| Carbon and water exchanges between the terrestrial ecosystem and atmosphere are the key processes of mass cycle and energy transportation. Long-term measurements of CO2, H2O and heat fluxes between the vegetation and atmosphere have facilitated the research on carbon and water cycle in terrestrial ecosystems and its response to global climatic changes. Global climate change projections suggested an increasing frequency of droughts and extreme rain events in the steppes of the Eurasian region. Using a four year data set of eddy covariance flux measurements of CO2 and H2O fluxes, above-ground biomass and meteorological driving forces, net ecosystem CO2 exchange (NEE) and evapotranspiration (ET) at different time scales were investigated at Songnen meadow steppe in northeastern China. We discussed the biological and environmental controls on the carbon sequestration and water balance, analyzed the seasonal and annual energy budgets and partitioning, as well as the water use efficiency (WUE) at leaf and ecosystem scales. Finally, a primary evaluation for the carbon sink/source strength and pattern of the Songnen meadow steppe has been made. The main results can be summarized as follows:(1) The carbon budget of Songnen meadow steppe in 2007, 2008, 2009 and 2010 (Jan. ~ Sep.) were -64.2, -160.5, -116.8 and -139.9 gC m-2, respectively. Even in an extreme drought year, the meadow steppe was still a net carbon sink, of which the strength is much greater than that of the typical steppes in Inner Mongolia.(2) In the growing season, frequent droughts strongly inhibited photosynthetic productivity of Songnen meadow steppe. Soil moisture from rainfall and under-ground water inputs determined the conversion of the ecosystem from carbon source to carbon sink.(3) Spring drought, not only led to a decline in productivity, but also changed the belowground/aboveground C allocation. Sufficient water contributed to both the accumulation of the underground biomass and the biomass transportation from the above- to under-ground, thereby enhancing the stability of the ecosystem(4) Rainfall distribution may determine not only the carbon sink strength, but also the direction of net carbon flow and energy partitioning in the meadow steppe. Low carbon uptake in the dry year confirmed that drought stress relied on drought intensity and duration, wherein more severe drought stresses more seriously affected the meadow steppe. Summer drought could further aggravate the severity of the spring drought, however, the negative consequence of the summer drought in reference to carbon storage in the meadow steppe was less than that of spring drought.(5) Precipitation and evapotranspiration was approximately balanced in wet years. However, irrespective of spring and summer drought year, the meadow steppe displayed a similar behavior, with accumulated rainfall being less than ET, which was different as compared to the water balance of typical steppes in Inner Mongolia and Central Mongolia. The difference between accumulated ET and the precipitation amount indicated that run-on is the dominant recharge of water. The water redistribution of rainfall and groundwater controlled the seasonal dynamics of energy fluxes.(6) Leaf-level water use efficiency (WUEplant) of the meadow steppe was higher than that of the typical steppes. Spring drought undoubtedly contributed to a decrease in plant transpiration (Tr), and total ecosystem evapotranspiration (ET). However, the suppression of ET was relatively small in comparison to gross ecosystem productivity (GEP) reduction. Therefore, spring drought reduced the ecosystem-level water use efficiency (WUEeco) primarily by suppressing carbon sequestration, which is different from the findings of typical steppe.(7) The data obtained by separating ET using Shuttleworth-Wallace model confirmed that spring drought caused a larger proportion of soil evaporation (Es) in the total ET, and a long-lasting low Tr during the growing season. Increasing soil dryness progressively decreases soil water potential (SWP). The short-term spring drought has long-term effects on water-absorbing capacity of soils within and after the growing season, due to the increasing the tendency of mineralization and accelerating topsoil salinization on the Songnen Plain.(8) WUEeco decrease under water stress correlated with an associated reduction in canopy conductance (Gc). A decrease of Gc would imply a loss of root and stem hydraulic conductivity to limit transpiration, which was considered as the mechanism of self-protection and self-regulation of plants to extreme environments. |
PDF Full Text Request