g - Thesis

Abstract:

Recent control of atmospheric S pollution is leading to important soil sulphur impoverishment (Varin, 2009, 2017). As a result, a decrease in soil sulphur content has appeared in the last decades in grasslands of Northern Europe. White clover (Trifolium repens L.) is a key species in grasslands and was chosen as a model species. These phD focused on clover response to sulphur availability.

Clover plasticity was assessed through the measurement of a set of functional traits along a gradient of nitrogen and sulphur fertilisation applied on a poor soil or a gradient of sulphate concentration when cultivated on a hydroponic system. A ¹⁵N / ³⁴S double labelling was used to measure nitrate and sulphate uptake, N₂ fixation an atmospheric S foliar deposition.

White clover traits showed a great phenotypic plasticity (Pp) (Varin et al., 2009a; Varin et al., 2009b, 2017), due to both morphological and physiological plasticity. Sulphur availability modify clover biomass as well as major functions such as the potential competitive ability, the vegetative reproduction, photosynthesis and sulphur and nitrogen acquisition (Varin et al., 2009a, 2017). Sulphur effect on clover is linked to N₂ fixation. N₂ fixation was drastically reduced in S deficient plants mainly as a consequence of a decreased nodule development, but also as a consequence of decreased protein synthesis in nodules (Varin et al., 2010). Nitrogenase and leghaemoglobin were lower in S deficient nodules. S derived from the atmospheric deposition (Sdad) through foliar deposition appeared to be a significant source of S for clover growth (Varin et al., 2013).Sdad:leaf area ratio increased with decreasing S concentration in the nutrient solution.

In contrast with N fertilisation, S fertilisation could help to maintain white clover in grassland.