited. We studied carbon cycling and net ecosystem production (NEP) over 4 years (2004-2008) in a cool-temperate deciduous forest at an early stage of secondary succession (18 years after clear-cutting). Net primary production of the 18-year-old forest in this study was 5.2 tC ha(-1 )year(-1), including below-ground coarse roots; this was partitioned into 2.5 tC ha(-1 )year(-1) biomass increment, 1.6 tC ha(-1 )year(-1) foliage litter, and 1.0 tC ha(-1 )year(-1) other woody detritus. The total amount of annual soil surface CO(2) efflux was 6.8 tC ha(-1 )year(-1), which included root respiration (1.9 tC ha(-1 )year(-1)) and heterotrophic respiration (RH) from soils (4.9 tC ha(-1 )year(-1)). The 18-year forest at this study site exhibited a great increase in biomass pool as a result of considerable total tree growth and low mortality of tree stems. In contrast, the soil organic matter (SOM) pool decreased markedly (-1.6 tC ha(-1 )year(-1)), although further study of below-ground detritus production and RH of SOM decomposition is needed. This young 18-year forest was a weak carbon sink (0.9 tC ha(-1 )year(-1)) at this stage of secondary succession. The NEP of this 18-year forest is likely to increase gradually because biomass increases with tree growth and with the improvement of the SOM pool through increasing litter and dead wood production with stand development.

No abstract

Quantification of carbon budgets and cycling in Japanese cedar (Cryptomeria japonica D. Don) plantations is essential for understanding forest functions in Japan because these plantations occupy about 20% of the total forested area. We conducted a biometric estimate of net ecosystem production (NEP) in a mature Japanese cedar plantation beneath a flux tower over a 4-year period. Net primary production (NPP) was 7.9 Mg C ha(-1) year(-1) and consisted mainly of tree biomass increment and aboveground litter production. Respiration was calculated as 6.8 (soil) and 3.3 (root) Mg C ha(-1) year(-1). Thus, NEP in the plantation was 4.3 Mg C ha(-1) year(-1). In agreement with the tower-based flux findings, this result suggests that the Japanese cedar plantation was a strong carbon sink. The biometric-based NEP was higher among most other types of Japanese forests studied. Carbon sequestration in the mature plantation was characterized by a larger increment in tree biomass and lower mortality than in natural forests. Land-use change from natural forest to Japanese cedar plantation might, therefore, stimulate carbon sequestration and change the carbon allocation of NPP from an increment in coarse woody debris to an increase in tree biomass.