Disentangling drivers of soil organic carbon storage in deltaic rice paddies from the Ebro Delta

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Paddy farming can potentially sequester carbon. However, agricultural practices may alter the stability of the soil organic carbon (SOC) stocks and thereby increase carbon mobilization in the topsoil and subsoil. We hypothesize that both agricultural practices and soil physical–chemical characteristics, mostly those related to the type of the parental soil where the paddy is established, play an important role in SOC stabilization and sequestration. To test this, we profiled the biochemical characteristics of soil organic matter (SOM) and the SOC concentrations in sediment cores of 10 rice fields from the Ebro Delta (Northeast Spain) representing former reclaimed habitats (i.e., peatlands, meadows, coastal lagoons, riverbanks and salt marshes). The effects of physical–chemical soil properties on SOM content were tested with Generalized Linear Models and the best models were selected using information-theoretic approach. The optical characteristics of SOM extracts were analyzed by UV–Visible and fluorescence spectrophotometry, and six fluorescence components were identified by Parallel Factor Analysis (PARAFAC). Higher clay and lower sand content explained the greater SOM presence in topsoil while higher clay content and salinity prevailed as the main explanatory factors in the subsoil showing a positive correlation with SOM content. The results revealed a correlated gradient of SOM quantity and quality where in SOM-rich soils both the aromaticity and the humification degree were higher while in soils with lower SOM content it was fresher and more microbially derived. Current agricultural practices favored SOM humification in the topsoil while the variability in SOM quality in subsoil is attributed to the combination of soil physical–chemical characteristics (pH, conductivity and clay content) and the previous habitat’s influence. This study represents a comprehensive empirical analysis on the carbon stocks in rice fields and the identification of soil physical–chemical factors, related to prior land uses, favoring soil carbon accumulation. These results may have major implications in decision-making process for climate change mitigation in vulnerable Mediterranean coastal paddies.