TY - JOUR
T1 - δ13C values of soil organic matter in semiarid grassland with mesquite (Prosopis) encroachment in southeastem Arizona
AU - Biggs, Thomas H.
AU - Quade, Jay
AU - Webb, Robert H.
N1 - Funding Information:
This study was funded by the Southwestern Borderlands Ecosystems Management Program of the USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, with partial funding supplied by the U.S. Geological Survey. Jerry Gottfried and Carl Edminster of the Forest Service were very supportive of the research. Chris Cochrane of the Natural Resources Conservation Service, Tucson, provided surficial soils information and access to some of the early aerial photography and Sheridan Stone of the Wildlife Office, Fort Huachuca, facilitated with access to the military reservation and arranged for personnel and equipment. Dave Dettman of the Department of Geosciences, University of Arizona, assisted with the mass spectrometer analyses. Tina Hayden and Laska Rohovit of the Department of Soil, Water and Environmental Science, University of Arizona, and Peter Griffiths of the U.S. Geological Survey assisted with soil analyses.
PY - 2002/11
Y1 - 2002/11
N2 - Over the past century, C3 woody plants and trees have increased in abundance in many semiarid ecosystems, displacing native C4 grasses. Livestock grazing, climatic fluctuations, and fire suppression are several reasons proposed for this shift. Soil carbon isotopic signatures are an ideal technique to evaluate carbon turnover rates in such ecosystems. On the gunnery ranges of Fort Huachuca in southeastern Arizona, study sites were established on homogeneous granitic alluvium to investigate the effects of fire frequency on δ13C values in surface soil organic matter (SOM). These ranges have had no livestock grazing for 50 years and a well-documented history of fires. Prosopis velutina Woot. (mesquite) trees have altered SOM δ13C pools by the concentration of plant nutrients and the addition of isotopically light litter. These soil carbon changes do not extend beyond canopy margins. Elevated total organic carbon (TOC), plant nutrient (N and P) concentrations, and depleted SOM δ13C values are associated with C3 Prosopis on an unburned plot, which enables recognition of former Prosopis-occupied sites on plots with recent fire histories. Elevated nutrient concentrations associated with former Prosopis are retained in SOM for many decades. Surface SOM δ13C values indicate the estimated minimum turnover time of C4-derived carbon beneath large mature Prosopis is about 100-300 years. In contrast, complete turnover of original C3 carbon to C4 carbon under grasslands is estimated to take a minimum of 150-500 years. Our study confirms that C4 grass cover has declined over the past 100 years, although isolated C3 trees or shrubs were not uncommon on the historic C4-dominated grasslands. We find evidence in surface soil layers for a modern C3 plant expansion reflected in the substantial shift of SOM δ13C values from C4 grasses to C3 shrublands.
AB - Over the past century, C3 woody plants and trees have increased in abundance in many semiarid ecosystems, displacing native C4 grasses. Livestock grazing, climatic fluctuations, and fire suppression are several reasons proposed for this shift. Soil carbon isotopic signatures are an ideal technique to evaluate carbon turnover rates in such ecosystems. On the gunnery ranges of Fort Huachuca in southeastern Arizona, study sites were established on homogeneous granitic alluvium to investigate the effects of fire frequency on δ13C values in surface soil organic matter (SOM). These ranges have had no livestock grazing for 50 years and a well-documented history of fires. Prosopis velutina Woot. (mesquite) trees have altered SOM δ13C pools by the concentration of plant nutrients and the addition of isotopically light litter. These soil carbon changes do not extend beyond canopy margins. Elevated total organic carbon (TOC), plant nutrient (N and P) concentrations, and depleted SOM δ13C values are associated with C3 Prosopis on an unburned plot, which enables recognition of former Prosopis-occupied sites on plots with recent fire histories. Elevated nutrient concentrations associated with former Prosopis are retained in SOM for many decades. Surface SOM δ13C values indicate the estimated minimum turnover time of C4-derived carbon beneath large mature Prosopis is about 100-300 years. In contrast, complete turnover of original C3 carbon to C4 carbon under grasslands is estimated to take a minimum of 150-500 years. Our study confirms that C4 grass cover has declined over the past 100 years, although isolated C3 trees or shrubs were not uncommon on the historic C4-dominated grasslands. We find evidence in surface soil layers for a modern C3 plant expansion reflected in the substantial shift of SOM δ13C values from C4 grasses to C3 shrublands.
KW - Desert grasslands
KW - Mesquite
KW - Soil organic matter
KW - Spatial distributions
KW - δC
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U2 - 10.1016/S0016-7061(02)00227-6
DO - 10.1016/S0016-7061(02)00227-6
M3 - Article
AN - SCOPUS:0036836895
VL - 110
SP - 109
EP - 130
JO - Geoderma
JF - Geoderma
SN - 0016-7061
IS - 1-2
ER -