Characterization of Humic Acid from No‐Tilled and Tilled Soils Using Carbon‐13 Nuclear Magnetic Resonance

Abstract

Chemical composition of humic acids is important due to their reactivity with soil‐applied pesticides and fertilizer amendments. Humic acids from continuously (7 yr) no‐tilled and tilled cotton, corn, and soybean plots in West Tennessee were characterized by 13C‐NMR from soils sampled at various depths. Humic acids were extracted with dilute NaOH solution and purified with KOH‐KCL and HCL‐HF solution, respectively. NMR solution spectra generated on a Jeol FX 90Q spectrometer (Jeol Co., Tokyo, Japan) were divided into six regions (0–40, 40–62, 62–105, 105–150, 150–170, and 170–190 ppm) and peak areas compared. Humic acid composition differed by soil depth and crop. Small differences were observed between tillage systems. Aliphatic and aromatic carbons ranged from 48 to 65% and 25 to 40% of total peak area, respectively. Carboxyl groups of humic acids as determined by 13C‐NMR ranged from 9 to 13%. Humic acids extracted from those soils containing larger amounts of C (surface no‐tilled treatments) had greater aliphatic to aromatic ratios, perhaps due to earlier stages of decomposition.

Pyrolysis-field ionization mass spectrometry of agricultural soils and humic substances: Effect of cropping systems and influence of the mineral matrix

“…Differences between cropping systems are observed far less in the HS spectra than in spectra of whole soil samples, in agreement with the smaller differences in their C and N concentrations (Table 1). This is also in line with results of Stearman et al (1989) who reported only minor effects of tillage on humic acid structures by 13C NMR, irrespective of clear differences in the Corg concentrations of whole soils.…”

Changes in soil organic carbon concentration, chemical composition and aggregate stability as influenced by tillage systems in the semi-arid and semi-humid area of North China

“…and aliphatic C (0-110 ppm). Other studies have reported similar findings (Stearman et al 1989;Arshad et al 1990;Dieckow et al 2009). The average accumulation of alkyl C under NTS (24.1%) and CTS (20.4%) relative to CT (16.2%) across the two soil layers may be attributable mainly by a selective preservation and an in situ synthesis of carbon (Sollins et al 1996;Baldock and Skjemstad 2000).…”

“…Tillage management affects the distribution of SOC and its chemical composition (Bayer et al 2002;Ding et al 2002;Du et al 2010;Zhao et al 2012;Piccoli et al 2016). The concentration of SOM in conventional tillage (e.g., moldboard plow) soils is often associated with a decrease in aliphatic C, and an increase in aromatic and carbonyl C and concentrations of semi-quinone free radicals (Stearman et al 1989;Bayer et al 2000), leading to greater SOM humification. In contrast, conservation tillage usually enhances accumulation of surface SOM as reflected by higher stratification ratios (SRs) (Sá and Lal 2009;Du et al 2010), which was defined by Franzluebbers (2002) as the value of quotient in the SOC at the surface and deeper layers.…”

Mean residence times and characteristics of humic substances extracted from a Taiwan soil

“…The freeze-dried humic fractions were subjected to solid state 13 C NMR analysis on a Bruker DSX 400 with cross polarization and magic-angle spinning (CPMAS), proton decoupling with contact time 2.00 ms, recycle time 5.00 s, spectral width 20000.0 Hz, spinning speed 3500 Hz, number of scans 6000, spectrometer frequency 50.330 MHz, and acquisition time 51.25 ms. According to the method recommended by Wilson (1981), Wershaw (1985), and Stearman et al (1989), the range of chemical shift of 0-190 ppm of the solid state CPMAS 13 C NMR spectra were divided into the ranges of 0-50, 50-90, 90-110, 110-140, 140-160, and 160-190 ppm. The areas covered by the spectra in these ranges were used to estimate the distribution percentages of characteristic carbons in the various samples.…”