Citation: Hasan, O.; Miko, S.; Mesi´ c, S.; Peh, Z. Chemical Weathering Rates of Soils Developed on Eocene Marls and Sandstones in a Mediterranean Catchment (Istria, Croatia). Land 2023, 12, 913. https:// doi.org/10.3390/land12040913 Academic Editor: Augusto Pérez-Alberti Received: 24 March 2023 Revised: 13 April 2023 Accepted: 17 April 2023 Published: 19 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). land Article Chemical Weathering Rates of Soils Developed on Eocene Marls and Sandstones in a Mediterranean Catchment (Istria, Croatia) Ozren Hasan , Slobodan Miko * , Saša Mesi´ c and Zoran Peh Croatian Geological Survey, Sachsova 2, 10000 Zagreb, Croatia; ohasan@hgi-cgs.hr (O.H.); zpeh@hgi-cgs.hr (Z.P.) * Correspondence: smiko@hgi-cgs.hr; Tel.: + 385-1616-0888 Abstract: Physical and chemical weathering, together with biological and biochemical processes, form soil from bedrock and strongly influence the chemical composition of natural waters. Erosive processes, primarily through the agents of running water and wind, remove the products of weath- ering from catchments. The aim was to determine the chemical weathering of minerals because of changes in land-use and natural forestation in two small neighboring catchments of the rivers Argilla and Bazuja. Agricultural land-use practice is very intense in the Argilla catchment, while the Bazuja catchment’s arable land is mostly abandoned, with progressive forestation. Chemical weathering in soils and sediments was evaluated with the aid of bulk chemistry analysis focused on major elements, trace elements, and zirconium. Weathering indices, mass balance, and strain were calculated. The abandonment of arable land and intense forestation in the Bazuja catchment caused increased chemical weathering with the loss of base cations (Ca and Mg) and enrichment of conservative elements (Zr and Ti) in surface horizons. EIC and MTF values are positive (enrichment) in areas with agricultural activities, while forested areas show negative values (loss). A comparison of the oldest and youngest parts of the overbank sediment profiles in the swallow hole zone and stream sediments shows that chemical and mechanical weathering in the Bazuja catchment was similar to present weathering in the Argilla catchment, while agriculture was active in the Bazuja catchment. The integrated knowledge gained in small catchment studies can be broadly applicable to larger systems. Keywords: chemical weathering; small catchment; land-use change; weathering index; mass balance; strain; forestation 1. Introduction Abandoning arable land and natural forestation has a significant effect on weathering. Two catchments in the Dragonja watershed were selected as representative sites to study the effects of natural reforestation and agriculture on chemical weathering and physical erosion. This watershed was selected because, in the short period of approximately ten years after World War II, significant changes in reforestation occurred [1,2]. Due to political but also economic conditions, there was an exodus of the local population, which caused abandonment or a change in the use of the land. The reforestation has led to a decrease in suspended sediment supply to the lower reaches of the Dragonja catchment [2–4]. Deforestation is an ongoing land cover change process in Europe [5] with the effects of soil erosion, degradation of soil properties, and nutrient losses [6–9]. The impacts of soil erosion go beyond the denudation of topsoil and reduction of soil fertility [6,10]. It results in major land and environmental degradation, and can lead to increased pollution and sedimentation in streams and rivers, causing declines in the ecosystem [10,11]. Erosion can create an increased sediment load, which directly impacts the flood-carrying capacity of streams and rivers and can eventually lead to flooding [11,12]. Since 1990, an opposite process, similar to the one in the Dragonja catchment, has been taking place in the EU, where forests increased by 10% between 1990 and 2020 [13]. Industrialized countries increased the Land 2023, 12, 913. https://doi.org/10.3390/land12040913 https://www.mdpi.com/journal/land