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Soil Science Society of America Journal 63:1848-1855 (1999)
© 1999 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

A Climate Change Scenario for Carbon Dioxide and Dissolved Organic Carbon Fluxes from a Temperate Forest Soil

Drought and Rewetting Effects

W. Borkena, Y.-J. Xub, R. Brummea and N. Lamersdorfa

a Inst. of Soil Science and Forest Nutrition, Univ. of Goettingen, Buesgenweg 2, 37077 Goettingen, Germany
b Dep. of Forestry, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061 USA

wborken{at}gwdg.de


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Our objective was to assess the effect of changes in rainfall amount and distribution on CO2 emissions and dissolved organic C (DOC) leaching. We manipulated soil moisture, using a roof constructed below the canopy of a 65-yr-old Norway spruce plantation [Picea abies (L.) Karst.] at Solling, Germany. We simulated two scenarios: a prolonged summer drought of 172 d followed by a rewetting period of 19 d and a shorter summer drought of 108 d followed by a rewetting period of 33 d. Soil CO2 emission, DOC, soil matric potential, and soil temperature were monitored in situ for 2 yr. On an annual basis no significant influence of the droughts on DOC leaching rates below the rhizosphere was observed. Although not significantly, the droughts tended to reduce soil respiration. Rewetting increased CO2 emissions in the first 30 d by 48% (P < 0.08) in 1993 and 144% (P < 0.01) in 1994. The CO2 flush during rewetting was highest at high soil temperatures and strongly affected the annual soil respiration rate. The annual emission rate from the drought plot was not affected by the drought and rewetting treatments in 1993 (2981 kg C ha-1 yr-1), but increased by 51% (P < 0.05) to 4813 kg C ha-1 yr-1 in 1994. Our results suggest that reduction of rainfall or changes in rainfall distribution due to climate change will affect soil CO2 emissions and possibly C storage in temperate forest ecosystems.

Abbreviations: DOC, dissolved organic carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
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.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Site
The study was conducted in a 65-yr-old Norway spruce plantation in the German Solling research area (51°31'N, 9°34'E, 510-m elevation). The area has an annual mean air temperature of 6.4°C, varying between -2 and 16°C monthly during a year, and an annual precipitation of {approx}1090 mm, evenly distributed throughout the year. Compared with the long-term average, the experimental years had slightly higher precipitation and air temperatures (1241 mm and 6.5°C in 1993, 1291 mm and 7.6°C in 1994). Meteorological data were collected at a height of 33 m (2–3 m above the canopy) from a tower constructed within a neighboring 118-yr-old Norway spruce plantation. In 1991 the forest in the study site was 20 m tall, with a density of 900 trees ha-1 and a basal area of {approx}50 m2 ha-1 (Dohrenbusch et al., 1993).

The soil is developed in 60- to 80-cm-thick solifluction deposits overlying weathered Triassic sandstone. The soil is classified as a Typic Dystrochrept according to U.S. soil taxonomy (Soil Survey Staff, 1994), with a pH (0.01 M CaCl2) gradient from 3.2 (0–10 cm) to 4.2 (20–40 cm) and a base saturation of <7% down to the 100-cm depth. Carbon storage at the site is estimated to be {approx}46 Mg ha-1 in the 6- to 9-cm-thick O horizon and 79 Mg ha-1 in the mineral soil down to 80 cm. The O horizon has a maximum water-holding capacity of 473% (g H2O g-1 dry matter), which may yield a maximum water storage of 48 L m-2.

Drought and Rewetting Experiment under Field Conditions
In the summer of 1991, two 300-m2 roofs made of transparent polycarbonate were constructed below the forest canopy, {approx}3.5 m above the forest floor. The roofs were part of the European EXMAN project conducted in several European countries (Bredemeier et al., 1998). One roof was used for the drought and rewetting experiment (drought plot), while the other roof served as an untreated control plot. During the drought experiments, only the soil was dried out but the canopy received precipitation. An ambient plot without a roof was selected in an adjacent spruce stand of the same age and on similar soil to quantify possible roof effects. A 1-m-wide trench was dug and sheathed with plastic foil to separate the soil of the roofed areas from the neighboring soil. A zone of 2 m within the plastic foil was demarcated in which no measurements were established. The high cost for the roof construction precluded replication of the roof treatment and therefore pseudoreplications were used to evaluate experimental treatments.

