Effects of soil type and tillage practice on atrazine transport through intact soil cores

Abstract

Agricultural systems of Argentina have increased herbicides inputs, mostly associated with adoption of no tillage (NT). Several studies have revealed presence of pesticides in groundwater. Therefore, research on the behaviour of herbicides in soils is driven by the need to manage and prevent possible contamination of groundwater. Soil organic carbon (OC) is the main soil component responsible of sorption, and consequently the main tool to reduce the leaching. However, in dynamic systems transport of organic chemicals depends on soil structural and hydraulic properties. Sorption controls the physical and biological availability of chemicals. Physical, heterogeneous flow domain, and chemical, kinetic reactions and molecular diffusion into aggregates, which are nonequilibrium processes that affect solute transport. The main objective of this paper was to evaluate the effects of soil texture and tillage system on atrazine transport through intact soil columns. The study focused on the identification of processes; and determination of parameters that control atrazine transport in the upper layer of soils. Balcarce (BAL, silty clay loam, fine, thermic, illitic, Typic Argiudoll)), Tres Arroyos (TAR, clay loam, fine, thermic, illitic, Typic Argiudoll) and Coronel Dorrego (DOR, loam, fine, thermic, mixed illitic–montmorillonitic, Typic Argiudoll) soils from the southeast of Buenos Aires Province (Argentina) were selected. The soils represent a wide range of OC content (BAL 35.5, TAR 28.8 and DOR 17.3 g kg − 1 ). At each site NT and conventional tillage (CT) systems were sampled. Four replicates of intact soils cores (15 × 8 cm) were removed from each combination of soil × tillage (BAL-NT, BAL-CT, TAR-NT, TAR-CT, DOR-NT, DOR-CT). Displacement studies were done using atrazine as the reactive solute and bromide as the nonreactive solute. Equilibrium and nonequilibrium transport models (CXTFIT 2.1) were employed to describe the breakthrough curves (BTCs). The software tool SMART was used to simulate atrazine transport under steady-state flow conditions. Atrazine BTCs were skewed to the right; exhibiting an asymmetric shape and tailing that implied nonequilibrium conditions during transport. Since physical nonequilibrium was assumed to be nearly negligible, the observed nonequilibrium was interpreted as a sorption-related process. The two-site nonequilibrium model showed an acceptable fit with the observed data (72 < R 2 < 86). Recovery percentages of atrazine in effluents were: BAL-CT 54.51%; BAL-NT 45.10%; TAR-CT 44.28%; TAR-NT 29.70%; DOR-CT 18.60%; DOR-NT 48.95%. The intraparticle diffusion model provided by SMART showed the best fit. In conclusion, intrinsic soil properties were more relevant for atrazine transport than those associated with tillage practices. However, no tillage produced early detection of atrazine in effluents, and favoured atrazine leaching in coarser soils with the lowest OC contents. However, the maximum loss of atrazine in the percolate took place in the soils with the highest OC level; with no effects of tillage practices. These soils had fine texture, and were well structured and aggregated. Intraparticle and intraorganic matter diffusion appear to be responsible for nonequilibrium sorption. Delayed sorption in aggregated soils leads to high concentration of atrazine available for leaching.