Improvement of physical, chemical and biological properties of saline soils and gossan waste through integrated biotechnological approach: Technosols and pasture development

In coming years, food demand will rise sharply in line with population growth. This demand will be met largely through the expansion of agricultural land, a limited and strained resource. Therefore, it is crucial to ensure a more productive but also sustainable agricultural system. The reclamation of marginal lands underused, such as saline and drought-prone lands, or even abandoned mining areas, could be a possible solution. In this sense, phytostabilisation is considered a suitable method for their rehabilitation and reconversion to crop and livestock activities. Some pasture plants can tolerate adverse growth conditions (e.g., high concentrations of potentially hazardous elements (PHE) and EC, low pH, organic C and nutrients, and poor structure). However, limited and slow plant growth can limit the 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, this study evaluated the improvement saline soils’ properties and gossan waste through Technosols and development of a biodiverse pasture. A Technosol was constructed using a saline Fluvisol of the Tagus Estuary and another one utilizing a gossan waste from the São Domingos mine, both with a mixture of organic and inorganic amendments. Four treatments were considered: (i) Fluvisol (VF), control of salinity; (ii) Technosol-Fluvisol (TVF); (iii) gossan waste (G), control of PHE contamination; and (iv) Technosol-Gossan (TG). After two months of pasture growing, samples from each treatment were characterised for soil properties, nutrient pool, soil enzyme activities, and aggregate stability, and compared to initial conditions. Both Technosols were effective in attenuating the main disturbances of these degraded environments. In TG, the acid pH of the gossan waste was neutralised (G: 3.8 - 4.1, TG: 6.5 – 6.7). Similarly, in TVF, salinity (EC) and exchangeable Na were reduced by about 65% and 60%, respectively. Nutrient pool in both Technosols was also enhanced by overall increases in organic C, N, P, K, Ca, Fe, Zn and Cu. Likewise, microbiological activity has been stimulated by Technosols and establishment of a pasture. For example, in G dehydrogenase activity was practically null, and in TG although it was also reduced/small, it increased 15 times more. While with the pasture, it increased 6-fold. In FV there was already some dehydrogenase activity, but in TVF it was six times higher and with the pasture even more elevated, reaching values like those of a healthy grassland. The other enzymes analysed (β-glucosidase, cellulase, acid phosphatase, urease, and protease) were also stimulated. Better aggregation was also observed in Technosols, as the number of larger aggregates (> 2 mm) was higher than in controls. In TG these represented 68% compared to 34.5% in G, and in TVF 93.6% compared to 32.8% in VF. Finally, all these improvements have allowed the establishment of a good pasture. Thus, the combined use of Technosols and pastures may be an effective green technology to convert marginal lands into food production areas (grazing or foraging).