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United States Department of Agriculture

Agricultural Research Service


Location: Range Management Research

Title: Denisty fractionation and 13C reveal changes in soil carbon following woody encroachment in a desert ecosystem

item Throop, Heather
item Lajtha, Kate
item Kramer, Marc

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2012
Publication Date: 3/1/2013
Citation: Throop, H.L., Lajtha, K., Kramer, M. 2013. Denisty fractionation and 13C reveal changes in soil carbon following woody encroachment in a desert ecosystem. Biogeochemistry. 112:409-422.

Interpretive Summary: Understanding how carbon moves between the soil and the atmosphere and being able to quantify these fluxes is very important because carbon cycling affects the climate through greenhouse gases like carbon dioxide. In order to use rangelands for carbon sequestration, it is necessary to understand their carbon dynamics and to identify the appropriate carbon pools (places in the ecosystem in which carbon is stored for a short period of time) and to understand how vegetation changes affect the process. This study shows that soil carbon dynamics in dryland ecosystems are complex and change when a grassland becomes a shrubland. The researchers identify 5 soil carbon pools that are important for modeling the soil carbon cycling.

Technical Abstract: Woody encroachment has dramatically changed land cover patterns in arid and semiarid systems (drylands) worldwide over the past 150 years. This change is known to influence bulk soil carbon (C) pools, but the implications for dynamics and stability of these pools are not well understood. Working in a Chihuahuan Desert C4 grassland encroached by C3 creosote bush (Larrea tridentata), we used two density fractionation techniques (2 and 7 pool density fractionations) and isotopic analysis to quantify changes in C pools and dynamics among vegetation microsites typical of an encroachment scenario (remnant intact grassland, shrub subcanopies, and in shrub intercanopy spaces within a shrubencroached area). The C concentration of bulk soils varied with microsite, with almost twice the C in shrub subcanopies as in intercanopy spaces or remnant grasslands. Estimated SOC accumulation rates from Larrea encroachment (4.79 g C m-2 year-1 under canopies and 1.75 g C m-2 year-1 when intercanopy losses were taken into account) were lower than reported for higher productivity Prosopis systems, but still represent a potentially large regional C sink. The composition of soil C varied among microsites, with the shrub subcanopy C composed of proportionally more light fraction C (\1.85 g cm-3) and C that was soluble in sodium polytungstate. Grassland soils contained very little carbonate C compared to shrub subcanopies or shrub intercanopy spaces. Stable isotope analyses indicate that inputs from C3 shrubs were incorporated into all density fractions, even in heavy fractions in which shrub inputs did not change overall C concentration. The seven density fractionation yielded unexpected d13C patterns, where the two heaviest fractions were strongly depleted in 13C, indicating strong fractionation following organic matter inputs. These results suggest that the utility of isotope mixing models for determining input sources may be limited in systems with similar fractionation patterns. We propose a five pool model for dryland soil C that includes a relatively dynamic light fraction, aggregate and heavy fractions that are stable in size but that reflect dynamic inputs and outputs, a potentially large and seasonally dynamic pool of soluble C, and a large pool of carbonate C. Combined, these results suggest that dryland soil C pools may be more dynamic than previously recognized.

Last Modified: 06/27/2017
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