Characteristics of humic acids in sod-podzolic soil under long-term exposure to different fertilization systems

The elemental composition and structure of humic acids in the soddy-podzolic heavy loamy soil of the Cis-Urals were studied during long-term application of organic, mineral and organo-mineral fertilizers under conditions of a long-term stationary experiment. Mineral fertilizers contributed to the formation of humic acids with increased content of aliphatic components, the ratio H : C = 1.45 and a high degree of oxidation (ω = 0.41). The low content of nitrogen in humic acids (from 2.0 to 2.6 at.%), with various fertilizer systems, is primarily due to its low content in the plant biomass of crops cultivated in crop rotation.The organic fertilizer system resulted in the enrichment of the structural aggregates of humic acids with aromatic groups and nitrogen. The high intensity of absorption of the >C=O group of carboxylic acids in the region of 1 717 cm-1 and double bonds of carbon atoms at 1 627 cm-1 confirms this position. Humic acids of the “manure 10 t/ha” and “manure 5 t/ha + equiv. NPK” have the largest number of carboxyl groups in their composition. The vibrations of the >C=O group of carboxylic acids are most weakly expressed in soil humic acids with a mineral fertilizer system. Organo-mineral fertilizer system demonstrates intermediate results in terms of the content of the main constitutional elements in humic acids. Mineral fertilizers application reduces the amount and thermal stability of the structural components of humic acids, thereby contributing to the degradation of the most stable part of macromolecule, which can lead to the loss of potential soil fertility. The use of mineral fertilizers on sod-podzolic heavy loamy soil led to the enrichment of humic acids with aliphatic fragments, and of organic ones - increased the content of stable components of aromatic nature, but lowered their thermal stability. According to thermographic analysis the ratio of weight loss in the low-temperature zone and the weight loss in the high-temperature zone (Z) during long-term use of mineral fertilizers is 0.9, i. e. the proportion of components of aliphatic nature in the structure of humic acids increases in comparison with unfertilized or manure fertilized soil.