GENERAL CIRCULATION MODELS have projected a human-induced global warming varying from 0.9 to 3.5°C by 2100 (Intergovernmental Panel on Climate Change, 1996). Changes in the global water cycle are expected as a result of this temperature increase, but the direction of change is unclear. On a global scale, Rind et al. (1990) pointed out that the likelihood of drought conditions will increase dramatically with increasing temperature. A study of Eurasian hydrology during the past 2500 to 3000 yr showed that the correlation of precipitation with temperature is negative in arid regions and positive in most other areas (Selivanov, 1994). MacCracken et al. (1991) predicted increases of summertime evaporation, which may cause a decrease in soil moisture, and of precipitation in winter and spring, which may cause an increase of soil moisture in spring, for the middle latitudes. On the other hand, elevated atmospheric CO2 increases the water use efficiency of vegetation because of decreasing transpiration rates (Bazzaz et al., 1990), which may compensate reduced water availability during droughts.
The amount of C stored in the soils of temperate forest ecosystems is estimated between 104 and 155 Pg (Houghton, 1995; Post et al., 1982; Taylor and Lloyd, 1992), with a mean residence time between 23 yr (Taylor and Lloyd, 1992) and 29 yr (Raich and Schlesinger, 1992). Soil respiration is very sensitive to changes in temperature and moisture. In their review of soil respiration rates from terrestrial ecosystems, Raich and Schlesinger (1992) showed that temperature was the single best predictor of annual soil respiration rates and that an inclusion of precipitation as an additional parameter considerably increased the model prediction. Laboratory studies of the effect of moisture on respiration rates of litter and soil organic matter generally show a wide moisture range with little effect on decomposition (Ino and Monsi, 1969; Linn and Doran, 1984; Skopp et al., 1990). Soil moisture conditions below and above this optimum range led to a reduction in soil respiration through water or O2 stress. Laboratory investigations simulating soil droughts resulted in decreased CO2 emission rates, while subsequent rewetting generally caused a CO2 flush (Birch, 1959; Seneviratne and Wild, 1985; Moore, 1986; Cabrera, 1993; Degens and Sparling, 1995).
Leaching of DOC represents another sensitive release of C from forest soils. Although the amount of C leaching as DOC is generally very small compared with that released by soil respiration, DOC production and transport may affect many biological and chemical processes in soils, such as activity of soil microorganisms and nutrient availability (Qualls and Haines, 1992). Concentrations of DOC in soil solutions may increase with increasing temperature because of promotion of microbial activity in the forest floor (Liechty et al., 1995).
Little is known about the effect of soil drought and rewetting on CO2 emission and DOC leaching under field conditions in temperate forest soils. The goal of our study was to investigate the effects of extended summer droughts and subsequent rewettings on soil CO2 emissions and DOC leaching. The study was conducted in situ in a mature Norway spruce plantation. At this site a roof below the canopy has been constructed to allow for manipulations of the amount of throughfall reaching the soil surface.