Evaluation of Three Field-Based Methods for Quantifying Soil Carbon

Roberto C Izaurralde,Ronny Harris,Allison M Thomson,Sudeep Mitra,Charles W Rice,Lucian Wielopolski,Kenneth D Sayre,Aaron G Rappaport,Gregory W Mccarty,James B Reeves,Jorge D Etchevers,Barry Francis,Michael H Ebinger,Bram Govaerts

doi:10.1371/journal.pone.0055560

Abstract

Three advanced technologies to measure soil carbon (C) density (g C m−2) are deployed in the field and the results compared against those obtained by the dry combustion (DC) method. The advanced methods are: a) Laser Induced Breakdown Spectroscopy (LIBS), b) Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS), and c) Inelastic Neutron Scattering (INS). The measurements and soil samples were acquired at Beltsville, MD, USA and at Centro International para el Mejoramiento del Maíz y el Trigo (CIMMYT) at El Batán, Mexico. At Beltsville, soil samples were extracted at three depth intervals (0–5, 5–15, and 15–30 cm) and processed for analysis in the field with the LIBS and DRIFTS instruments. The INS instrument determined soil C density to a depth of 30 cm via scanning and stationary measurements. Subsequently, soil core samples were analyzed in the laboratory for soil bulk density (kg m−3), C concentration (g kg−1) by DC, and results reported as soil C density (kg m−2). Results from each technique were derived independently and contributed to a blind test against results from the reference (DC) method. A similar procedure was employed at CIMMYT in Mexico employing but only with the LIBS and DRIFTS instruments. Following conversion to common units, we found that the LIBS, DRIFTS, and INS results can be compared directly with those obtained by the DC method. The first two methods and the standard DC require soil sampling and need soil bulk density information to convert soil C concentrations to soil C densities while the INS method does not require soil sampling. We conclude that, in comparison with the DC method, the three instruments (a) showed acceptable performances although further work is needed to improve calibration techniques and (b) demonstrated their portability and their capacity to perform under field conditions.

Highlights

Terrestrial C sequestration through planned changes in land use and management practices has been identified as an early adoption technology to mitigate the buildup of atmospheric CO2 [1,2,3]
Use of a ‘‘universal’’ calibration produced a mean estimate of soil C density about 37% lower than that obtained by the DC method
Using the same 11 samples to construct their new calibration curve, and using PLS methods, the LIBS team improved their instrument’s R2 to 0.919 they were only able to use 30 of the 101 samples due to software limitations (Figure 5). These results demonstrate that there is considerable opportunity to improve the predictability of both instruments by using DC results from a small number of local samples

Summary

Introduction

Terrestrial C sequestration through planned changes in land use and management practices has been identified as an early adoption technology to mitigate the buildup of atmospheric CO2 [1,2,3]. The results are presented on a mass basis C per unit mass [g C kg21] or per unit volume [kg C m23]; alternatively they are reported as C density per unit area to a depth of 30 cm, [kg C m22]) [9,10]. This procedure produces excellent results, it must be done in the laboratory increasing the time, efforts and costs that restrict its routine use in agricultural C sequestration projects, in developing countries. There is a need to develop portable, rapid, precise, and cost-efficient methods for measuring soil C changes in the field [10,11]

Objectives

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Results

Conclusion