Below the drought roof, dry periods were simulated between 1 April and 19 Sept. 1993 and between 1 April and 17 July 1994 (Table 1) . Throughfall intercepted by the roofs was piped into several water tanks and stored for 172 d in 1993 and for 108 d in 1994. The soil was rewetted with the collected throughfall, using a sprinkler system installed underneath the roof. Water was applied with an intensity of 1 to 2 mm h-1 for 19 d in September 1993 and 33 d in 1994. The total water input on the drought plot was reduced by 475 mm in 1993 and 152 mm in 1994 compared with the throughfall in the ambient plot. After the rewetting periods, both control and treated plots received the same amount of throughfall. Litter collected by the roofs was redistributed onto the forest floor. The amounts of litter in the ambient plot were 1.89 Mg C ha-1 in 1993 and 1.92 Mg C ha-1 in 1994 and were not different from those in the drought and control roof plot.


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  		Table 1 Throughfall water for different time periods at the ambient, control roof, and drought roof plot in a temperate Norway spruce plantation at Solling, Germany

 
Carbon dioxide emissions were measured on the drought (n = 4) and the ambient plots (n = 3) from September to November 1993 and from April to November 1994 by an automated chamber system (Brumme and Beese, 1995). For other periods between 1993 and 1994, manual gas samples were taken weekly and analyzed in the laboratory (Loftfield et al., 1997). In the control plot (n = 4), soil respiration was measured only weekly during 1993. All chambers covered an area of 0.25 m2 and were inserted into the O horizon down to the 5-cm depth.

The automated chambers were closed four to five times per day, and gas samples were taken at 0, 30, and 60 min after closure. Carbon dioxide was analyzed on a gas chromatograph (GC 6000, Vegas Series 2, Carlo Erba Instruments, Milan, Italy) equipped with an electron capture detector. The system was interfaced to a personal computer with the software BONANOX (Messwert GmbH, Goettingen, Germany), which controlled the sampling and analysis of gases, monitored the gas chromatograph detector signal, air pressure (temperature compensated silicon piezo-resistive sensor 142-SC-15A, Sensym, Rugby, UK) and air temperature (PTC, Siemens, Munich, Germany) within the chambers. Air temperature and air pressure were measured at 10-min intervals and recorded as hourly averages. Four certified CO2 standards (350, 750, 1200, and 1800 µL L-1 CO2 in N2; Messer Griesheim, Krefeld, Germany) were used for calibration every 2 h. Repeated measuring of certified CO2 standards resulted in an accuracy of 0.5% for our system.

Manual gas sampling was done with evacuated glass bottles (0.1 L) and a sampling device that checks the vacuum in the glass bottles and takes the gas sample. Before sampling, the hose connecting the chamber with the glass bottle was flushed with gas sample from the chamber. Gas samples were analyzed in the laboratory using an automated gas chromatograph system similar to the automated field system (Loftfield et al., 1997).

Soil temperature (n = 3) at the 0-cm mineral soil depth and soil matric potential at the 10-cm soil depth (n = 3) were automatically (IMKO GmbH, Munich, Germany) recorded every 15 min at both the drought and ambient plots throughout the experiment. Soil temperature was measured using standard Pt100 sensors (Siemens). Soil matric potential was measured with tensiometers consisting of ceramic cups (5-cm length) and a temperature-compensated silicon piezo-resistive pressure transducer (Schmidt, 1993). During drought periods, soil matric potential exceeded the measurable range of the monitoring system. The missing values were substituted with the potentials estimated using a soil water balance model (SOW; Xu et al., 1998), which is a deterministic model that calculates actual evapotranspiration and soil water fluxes using Penman-Montheith and Richards' equation with an empirical reduction function for root water uptake. The validity of the model was tested by comparing predicted matric potential values with measured values at varied soil depths. Soil solution was sampled using suction lysimeters (ceramic P-80 cups) installed with five replicates in the mineral soil at the 10- and 100-cm depths. Samples were collected monthly but more frequently (daily to weekly) during the rewetting events. Dissolved organic carbon was determined using a total organic C analyzer (Shimadzu-5050, Shimadzu Scientific, Columbia, MD). Annual DOC fluxes were calculated by multiplying seasonal concentrations with corresponding water fluxes.

