Storage of persistent organic matter in temperate gypsum soils –Relevance of the parent material and vegetation cover

Abstract

By storing large amounts of terrestrial carbon (C), soils prevent it from being released into the atmosphere as carbon dioxide, the main greenhouse gas. We examined soils derived from gypsum and limestone, assuming the former had a special ability to accumulate and stabilise soil organic matter (SOM) under temperate climatic conditions when compared to limestone-derived soil. We compared the benefit of gypsum or limestone as a parent material and the effect of vegetation cover on the quantity and quality of SOM. Properties of the soil humus horizons were determined, including texture, available potassium (K) and phosphorus (P), pH, amount of gypsum and calcite, enzyme activity, and vegetation cover (biomass, K, P, sodium, calcium (Ca), and amount of lignin in the plant upper parts, coarse roots and fine roots). The structure and composition of humic acids (HAs), as indicators of SOM quality, were also studied. Stocks of organic carbon accumulated in 0–20 cm depth were much higher in gypsum soils (9.72 and 10.09 kg/m2) than in limestone soils (5.02 and 6.36 kg/m2). We found that the SOM in the gypsum soils was more mature, and contained more aromatic HAs, than in the limestone soils. Also, the gypsum soils had higher enzyme activity and a greater mass of vegetation cover, the fine roots providing higher amounts of lignin than the limestone soils. The factors that shaped the soil properties were identified and the interaction between the parent material and the vegetation cover and its effect on SOM storage was determined, indicating the importance of fine-root composition (lignin, Ca, K) and enzyme activity on C accumulation. Gypsum as a parent material favoured SOM accumulation through its physical stabilization, by providing active Ca cations to protect the SOM together with silt and clay, and its biochemical stabilization, through high enzyme activity and the quantity and quality of the biomass. Our results show that gypsum soils are important as a response to global warming through their ability to preserve SOM over the long term.