Carbon and oxygen isotope composition in soil carbon dioxide and free oxygen within deep ultisols at the Calhoun CZO, South Carolina, USA

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2018-10-01

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Abstract

In order to evaluate effects of three land uses on isotopic compositions of CO and O2 of soil air to 5 m soil depth, a field study was conducted in the Calhoun Critical Zone Observatory, located in the subtropical climate of the Southern Piedmont of South Carolina, USA. Soil gas reservoirs were installed in ecosystems with three different land uses, each replicated three times: (i) reference hardwood stands that were never cultivated; (ii) currently cultivated plots; (iii) pine stands, which had been used for growing cotton in 19th century but were abandoned in about the 1930s and 1940s when they were regenerated with pines that are today 70-80 yr old. In addition to soil CO and O2 concentration measurements, soil gas samples were analyzed for Δ C, δ C, and δ O. Stable carbon isotopic composition becomes lighter with the depth in soils of all three land uses: in the cultivated site δ C decreases from -18‰ at 0.5 m to -21‰ at 5 m, in pine site from -22 to -25‰, and in hardwood from -21.5 to -24.5‰, respectively. Δ C increased with depth from 40 to 60‰ in the top 0.5 m to about 80-140‰ at 5 m depending on land use. While surficial soils had relatively similar Δ C in CO2, between 40 to 60‰ at 0.5 m, at 3 and 5 m, cultivated soils had the highest Δ C, hardwood the lowest, and pine in between, a pattern that emphasizes the importance of contemporary respired CO in hardwood stands. Oxygen isotopic composition of CO did not change with depth, whereas free O2 was greatly enriched in lower horizons of forest soils, which we attribute to strong fractionation by respiration. 2 2 2 2 14 13 18 13 14 14 14

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10.1017/RDC.2018.99

Scholars@Duke

Richter

Daniel D. Richter

Professor in the Division of Earth and Climate Science

Richter’s research and teaching links soils with ecosystems and the wider environment, most recently Earth scientists’ Critical Zone.  He focuses on how humanity is transforming Earth’s soils from natural to human-natural systems, specifically how land-uses alter soil processes and properties on time scales of decades, centuries, and millennia.  Richter's book, Understanding Soil Change (Cambridge University Press), co-authored with his former PhD student Daniel Markewitz (Professor at University of Georgia), explores a legacy of soil change across the Southern Piedmont of North America, from the acidic soils of primary hardwood forests that covered the region until 1800, through the marked transformations affected by long-cultivated cotton, to contemporary soils of rapidly growing and intensively managed pine forests.  Richter and colleagues work to expand the concept of soil as the full biogeochemical weathering system of the Earth’s crust, ie, the Earth’s belowground Critical Zone, which can be tens of meters deep.  The research examines decadal to millennial changes in the chemistry and cycling of soil C, N, P, Ca, K, Mg, and trace elements B, Fe, Mn, Cu, Be, Zr, and Zn across full soil profiles as deep at 30-m.  Since 1988, Richter has worked at and directed the Long-Term Calhoun Soil-Ecosystem Experiment (LTSE) in the Piedmont of South Carolina, a collaborative study with the USDA Forest Service that quantifies how soils form as natural bodies and are transformed by human action, and a study that has grown to become an international model for such long-term soil and ecosystem studies.  In 2005, Richter and students initiated the first comprehensive international inventory project of the world’s LTSEs, using an advanced-format website that has networked metadata from 250 LTSEs.  The LTSEs project has held three workshops at Duke University, NCSU's Center for Environmental Farming Systems, and the USDA Forest Service's Calhoun Experimental Forest and Coweeta Hydrologic Laboratory, hosting representatives from Africa, Asia, Australia, Europe, and the Americas.  Richter's 60-year old Long Term Calhoun Soil and Ecosystem Experiment is linked to similar experiments and platforms around the world via the ‘Long-Term Soil-Ecosystem Experiments Global Inventory’, assembled by Dan Richter, Pete Smith, and Mike Hofmockel."He is an active member of the International Commission on Stratigraphy’s Working Group on the Anthropocene.  Richter has written in the peer-reviewed literature about all of these projects, and in November 2014 his soils research at the Calhoun and his soils teaching were featured in Science magazine.


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