Model Development
Previous investigations have shown that temperature and water potential interact in a nonlinear way with soil respiration rate (Moore, 1986). In temperate forest soils, the osmotic potential is negligible compared with the matric potential. We therefore used soil matric potential as an indicator for water availability to microorganisms and roots. The temperature dependence of soil respiration has been described using an Arrhenius equation (e.g., Lloyd and Taylor, 1994). The magnitude of the influence of temperature and matric potential on CO2 emission depends on which is the limiting factor. We modified the Arrhenius equation as follows:

(1)
where A is an Arrhenius constant, E is the apparent activation energy, R is the universal gas constant, and T is the soil temperature (K). The a is an empirical fitting parameter that describes the influence of soil matric potential (kPa) on CO2 emission, and {psi} is the soil matric potential (kPa). The term (1 + a{psi}) may be described as a moisture regulator and is only valid for soils under unsaturated conditions, in which soil respiration is not limited by O2 stress. The parameters A, E, and a were calculated using daily averages of CO2 emission, soil temperature, and soil matric potential. Q10 values were determined using the calculated apparent activation energy E:

(2)

Statistics
Data were analyzed using SAS software (SAS Institute, 1996). Nonlinear regression analyses were performed to fit mean daily CO2 emission rates of the ambient and the drought plots to daily averages of soil temperature and soil matric potential. The effects of drought and rewetting on CO2 emission rates and DOC concentrations at the 10- and 100-cm soil depths were analyzed by performing t tests using means of pseudoreplications from the ambient and the drought plots. The roof effect on CO2 emission was analyzed by comparing emission rates of the control and drought plots with the same statistical procedure. Standard deviations are given for spatial variation in CO2 emission rates and DOC concentrations.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Carbon Dioxide Emissions in Ambient and Control Plots
Carbon dioxide emission (weekly average) (Fig. 1a) showed a clear seasonal pattern: it increased at the beginning of spring and decreased in fall, following the pattern in soil temperature (Fig. 1b). In 1993, the highest emissions (1.55 ± 0.23 g C m-2 d-1) occurred from August to early October. During this year no severe drought occurred except for a short dry period in July with the lowest matric potential of -61 kPa (Fig. 1c). This natural drought was accompanied by a slight drop of 16% in CO2 emission rates compared with June, although soil temperature increased by 1.0°C. In 1994, the highest emissions were observed from July to early September (1.53 ± 0.22 g C m-2 d-1). Low throughfall occurred from July to August 1994 (Fig. 1d), resulting in a soil drought for several weeks with low matric potential below -120 kPa (Fig. 1c). Concurrently, CO2 emissions (1.35 ± 0.21 g C m-2 d-1) decreased from the end of July to August when soil temperature reached its maximum of 15.7°C. More than 75% of the annual total emission occurred during April to October.



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  		Fig. 1 Seasonal changes of (a) measured and modeled CO2 emission, (b) soil temperature at 0-cm depth, (c) soil matric potential at 10-cm depth, and (d) throughfall at the ambient plot

 
Cumulative CO2 emission from the ambient plot was 3015 kg C ha-1 in 1993 and 3192 kg C ha-1 in 1994 (Table 2) . This difference may be explained by the difference in mean soil temperatures of 6.0°C in 1993 and 6.8°C in 1994. Cumulative CO2 emission from the control plot (3205 kg C ha-1) was not significantly different from the ambient plot in 1993. Although throughfall was reduced by 14% in 1993 in the control plot (Table 1), we observed no roof effect on soil respiration.


