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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 17, NO. 2, 1046, doi:10.1029/2001GB001821, 2003

Elevated atmospheric CO₂ effects and soil water feedbacks on soil respiration components in a Colorado grassland

E. Pendall

Institute for Arctic and Alpine Research, Boulder, Colorado, USA

S. Del Grosso

Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado, USA

J. Y. King

USDA-ARS, Soil-Plant-Nutrition Research, Fort Collins, Colorado, USA

D. R. LeCain

USDA-ARS, Crops Research Laboratory, Fort Collins, Colorado, USA

D. G. Milchunas

USDA-ARS, Crops Research Laboratory, Fort Collins, Colorado, USA

J. A. Morgan

USDA-ARS, Crops Research Laboratory, Fort Collins, Colorado, USA

A. R. Mosier

USDA-ARS, Soil-Plant-Nutrition Research, Fort Collins, Colorado, USA

D. S. Ojima

Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado, USA

W. A. Parton

Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado, USA

P. P. Tans

NOAA-CMDL, Boulder, Colorado, USA

J. W. C. White

Institute for Arctic and Alpine Research, Boulder, Colorado, USA

Abstract

The shortgrass steppe is a semi-arid grassland, where elevated CO₂ reduces stomatal conductance and promotes soil moisture storage. Enhanced biomass growth from elevated CO₂ has been attributed in part to soil moisture effects. However, the influence of this soil moisture feedback on C cycling has received little attention. We used open-top chambers to increase atmospheric CO₂ concentrations to twice-ambient for four growing seasons. Soil respiration rates and stable C isotopes of soil CO₂ were measured during the third and fourth seasons. Elevated CO₂ increased soil respiration rates by ∼25% in a moist growing season and by ∼85% in a dry season. Stable C isotope partitioning of soil respiration into its components of decomposition and rhizosphere respiration was facilitated on all treatments by a ¹³C disequilibrium between currently growing plants and soil organic matter. Decomposition rates were more than doubled by elevated CO₂, whereas rhizosphere respiration rates were not changed. In general, decomposition rates were most significantly correlated with soil temperature, and rhizosphere respiration rates were best predicted by soil moisture content. Model simulations suggested that soil moisture feedbacks, rather than differences in substrate availability, were primarily responsible for higher total respiration rates under elevated CO₂. By contrast, modeling demonstrated that substrate availability was at least as important as soil moisture in driving CO₂ treatment differences in soil organic matter decomposition rates.

Received 28 October 2001; accepted 20 November 2002; published 15 May 2003.

Index Terms: 1610 Global Change: Atmosphere (0315, 0325); 1615 Global Change: Biogeochemical processes (4805); 1851 Hydrology: Plant ecology; 1866 Hydrology: Soil moisture.

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Citation: Pendall, E., et al. (2003), Elevated atmospheric CO₂ effects and soil water feedbacks on soil respiration components in a Colorado grassland, Global Biogeochem. Cycles, 17(2), 1046, doi:10.1029/2001GB001821.

Copyright 2003 by the American Geophysical Union.