Emerging techniques for soil carbon measurements

Soil carbon sequestration is one approach to mitigate greenhouse gases. However, to reliably assess the quantities sequestered as well as the chemical structure of the soil carbon, new methods and equipment are needed. These methods and equipment must allow large scale measurements and the construction of dynamic maps. This paper presents results from some emerging techniques to measure carbon quantity and stability. Each methodology has specific capabilities and their combined use along with other analytical tools will improve soil organic matter research. New opportunities arise with the development and application of portable equipment, based on spectroscopic methods, as laser-induced fluorescence, laser-induced breakdown spectroscopy and near infrared, for in situ carbon measurements in different ecosystems. These apparatus could provide faster and lower cost field analyses thus improving soil carbon contents and quality databases. Improved databases are essential to model carbon balance, thus reducing the uncertainties generated through the extrapolation of limited data

Quantifying soil carbon stocks and humification through spectroscopic methods: a scoping assessment in EMBU-Kenya

Published online: 12 Jan 2019A soil carbon assessment was performed comparing agricultural cropping systems with natural vegetation along a sampling transect spanning different agro-ecologies on the eastern foot slopes of Mount Kenya in Embu county, 125 km from Nairobi, Kenya. The aim was to determine differences in soil carbon stocks and carbon recalcitrance and relate these to soil textural class, altitude, climatic parameters and land use. Soils from main agricultural systems as tea, coffee and maize-based intercropping, as well as from natural vegetation cover were sampled in triplicates, in five layers from 0 to 30 cm in depth and processed for total carbon analysis. The whole soil samples were also analysed using Laser-Induced Fluorescence Spectroscopy (LIFS) to assess carbon humification. Prototype portable equipment intended for future in situ analysis was used in the lab to ascertain the structure of the most recalcitrant and stable carbon present in different agro-ecosystems. In addition, Near Infrared Spectroscopy (NIRS) was tested for the quantitative analysis of soil carbon, showing that it is a reproducible and low-cost method that provided satisfactory results under the processing conditions of the samples. Results showed wide variation in the level and quality of carbon stored in the soils, depending on soil texture, land use, elevation, climate, agricultural practices and land use history. Considering the heterogeneous nature of sampled soils and the performance of NIRS and LIFS, these results can be used as a basis for the development of fully portable systems able to provide rapid, clean and potentially cost-effective relevant information for soil management

Characterization of Peatland Soils from the High Andes through 13C Nuclear Magnetic Resonance Spectroscopy

Whole soil samples from Peruvian bofedales (highland peatlands), located at an average altitude of 3881 m above sea level, were analyzed through 13C solid-state nuclear magnetic resonance (13C SSNMR) spectroscopy. The objective was to make a semi-quantitative characterization of the predominant organic chemical structures and to compare the organic matter from permanently and seasonally flooded peatlands soils as well as to characterize the changes throughout the soil profile using principal component analysis (PCA) of 13C-SSNMR spectra. Results indicated a relative accumulation of recalcitrant organic compounds as a function of depth that could be due to the constant input of fresh material to the soil surface. Notwithstanding, the results were different for each soil type. In seasonally flooded bofedales, the accumulated recalcitrant material was mainly composed of carboxylated aromatic moieties, whereas in permanently flooded bofedales, the accumulated material presented crystalline polymethylene, being the main difference the anoxic condition of permanently waterlogged soils. On the other hand, the degradable (labile) material was similar in both soils (i.e., mainly cellulose and partially oxidized cellulose). Another interesting feature was that the results seem to corroborate paleobotanical findings, pointing out to an ancient dominance of C4 taxa in Andean grasslands (deep layers in bofedales samples) whose lignin had more coumaryl alcohol at the expense of guaiacyl and syringyl unit