Carbon Quantification in Forest Soils Using Low Resolution Laser-Induced Breakdown Spectroscopy

The C cycle in the Brazilian forests is very important for issues related to climate changes. Assessing and understanding C dynamics in Amazonian soils can help scientists to improve models and anticipate scenarios. Development of new methods for soil C measurements in situ is crucial in this region due to the financial costs and times needed for collecting and sending soil samples from the forest to the research laboratory. Laser-induced breakdown spectroscopy (LIBS) is a type of atomic emission spectroscopy which uses a highly energetic laser pulse for the plasma production. In this technique sample preparation is typically minimized or unnecessary. Furthermore, LIBS is able to perform a simultaneous semi-quantitative and multi-element analysis in few seconds, thus allowing the monitoring and comprehensive analysis of soil in situ. In this study, the feasibility of a portable and low resolution LIBS system for the in situ quantitative analysis of soil C was investigated in two typical forest Brazilian soils, a Spodosol (Amazon Forest) and an Oxisol (Atlantic Forest). The intensities of LIBS C lines were analyzed in fifty-three soil samples. In particular, two C lines were evaluated, at 193.03 nm and 247.86 nm. The line at 247.86 nm showed a very strong interference with iron and titanium lines, which made impossible any quantitative analysis. The line at 193.03 nm showed interference by atomic and ionic aluminum emission lines, but the analytical conditions were better than those at 247 nm. To solve this problem a correction method was proposed and tested for the C line at 193.03 nm. A strong correlation was found between the C data measured by LIBS and those measured by the reference technique (CHN elemental analysis). The Pearson's coefficient value of R = 0.91 supported the efficiency of the used correction.