Abstract
Author(s): Sulman, BN; Harden, J; He, Y; Treat, C; Koven, C; Mishra, U; O’Donnell, JA; Nave, LE | Abstract: Soils contain a large and dynamic fraction of global terrestrial carbon stocks. The distribution of soil carbon (SC) with depth varies among ecosystems and land uses and is an important factor in calculating SC stocks and their vulnerabilities. Systematic analysis of SC depth distributions across databases of SC profiles has been challenging due to the heterogeneity of soil profile measurements, which vary in depth sampling. Here, we fit over 40,000 SC depth profiles to an exponential decline relationship with depth to determine SC concentration at the top of the mineral soil, minimum SC concentration at depth, and the characteristic “length” of SC concentration decline with depth. Fitting these parameters allowed profile characteristics to be analyzed across a large and heterogeneous dataset. We then assessed the differences in these depth parameters across soil orders and land cover types and between soil profiles with or without a history of tillage, as represented by the presence of an Ap horizon. We found that historically tilled soils had more gradual decreases of SC with depth (greater e-folding depth or Z∗), deeper SC profiles, lower SC concentrations at the top of the mineral soil, and lower total SC stocks integrated to 30 cm. The large database of profiles allowed these results to be confirmed across different land cover types and spatial areas within the Continental United States, providing robust evidence for systematic impacts of historical tillage on SC stocks and depth distributions.
Highlights
Soils contain a substantial fraction of global terrestrial carbon (C) stocks, accounting for more than the C content of vegetation and the atmosphere combined (Jobbágy and Jackson, 2000; Ciais et al, 2013)
Because the majority of fitted profiles were located in the Continental United States, and the National Land Cover Dataset (NLCD) dataset is limited to the Continental United States, we focused our analysis on that region
Statistical Analyses We focused our statistical analyses on the distribution of soil profile parameters as they varied across soil order, spatial areas, and landcover types
Summary
Soils contain a substantial fraction of global terrestrial carbon (C) stocks, accounting for more than the C content of vegetation and the atmosphere combined (Jobbágy and Jackson, 2000; Ciais et al, 2013). The vertical distribution of soil C with depth may hold critical information about the vulnerability of SC to land cover change, land management practices, and other disturbances (Rumpel and Kögel-Knabner, 2011). Historical changes in SC stocks due to land management practices such as deforestation and agricultural tillage have been responsible for a significant fraction of historical anthropogenic carbon dioxide (CO2) emissions to the atmosphere (Eglin et al, 2010; Houghton et al, 2012). Tillage redistributes SC through the soil profile, potentially offsetting SC losses in upper layers by increasing storage in deeper soil layers (Baker et al, 2007; Angers and Eriksen-Hamel, 2008)
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