Calcium Humate Fractions Research Articles

Analysis of Carbon Functional Groups in Mobile Humic Acid and Recalcitrant Calcium Humate Extracted from Eight US Soils

Solid state 13C nuclear magnetic resonance (NMR) spectroscopy is a common tool to study the structure of soil humic fractions; however, knowledge regarding carbon structural relationships in humic fractions is limited. In this study, mobile humic acid (MHA) and recalcitrant calcium humate (CaHA) fractions were extracted from eight soils collected from six US states and representing a variety of soils and ecoregions, characterized by this spectroscopic technique and analyzed for statistical significance at P = 0.05. We found that the abundances of COO and N-C=O functional groups in the MHA fractions were negatively correlated to soil sand content, but were positively correlated to silt, total N and soil organic carbon contents. In contrast, the abundances of the COO and N-C=O functional groups were only positively correlated to the content of clay in the CaHA fractions, indicating that the two humic fractions were associated with different soil components. The two 13C NMR peaks representing alkyls and OCH3/NCH were negatively correlated to the peaks representing aromatics, aromatic C-O and N-C=O/COO. Comparison of the sets of data from 13C NMR spectroscopy and ultrahigh resolution mass spectrometry revealed that the aromatic components identified by the two methods were highly consistent. The comparison further revealed that protein in MHA was associated with, or bound to, the nonpolar alkyl groups, but a component competitively against (or complementary to) aromatic groups in the MHA composition. These observations provided insight on the internal correlations of the functional groups of soil humic fractions.

Solid‐State NMR Analysis of Soil Organic Matter Fractions from Integrated Physical–Chemical Extraction

Fractions of soil organic matter (SOM) were extracted by an integrated physical–chemical procedure and their chemical natures were characterized through 13C nuclear magnetic resonance (NMR) spectroscopy. For the 0‐ to 5‐cm depth of a corn (Zea mays L.)–soybean [Glycine max. (L.) Merr.] soil in Iowa, we extracted in sequence the light fraction, two size fractions of particulate organic matter (POM), and two NaOH‐extractable humic acid fractions based on their binding to soil Ca2+: the unbound mobile humic acid fraction and the calcium humate fraction. Whole SOM was obtained by dissolving the soil mineral component through HF washes. All samples were analyzed by advanced 13C NMR techniques, including quantitative direct polarization/magic angle spinning, spectral‐editing techniques, and two‐dimensional 1H–13C heteronuclear correlation NMR. The NMR spectra were comparable for the light fraction and two POM fractions and were dominated by carbohydrates and to a lesser extent lignins or their residues, with appreciable proteins or peptides. By contrast, spectra of the two humic fractions were dominated by aromatic C and COO/N–C=O groups, with smaller proportions of carbohydrates and NCH/OCH3 groups, indicative of more humified material. This trend was yet more pronounced in the calcium humate fraction. The spectrum for whole SOM had signals intermediate between these two groups of SOM fractions, suggesting contributions from both groups. Our results for this soil suggest that either chemical or physical fractions alone will partially represent whole SOM, and their integrated use is likely to provide greater insight into SOM structure and possibly function, depending on the research issue.

Influence of animal manure application on the chemical structures of soil organic matter as investigated by advanced solid-state NMR and FT-IR spectroscopy

Annual application of cattle manure to a farmer's field in eastern Nebraska for 5 yr caused improved soil N and P supply and increased corn ( Zea mays L.) yield in less productive portions of the field compared to another field treatment receiving the same amount of N as inorganic fertilizer. As a first step toward identifying the soil processes that led to these changes, the effects of manure addition on the chemical nature of soil organic matter were investigated by extracting two humic fractions—mobile humic acid (MHA) and calcium humate (CaHA)—from the field soil for subsequent analysis by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier Transform-infrared (FT-IR) spectroscopy. Functional groups were selectively identified and quantified through NMR techniques that included quantitative direct polarization, cross polarization/total suppression of sidebands, 1H- 13C heteronuclear correlation NMR, 13C chemical shift anisotropy filtering, CH-signal selection, and other spectral-editing techniques. The CaHAs exhibited high aromaticity and line shapes typical of Mollisols rich in oxidized charcoal, while the MHAs had lower aromaticity and greater contributions from lignin aromatic ethers. Results show that the chemical nature of each humic fraction did not differ significantly between an unfertilized control and the treatment receiving N as inorganic fertilizer. The FT-IR results were consistent with all these findings. The NMR results further showed that the CaHA fraction from the manure treatment was clearly depleted in aromatic rings and enriched in nonpolar alkyl compounds, most likely fatty acids, compared to the other two fertilizer treatments. Selective spectra further show strong indications of enhanced peptide contributions in the manure CaHA, consistent with the improved soil N supply associated with the manure treatment. The peptide enrichment was partially masked in unselective spectra by a simultaneous decrease in overlapping signals for OCH 3 and COO groups that are associated with aromatic components, demonstrating the improved accuracy of the selective spectral-editing NMR techniques.

