A coupled carbon, aggregation, and structure turnover (CAST) model for topsoils

Abstract

The current multi-pool soil organic carbon (SOC) models, although a major improvement over the single pool ones, are not always able to capture soil saturation capacity and give reliable predictions for climate change effects, since they do not account for environmental constraints, like physical protection. In this work, we developed a soil carbon, aggregation, and structure (CAST) turnover model based on the concept suggested by many authors in the scientific literature that macroaggregates are formed around particulate organic matter, followed by the release of micro-aggregates. A simplified mechanistic Nitrogen model was also developed. The CAST model was evaluated by field data of cropland to set aside conversions of Critical Zones Observatories in Greece (fine textured Mediterranean) and Iowa (coarse textured humid continental). The model was able to capture the carbon content and the C-to-N ratio content of the pools comprising the three aggregate types (macroaggregates: >250 μm, micro-aggregates: 53-250 μm, silt-clay sized aggregates: <53 μm) in both sites. The soil system reached maximum macro-aggregation/porosity and minimum bulk density after 7 and 14 years in Greece and Iowa, respectively. Afterwards, macro-aggregate disruption presented a constant seasonal pattern and any further SOC increase was due to micro-aggregation resulting in the increase of bulk density and decrease of porosity towards to a stable value. The CAST model can assist in revealing primary factors determine organic matter, aggregation, and structure turnover in different ecosystems and in describing the response of the soil system to management practices, land use changes, and climate change in order to design and optimize the appropriate measures/practices.