The Content and Stocks of Organic Carbon and Labile Carbon Fractions Are Key Properties for the Sustainable Use of Terrestrialized Former Mill Ponds

The basins of former mill ponds are a common element of the cultural landscape, while soil science studies focusing on mill pond sediments remain relatively rare. One of the key properties relevant to the sustainable management of these sites is the content and stocks of organic carbon in the soils developed from the mill pond sediments. Therefore, the study aimed to analyze the total organic carbon (TOC) and total nitrogen content, as well as its stocks in the studied soils, the share of the labile fractions of these compounds in relation to the environmental factors influencing these properties and finally their potential changes over time (stability). The obtained results confirmed that the studied soils (mostly classified as Gleyic Fluvisols and Fluvic Phaeozems) are characterized by a high amount of TOC. This manifests in the large TOC stocks in the upper 50 cm (mean = 18.1; min = 8.35; max = 35.5 kg m−2) and in the whole soil profiles (mean = 43.7; min = 10.6; max = 93.6 in kg m−2). The dissolved organic carbon share in TOC content was slightly over 1% on average (mean = 1.06; min = 0.19; max = 4.75) and the specific UV absorbance averaged about 2.40 L mg C−1 m−1, indicating the relatively large proportion of recalcitrant organic fractions and low potential for further degradation.