Seasonal Sulfur Over-Accumulation and Coordinated Nutrient Cycling in Ononis Species With Contrasting Gypsum Affinity

Gypsum soils present extreme nutrient imbalances, with scarcity of nutrients like N, P, and K and excess of Ca and S, seriously limiting plant growth. Plant species differing in their affinity for gypsum soils display contrasting nutritional strategies. Gypsum-exclusive species (gypsophiles) typically accumulate large amounts of S, Ca, and Mg throughout their tissues, whereas gypsum-tolerant species show restricted translocation of these elements aboveground, resulting in lower foliar concentrations. These contrasting strategies may influence seasonal nutrient cycling in different plant organs, yet this has not been previously investigated. We tested the hypothesis that seasonal macronutrient cycling (N, P, K, S, Ca, and Mg) and foliar nutrient resorption from senescing leaves would differ in natural populations of two closely related Ononis species (Fabaceae) with contrasting gypsum affinity: the gypsophile Ononis tridentata L. and the gypsum-tolerant Ononis fruticosa L. For O. fruticosa, we additionally compared seasonal nutrient cycling between populations growing on and off gypsum. Our results reveal a pronounced capacity of the gypsophile species to over-accumulate S and accrete it in leaf litter. Moreover, we identify a striking intraspecific variation in the seasonal S cycling of gypsum-tolerant O. fruticosa, with populations at the gypsum site showing patterns that converged towards those observed in O. tridentata. These findings suggest a potential role for foliar S over-accumulation and coordinated seasonal dynamics of S, Ca, and Mg in plant adaptation to gypsum soils. They further indicate that nutrient accumulation and cycling are associated with local site conditions and not exclusively determined by fixed species traits.