Iron-bound organic carbon in forest soils: quantification and characterization

Qian Zhao,Daniel Obrist,James J Dynes,Yu Yang,Samira Sumaila,Joyce M Mcbeth,Simon R Poulson

doi:10.5194/bg-13-4777-2016

Abstract

Abstract. Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to understand the amount, spatial variability, and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States and determined the impact of ecogeographical variables and soil physicochemical properties on the association of OC and Fe minerals. On average, Fe-bound OC contributed 37.8 % of total OC (TOC) in forest soils. Atomic ratios of OC : Fe ranged from 0.56 to 17.7, with values of 1–10 for most samples, and the ratios indicate the importance of both sorptive and incorporative interactions. The fraction of Fe-bound OC in TOC (fFe-OC) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and fFe-OC increased with latitude and reached peak values at a site with a mean annual temperature of 6.6 °C. Attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in 13C compared to the non-Fe-bound OC, but C ∕ N ratios did not differ substantially. In summary, 13C-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of fFe-OC being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils and uncovers the governing factors for the spatial variability and characteristics of Fe-bound OC.

Highlights

Soil organic carbon (OC) in forests is a vital component of C biogeochemical cycles (Eswaran et al, 1999)
This study provided a comprehensive investigation into the amount and characteristics of Fe-bound OC in forest soils as well as the impact of soil physicochemical properwww.biogeosciences.net/13/4777/2016/
Fe-bound OC contributed to 37.8 % of total OC (TOC) in forest soils, composing an important component of C cycles in terrestrial ecosystem

Summary

Introduction

Soil organic carbon (OC) in forests is a vital component of C biogeochemical cycles (Eswaran et al, 1999). Global warming can potentially accelerate the decomposition of forest soil OC, contributing to greenhouse gas emissions (Steffen et al, 1998). Forest soils can act as strong sinks for OC, if appropriate management is implemented, such as forest harvesting and fire treatment (Eswaran et al, 1999; Johnson and Curtis, 2001). Understanding the fate and stability of forest OC is important for evaluating and managing the global C cycle under the framework of climate change. There is an information gap concerning the stability and residence time of OC, contributing to the problem that the residence time of OC (ranging from months to hundreds of years) is a major source of uncertainty in modeling and prediction of C cycles (Schmidt et al, 2011; Riley et al, 2014). Many concepts have been proposed to account for OC stabilization and residence times, including molecular recalcitrance, physical occlusion, and chemical protec-

Objectives

Methods

Findings

Discussion

Conclusion