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Biochar application as a win-win strategy to mitigate soil nitrate pollution without compromising crop yields: a case study in a Mediterranean calcareous soil

Martos, Soledad, Mattana, Stefania, Ribas, Angela, Albanell, Elena, Domene, Xavier
Journal of soils and sediments 2020 v.20 no.1 pp. 220-233
Hordeum vulgare, Mediterranean climate, agroecosystems, alkaline soils, barley, biochar, biomass, calcareous soils, case studies, chlorides, crop yield, ecophysiology, ecosystem services, electrical conductivity, fertilizer application, functional diversity, gasification, genes, global change, magnesium, microbial activity, nitrates, nitrogen, nitrogen content, nitrogen fertilizers, nutrient content, nutrition, pH, photosynthesis, plant nitrogen content, pollution, pollution control, potassium, soil water, soil water retention, stomatal conductance, sulfates, wood
PURPOSE: The environmental benefits of biochar application, ranging from improvements in crop yield to global change mitigation, have been extensively studied in the last decade. However, such benefits have not been profusely demonstrated under a Mediterranean climate and still less in combination with high pH soils. In our study, the short to medium effects of biochar application on a soil-plant system under Mediterranean conditions in an alkaline soil were assessed. MATERIAL AND METHODS: Barley plants were grown in field mesocosms during three agronomical years at three biochar addition rates (0, 5, and 30 t ha⁻¹). Related to soil, different physicochemical parameters were analyzed as well as microbial respiration, biomass, and functional diversity. In the plant domain, in vivo ecophysiology variables such as leaf transpiration rate, stomatal conductance, and photosynthesis rate were determined while photosynthetic pigment content and soluble protein concentrations were measured in the laboratory. Additionally, crop yield and nutrient composition were also analyzed. The soil-plant connection was investigated by the N content ratio in both fractions establishing the nitrogen efficiency in the system. RESULTS AND DISCUSSION: The highest rate of biochar amendment enhanced soil moisture and electrical conductivity combined with an increase of SO₄²⁻, Cl⁻, Mg²⁺, and K⁺, and decrease of NO₃⁻ and HPO₄⁻. Notable variations regarding nutrition and moisture were induced in this Mediterranean alkaline soil after biochar addition although pH remained stable. Contrastingly, there were no major effects on microbial activity, but a lower abundance of the nosZ functional gene was found. Similarly, plant parameters were unaffected regarding chemical composition and ecophysiology although biochar induced a higher efficiency in the plant nitrogen uptake without increasing crop yield. CONCLUSIONS: Biochar addition at the highest rate (30 t ha⁻¹) reduced soil-soluble nitrate although N uptake by the plant remained invariable, in turn coupled to no effects on crop productivity. Our study showed that, in a Mediterranean agroecosystem, a wood biochar produced by gasification was unable to increase crop yield, but enhanced soil water retention, decreased the need for N fertilization, and decreased soil-soluble nitrate concentrations, something that could help to mitigate the excessive nitrate levels associated with over-fertilization.