Deep Autotrophic Soil Respiration in Shrubland and Woodland Ecosystems in Central New Mexico

Ecosystems, Jan 2012

Quantifying the controls on soil respiration is important for understanding ecosystem physiology and for predicting the response of soil carbon reservoirs to climate change. The majority of soil respiration is typically considered to occur in the top 20–30 cm of soils. In desert soils, where organic matter concentrations tend to be low and plants are deeply rooted, deeper respiration might be expected. However, little is known about the depth distribution of respiration in dryland soils. Here we show that the average depth of soil respiration between pulse precipitation events is almost always greater than 20 cm and is frequently greater than 50 cm in two central New Mexico desert shrublands. The average depth of soil respiration in a piñon-juniper woodland was shallower, between 5 and 40 cm. In the shrublands, 8‰ seasonal variations in the carbon isotope composition of soil-respired CO2 (δ13Cr-soil) that correlate with vapor pressure deficit support root/rhizosphere respiration as the dominant source of soil CO2. Such deep autotrophic respiration indicates that shrubs preferentially allocate photosynthate to deep roots when conditions near the surface are unfavorable. Therefore, respiration rates in these soils are not necessarily correlated with root biomass. The δ13Cr-soil values provide no evidence for CO2 evolved from soil inorganic carbon. Our results also suggest that organic carbon cycling is rapid and efficient in these soils and that the δ13C value of CO2 respired from soils in much of the southwestern US, and perhaps in other semiarid regions, varies seasonally by at least 4‰.

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Deep Autotrophic Soil Respiration in Shrubland and Woodland Ecosystems in Central New Mexico

D. O. Breecker 2 L. D. McFadden 1 Z. D. Sharp 1 M. Martinez 2 M. E. Litvak 0 0 Department of Biology, The University of New Mexico , Albuquerque, New Mexico 87131, USA 1 Department of Earth and Planetary Sciences, The University of New Mexico , Albuquerque, New Mexico 87131, USA 2 Department of Geological Sciences, The University of Texas at Austin , Austin, Texas 78712, USA - Quantifying the controls on soil respiration is important for understanding ecosystem physiology and for predicting the response of soil carbon reservoirs to climate change. The majority of soil respiration is typically considered to occur in the top 2030 cm of soils. In desert soils, where organic matter concentrations tend to be low and plants are deeply rooted, deeper respiration might be expected. However, little is known about the depth distribution of respiration in dryland soils. Here we show that the average depth of soil respiration between pulse precipitation events is almost always greater than 20 cm and is frequently greater than 50 cm in two central New Mexico desert shrublands. The average depth of soil respiration in a pin on-juniper woodland was shallower, between 5 and 40 cm. In the shrublands, 8& seasonal variations in the carbon isotope composition of soilrespired CO2 (d13Cr-soil) that correlate with vapor Received 16 April 2011; accepted 21 September 2011; published online 19 October 2011 Author Contributions: D. Breecker: conceived and designed study, selected study locations, performed research, interpreted results, and wrote article; Z. Sharp: conceived study, helped to write the article, and oversaw stable isotope analyses; L. McFadden: conceived study, helped select study location, and helped to write the article; M. Litvak: interpreted results and helped to write the article; M. Martinez: coded the numerical CO2 model and performed research using the model. *Corresponding author; e-mail: pressure deficit support root/rhizosphere respiration as the dominant source of soil CO2. Such deep autotrophic respiration indicates that shrubs preferentially allocate photosynthate to deep roots when conditions near the surface are unfavorable. Therefore, respiration rates in these soils are not necessarily correlated with root biomass. The d13 Cr-soil values provide no evidence for CO2 evolved from soil inorganic carbon. Our results also suggest that organic carbon cycling is rapid and efficient in these soils and that the d13C value of CO2 respired from soils in much of the southwestern US, and perhaps in other semiarid regions, varies seasonally by at least 4&. INTRODUCTION Carbon dioxide (CO2) emitted from soils to the atmosphere constitutes one of the largest fluxes of carbon to the atmosphere (Raich and Schlesinger 1992). Small but sustained perturbations in the flux of soil-respired carbon could, therefore, drastically alter the CO2 concentration of Earths atmosphere. Debate surrounding the sensitivity of soil carbon stocks to global change (for example, Davidson and Janssens 2006) must be resolved to constrain future carbon budgets and predict future climate conditions. Scaling up from individual sites to the global scale will require a mechanistic understanding of soil respiration, which we help to develop by studying the origin of CO2 in central New Mexican soils. The CO2 flux from dryland (arid and semiarid region) soils, although relatively small on a unit area basis, constitutes a significant portion of the global carbon cycle because drylands cover approximately 40% of Earths land surface (Taylor and Lloyd 1992; Shen and others 2008). In dryland soils, uncertainty exists in relative contribution to total soil CO2 efflux from root/rhizosphere respiration (autotrophic respiration), from decomposition of soil organic matter (heterotrophic respiration) and from abiotic sources (for example, calcium carbonate in soils). This uncertainty masks the processes important in the transfer of CO2 from soils to the atmosphere. Investigating the sources of CO2 emitted from dryland soils is therefore important for quantifying the global carbon cycle and, on a smaller scale, for understanding ecosystem carbon exchange in these biomes. In drylands, pulses of biological activity caused by precipitation events punctuate background between-pulse levels of biological activity (Noy-Meir 1973). We studied the origin (depth and source) of the between-pulse soil-respired CO2 (with the intention to investigate pulse events in the future) to help develop a mechanistic understanding of dryland soil respiration. BACKGROUND Biological CO2 is produced in soils by respiration in the rhizosphere (by plant roots and by associated heterotrophic microorganisms) and by the nonrhizosphere microbial oxidation of organic matter (decomposition). The accumulation of CO2 in soil pore spaces (soil CO2) causes the development of soil-atmosphere concentration gradients, which result in net CO2 diffusion into the atmosphere, a flux typically termed s (...truncated)


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D. O. Breecker, L. D. McFadden, Z. D. Sharp, M. Martinez, M. E. Litvak. Deep Autotrophic Soil Respiration in Shrubland and Woodland Ecosystems in Central New Mexico, Ecosystems, 2012, pp. 83-96, Volume 15, Issue 1, DOI: 10.1007/s10021-011-9495-x