Soil Respiration Of Larix Gmelinii Forests Transplanted From A Latitudinal Transect

Boreal forest plays an important role in both local and national carbon budgets in northeastern china because of the large area and huge amount of carbon storage. The spatial and temporal changes in soil surface CO₂ flux (R_S), the major CO₂ source to the atmosphere from terrestrial ecosystems, directly influence the local and regional carbon budgets. Dahurian larch (Larix gmelinii Rupr.) is a dominating species of boreal forest in china, and its distribution range is sensitive for the climate change. In this study, we developed a latitudinal transect, comprising of 4 sites across the distribution range of boreal Dahurian larch in northeastern China in the fall of 2004. The sites were matched as well as possible to have the same stand characteristics and site conditions. Four 10×10 larch ecosystems (trees+ understory + ground cover + soil A and B horizons) at each site were transplanted to the southern boundary of its distribution range (127°30′E, 45°20′N). The experiment was a completely randomized design including 4 treatments (sites) by 4 replicate plots. We used root exclusion method and an infrared gas exchange analyzer to measure R_S and heterotrophic respiration (R_H) for the transplanted and background larch forest ecosystems during the period of soil freezing-thawing(between March and May 2006) and the growing season.During the period of soil freezing-thawing, the temperature dependency of soil respiration was related to soil thawing stages. The diurnal patterns of R_S and R_H did not differ significantly among the four treatments, but did at various thawing periods. The diurnal patterns of R_S and R_H were uncoupled with those of temperature before the soil was completely thawed, but strongly depended on soil temperature afterwards. Pooling all data across the whole soil thawing process, we found that the R_S and R_H were significantly affected by soil temperature and water content across the four treatments (R²=0.569-0.743, P＜0.001). The soil surface CO₂ flux was predominantly composed of R_H up till the end of April when the rhizospheric respiration (R_R) occurred in this area. The treatment, thawing period and their interaction significantly affected both R_S and R_H. The mean values of R_S and R_H and their differences increased with the soil thawing proceeding. During the soil thawing process, the R_S and R_H varied from 0.50-3.30μmolsCO₂m^-2s^-1and 0.52-3.04μmolsCO₂m^-2s^-1, respectively. Under the same climate conditions, the R_S and its sensitivity to soil temperature and water content tended to increase with the latitude across the four treatments, wherein the R_R component showed a greater sensitivity.During the growing season, we found that the soil temperature and water content interpret the R_S and R_H from average 91% for the different treatments in background plots to average 64% for the different treatments in transplanted plots, at the same time, the R_S were uncoupled with soil water content in some treatments of transplanted plots compared with the background plots. The sensitive coefficient of R_S and R_H to soil temperature in 10cm depth varied from 2.03±1.06 to 5.17±1.03 in all treatments. In transplanted plots, the R_S and R_H varied from 1.34- 5.32μmolsCO₂m^-2s^-1 and 1.39-5.13μmolsCO₂m^-2s^-1, respectively; the seasonal change of R_S and R_H showed a single apex curve pattern according to the soil temperature. The varied range of the annual and growing season fluxes of R_S is 576±19-908±27gCm^-2a^-1, the proportion of the growing season fluxes of R_S to the annual fluxes varied from 86.9-90.7% in transplanted plots.Our results imply a stronger response of soil respiration during the soil thawing process and growing season for more northern Larix gmelinii forests under climate warming scenarios.