Evaluation of optical techniques for characterising soil organic matter quality in agricultural soils
Introduction
Soil organic matter (SOM) is composed of organic residues that are originated from plant and animal remains and microbial products at different stages of decomposition or humification (Hur et al., 2013). Additionally, it is one of the main global carbon pools, storing three times more carbon than living organisms or the atmosphere (Fischlin et al., 2007, Brevik, 2012). Aside from carbon sequestration, SOM is also a measure of soil quality because of the beneficial function it has on a variety of soil processes. For instance, it reduces erosion and, therefore, increases crop production by increasing the elasticity and resistance to deformation and compactability as well as porosity and water retention (Sellami et al., 2008, Paradelo and Barral, 2013). Increased water retention decreases potential runoffs by improving water infiltration in to soils and provides a store of water for plant uptake, buffering against moisture and rainfall fluctuations (Lal, 2004). This is of importance considering that the lack of water retention leads to a change in the hydrological patterns of agricultural areas and promotes the quantity and severity of floods and water-led erosion. Also, SOM leads to an increased vegetative cover, which ultimately reduces soil erosion (Cerdà, 1998, Cerdà, 2000, Novara et al., 2011, Zhao et al., 2013). Carbon mineralisation is crucial in SOM dynamics and along with carbon input, determines how much carbon accumulates in soil and releases nutrients that are essential for plant growth. Factors that affect mineralisation are the size of labile carbon, environmental conditions and the local microbial community (Zhao et al., 2008, Li et al., 2013). SOM and soil assemblage; SOM decomposition and transport by organisms contribute to soil stabilisation and the improvement of soil structure (Brevik et al., 2015). Moreover, SOM quantity has been directly related to the preservation of soil aggregates, which in turn reduces soil erodibility (Novara et al., 2011). Also, the direct processing of SOM along with its decomposition contribute to the improvement of soil chemical properties and stability (Brevik et al., 2015). Therefore, optimal quantities of SOM improve structure, water retention, and nutrient holding capacity of soils, which has an effect in multiple aspects of the soil system. These are essential considering the wider context of Earth System, as SOM conservation techniques have been proven to improve the fertility of degraded soils of a wide variety of ecosystems that are the main resource of large communities of developing areas of our planet, as well as have an influence on biogeochemical cycles and climate change mitigation (Batjes, 2014, Saha et al., 2014, Srinivasarao et al., 2014).
SOM has been conceptualised as containing three pools, with different residence/turnover times (Trumbore, 2000). These pools are the active SOM (living biomass of microorganisms and partially decomposed residues; associated with 1 year turnover); the slow SOM (resistant plant material; associated to a turnover from years to centuries); and passive SOM (humic substances and inert organic matter), which has been traditionally associated with longer residence time (thousands of years) and more stability. Some authors consider that the inert organic matter should not be considered as part of the passive pool, but as a fourth pool (Trumbore, 1997, Ohno, 2002, Agren and Bosatta, 2002, Sparks, 2003, Bell and Lawrence, 2009, Dungait et al., 2012). Although the traditional view has been that decomposition led to complex molecules that were very stable as a result of their structure; it has recently been known that environmental conditions, organo-mineral associations and other processes influence more in SOM stability than structure, which only plays a secondary role. As a result of this new view, recent research has found that humic substances, which have always been considered high molecular mass polymers, could be simpler than originally thought (Kleber and Johnson, 2010, Schmidt et al., 2011). Still, their structure is on discussion and the separation of SOM into fractions with different turnovers remains a major challenge (Kleber, 2010, Schmidt et al., 2011, Schrumpf and Kaiser, 2015).
Non-humic substances are composed by microbial biomass, decomposable plant material (active SOM); and resistant plant material, mainly waxes, lignified tissues and polyphenols (slow SOM) (Dungait et al., 2012). Microbial biomass has been used for comparing natural and degraded ecosystems and as an early indicator of soil processes, fertility and health (García-Gil et al., 2000, Brevik, 2009, Chen et al., 2013).
Measures such as % organic matter measured by loss of ignition (LOI) are useful, popular and inexpensive methods to determine bulk SOM (Luke et al., 2009, Salehi et al., 2011). However, some studies have concluded that bulk SOM measurements cannot be used, on their own, as a representative indicator of carbon in soil due to their limitations (Koarashi et al., 2005, Salehi et al., 2011).