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  		Table 2 Carbon dioxide fluxes for different time intervals at the ambient, control, and drought plots.{dagger}

 
Drought Effects on Carbon Dioxide Emission
Compared with the annual amount of throughfall at the ambient plot, the amount of throughfall at the drought plot was reduced by 45% in 1993 and 14% in 1994 (Table 1). After the drought experiment started in April 1993, the soil matric potential at the 10-cm depth sharply decreased from -5 to -126 kPa and maintained at this level throughout the 172-d drought period (Fig. 2c) . Compared with the treatment period in 1993, the drought in 1994 was less prolonged and matric potential remained at a level below -100 kPa for only 4 wk. The summer drought experiments only slightly affected soil CO2 emissions. Emission rates slowly decreased when the droughts started (Fig. 2a and 2c). During the 172-d drought period in 1993, the soil emitted 1639 kg C ha-1 (Table 2). This rate was 23.3% (P < 0.05) lower than that of the control plot and 12.4% lower (P > 0.09) than that of the ambient plot. During the less intense 108-d drought period in 1994, the emission rate was not reduced (-1.2%). The stronger reduction in 1993 was due to the long and severe drought induced at the drought plot compared with the ambient plot, which received a throughfall input of 483 L m-2 (Table 1).



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  		Fig. 2 Seasonal changes of (a) measured and modeled CO2 emission, (b) soil temperature at 0-cm depth, (c) soil matric potential at 10-cm depth, and (d) throughfall at the drought plot

 
Rewetting Effects on Carbon Dioxide Emission
Rewetting resulted in an immediate increase of soil emission in both treatment years. The emission rates continued to increase for 3 wk and reached their maximum in October 1993 (2.24 g C m-2 d-1) and August 1994 (4.94 g C m-2 d-1). During rewetting, mean soil temperature was 9.2°C in 1993 and 13.8°C in 1994. The drop in CO2 emission after the peak flush was accompanied by a decrease in soil temperature.

Rewetting lasted for a shorter time in 1993 than in 1994, but the intensity of rewetting was higher in 1993 (193 L m-2 within 19 d) than in 1994 (184 L m-2 within 33 d, Table 1). During the first 30 d after the start of rewetting, CO2 emission was 504 kg C ha-1 in 1993 and 1088 kg C ha-1 in 1994 (Table 2). This corresponded with about one-sixth of the annual C release in 1993 and one-fifth in 1994. Compared with the C release at the ambient plot during these 30 d, emission rates at the drought plot were 48% (P < 0.08) higher in 1993 and 144% (P < 0.01) higher in 1994.

Both duration of the summer drought and soil temperature during the rewetting had an effect on the annual CO2 release. The drought period and subsequent rewetting led to an insignificantly lower annual rate in 1993. By contrast, the drought period and rewetting event in 1994 increased the annual CO2 emission rate significantly (P < 0.05) by 51% compared with the ambient plot.

Quantitative Analyses of Effects of Soil Temperature and Matric Potential on Carbon Dioxide Emission
Carbon dioxide emission rates increased exponentially with increasing temperature from -1 to {approx}16°C during the untreated and rewetting periods (Fig. 3) . It is apparent that soil temperature was the dominant factor for soil CO2 emissions when matric potential was higher than -20 kPa (mostly during the untreated and rewetting period, Fig. 2c). During the drought periods, CO2 emission rates were below 50 mg C m-2 h-1. Apparently a moisture-dependent threshold value existed for soil respiration below which temperature effects become virtually zero.



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  		Fig. 3 Relationship between soil temperature at 0-cm depth and CO2 emission (n = 964) for one representative chamber at the drought plot from 1993 to 1994

 
The calculated CO2 emission rates using Eq. [1] and [2] strongly correlated with those measured at both the ambient (r2 = 0.85) and drought plots (r2 = 0.95) (Fig. 1a and 2a). The parameters in Eq. [1] were fitted as follows: E = 85.0 kJ mol-1, A = 2.57 x 1017, and a = 0.0029 for the control plot; and E = 109 kJ mol-1, A = 1.02 x 1022, and a = 0.0049 for the drought plot. Calculated Q10 values were 3.9 for the ambient plot and 5.7 for the drought plot. The high values for E and a indicate a much stronger influence of soil temperature and matric potential on soil respiration at the drought plot. Disregarding soil matric potential resulted in lower correlations at the ambient (r2 = 0.77) and drought plots (r2 = 0.63). The parameters were fitted as follows: E = 65.8 kJ mol-1 and A = 6.83 x 1013 for the ambient plot, and E = 102 kJ mol-1 and A = 4.05 x 1020 for the drought plot. The calculated Q10 values of 2.87 for the ambient plot and of 5.11 for the drought plot suggested a smaller temperature effect than when matric potential was included.

