FTIR indicators of soil organic matter quality across a landscape of organic management
The chemical composition of soil organic matter (SOM) can be an important determinant of the nutrient supply capacity of soils. This is especially relevant for low-input or organic agricultural systems dependent on SOM mineralization for crop nutrient supply. Organic management can increase labile C in the short-term and total soil C (TSC) in the longer-term, but less is known about how management practices may affect SOM chemical composition. On-farm research conducted across a landscape may provide a large range of SOM values, which may be related to the diversity of input types, and thus show differences in measures of SOM composition, measurable by spectroscopic techniques like Fourier-transform infrared (FTIR), as related to nutrient pools. FTIR markers of SOM composition could be combined with soil C measurements (e.g., TSC) as a basis for improving SOM quality.
Work in our laboratory evaluated FTIR spectroscopy as a rapid, low-cost approach for characterizing SOM composition. FTIR spectroscopy can provide insight to SOM quality because it enables identification of the chemical building blocks that make up SOM (i.e. moieties), including labile C pools relevant to nutrient mineralization. FTIR spectroscopy measures absorbance of infrared light at frequencies specific to organic matter bond type and excitation upon absorption (e.g., stretching, bending) (Fig. 1). Since FTIR is a rapid, low-cost, and information-rich measurement, it may be an attractive option to complement or replace more expensive and laborious soil analyses, and could ultimately be integrated into soil testing labs. Absorbance intensity in the mid-infrared region (4000-400 cm-1) can be used to fingerprint SOM composition, and specific spectral features (e.g., bands) offer a means to monitor compositional changes induced by management, including amendments, crop rotations, and land use history.
Bulk soil samples are often used in FTIR analysis of SOM but the predominance of mineral absorbances can limit the extent and accuracy of SOM characterization. An alternative strategy for FTIR of SOM is the calculation of subtraction spectra, in which the spectrum of a soil from which the organic matter has been removed (e.g., ashing, chemical oxidation) is subtracted from the original soil spectrum (Parikh et al., 2014; Margenot et al., 2015; Margenot et al., 2016). This can enhance organic absorbances by mathematical removal of mineral absorbances, but may cause artifacts of subtraction that can compromise the accuracy of spectral analyses of SOM.
In order to evaluate its potential for use in organic systems, more research is needed both to improve methods of FTIR analysis of SOM, and compare these with traditional soil measures across a range of organic management practices and soil characteristics. We evaluated the potential of subtractive FTIR spectroscopy as an improvement on bulk soil spectroscopy for characterization of SOM, in an on-farm approach that captures a diversity of soil and management factors known to influence SOM composition. In addressing this, FTIR spectroscopy was used to characterize SOM from 13 organically-managed Roma-type tomato fields on soils of similar texture and mixed mineralogy in the Sacramento Valley of California. These fields constituted a three-fold gradient of TSC and had distinct profiles of soil enzyme potential activities driven by C- and-N cycling enzymes (for more detail, see Nutrient Cycling on Organic Farms across a California Landscape), suggesting that SOM composition influenced microbial activity. FTIR spectra were collected for bulk soils, and subtraction spectra of SOM obtained by ashing and oxidation were compared for artifacts of subtraction. Spectral features (e.g., absorbance intensities, bands) representing SOM moieties of differing chemical lability (Fig. 1)were then related to soil C and N pools representing different availabilities.
Spectral features representing organic moieties showed greater accentuation in subtraction spectra and stronger relationships with soil C and N pools relative to bulk soil spectra (Margenot et al., 2015). Subtraction spectra calculated with oxidized soils compared to ash soils had fewer to no artifacts of subtraction. Using the oxidized subtraction spectra as an improved FTIR method for the analysis of SOM, areas of bands representing a variety of moieties (e.g., amides, aliphatics, carbohydrates) were then quantified for the 13 fields. Band areas representing aliphatic and aromatic moieties of SOM (Fig. 2), considered markers of labile and recalcitrant OM, respectively, showed greatest differences among fields, and were related to the more available SOM pools such as microbial biomass carbon and nitrogen (MBC, MBN), potentially-mineralizable nitrogen (PMN), and permanganate-oxidizable carbon (POXC). Increased TSC was strongly associated with an increase in aliphatic band areas and a concurrent decrease in aromatic band areas.
By examining relationships between FTIR regions and other soil analyses in an on-farm study encompassing a range of organic management practices, this work illustrates the potential for several spectral indicators (e.g. band areas at 2930, 1620, 1420, 815 cm-1, Fig. 2) to improve the use of common measurements like TSC in guiding organic management. Furthermore, this work shows that new types of pre-treatments of soil prior to FTIR analysis can serve to better describe SOM composition, which is a pre-requisite to its use as a soil testing tool in the future, and demonstrates its potential applicability for SOM management in on-farm contexts.
Figure 1. FTIR spectroscopy of whole soil samples for characterization of SOM chemical composition. Absorption of infrared light (4000-400 cm-1) by bonds of moieties that constitute SOM could be used to develop spectral indicators of SOM quality relevant to agroecosystem management. A band is considered a region of absorbance and is referred to by the wavenumber (x-axis). Absorbance intensity is the y-value, or absorbance ‘height’, and can be used to semi-quantify relative differences among moieties. A peak is considered a local maximum of absorbance intensity.
Figure 2. FTIR subtraction spectra representing SOM of two organic tomato fields on similar soil type from a California landscape of organic management. The TSC content of the fields are 18 g C kg-1 (high) and 7 g C kg-1 (low). Spectra are calculated by difference between bulk soil and hypochlorite oxidized soil. Areas of bands representing chemically labile aliphatic moieties at 2930 cm-1 and 1420 cm-1 and chemically refractory aromatic moieties at 1620 cm-1 and 815 cm-1 indicate greater relative chemical lability of SOM in the higher TSC field. Aliphatic band areas show strong, positive relationships with the size of labile soil C and N pools such as MBC, POXC, and PMN, whereas aromatic band areas pools have negative relationships with these pools. The relative increase in chemically labile to refractory band areas associated with increasing TSC is greater at lower (<10 g C kg-1) compared to higher (>14 g C kg-1) TSC contents.
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