Changes in the Fractional Composition of Organic Matter in the Soils of the Forest–Steppe Zone during Their Postagrogenic Evolution

Abstract

The postagrogenic dynamics of organic carbon (Corg), total nitrogen (Ntot), and density fractions of organic matter (OM) in the dark gray soil (Haplic Phaeozem, Belgorod oblast, Les na Vorskle (Forest on Vorskla) Reserve) and migrational–mycellary Chernozem (Haplic Chernozem, Kursk oblast, Streletskaya Steppe Reserve) were examined. The most significant postagrogenic changes in all the studied soil properties were observed in the top 0–5 cm layer. The contents of Corg and Ntot in this layer of the Phaeozem chronosequence increased by 1.2–1.3 times over 45 years after cessation of farming. The contents of Corg and Ntot in the top 0–5 cm layer of Chernozem chronosequence increased by 1.9–2.0 times over 60 years of the postagrogenic development. The accumulation of Corg took place in all density fractions. In both chronosequences, the free fraction of organic matter (density <1.6 g cm–3) increased, while the occluded fraction (density <2.0 g cm–3) virtually did not change. In 60 years after the cessation of farming on Chernozem, all the studied parameters became close to those in the Chernozem under natural steppe. In the Phaeozem, these parameters recovered by no more than 60% of the level typical for the natural soil over 45 years after the cessation of farming. Thus, the postagrogenic restoration of organic matter and all its fractions in the forest-steppe zone was much faster in the Chernozem in comparison with the Phaeozem.