Herbicide and conservative tracer movement through the soil profile and to subsurface drains under no-till and chisel plow systems

Abstract

<p>Adoption of conservation tillage has increased the role of macropore flow on the transport of agricultural chemicals to groundwater. Therefore, it is important to understand the role of macropore flow in the transport of water through the vadose zone and to the shallow groundwater. Two field studies were conducted at Iowa State University's Agricultural Research Center near Boone, Iowa, to determine the effect of two tillage practices (chisel plow and no-till) on the transport of herbicides and tracers, with applied water, through the soil profile and to the shallow groundwater. Two herbicides, atrazine [(6-chloro-N-ethyl-N-(1-methylethyl-1,3,5 triazine)-2-4-diamine] and alachlor [2-chloro-N-(2,6-diethylphenyl-N-(methoxymethyl) acetamide], and a variety of conservative tracers such as chloride (Cl--), bromide (Br--), pentafluorobenzoate (PFBA), o -trifluoromethylbenzoate (o-TFMBA), and 2,6-difluorobenzoate (2,6-DFBA) were applied on the cropland before and during irrigation events to investigate the effect of no-till and chisel plow systems on the movement of chemicals and water to shallow groundwater. Water samples were collected using suction lysimeters and continuous monitoring of subsurface drain flow during and after various irrigation events. Soil samples were collected before and after the irrigation events and were analyzed for various chemical concentrations. The results of the study showed that significant amount of herbicides and tracers were not found to be lost with subsurface drain flow under either tillage system. Also, it was found that macropore flow did not contribute significantly in the transport of solutes through the soil under chisel plow system, however, it contributed significantly under no-till system. Atrazine leaching losses were found to be higher than alachlor under chisel plow and no-till systems. First irrigation or simulated rainfall event immediately after chemical application resulted in higher peak concentrations of tracers and herbicides in the subsurface drain water under both tillage systems compared to subsequent irrigation events. The LEACHP model was also evaluated in predicting the chemical movement through the soil profile. The LEACHP, on the average, predicted the transport of water and solutes through the soil profile reasonably well. The LEACHP model could not accurately predict the effect of rainfall events on the subsurface drain flows, although predicted flow for the no-till system was closer to the observed values. Predicted chemical losses with subsurface drain flow were correlated to the amount of water drained as subsurface drain flow.</p>