Molecular and elemental compositions of humic acids of arable soils in north forest-steppe zone of Russia

The degradation of arable lands is becoming widespread and leads to significant loss of soil organic matter (SOM), expressed both in quantitative losses and in qualitative changes in the molecular and elemental composition of SOM. In this regard, a study of soils in the forest-steppe zone of Russia was conducted on the territory of the proposed Carbon Polygon in the Tambov Region and its humus state was characterised. Four contrasting types of land use were selected: long-term arable land, fallow land (60 years of self-restoration), a shelterbelt site (as a zonal reference) and an eroded site with a buried arable horizon. Carbon and nitrogen stocks were investigated, the element and molecular composition of humic acids (HAs) being analysed. This set of sites allows us to analyse different stages of SOM transformation under varying scenarios of land use, ranging from active degradation to post-agrogenic recovery. To analyse the molecular structure, the method of 13 C NMR spectroscopy was used. As a result of the study, it was found that the type of land use determines both the quantitative indicators of SOM and nitrogen stocks and the molecular structure of soil HAs. It was established that, in soils of fallow lands, a significant amount of SOM accumulates (up to 50.81 ± 0.96 kg/m 2 ), which possesses a relatively high level of resistance to biodegradation, determined by a high concentration of aromatic structural fragments. It was noted that arable and fallow soils may have high carbon sequestration potential compared to zonal soil and eroded site. Long-term agricultural practices lead to the formation of relatively stable molecular complexes of HAs, which show signs of oxidative degradation of SOM due to an increased proportion of carboxyl structural fragments.