Does elevated atmospheric CO2affect soil carbon burial and soil weathering in a forest ecosystem?

Miquel A Gonzalez-Meler,Armen Poghosyan,Neil C Sturchio,Eduardo Dias De Olivera,Yaniria Sanchez-De Leon,Richard J Norby

doi:10.7717/peerj.5356

Abstract

Most experimental studies measuring the effects of climate change on terrestrial C cycling have focused on processes that occur at relatively short time scales (up to a few years). However, climate-soil C interactions are influenced over much longer time scales by bioturbation and soil weathering affecting soil fertility, ecosystem productivity, and C storage. Elevated CO2can increase belowground C inputs and stimulate soil biota, potentially affecting bioturbation, and can decrease soil pH which could accelerate soil weathering rates. To determine whether we could resolve any changes in bioturbation or C storage, we investigated soil profiles collected from ambient and elevated-CO2plots at the Free-Air Carbon-Dioxide Enrichment (FACE) forest site at Oak Ridge National Laboratory after 11 years of 13C-depleted CO2 release. Profiles of organic carbon concentration, δ13C values, and activities of 137Cs, 210Pb, and 226Ra were measured to ∼30 cm depth in replicated soil cores to evaluate the effects of elevated CO2 on these parameters. Bioturbation models based on fitting advection-diffusion equations to 137Cs and 210Pb profiles showed that ambient and elevated-CO2 plots had indistinguishable ranges of apparent biodiffusion constants, advection rates, and soil mixing times, although apparent biodiffusion constants and advection rates were larger for 137Cs than for 210Pb as is generally observed in soils. Temporal changes in profiles of δ13C values of soil organic carbon (SOC) suggest that addition of new SOC at depth was occurring at a faster rate than that implied by the net advection term of the bioturbation model. Ratios of (210Pb/226Ra) may indicate apparent soil mixing cells that are consistent with biological mechanisms, possibly earthworms and root proliferation, driving C addition and the mixing of soil between ∼4 cm and ∼18 cm depth. Burial of SOC by soil mixing processes could substantially increase the net long-term storage of soil C and should be incorporated in soil-atmosphere interaction models.

Highlights

Soils contain most of the organic carbon in Earth’s ‘‘critical zone’’, formation, transport and degradation of soil organic carbon (SOC) are key factors in the global carbon cycle (Hopkins et al, 2013)
Soil organic carbon content was highest at the surface and decreased with depth (Fig. 1A)
The SOC being deposited at the soil surface has a δ13C value of about −38 in the elevated-CO2 plot compared with −28 in the ambient plot (Fig. 2)

Summary

Introduction

Soils contain most of the organic carbon in Earth’s ‘‘critical zone’’, formation, transport and degradation of soil organic carbon (SOC) are key factors in the global carbon cycle (Hopkins et al, 2013). Fixation of atmospheric CO2 by plant photosynthesis and the consequent decomposition and release of this organic carbon as CO2 by soil biota are principal factors in the evolution of the SOC pool and the atmospheric concentration of CO2. Soil organic carbon decomposition depends on vegetation, microbial community, molecular composition of the organic matter, mineralogy, moisture, and temperature (Jastrow, 1996; Jastrow, Amonette & Bailey, 2007; O’Brien et al, 2010; Cheng et al, 2014). Transport of SOC within the soil C matrix is difficult to measure but SOC burial has been recognized in playing a role in the responses of the soil C pool to climatic factors (Lehmann & Kleber, 2015)

Methods

Results

Discussion

Conclusion