Humic substances have also been measured to determine soil quality, as their presence has been associated with a higher quality of soils as stated in Section 1.2. Their study is relevant in agricultural soils, as they increase crop yield and root dry weight, although this response is not fully understood (Rose et al., 2014). Therefore, the determination and characterisation of humic substances gives essential information of the maturity and stresses of soils as well as of their health.
Traditionally, alkali and acid abstraction methods have been used, to later interpret the chemistry of the extracted functional groups (Olk and Gregorich, 2006). Afterwards, these were combined with other complex techniques that enabled scientists to obtain new information on the structure and dynamic associations of humic substances (Sutton and Sposito, 2005, Schmidt et al., 2011). Despite these advances, SOM dynamics and cycling still have many questions to answer, with models differing in SOM fluxes results for the future, due to their sensitivity to SOM turnover time assumptions (Schmidt et al., 2011). There are a number of powerful but complex and expensive techniques that have been used for the study of soil fluxes (Helal et al., 2011). The economic resources needed, along with the time required to prepare the samples and conduct the analyses, make its use with a large number of samples difficult and delays experiments, while more work is still needed to accurately determine and define the molecular structures and linkages between the SOM components (Weishar et al., 2003; Helal et al., 2011 Helal et al., 2011).
Nuclear magnetic resonance (NMR) spectroscopy is a non-destructive technique that is valuable for the characterisation of SOM and humification processes, providing information on static and dynamic properties of molecules. This is due to its high performance to assess intermolecular interactions. The relationship between SOM, contaminants and metals can also be studied with NMR (Cardoza et al., 2004). Of the various variants that exist, 1H NMR spectroscopy was used in this study. This technique analyses humic and fulvic acids dissolved in neutral or alkaline solutions to characterise the components of the substance, and gives a semi-quantitative notion of aromatic, aliphatic and carboxylic groups (Hemminga and Buurman, 1997). One of the main drawbacks of this technique is the quantity of economic resources that are necessary for its regular application in research laboratories/centres. This is due to the expensive deuterated solvents and NMR tubes, as well as the expensive equipment and significant sample preparation that are required (Weishar et al., 2003 Cardoza et al., 2004, MIT, 2008). Also, the technique is time consuming not only when measuring, but when interpreting 2-D or 3-D data resulting from it (Cardoza et al., 2004). Simpler methods for the characterisation of SOM are required.
Water extractable carbon is the most active component in the carbon cycle. Its quantity and biological nature is affected by the extraction temperature (Bu et al., 2010). Hot-water extractable carbon (HWC) contains simple compounds such as microorganisms, soluble carbohydrates and other compounds that account for the labile fraction of SOM (Ghani et al., 2003). HWC responds to land use changes in the short term and has been used to detect the effects of different land management practices and for determining the effects of soil amendments such as biochar or agricultural residues (Leifeld and Kogel-Knabner, 2005, Uchida et al., 2012, Alburquerque et al., 2014, Fernández-Romero et al., 2014). For these reasons, it has been proven useful to obtain information about soil quality (Ghani et al., 2003, Xue et al., 2013).
Fluorescence has become popular because of its potential to characterise SOM and study humic substances, as it is non-destructive, simple, non-separative and accurate. As a result, it has been used for determining the compositional and structural properties of SOM (Chen et al., 2003, Senesi and D’Orazio, 2005, Sun et al., 2007, Kwiatkowska et al., 2008, Henderson et al., 2009, Hur and Kim, 2009, Tang et al., 2011). The intensity and position of the peaks detected in the spectra are unique to each substance structural and functional characteristics. For instance, higher fluorescence intensities are related to a higher humification (Martins et al., 2011).
The fluorescence index (FI) was developed to assess different properties of dissolved organic matter (DOM). It was defined by McKnight et al. (2001) as the ratio of emission intensities at 450–500 nm excited at 370 nm. The 450 nm point was chosen for specific characteristics of the experiment. Later, Cory et al. (2010) modified the ratio to 470–520 nm to reflect corrections specific to the instruments used. This index has been correlated to the aromaticity of DOM (Korak et al., 2014).
Fluorescence spectroscopy can be used in combination with UV–vis spectroscopy to characterise humic substances, as absorbance measures transitions from the ground state to the excited state, as opposed to fluorescence spectroscopy (Skoog et al., 2007). Its spectra are usually uniform and provide with qualitative data when a specific wavelength is selected (Hassouna et al., 2012). Also the specific absorbance at 254 nm (SUVA-254) has been recognised as a method to determine SOM aromaticity (Fuentes et al., 2006, Chow, 2006). This parameter is very useful for assessing the nature of the general composition of dissolved organic carbon (DOC), due to its high correlation with it (Weishar et al., 2003).
