Characterization of Humic Carbon in Soil Aggregates in a Long‐term Experiment with Manure and Mineral Fertilization

Abstract

Carbon stabilization in soil aggregates has been proposed as an important mechanism for carbon sequestration in agricultural soils. However, few studies have investigated how long-term strategies for accumulating C In the soil (e.g., organic fertilizers) can affect humic substance composition in aggregate-size fractions. The aim of this work was to investigate the effects of long-term application of farmyard manure (FYM) and mineral (MIN) fertilization on the humic carhon (HC) chemical structure extracted from macroaggregates (MA) (2000-250^lm), microaggregates (Ml) (53-250 \¡m) and silt-clay (SC) (<53 pm) fractions of two different soils (clay and peaty) hy using ô^-^C, solid-state 13C cross-polarization-magic angle spinning nuclear magnetic resonance (CP MAS 13C NMR) and diffuse reflectance infrared Fourier transform (DRIFT) analyses. Results showed that FYM had improved the HC concentration compared with the unfertilized and MIN fertilization. In clay soil, MIN fertilization led to a significantly heavier isotope 13C enrichment in the SC than macro-and microaggregate fractions, suggesting that HC in this fraction was the oldest and most stabilized. This was not so evident in peaty soil, where the HC was prohably in an early stage of the humification process. In addition, the HC structure changed in both soils as a consequence of the treatments. The FYM amendment caused a decrease in O, N-alkyl-C, and alkyl C from macroaggregate to SC fractions, suggesting an advanced state of humic component degradation. However, FYM led to a considerably positive effect on the aromatic C content in both soils. This effect was magnified in the microaggregates of clay soil. These results revealed that the structural changes of HC in different soil aggregates make it possible to understand the effects of mineral and organic fertilization on cultivated soils.

Soil Chemical Insights Provided through Vibrational Spectroscopy

“…moisture content, soil mineralogy). For instance, whole soil samples have been employed in indices of hydrophobicity-hydrophilicity contributed to by OM (Ellerbrock and Gerke, 2004;Ellerbrock et al, 2005;Matejkova and Simon, 2012;Simon, 2007;Šimon et al, 2009;Simonetti et al, 2012). Areas of the aliphatic vs(C-H) band at 3000-2800 cm −1 as the hydrophobic region and a hydrophilic region of vs(C]O) at 1740-1600 cm −1 of whole soil spectra provided hydrophobic/hydrophilic ratios.…”

“…As a result, researchers commonly use extractions of representative or C-enriched fractions in order to obtain detailed FTIR data on the organic component of soil (Simonetti et al, 2012). SOM fractions can be highly enriched in OM or are dominantly OM, allowing for better detection.…”

“…Chemical and physical fractionation can be coupled to increase the specificity of FTIR data, as was done by Simonetti et al by extracting humic acids from macroaggregates, micoraggregates, and the silt + clay fraction (Simonetti et al, 2012). FTIR analysis of SOM using chemical and physical fractionations are discussed below.…”

“…HS have been traditionally understood as a pool of stabilized, recalcitrant SOM Schnitzer and Monreal, 2011). HS are an alkaline-soluble fraction of SOM, and FTIR is performed on further fractions based on solubility in acid and base (Kerek et al, 2003;Mao et al, 2008;Senesi et al, 2007;Simonetti et al, 2012;Watanabe et al, 2007). These operationally defined components of HS include humic acids (HA), fulvic acids (FA) and humin, with similar postulated structures but differing and characteristic molecular weight, elemental composition, and functional group content (Khan, 1975).…”

Humified Fraction of Organic Matter Due to Plant Mixture Cultivation

“…The application of spectroscopy in the infrared region using diffuse reflectance infrared Fourier transform (DRIFT) on soil samples is comparatively cheaper than 13 C NMR, easy to prepare and fast performing (a few minutes per sample) (LIEFELD, 2006;FRANCIOSO et al, 2009;CARLETTI et al, 2010;FERRARI et al, 2011;SIMONETTI et al, 2012), including the assessment of resulting changes from management.…”