Trifolium subterraneum as a potential nurse plant for restoring soil and Mediterranean grasslands after quarry exploitation in Lebanon

Destruction of Mediterranean herbaceous communities resulting from quarry exploitation exacerbates the need for their ecological restoration. By facilitating the growth and development of other plant species, nurse plants can be used to speed up regeneration processes. Nevertheless, differences in soil properties and co-existing species may change plant interactions from facilitation to competition. This study evaluated interactions of a potential nurse species, Trifolium subterraneum L., in a 2-years ex situ mesocosm experiment. During the first year, soils collected from a former quarry and a nearby dry-grassland were planted in pots with T. subterraneum alone or with four individuals of four target species (Aegilops triuncialis L., Echinops viscosus L., Eryngium creticum Lam., Hyoscyamus aureus L.) characteristics of Mediterranean grasslands. In the second year, the same experiment was repeated following the incorporation of dried T. subterraneum biomass into the soil. Soil analyses were done before initiating the experiment from both reference and degraded soil, after the first and the second growing season. Index of Relative Interaction Intensity (RII) was calculated for several plant traits enabling the measuring of the plants' competitive ability. Our results show that, before starting the experiment, soils collected from both the reference and degraded sites were significantly different (e.g. CEC, pH, P₂O₅, MgO, N, C, Clay). These differences were still notable throughout the first growing season. Nevertheless, after the second growing season and whendried. T. subterraneum biomass was incorporated into the soil, a significant increase in soil fertility (e.g. N, C, C:N, P₂O₅) was measured. For plants, during the first growing season, RII figures amidst competition dominating interactions. Facilitation was apparent only between the biomass of T. subterraneum pure seed mix in both degraded and reference soil. During the second growing season, T. subterraneum also performed better alone, especially on degraded soils. Nevertheless, because of its facilitative effects on biomass and surface cover when cultivated as a pure stand, T. subterraneum can be recommended as a nurse species for re-establishing soil fertility of degraded quarries as a first step of ecological restoration.