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)