Considering what has been described in Sections 1.3 and 1.4, both the fluorescence and NMR techniques can be used in combination to determine the humic substances properties and the degree of aromaticity; while HWC could contribute further to the understanding of soil quality, given its usefulness to detect the biodegradation of soil biochemical properties (Ghani et al., 2003, Saab and Martin-Neto, 2007, González-Pérez et al., 2007).
As an illustrative and additional way to characterise and represent some of the analyses conducted and the results obtained, excitation–emission matrix (EEM) spectra have been plotted. These provide information on the relative intensity of fluorescence at different excitation and emission wavelengths regions in a fast manner that is also easy to interpret (Coble, 1996). Several peaks have been identified that are used to describe EEM fluorescence spectra. Peak A and C refer to humic peaks. Peak A is referred to as UVC-excited and is located at an excitation wavelength (λEx) between 240 and 260 nm and an emission wavelength (λEm) between 400 and 460 nm. Peak C, also referred to as UVA-excited; is located at a λEx between 320 and 360 nm and a λEm between 420 and 460 nm. There are also peaks that indicate biological activity material (peaks B and T, which are defined as tyrosine-like and tryptophan-like peaks, respectively). B has λEx of 270–280 nm and λEm of 300–315 nm whereas T has λEx between 270 and 280 nm and λEm of 345–360 nm (Birdwell and Engel, 2010).
The aim of this study was to evaluate the use of fluorescence spectroscopy to measure SOM quality (specifically the grade of humification). The specific objectives were: (1) characterise water extractable SOM quality using liquid state 1H NMR; (2) characterise the quality of water extractable organic matter using fluorescence spectroscopy and UV–vis; (3) compare measures of quantity and quality of water extractable organic matter with the specific organic matter fractions measured by 1H NMR like aromaticity. If robust relationships and similarities between optical measures and 1H NMR are found, there may be potential for fluorescence spectroscopy as a fast and more cost-effective method of organic matter characterisation.
Section snippets
Field sites description
Cambisols were sampled in two regions with contrasting climatic conditions; Andalusia (South Spain) and Berkshire (South East England) (Table 1). Berkshire has a temperate climate, characterised generally by relatively mild winters and summers and rainfall throughout the year. The annual mean temperature ranges from 6.7 °C and 14.5 °C (30 years, annual mean temperature: 10.5 °C) and the average annual rainfall is 635.4 mm (UK Met Office, 2014).
Andalusia has a Mediterranean climate, which is
Soil characteristics
The majority of CC-UK soils had sandy texture, with a relatively high proportion of silt (Table 2). The only exception was CC-UK-2, although the sand proportion was quite close to that of silt. This texture was similar to that of CC-ES-1 and CC-ES-2, although CC-ES-3 had a higher proportion of silt and higher proportion of clay than of sand. All the OG soils presented a texture of a majority of silt (56.7–59.5%), followed by clay. It is worth considering that silt is the most erodible fraction (
Conclusions
Different techniques to measure the quantity and quality of SOM were tested in cambisols from very different climatic locations and under different cropping systems. HWC extracted a higher amount and carbon than CWC, and correlated better with the %SOM (LOI) than CWC.
SUVA-254 and HWC correlated significantly with the proportion of aromatics measured with 1H NMR, demonstrating their complementary nature. Linear regression models fitted to the data were able to explain the relationship between
Acknowledgments
We thank J.M. Calero and M. Bell for their contribution to improve this paper.
References (90)
- et al.
Reconciling differences in predictions of temperature response of soil organic matter
Soil Biol. Biochem.
(2002) - et al.
Characterization of dissolved organic matter in cave and spring waters using UV–vis absorbance and fluorescence spectroscopy
Org. Geochem.
(2010) - et al.
Spectroscopic characterization of hot-water extractable organic matter from soils under four different vegetation types along an elevation gradient in the Wuyi Mountains
Geoderma
(2010) - et al.
Applications of NMR spectroscopy in environmental science
Prog. Nucl. Mag. Res Sp.
(2004) Aggregate stability against water forces under different climates on agriculture land and scrubland in southern Bolivia
Soil Till. Res.
