IMPACTS OF ANTECEDENT MOISTURE AND SOIL SURFACE MULCH COVERAGE ON WATER AND CHEMICAL-TRANSPORT THROUGH A NO-TILL SOIL

Flow in macropores of no-tillage soils is often implicated as a princi pal mechanism responsible for accelerated movement of agrochemicals in to groundwater. The objective of this study was to assess the impact o f a surface mulch coverage and antecedent water content on water and c hemical transport characteristics in a Typic Hapludult soil. SrBr2.6H( 2)O and atrazine were surface-applied to four undisturbed 0.3 m X 0.3 m X 0.3 m surface soil blocks. Three simulated 30 mm rains were applie d to the block surfaces, and leachate was collected from 64 cells at t he bottom of each block. Leachate volume, chemical amounts, and conduc ting macropore areas were determined for each cell and block. A parame ter, m, found by fitting sorted cumulative outflow curves to an expone ntial function, was used to describe the degree of flow preference in a block. The dominant factor producing transport differences between t he four blocks was pre-rain moisture content, which correlated negativ ely with degree of flow preference and positively with total leachate volume in each block. In a drier soil only the more rapid flow pathway s, marked by high cell leachate volumes, contributed to the flow, whil e the slower pathways having greater interaction with the bulk soil we re mostly truncated. This resulted in a higher degree of flow preferen ce, smaller total leachate volumes and smaller block-averaged concentr ations of Br, Sr and atrazine in soil with lower pre-rain moisture con tent. The peak of chemical transport was observed after the first simu lated rain regardless of pre-rain moisture and surface mulch coverage. Following the second and third rains the chemical transport was reduc ed 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 chemica ls by subsequent leaching events. Mulch had little effect on water mov ement, but slightly enhanced the Sr and atrazine transport through the blocks, most likely by prolonging the chemical contact with infiltrat ing water at the soil surface.