Development capacity of a biodiverse pasture on Technosols for the rehabilitation of marginal lands (saline soils and mining waste)

In the coming years, food demand will increase sharply in line with population growth. So, production will have to increase mostly by area expansion, which could lead to biodiversity loss and an increase in greenhouse gas emissions. Furthermore, both land and water resources are limited and already under severe pressure, making it imperative to ensure a more productive but also sustainable agricultural system. A possible solution to this could be the reclamation of marginal lands, such as saline and drought-prone lands, or even abandoned mining areas. In this sense, phytostabilisation is considered a suitable method for their rehabilitation and reconversion to agricultural and livestock activities while protecting the food chain. Some pasture plants can tolerate adverse growth conditions, such as mine waste or soil conditions (e.g. high concentrations of potentially hazardous elements (PHE) and EC, low pH, organic C and nutrients, and poor structure and water holding capacity). However, low and slow plant growth can limit environmental rehabilitation success. The combined use of Technosols and pastures may be an effective green technology towards reclaiming these marginal areas for food production. To verify this hypothesis, we studied the development of a biodiverse pasture in two biogeochemically distinct Technosols. The pasture is composed of leguminous plants of the genus Trifolium sp. (T. michelianum var. paradana Savi, T. vesiculosum var. cefala Savi, T. resupinatum var. nitrofolus L., and T. squarrosum L.) and Medicago sp., and gramineous plants such as Lolium multiflorum Lam. Two Technosols have been built using a saline Fluvisol collected in the Tagus Estuary and a gossan waste from the São Domingos mine together with a mixture of organic and inorganic amendments. The microcosm assay consisted of four treatments set up in pots of 1.5 dm³ volume (four replicates): (i) Fluvisol (VF), control of salinity affection (EC: 7.9 dS/m; exchangeable sodium 25%); (ii) Technosol-Fluvisol (TVF); (iii) gossan waste (G), control of PHE contamination (g/kg As: 9.1; Pb: 29.6) ; and (iv) Technosol-Gossan (TG). One month after sowing (5 g seeds per pot), the pasture biomass generated so far was mowed to simulate livestock grazing. Pasture in FV showed no seed germination, thus no plant growth. In contrast, in G (10 cm stem length and 1.41 g DW), was observed seed germination followed by plant growth, with gramineous dominating over leguminous plants. In the Technosols, pasture growth improved extraordinarily, with plants reaching more than 15 cm stem length and higher biomass with 2.59 g DW on TVF and 3.8 g DW on TG. Moreover, the pasture was more biodiverse, with the presence of as many leguminous as gramineous. Upon the first cut, the pasture was left for another month for the plants to regrow, and then mowed for the last time. During this time, L. multiflorum has kept growing to similar sizes to the first mowing, while the leguminous have not regrown as effectively. Thus, an integrated biotechnological approach involving Technosols and pastures could be a useful green technology to convert marginal lands into food production areas (grazing or foraging).