Long-term biosolids use in agricultural lands : effects on selected soil chemical properties, accumulation of triclosan and sulfamethoxazole in the soil and maize plant

Abstract

Biosolids are the transformed products of sewage sludge produced through various sludge stabilization processes including drying, dosing and digestion. Biosolids are commonly applied to agricultural lands to improve soil organic matter, supply plant nutrients, and thus rehabilitate degraded lands. Hence, improving crop production and playing a role in poverty alleviation. However, there is little or no information on the long-term effects of biosolid use in agricultural lands on soil chemical parameters and accumulation of emerging contaminants in sub-tropical Africa in general and in South Africa in particular. The aim of this study was, therefore, to investigate the long-term (> 15 years) effects of biosolid use in agricultural lands on a) soil organic matter, nitrogen, salinity, and soil pH, b) the accumulation of sulfamethoxazole and triclosan in the top 30 cm soil layer, and c) uptake of sulfamethoxazole and triclosan by maize crop. It was hypothesized that long-term (> 15 year) biosolids use in agricultural lands will, a) improve soil organic matter and nitrogen content, b) not compromise soil productivity through salt accumulation as long as soil pH is maintained through liming, c) not lead to significant accumulation of sulfamethoxazole and triclosan in the top 30 cm soil layer to compromise soil quality, and d) not lead to significant uptake of sulfamethoxazole and triclosan by plants to cause food safety related health concerns. This study was conducted on a controlled long-term field trial (> 15 years) consisting of rainfed-irrigated and rainfed maize plots arranged in a completely randomized block design. Both rainfed and rainfed-irrigated plots were treated annually with three biosolid application rates (4, 8, and 16 t ha-1), one commercial inorganic fertilizer (according to soil analysis and crop nutrient requirements) and a zero control (without biosolid and commercial inorganic fertilizer). At crop maturity, maize plants and soil samples were collected and prepared for selected soil chemical parameters (soil organic carbon, pH, electrical conductivity and total nitrogen) and emerging contaminants (triclosan and sulfamethoxazole) analyses. Soil samples were collected only from the top 30 cm soil layer (the plough layer (20 cm) plus 10 cm below the plough layer considering possible illuviation) due to high analytical costs. Soil organic matter was determined by oxidation method following Walkley-Black’s procedure. Soil pH and electrical conductivity were measured in soil-water suspension of 1:2.5. Total soil nitrogen was analysed by dry combustion method using Carlo Erba NA1500 C/N analyser. Extraction of triclosan and sulfamethoxazole from soil and maize samples was performed using Ultrasound Assisted Extraction (UAE) method. Prior to extraction, samples were spiked with a known concentration of a surrogate standard (triclosan, TCS 13C12) to assess the method performance. The mean recovered concentrations of TCS 13C12 ranged from 75 % (in leaves) to 105 % (in soil). Clean-up was performed using SupelcleanTM ENVITM-18 cartridges. Identification and quantification of the two targeted compounds, triclosan and sulfamethoxazole, was achieved with the aid of Shimadzu Liquid Chromatograph Mass Spectrometer (LC-MS/MS). Long-term biosolid applications to agricultural land significantly (p ≤ 0.0001) improved soil organic matter and total nitrogen in the uppermost 30 cm soil layer under both rainfed-irrigated and rainfed maize cropping systems. This was, however, associated with a significant (p ≤ 0.0001) increase in soil acidity (declining soil pH) and salinity (EC), which may limit soil productivity subsequently lowering crop production. The level of increment in soil salinity was, however, too low to cause concerns in soil productivity. Therefore, hypotheses a and b were accepted. Long-term biosolid applications to agricultural lands at 16 t ha-1 yr-1 and lower rates did not result in traceable concentration levels of triclosan and sulfamethoxazole in the top 30 cm soil layer under both rainfed and rainfed-irrigated maize cropping systems (i.e. below the detection limits, which were 1.963 ng/g for triclosan and 0.64 ng/g for sulfamethoxazole). Similarly, sulfamethoxazole concentration in maize crop planted to long-term biosolid amended soils was below the detection limits (i.e. 14.30 ng/g). However, triclosan was detected in different above ground biomass parts of maize planted to soils that received 8 and 16 t ha-1 biosolids annually under both rainfed-irrigated and rainfed cropping systems. Maize stems recorded the highest triclosan concentration (666 - 892 ng/g dry weight), whereas maize grains recorded the least concentration (35.5 - 42.8 ng/g dry weight). The estimated daily triclosan intakes (EDIs) from the current study ranged between 0.21 µg Kg-1 day-1 for adults (individuals older than 18 years) and 1.25 µg Kg-1 day-1 for toddlers (children younger than 5 years), which was well below the Acceptable Daily Intake (ADI) of triclosan (83 µg/Kg/day). It can be concluded that, long-term biosolids use in agricultural lands could serve as a climate change action strategy through carbon sequestration, as a poverty alleviation by improving soil fertility, as a land rehabilitation strategy by increasing soil organic matter, and for cleaner water and environment by reducing nitrate and phosphate contamination of surface and ground water. Therefore, biosolids of similar quality with the current study could be applied at rates of 16 t ha-1 yr-1 and lower in agricultural lands under similar agro-ecological zones without contaminating the soil with triclosan and sulfamethoxazole and causing concerns in human health risks via dietary intake. It is recommended that biosolid application practices be synchronised with proper lime application strategies to remediate soil acidification associated with biosolids use in agricultural lands. To clearly understand the fate of triclosan and sulfamethoxazole in the soil-plant system, future studies should focus on monitoring the dynamics of triclosan, sulfamethoxazole as well as their transformation products in the whole soil profile and plant biomass.

Keywords: Biosolids, Triclosan, Sulfamethoxazole, Rainfed-Irrigated, Rainfed.