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

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 carbon (HC) chemical structure extracted from macroaggregates (MA) (2000-250 mu m), microaggregates (MI) (53-250 mu m) and silt-clay (SC) (< 53 mu m) fractions of two different soils (clay and peaty) by using delta C-13, 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 probably 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.