Impacts of antecedant moisture and soil surface mulch coverage on water and chemical transport through a no-till soil

Abstract

Flow in macropores of no-tillage soils is often implicated as a principal mechanism responsible for accelerated movement of agrochemcials into groundwater. The objective of this study was to assess the impact of a surface mulch coverage and antecedent water content on water and chemical transport characteristics in a Typic Hapludult soil. SrBr 2·6H 2O and atrazine were surface-applied to four undisturbed 0.3 m × 0.3 m × 0.3 m surface soil blocks. Three simulated 30 mm rains were applied to the block surfaces, and leachate was collected from 64 cells at the bottom of each block. Leachate volume, chemical amounts, and conducting macropore areas were determined for each cell and block. A parameter, m, found by fitting sorted cumulative outflow curves to an exponential function, was used to desctibe the degree of flow preference in a block. The dominant factor producing transport differences betweent the four blocks was pre-rain moisture content, which correlated negatively with degree of flow preference and positively with total leachate volume in each block. In a drier soil only the more rapid flow pathways, marked by high cell leachate volumes, contributed to the flow, while the slower pathways having greater interaction with the bulk soil were mostly truncated. This resulted in a higher degree of flow preference, smaller total leachate volumes and smaller block-averaged concentrations of Br, Sr and atrazine in soil with lower pre-rain moisture content. The peak of chemical transport was observed after the first simulated rain regardless of pre-rain moisture and surface mulch coverage. Following the second and third rains the chemical transport was reduced twofold for the less reactive Br, three-fold for the more reactive atrazine and ten-fold for Sr, apparently due to the by-pass of chemicals by subsequent leaching events. Much had little effect on water movement, but slightly enhanced the Sr and atrazine transport through the block, most likely by prolonging the chemical contact with infiltrating water at the soil surface.