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Drying and Wetting Effects on Carbon Dioxide Release from Organic Horizons

^a Woods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543
^b Dep. of Environmental Studies, Univ. of California, Santa Cruz, CA 95064
^c Dep. of Earth System Science, Univ. of California, Irvine, CA 92697
^d BITÖK, Bayreuther Institut für terrestrische Ökosystemforschung, Bayreuth, Germany

View larger version (8K): [in a new window]   Fig. 1. General description of the direct current half-bridge sensor. The data logger and multiplexer are used to monitor the change in resistance of a piece of basswood placed in the O horizon as a function of water content.

View larger version (28K): [in a new window]   Fig. 2. Relationship between gravimetric water content (n = 5) and volt signal from direct current half-bridge sensors installed in the (2a; n = 12) Oi horizon and (2b; n = 24) Oe/Oa horizon. Error bars represent the standard error of the mean.

View larger version (25K): [in a new window]   Fig. 3. (a) Mean soil respiration (squares) and mean soil temperature at the 10-cm depth (triangles) (b) predicted gravimetric water contents in the Oi and Oe/Oa horizons, (c) volumetric soil water content at the 5-cm mineral soil depth, and (d) daily precipitation of a mixed deciduous forest at Harvard Forest from June to October 2001. Error bars represent the standard error of mean soil respiration rate (n = 3).

View larger version (17K): [in a new window]   Fig. 4. Relationship between soil respiration and soil temperature at the 10-cm depth using the Arrhenius equation (n = 21). Symbols A, E, R, T of the Arrhenius equation stand for Arrhenius constant (dimensionless), apparent activation energy (kJ mol-1), gas constant (kJ mol-1 K-1), and soil temperature (K), respectively.

View larger version (22K): [in a new window]   Fig. 5. Residuals of the Arrhenius equation versus (a) gravimetric water contents predicted by direct current half-bridge sensors in the Oi and Oe/Oa horizons and (b) volumetric water content of the mineral soil at the 5-cm depth measured with time domain reflectometry sensors. Positive residuals indicate an underestimation of soil respiration by the temperature function when water contents are high and negative residuals indicate an overestimation of soil respiration by the temperature function when water contents are low. Data were obtained from field measurements at the Harvard Forest.

View larger version (28K): [in a new window]   Fig. 6. Carbon dioxide release of O horizon, and water contents of leaf litter layer, the 2- and 5-cm depth during drying and wetting cycles. The O horizon material was initially air-dry. Stepwise additions of 0.5, 1, 2, 4, 8, and 0.5 mm of water were applied sequentially over 79 d in laboratory microcosm experiments. Error bars represent the standard error of the mean of three replicate microcosms (n = 3).

View larger version (28K): [in a new window]   Fig. 7. Carbon dioxide release from the O horizon and water contents in the leaf litter layer, and at the 2- and 5-cm depths in a second laboratory microcosm experiment. O horizon material was initially air-dried. Stepwise additions of 2, 2, 4, 4, and 8 mm of water were applied sequentially over 5 d. Error bars represent the standard error of the mean of three replicate microcosms (n = 3).

View larger version (24K): [in a new window]   Fig. 8. Relationship between CO2 release and water content of (a) leaf litter layer, (b) the 2- and (c) 5-cm depth of O horizon from Harvard Forest in laboratory microcosm drying and wetting experiments. This figure includes all data from Fig. 6 and 7.