Interactive effects of Ag nanoparticles/nitrate and plant root systems on quality indicators and aggregate stability of two texturally-different soils

Abstract

Widespread sources of silver nanoparticles (AgNPs) might threaten soil ecosystems. Most studies on NPs have been carried out in plant-free soils, which do not represent natural conditions. Monitoring the fate and possible effects of nanoparticles (NPs) in soil-plant systems is crucial for predicting their environmental consequences. Plant root systems might respond differently to Ag types/concentrations, associated with changes in microbially-induced soil structural stability as well as soil C pools but evidence is not available. Therefore, a greenhouse experiment was conducted in a factorial arrangement of treatments within a randomized block design. The treatments included: 1) soil types (loamy sand and sandy loam), 2) root systems (non-planted, wheat with fibrous roots and safflower with taproot), 3) Ag types (no-Ag added, AgNPs of mean size 38.6 nm, and AgNO₃), and 4) Ag concentrations (50 and 100 mg kg^–1 soil). Soil samples were collected from root zone and non-planted soil 110 days after sowing. Soil quality indicators including high energy moisture characteristic (HEMC) indicators, percent of water-stable aggregates (WSA), water-dispersible clay (DC), substrate-induced respiration (SIR), microbial biomass carbon (MBC) and metabolic quotient (q_CO2) were determined. The results showed that the soil structure was improved in the presence of Ag and plants. Structural stability indicators were greater in the safflower root zone followed by the wheat root zone and the non-planted soil. A clear effect of Ag on HEMC was observed in the 100 mg AgNPs kg^–1 treatment. The stability ratio (SR, ratio of fast-wetting to slow-wetting structural indexes) of the AgNPs-treated soils (SR = 0.79) was significantly greater than that of the AgNO₃-treated soils (SR = 0.78) followed by the control (no-Ag) soils (SR = 0.74). In the AgNO₃-treated soils, the SIR was significantly lower than in the AgNPs-treated soils. The SIR of the 50 mg kg^–1 Ag treatment (232 mg CO₂-C kg^–1) was higher than the 100 mg kg^–1 (227 mg CO₂-C kg^–1). Microbial biomass was significantly affected by Ag types/concentrations and all Ag-treated soils exhibited significantly lower MBC than control. The q_CO2, the index of stress to microbial community, was significantly greater in the Ag-treated soils. Scanning electron microscope images confirmed that AgNPs altered the arrangement of particles which was greater in the higher AgNPs concentration. These results imply that multiple factors (root systems, soil texture, Ag type/concentration) may combine additively/regressively to affect soil quality indicators, which may have important consequences for soil ecosystem services.