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

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 piñ on-juniper woodland was shallower, between 5 and 40 cm. In the shrublands, 8& seasonal variations in the carbon isotope composition of soilrespired CO 2 (d 13 C r-soil) that correlate with vapor pressure deficit support root/rhizosphere respiration as the dominant source of soil CO 2. 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 d 13 C r-soil values provide no evidence for CO 2 evolved from soil inorganic carbon. Our results also suggest that organic carbon cycling is rapid and efficient in these soils and that the d 13 C value of CO 2 respired from soils in much of the southwestern US, and perhaps in other semiarid regions, varies seasonally by at least 4&.

Isotopic fractionation corrections for the radiocarbon composition of CO<sub>2</sub> in the soil gas environment must include diffusion and mixing

“…Diffusion-and production-controlled soil gas profiles, similar to those presented in Fig. 3, have been shown in other papers 25 for δ 13 C (e.g., Bowling et al, 2015;Breecker et al, 2012a;Cerling, 1984;Cerling et al, 1991;Davidson, 1995;Nickerson and Risk, 2009b). Given that we based our gas transport corrected Δ 14 C calculation on the same equations, this is what we expected, where values of Δ 14 C of soil CO 2 differed in the soil profile (shown in Fig.…”

Deep Unsaturated Zone Contributions to Carbon Cycling in Semiarid Environments

“…Further application of the diffusion gradient analysis indicates that a large portion (30% annually) of the CO 2 emissions at the soil surface originated from below 1.0 and above the water table over the course of the year, and that the low winter time respiration has an even higher relative contribution (60%) from this deeper zone (Figure d). Roots are sparsely distributed within the 1‐ to 2‐m depth interval and contribute directly to CO 2 production in the deeper zone through root respiration (Breecker et al, ), as well as indirectly through microbial degradation of exudates and dead tissue.…”

“…However, much less is known about carbon fluxes through deeper strata where about half of the Earth's terrestrial carbon inventory resides (Davidson et al, ), especially in semiarid and arid regions, which represent about 40% of the Earth's land surface (Ahlstrom et al, ). In desert shrublands, the average depth of soil respiration can exceed 0.5 m and be primarily attributable to roots (Breecker et al, ). In spite of the importance of soil carbon dynamics to climate change over the next century (Friedlingstein et al, ), predictions of soil carbon cycling using Earth System Models (ESMs) remain uncertain (Todd‐Brown et al, ) with simplified representation of key processes, for example.…”

Soil organic and inorganic carbon and stable carbon isotopes in the Yanqi Basin of northwestern China

“…It is known that soil respiration occurs mainly in the upper 30 cm in agricultural soils, which would cause more negative δ 13 C values in SIC of the topsoils. On the other hand, root respiration may be substantial in the shrub subsoils (Breecker et al , ), which may lead to more negative δ 13 C values in SIC.…”