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DIVISION S-7—FOREST & RANGE SOILS

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

^* Corresponding author (werner.borken{at}bitoek.uni-bayreuth.de).

Drying and wetting cycles of O horizon in forest soils have not received much attention, partly due to methodological limitations for nondestructive monitoring of the O horizon water content. The objective of this study was to determine the importance of moisture limitations in the O horizon of a temperate forest on summertime soil respiration. We measured soil respiration in three replicated plots in a mixed deciduous forest at Harvard Forest, Massachusetts, weekly from May to October 2001. Direct Current (DC) half-bridge sensors that had been calibrated using destructive samples of the Oi and Oe/Oa horizons were placed in the Oi and Oe/Oa horizons to record hourly changes of gravimetric water contents. Soil temperature explained 47% of the variation in soil respiration using the Arrhenius equation. The residuals of the temperature model were linearly correlated with gravimetric water content of the Oi horizon (r² = 0.72, P < 0.0001) and Oe/Oa horizon (r² = 0.56, P < 0.001), indicating that temporal variation in soil respiration can be partly explained by water content of the O horizon. Additionally, a laboratory study was performed to evaluate drying/wetting cycles of the O horizon at constant temperature. Even small simulated rainfall amounts of 0.5 mm significantly increase CO₂ flux from dry O horizon within a few minutes. The duration of CO₂ pulses increased with the amount of applied water, lasting from a few hours to days. A strong correlation between CO₂ release and water content of the O horizons demonstrates a clear regulatory role of litter water content on decomposition within the O horizons.

Abbreviations: DC, direct current • IRGA, infrared gas analyzer • SOC, soil organic carbon • TDR, Time domain reflectometry