Effects of Sorption and Desorption on Bioavailability of Atrazine in Soils Amended with Crop Residue Derived Char

Abstract

The bioavailability of pesticides in soils is affected by the addition of crop residue-derived char, which alters the sorption and desorption characteristics of the soil. Sorption, desorption and bioavailability experiments were performed using two types of soils (Hartsells and Grady) in the presence and absence of a wheat char (Triticum aestivum L.), and char only systems with atrazine as the sorbate. The sorption isotherms for the two soils were linear, suggesting that partitioning into organic matter was the primary sorption mechanism. The isotherms for the 1% char-amended soils showed an increased sorption nonlinearity and sorption capacity. The 1% char amendment to both soils produced similar isotherms, indicating that the presence of a small amount of char may dominate the overall sorption process. The amount of atrazine sorbed by char was 800 – 3800 times greater than that sorbed by the soils alone. The fraction resistant to desorption was 38% in both the Hartsells soil and char-amended Hartsells. The Grady soil showed a slightly less resistant fraction of 33%, which increased to 38% with the addition of char. The resistant fraction in char was 33%. Pseudomonas sp. strain ADP, which is capable of utilizing atrazine as its sole nitrogen source, was used in the bioavailability assay. The presence of char in the system reduced the CO2 production by 11% and 20% in the char amended soils and char, respectively. A desorption-biodegradation-mineralization (DBM) model was used to assess the bioavailability of atrazine in the test systems. The model predicted the sorbed phase bioavailability in soils and soil amended with char systems, but not in char. Multistep kinetic and equilibrium desorption experiments showed that the site fractions varied with atrazine concentration. Hence, a dynamic DBM model incorporating this variation in site fractions with atrazine concentration was developed. The dynamic DBM model confirmed that sorbed phase atrazine was not utilized in char slurries. Further, the results indicate that sorbed atrazine in both the equilibrium and non-equilibrium fractions were bioavailable in soils and soils amended with char, but not in the non-desorbed fraction. This study shows that the presence of char in soil affects the sorption and desorption processes and, ultimately, the bioavailability of atrazine.