Dissolved Organic Carbon Fluxes
During the growing season, mean DOC concentrations in the 10- and 100-cm soil depths were higher than in winter at the ambient and drought plots (Fig. 4) . The DOC concentrations in the top soil of the ambient plot (21.5 ± 5.2 mg L-1) and the drought plot (18.5 ± 4.1 mg L-1) were significantly higher (P < 0.05) than below the rhizosphere (6.3 ± 4.1 mg L-1 at ambient plot and 5.9 ± 3.3 mg L-1 at drought plot). In both years and at both depths, DOC concentrations peaked following rewetting both at the drought and ambient plots. The DOC concentrations in the drought plot were not significantly different from the ambient plot.



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  		Fig. 4 Dissolved organic C (DOC) concentrations at 10- and 100-cm soil depths (n = 5) at the ambient and the drought plots from 1993 to 1994

 
The annual rates of DOC input by throughfall at the ambient plot were 106 kg ha-1 in 1993 and 93 kg ha-1 in 1994, while the drought and control plots had considerably lower input rates (Table 3) . The large difference in DOC input to the soil was mainly caused by the lower water input at the drought and control plots. Using the water flow results modeled by Xu et al. (1998), we calculated DOC fluxes between 91 and 176 kg ha-1 at the 10-cm depth (Table 3). This higher DOC flux at 10 cm compared with the input indicated a mobilization of DOC in the upper soil at all plots. The difference between DOC input and output soil flux at 10 cm was highest at the ambient plot and lowest at the control plot. Much lower rates of DOC leaching at the 100-cm soil depth were calculated for all plots (28–46 kg ha-1).


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  		Table 3 Dissolved organic C (DOC) fluxes for throughfall and seepage water at 10- and 100-cm mineral soil depths at the ambient, control, and drought plots.{dagger}

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
We found no significant differences in CO2 emission between the ambient and control plots and therefore used the measurements of the ambient plot to evaluate our drought and rewetting effects. Dissolved organic C fluxes were different between ambient and control plots, but patterns were similar, which is enough to justify comparison of the drought with the ambient plot. Annual soil respiration rates were 3015 kg C ha-1 yr-1 in 1993 and 3192 kg C ha-1 yr-1 in 1994 at the ambient plot. These rates are low compared with the mean flux rate of 6810 ± 950 kg C ha-1 yr-1 in a wide range of temperate coniferous forests summarized by Raich and Schlesinger (1992). The prolonged summer drought and subsequent rewetting of the spruce forest soil at the drought plot did not affect the annual CO2 emission rate in 1993, but caused an increase of 51% in 1994. We explain this discrepancy between the years by the duration of the drought combined with the level of soil temperature during rewetting.

Drought Effects on Carbon Dioxide Emission
Soil respiration was not significantly reduced by drought treatments. However, our study suggests that the cumulative respiration was lower when length of drought increased. Natural droughts at the ambient plot may have reduced the respiration rate of the forest floor, although matric potential at the 10-cm mineral soil depth indicated sufficient water availability. Carlyle and U Ba Than (1988) reported a strong decline in soil respiration of {approx}70% with decreasing soil moisture in a Monterey pine (Pinus radiata D. Don) stand in southeastern Australia, but that drought was much more severe than in our study. In agreement with our results, they also reported no correlation with temperature during the drought (Fig. 3).