Accumulation of heterocyclic nitrogen in humified organic matter: a 15N‐NMR study of lowland rice soils

SummaryRecent intensification of cropping and the attendant longer submergence of the soil for lowland rice in tropical Asia appear to have altered the nature of the soil organic matter, and perhaps also nutrient cycling. To identify the dominant forms of organic nitrogen in the soils we extracted the labile mobile humic acid (MHA) and the more recalcitrant calcium humate (CaHA) fractions from soils under several long‐term field experiments in the Philippines and analysed them by 15N‐nuclear magnetic resonance spectroscopy. Amide N dominated the spectra of all humic acid (HA) samples (60–80% of total peak area). Its proportion of total spectral area increased with increasing intensity of cropping and length of time during which the soil was flooded and was greater in the MHA fraction than in the CaHA fraction. Simultaneously the spectral proportion of free amino N and other chemical shift regions decreased slightly with increasing length of submergence. Heterocyclic N was detected at modest proportions (7–22%) and was more prevalent in more humified samples, especially in the CaHA of aerated soils. Correlations of spectral proportions of heterocyclic N with other properties of the HA, reported elsewhere, were highly significant. Correlations were positive with visible light absorption (r= 0.86) and concentration of free radicals (r= 0.85), both of which are indices of humification, and negative with concentration of H (r= −0.86), a negative index of humification. Correlations of spectral proportions of amide N with these properties were also highly significant but in each case of opposite sign to that of heterocyclic N. Proportions of heterocyclic N declined with increasing duration of submergence. The results suggest that (i) 15N‐NMR can reproducibly measure some portion of heterocyclic N, (ii) formation of heterocyclic N is associated solely with gradual humification occurring over many years, and (iii) the abundant phenols in the submerged rice soils did not promote formation of heterocyclic N, and hence some other process is responsible for a substantial decrease in the availability of native N associated with intensive rice cropping.

Accumulation of heterocyclic nitrogen in humified organic matter: a 15N-NMR study of lowland rice soils

Summary Recent intensification of cropping and the attendant longer submergence of the soil for lowland rice in tropical Asia appear to have altered the nature of the soil organic matter, and perhaps also nutrient cycling. To identify the dominant forms of organic nitrogen in the soils we extracted the labile mobile humic acid (MHA) and the more recalcitrant calcium humate (CaHA) fractions from soils under several long-term field experiments in the Philippines and analysed them by 15N-nuclear magnetic resonance spectroscopy. Amide N dominated the spectra of all humic acid (HA) samples (60–80% of total peak area). Its proportion of total spectral area increased with increasing intensity of cropping and length of time during which the soil was flooded and was greater in the MHA fraction than in the CaHA fraction. Simultaneously the spectral proportion of free amino N and other chemical shift regions decreased slightly with increasing length of submergence. Heterocyclic N was detected at modest proportions (7–22%) and was more prevalent in more humified samples, especially in the CaHA of aerated soils. Correlations of spectral proportions of heterocyclic N with other properties of the HA, reported elsewhere, were highly significant. Correlations were positive with visible light absorption (r= 0.86) and concentration of free radicals (r= 0.85), both of which are indices of humification, and negative with concentration of H (r= −0.86), a negative index of humification. Correlations of spectral proportions of amide N with these properties were also highly significant but in each case of opposite sign to that of heterocyclic N. Proportions of heterocyclic N declined with increasing duration of submergence. The results suggest that (i) 15N-NMR can reproducibly measure some portion of heterocyclic N, (ii) formation of heterocyclic N is associated solely with gradual humification occurring over many years, and (iii) the abundant phenols in the submerged rice soils did not promote formation of heterocyclic N, and hence some other process is responsible for a substantial decrease in the availability of native N associated with intensive rice cropping.