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 
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 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 is estimated to be 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, 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.
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 CO
2 emission, and
is the soil matric potential (kPa). The term (1 +
a) may be described as a moisture regulator and is only valid for soils under unsaturated conditions, in which soil respiration is not limited by O
2 stress. The parameters
A,
E, and
a were calculated using daily averages of CO
2 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.
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