Several studies in temperate forests summarized by Singh and Gupta (1977) have shown a reduction of respiration in the litter layer during dry summer months. Particularly, soils with a thick forest floor are sensitive to changes in temperature and water availability because this layer contains a large pool of labile organic matter and a large contribution of living roots. We could not differentiate between microbial activity and root respiration, the two major sources for soil CO2 emission. We assume that root respiration was not decreased in our drought treatments since fine root growth was not severely affected (Murach, 1999, personal communication). However, root respiration may have shifted to deeper soil layers to ensure the water supply of the trees (Feil et al., 1988). Consequently, the reduction in CO2 emission was most likely caused by a decrease in heterotrophic respiration in the dried soil. The relatively small reduction in CO2 emission caused by droughts points at a dominant role of fungi in acid forest soils instead of bacteria as primary decomposers (Alexander, 1980). Soil fungi are active down to a water potential of -15 MPa, whereas bacteria are already inactive below -1.0 to -1.5 MPa (Swift et al., 1979). However, bacteria maintain a basic metabolism at low moisture content and may also contribute to the almost constant CO2 emissions during the drought treatments.

Rewetting Effects on Carbon Dioxide Emission
Rewetting generally caused a strong increase in soil CO2 emission, agreeing well with the laboratory observations of Orchard and Cook (1983), Seneviratne and Wild (1985), and Degens and Sparling (1995). It is unclear whether this strong increase in soil CO2 emission is caused by an increase in root or heterotrophic respiration. Murach (personal communication, 1999) found an increase of root growth in the topsoil at the drought plot with a delay of 4 to 6 wk after rewetting. The initial CO2 flush following rewetting was therefore most likely caused by enhanced activity of the decomposer community. Several factors can contribute to this CO2 pulse after a rewetting. A considerable proportion of soil microorganisms die during drought (van Gestel et al., 1991), and the dead cells can be decomposed quickly during a rewetting. In addition, availability of organic substrates can increase through desorption from the soil matrix (Seneviratne and Wild, 1985) and through increased exposure of organic surfaces to microorganisms (Birch, 1959). Compared with the sharp peak of CO2 after rewetting in the laboratory studies, the peak of CO2 emission of our field study was delayed and reached a maximum after 2 to 3 wk. This difference may be caused by the slower rewetting in our field study. In addition, soils in laboratory studies are disturbed (cutting roots, sieving soils, etc.) which may increase the availability of C.

The higher CO2 release in 1994 than in 1993 is probably associated with the higher temperature level. In 1993, the rewetting was between September and October, a period where soil temperature dropped from 11.1 to 5.7°C (Fig. 2b). In 1994, soil temperature during the first 30 d following rewetting varied between 10.9 and 15.7°C. In this year, both soil temperature and moisture conditions were optimal, so that CO2 release was 144% higher than the ambient plot. Obviously, the length of the drought period is less important for the CO2 release than the moisture and temperature conditions during rewetting.

Q10 Values
Q10 values are used to describe the temperature dependence of soil respiration. Carlyle and U Ba Than (1988) showed that during a natural drought period the Q10 value was low and increased from 0.77 to 2.60 with moisture availability in a soil temperature range of {approx}5 to 20°C. An inclusion of a moisture-dependent Q10 term in their model, FRESP, resulted in a strong agreement between measured and fitted values (r2 = 0.85). We obtained higher Q10 values of 3.9 for the ambient plot and 5.7 for the drought plot using the modified Arrhenius equation including the water term. These Q10 values may represent temperature dependence of soil respiration under optimum moisture condition. Our Q10 values are considerably higher than the median Q10 value of 2.4 found by Raich and Schlesinger (1992) from seasonal changes in soil temperature and soil respiration rates for various soils under field condition. The Q10 values in their literature review may include possible moisture limitations and therefore do not only represent a temperature but also a moisture dependency. In addition, Q10 values from the literature may be underestimated because Kicklighter et al. (1994) obtained lower Q10 values with air temperature (1.99) than with soil temperature (3.08). Generally, Q10 values appear to be higher in cold regimes and lower under warm regimes. Kirschbaum (1995) found a temperature dependence for Q10 values obtained from CO2 emissions of soil and litter of various climate regions. In this study, the fitted Q10 values decreased nonlinear from 8 at 0°C to {approx}4.5 at 10°C and 2.5 at 20°C. In cold regions, microorganisms show a stronger temperature reaction compared with temperate or tropical regions because of a high substrate availability during the few summer months when the soils are thawed. For instance, tundra microorganisms are adapted to low temperatures, but respond to temperature increases like microorganisms elsewhere (Flanagan and Veum, 1974). The results of Kirschbaum (1995) are also in agreement with our model results because under field conditions water availability at low temperatures is normally higher than at high temperatures.

