Deep Autotrophic Soil Respiration in Shrubland and

Authors: D O Breecker L D McFadden Z D Sharp M Martinez M E Litvak

Publish Date: 2011/10/19

Volume: 15, Issue: 1, Pages: 83-96

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Abstract

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 piñonjuniper woodland was shallower between 5 and 40 cm In the shrublands 8‰ seasonal variations in the carbon isotope composition of soilrespired CO2 δ13Crsoil 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 δ13Crsoil 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‰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 modelCarbon 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 soilrespired carbon could therefore drastically alter the CO2 concentration of Earth’s 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 soilsThe 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 Earth’s 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 “betweenpulse” levels of biological activity NoyMeir 1973 We studied the origin depth and source of the “betweenpulse” soilrespired CO2 with the intention to investigate pulse events in the future to help develop a mechanistic understanding of dryland soil respirationBiological 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 soilatmosphere concentration gradients which result in net CO2 diffusion into the atmosphere a flux typically termed soilrespired CO2 The flux of CO2 from soils is known to be sensitive to soil moisture and temperature among other variables but a mechanistic understanding useful for modeling extrapolation and prediction is lackingA mechanistic understanding of soil respiration must involve spatial distribution For instance soil respiration rates at discrete depths should ideally be compared with soil temperatures soil moisture and so on at those depths However the number of studies in which the depth distribution of soil respiration has been investigated is very small compared with the number of studies in which the flux across the soilatmosphere interface was the only CO2 measurement made It is typically assumed that soil respiration primarily occurs in the top 20–30 cm of soils and that soil respiration rates below this depth are negligibly small The concentration of soil organic matter is highest in the near surface soil O and A horizons and decreases exponentially with increasing depth in most soils Therefore the assumption that soil respiration is primarily confined to the top several decimeters is probably true for most temperate forest and prairie soils as suggested by previous studies of soil CO2 profiles de Jong and Schappert 1972 Dörr and Münnich 1990 Drewitt and others 2005 Hashimoto and others 2007 However considerable soil respiration below 20 cm soil depth has been documented in some soils Hirsch and others 2002 Davidson and others 2006 and average depths of soil respiration up to 40 cm have been observed during droughts Fierer and others 2005 Hashimoto and others 2007The depth distribution of roots is also an important consideration for understanding soil respiration especially in soils with low organic matter contents such as desert soils Desert shrub roots are known to extend to depths below 5 m Gile and others 1998 even in soils with petrocalcic horizons Gibbens and Lenz 2001 These deep roots are known to uptake P Hartle and others 2006 but their contribution to soil respiration is poorly understood The average depth of soil respiration varied between 5 and 40 cm in a sand dune with low soil organic matter content planted with a Pinus radiata tree Cook and others 1998 but soil water was maintained at field capacity in this study and so natural variations were not observed Naturally occurring depths of soil respiration in desert soils are essentially unknown despite repeated documentation of such deep rooting systems The fist objective of this study was to determine the depth of soil respiration in some central New Mexican woodland and shrublandsA mechanistic understanding of soil respiration must also include an understanding of the relative contribution from different carbon sources to total CO2 efflux The second objective of this study was to use variations in carbon isotope composition of CO2 produced in soils δ13Crsoil to identify the source of soilrespired CO2 Changes in the value of δ13Crsoil are controlled by multiple mechanisms which can be divided into two broad categories

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