Cropping History and Cropland Abandonment Period Influenced Levels of Soil Organic Carbon, Distribution of Aggregates and Aggregate Associated Carbon

Abstract

Knowledge gaps remain regarding soil organic carbon (SOC) storage and aggregate dynamics under varying cropland abandonment periods, particularly within the context of degraded and low-SOC soils. This study aimed to assess the influence of cropping history and the period of cropland abandonment period on SOC, aggregate distribution, and aggregate associated carbon in the Eastern Cape, South Africa. Two abandoned cropland sites, namely 2 LUC (abandoned since 2021) and 6 LUC (abandoned since 2017), were examined. Soil samples were collected at depth intervals of 0–10 cm, 10–20 cm, and 20–30 cm. The samples were analyzed for soil texture, pH, organic carbon, humic and fulvic acids, exchangeable Ca, Mg, and K, wet and dry aggregate distribution, mean weight diameter (MWD), and aggregate associated organic carbon. Dry and water stable microaggregates plus individual particles dominated in 2 LUC, while 6 LUC showed an even distribution of large and small macroaggregates, along with microaggregates plus individual particles. Wet and dry MWD were significantly higher in 6 LUC across all depths. Macroaggregate associated organic carbon did not differ significantly between the sites and depths, whereas microaggregate associated organic carbon was consistent between sites but decreased with depth from 0–10 cm to 10–20 cm. The results highlight the complex interaction between soil structure, aggregate stability, and carbon content, suggesting that a combination of factors such as cropping history, soil disturbance, and biological activity shape aggregate characteristics, in addition to the effects of SOC. Additionally, the duration of cropland abandonment plays a key role in overcoming the legacy effects of prior land use.