The high Q10 values of 5.7 and 5.1 calculated for the drought plot a direct temperature dependence of soil respiration but might also be the result of increased C availability during rewetting. As a considerable part of the CO2 emission is produced in the forest floor, extended summer droughts and rewetting events at high temperatures may increase soil CO2 emission rates from forest soils, especially those with a thick O horizon.

Dissolved Organic Carbon Fluxes
The DOC input by throughfall and DOC fluxes at the 10- and 100-cm soil depth were reduced mainly by lower water input at the drought plot (Tables 1 and 3). Our drought and rewetting treatment only slightly reduced the mobilization and degradation of DOC in the upper soil as indicated by the mobilization rate calculated from the difference of DOC input by throughfall and DOC fluxes at the 10-cm soil depth. Natural drought and rewetting events at the ambient plot led to a similar pattern in DOC concentration at the 10-cm soil depth (Fig. 3). Although the DOC flux at the 10-cm soil depth at the ambient plot was higher than at the drought plot, the annual DOC outputs at 100 cm were similar. Dissolved organic C leaching from the 10- to 100-cm soil depth increased at the drought plot because of water flux in macropores during rewetting (Lamersdorf et al., 1998). However, the much lower rates of DOC output below the rhizosphere at both plots suggest that soil has the potential to accumulate C in its sublayers. There is some evidence that DOC is absorbed by sesquioxides in the B horizons and that DOC may play a significant role for carbon storage in the mineral soil of spruce forests (Zech et al., 1994). This is in agreement with the results of Qualls and Haines (1992) that showed that the capacity of microorganisms to degrade DOC is limited and temperature increase does not affect microbial degradation of DOC. Our results suggest that droughts and rewettings under changing climate will have only a limited impact on DOC concentrations in groundwater.

Potential Impacts on Carbon Storage
An inventory of tree and root growth at the site showed no clear difference in litterfall, fine root biomass, and tree diameter growth between the ambient plot and the drought plot for both treatment years (Bredemeier et al., 1998). However, a reduced height growth rate of trees was measured during the drought period in 1993 and 1994 (Dohrenbusch et al., 1999). Although litterfall was not reduced, a decrease of litterfall may be expected in the following years because of the long life span of Norway spruce needles. In the long run, a decrease of litter production and an increase of the decomposition rate may reduce the C storage of this forest ecosystem. On the other hand, elevated atmospheric CO2 concentrations have direct fertilization effects on tree and root growth (Bazzaz et al., 1990; Johnson et al., 1996; Norby et al., 1992). All of these processes will influence the soil C balance, which may have a direct feedback to the atmospheric CO2 concentration.

Presently, northern hemisphere temperate forests are considered to be a substantial sink of C (Kauppi et al., 1992; Sedjo, 1992; Birdsey et al., 1993; Nakane and Lee, 1995). Taylor and Lloyd (1992) estimated a net sink effect of 0.33 Pg C yr-1, based on an inventory of temperate forests in 1985. In the long run, prolonged summer droughts may reduce the C storage in temperate, coniferous forest soils as a result of lower net primary production and a larger CO2 release during subsequent rewetting.Carlyle Ba Than 1988


   ACKNOWLEDGMENTS
 
We would like to thank Professor E. Veldkamp for invaluable discussions and helpful comments. We also thank the Institute of Bioclimatology of the University of Goettingen for meterological data we used for hydrological modeling of water fluxes. The study was a part of the Environmental Program Project CT91-0052 financially supported by the Commission of European Communities.

Received for publication July 29, 1998.


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