(2000)- et al.
Fluorescence spectroscopic studies of natural organic matter fractions
Chemosphere
(2003) - et al.
Changes in soil carbon pools and microbial biomass from urban land development and subsequent post-development soil rehabilitation
Soil Biol. Biochem.
(2013) Disinfection byproduct reactivity of aquatic humic substances derived from soils
Water Res.
(2006)Characterization of marine and terrestrial DOM in seawater using excitation–emission matrix spectroscopy
Mar. Chem.
(1996)- et al.
The usefulness of UV–visible and fluorescence spectroscopies to study the chemical nature of humic substances from soils and composts
Org. Geochem.
(2006)
Long-term effects of municipal solid waste compost application on soil enzyme activities and microbial biomass
Soil Biol. Biochem.
Hot-water extractable carbon in soils: a sensitive measurement for determining impacts of fertilization, grazing and cultivation
Soil Biol. Biochem.
A laser-induced fluorescence spectroscopic study of organic matter in a Brazilian Oxisol under different tillage systems
Geoderma
Biodegradability of soluble organic matter in maize-cropped soils
Geoderma
Production and elimination of water extractable organic matter in a calcareous soil as assessed by UV/Vis absorption and fluorescence spectroscopy of its fractions isolated on XAD-8/4 resins
Geoderma
Characterization of different humic materials by various analytical techniques
Arab. J. Chem.
Editorial: NMR in soil science
Geoderma
Fluorescence as a potential monitoring tool for recycled water systems: a review
Water Res.
Properties of fluorescent dissolved organic matter in the Gironde Estuary
Org. Geochem.
Comparison of the heterogeneity within bulk sediment humic substances from a stream and reservoir via selected operational descriptors
Chemosphere
Enhanced binding of hydrophobic organic contaminants by microwave-assisted humification of soil organic matter
Chemosphere
Characterization of biochar-derived dissolved organic matter using UV–visible absorption and excitation–emission fluorescence spectroscopies
Chemosphere
Biodegradation of soil derived dissolved organic matter as related to its properties
Geoderma
Multi-method study of the characteristic chemical nature of aquatic humic substances isolated from the Han River, Korea
Appl. Geochem.
Advances in understanding the molecular structure of soil organic matter: implications for interactions in the environment
Adv. Agron.
Effects of past fly ash deposition on the forest floor humus chemistry of pine stands in Northeastern Germany
Forest Ecol. Manage.
Radiocarbon and stable carbon isotope compositions of chemically fractionated soil organic matter in a temperate-zone forest
J. Environ. Radioact.
Critical analysis of commonly used fluorescence metrics to characterize dissolved organic matter
Water Res.
Selective adsorption of dissolved organic matter to mineral soils
Geoderma
Long term effects of a brown coal-based amendment on the properties of soil humic acids
Geoderma
Soil carbon sequestration to mitigate climate change
Geoderma
Soil organic matter fractions as early indicators for carbon stock changes under different land-use?
Geoderma
Carbon and nitrogen mineralization patterns of two contrasting crop residues in a Mollisol: effects of residue type and placement in soils
Eur. J. Soil Biol.
Soil organic matter humification under different tillage managements evaluated by laser induced fluorescence (LIF) and C/N ratio
Soil Till. Res.
Soil erosion assessment on tillage and alternative soil managements in a Sicilian vineyard
Soil Till. Res.
Influence of organic matter and texture on the compactability of Technosols
Catena
A comprehensive structural evaluation of humic substances using several fluorescence techniques before and after ozonation. Part I: structural characterization of humic substances
Sci. Total Environ.
Chapter two—a meta-analysis and review of plant-growth response to humic substances: practical implications for agriculture
Adv. Agron.
Refining soil organic matter determination by loss-on-ignition
Pedosphere
Large differences in estimates of soil organic carbon turnover in density fractions by using single and repeated radiocarbon inventories
Geoderma
Maturity assessment of composted olive mill wastes using UV spectra and humification parameters
Bioresour. Technol.
Fluorescence spectroscopy
Fluorescence of sediment humic substance and its effect on the sorption of selected endocrine disruptors
Chemosphere
Different analysis techniques for fluorescence excitation–emission matrix spectroscopy to assess compost maturity
Chemosphere
The relationship of water-soluble carbon and hot-water-soluble carbon with soil respiration in agricultural fields
Agric. Ecosyst. Environ.
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