Temporal And Spatial Variations Of Soil Respiration And Its Controlling Factors In Warm-temperate Oak (Quercus Acutidentata) Forests

Soil respiration is one of the atmosperic CO₂ sources, which plays an improtant role in determining whether an ecosystem is a carbon sink or source to the atmosphere. A good understanding of the machanisms underlying soil respiration will help us to better ascertain the global land carbon budget. However, the temporal and spatial variations of soil respiration and their controlling factors varied largely with forest succussional stages and types. This thesis was designed to examine soil respiration along with its controlling factors in oak forests at the Baotianman Natural Reserve in China in order to assess the regional carbon budget.1) The temporal and spatial variations of soil respiration (RS) was investigated in a warm-temperate oak chronosequence, in China. The oak choronosequence included a 40-year-old young oak forest (YO), a 48-year-old intermediate oak forest (IO), a 80-year-old mature oak forest (MO) and a 143-year-old old growth oak forest (OGO). Temporal variations of RS of the four forests largely depended on soil temperature at 5cm depth (T5), which explained 73.8～82.5% of the temporal variation of RS. In IO forest, soil water content (SWC) had a weak effect on the temporal variation of RS. The seasonal patterns of standard deviation (SD) of RS showed the similar trend with T5, while no seasonal trends of variation coefficients (CV) in RS was found for the four forests. In YO and MO forests, spatial variation coefficients of SWC correlated significantly positive with the spatial variation coefficients of RS. The spatial variation of RS was the highest in OGO among the oak chronosequence. Therefore, more sampling points were needed in OGO forest in order to obtain an average rate of RS within 20% of its actual value at the 95% confidence level. Top soil organic carbon (SOC), especially soil light fraction organic carbon (LFOC) well explained the variation of cumulative RS among the stands. We found total porosity (TP) at 0-5cm soil depth correlated negatively with the cumulative RS, this may be due to the limitation of capillary porosity (CP) on RS. Soil labile carbon storage and soil physical properties varied with the forest succession, which influenced the CO₂ efflux. Furthermore, forest age had a positive effect on spatial varation of RS.2) Plot trenching and root decomposition experiments were conducted to partion soil respiration components in a warm-temperate oak chronosequence (YO, IO, MO and OGO forests) in China. Total soil surface CO₂ efflux (RS) was partitioned into rhizospheric (RR) and heterotrophic respiration (RH) across growing season of 2009. It was found that the temporal variations of RR and RH can be well explained by soil temperature at 5cm depth (T5) using exponential equation. However, RR of YO and IO forests peaked in September, while their T5 peaks advanced 30 days (in August). Also, unlike MO and OGO forests, the contribution of RR to RS (RC) of YO and IO forests presented the second peak in September. There were significant differences in the cumulative RH and RR fluxes during the growing season among the four forests. The estimated RH values for YO, IO, MO and OGO forests averaged 431.72, 452.02, 484.62 and 678.93 g C m-2, respectively, while their corresponding RR averaged 191.94, 206.51, 321.13 and 153.03 g C m-2, respectively. The estimated RC increased from 30.78% in the YO forest to 39.85% in the MO forest and then declined to 18.39% in the OGO forest. There was significant correlation between soil organic carbon (SOC), especially the labile organic carbon (LFOC) storage of 0-10cm soil depth and the cumulatve RH during the growing season. There was no significant relationship between RR and fine root biomass regardless of the stand age. Apparent temperature sensitivity (Q₁₀) of RH (3.93±0.27) is significantly higher than that of RR (2.78±0.73). The capillary porosity decreased as the stand age increased, which accounted for the differences in cumulative RS among the four chronosequence. It was concluded that the positive effect of forest age on RS attributed mainly to the increasing proportion of RH with age. The seasonal patterns of RR varied with forest age. Our results emphasized the importance of respiration components partitioning when evaluating the age effect on soil respiration and its significance to future model construction.3) Factors that control spatiotemporal variations of soil respiration (RS) were assessed in a natural regenerated oak forest (OF) and a nearby pine plantation (PP) in warm-temperate area of China. RS, soil properties and stand structure were measured at 10m intervals in two 40×60m plots (35 grid points) for OF and PP from Oct. 2008 to Oct. 2009, respectively. The observed spatial pattern kept remarkably stable throughout the growing season. Compared to OF, PP showed relatively lower spatial variations of RS across the growing season. The spatial relationships between RS and soil water content (SWC) were found to be negative. However, the restriction of gas diffusivity in water-saturated soil was not the primary cause of the low RS in wetter regions. Compared to SWC, water filled pore space (WFPS) might be a better parameter to explain the spatial variation of RS. The stand structure parameters, such as basal area (BA), max diameter at breast height (max DBH) and mean DBH within 4 or 5m of the measurement points accounted well for the spatial variation of RS in OF. However, no similar correlation was found in PP. Multilinear regression results showed that light fraction organic carbon (LFOC) and water hold capacity (WHC) explained 49.6% of the variation of RS in PP, while WHC, Max DBH(4) and total porosity (TP) explained 64.2% of the variation of RS in OF. This suggested that biotic and abiotic factors played different roles in controlling spatial variations of RS between OF and PP. Regardless of the stand, spatial distribution of carbon pool lability (LLFOC) and fine root biomass (FR) correlated positively with the spatial variation of apparent temperature sensitivity of RS (Q₁₀), while SWC negatively correlated with the spatial variation of Q₁₀. The significant higher Q₁₀ of PP compared to OF may due to the decreased SWC. Our findings ascertain the spatio-temporal variations of RS between plantation and naturally regenerated forests, which is useful to make an accurate estimation of regional carbon fluxes.4) Few research has been conducted on how climate change may affect the soil C and N processes of warm-temperate oak forest in China. Along the slope of the Funiu mountains, China, intact soil monoliths from a 1400m (Quercus acutidentata) oak forest were translocated to a 620m (Quercus variabilis) oak forest and vice versa. Through the intact soil monoliths reciprocal translocation experiment, the likely impacts of climate change on soil C and N processes were explored. The results showed the mean annual soil temperature at 5cm depth (T5) was about 3℃higher in the low-elevation site than that in the high-elevation site, while the soil water content (SWC) was about 6% lower in the low-elevation site than that in the high-elevation site. Net rates of N transformations and CO₂ fluxes were measured in high-elevation soil monoliths incubated in situ and soil monoliths transferred to the low-elevation site and vice versa. Net N mineralization and nitrification increased about 254% and 67% in transferred soil cores compared with in situ soil cores. Soil transfer significantly increased CO2 efflux (52%) compared to fluxes from soil monoliths incubated in situ. Soil transfer resulted in a relatively stable temperature increase and this warming effect on soil CO2 efflux increased as the weather get warmer. Soil microbial biomass carbon (MBC) decreased in transferred soil monoliths compared to in situ soil monoliths after one year incubation period (from 1.14 to 0.73), while dissolved organic carbon (DOC) increased (from 0.23 to 0.27). Furthermore, transferred soil monoliths from the high-elevation to the low-elevation raised soil respiration temperature sensitivity (Q₁₀) (from 2.73 to 3.42), while reduced soil basal respiration (R0) (from 0.42 to 0.29) compared to soil monoliths incubated in situ. In contrast, transferred soil monoliths from the low-elevation to the high-elevation site (i.e. simulated global cooling) produced weak effects on soil C process as no significant changes in soil respiration parameters and soil labile carbon content were found. Our results suggested that short term soil translocation would lead to large impacts on soil C and N processes, and the soil substrate availability as well.