Characterization of Tillage and Traffic Effects on Unconfined Infiltration Measurements

Tillage and wheel-traffic compaction alter pore structure and hydraulic properties of agricultural soils. These alterations will affect root growth and movement of water and solutes. This study was conducted to develop new methods needed to characterize the effects of wheel traffic and tillage on pore structure as measured by water flow through macropores. Unconfined (three-dimensional) saturated and unsaturated infiltration measurements (0-, 30-, 60-, and 150mm water tension) at the soil surface were taken sequentially at field sites to determine the steady-state rate of water flow through different pore-size classes on a Tama silty clay loam (fine-silty, mixed, mesic Typic Argiudoll). A tension infiltrometer was used to obtain the unsaturated infiltration rates. Sites were selected on trafficked and untrafficked interrow positions in two tillage systems (chisel plow and no-till). Steady-state infiltration rates from 2 to 300 ion s" were measured. Increasing the tension of applied water resulted in decreasing infiltration rates for both tillages and traffic treatments because, as tensions increased, larger pores emptied. Wheel traffic reduced infiltration rates in both tillages, but caused a greater decrease in infiltration rates in the chisel-plow system than in no-till. Increasing tension caused proportionately smaller decreases in infiltration rates for wheel-trafficked positions. This suggests that larger macropores were transporting a greater proportion of the total water flow in untrafficked soil than in trafficked soil and reinforces the concept that larger, as opposed to smaller, pores are more easily destroyed by wheel traffic. Unconfined infiltration measurements were shown to be useful in quantifying the effects of tillage and compaction on soil macropores. I OF WATER INTO SOIL is directly related to soil macroporosity. Macropores are also important for root growth (Wang et al., 1986) and for solute movement (Beven and Germann, 1982). Tillage and compaction, however, can alter soil macroporosity. Thus, to elucidate the effects of tillage and compaction on soil macroporosity, we measured the rate of water infiltration into macropores for different tillage and wheel-traffic treatments. Macropores have been denned in several ways (Luxmoore, 1981). In this study, we define macropores as pores than empty at 0.2 mm nominal diameter or larger will drain at 150 mm tension. This pore-size range was selected because it includes the pore sizes important for (i) root growth and (ii) preferential solute flow. Secondary laterals of cereal root tips have an average diameter of ~0.2 mm (Hackett, 1969). A root tip growing into a pore smaller than its own diameter undergoes mechanical stress (Russell, 1977). Therefore, an estimate of pores with diameter of >0.2 mm should be correlated with unrestricted root extension, in a soil of moderate Joint contribution from the Natl. Soil Tilth Lab., USDA-ARS, and Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Journal Paper no. J-13513 of the Iowa Agric. and Home Economics Exp. Stn., Ames, IA; Project no. 2878. Received 8 June 1989. •Corresponding author. Published in Soil Sci. Soc. Am. J. 54:837-840 (1990). strength. Preferential solute flow also occurs in large soil pores. Scotter (1978) calculated that significant preferential flow of both strongly and weakly absorbed solutes could occur in continuous macropores with diameters >0.2 mm. Tension infiltrometers have been used to estimate soil pore and hydraulic properties. Clothier and White (1981) and Walker and Chong (1986) used tension infiltrometer measurements to estimate sorptivity. Moore et al. (1986) and Wilson and Luxmoore (1988) measured one-dimensional infiltration rates on forest soils to estimate macroporosity and unsaturated hydraulic conductivity. Control of tension at the soil surface by a tension infiltrometer limits the size of pores that are conducting water (Clothier and White, 1981) and allows measurement of unsaturated infiltration. Imposition of sequentially greater tensions leads to the incremental draining of smaller and smaller pores. Infiltration rates decrease as more of the water-conducting pores empty. Therefore, by comparing infiltration rates at increasing tensions, the relative contributions to water flow by various pore sizes can be evaluated. Greater relative water-flow rates are assumed to indicate more and/or better-connected pores within a pore-size class. This study was conducted to determine the effects of wheel traffic and tillage on pore structure as measured by water flow through macropores. MATERIALS AND METHODS Unconfined infiltration rates at selected tensions were measured, using a tension infiltrometer. Measurement at multiple tensions allowed evaluation of treatment effects on different pore sizes. Three-dimensional flow measurement avoids two problems inherent in one-dimensional measurement: (i) truncation or destruction of pores caused by driving a ring or isolating a soil monolith and (ii) wall flow along the edge of the soil sample. Field plots were established in the fall of 1984 on a Tama soil 12 km west of Marshalltown, I A. Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were grown in rotation on the site beginning in 1985. Corn was grown in 1988 on the areas where infiltration measurements were taken. Three tillage systems (no-till, ridge, and chisel plow) with controlled wheel traffic and four replications had been established on the site. Infiltration measurements were taken only on the no-till (NT) and chisel-plow (CP) tillage systems. The NT plots received no primary tillage and were cultivated once a year; CP plots were chiseled in the fall, disked shortly before planting, and cultivated. Plots were arranged in a five-row configuration, with 76-cm rows. Foot traffic was confined to trafficked interrows throughout the year. Infiltration measurements were made in mid-June, shortly before the 1988 cultivation. Infiltration measurements were taken in the center of trafficked and untrafficked interrows at two sites within each tillage replication. Thus, eight sites were measured in each tillage/traffic combination, for a total of 32 sites. At each of the 32 sites, steady-state (unconfined) infiltration rates were measured at four tensions: 0-, 30-, 60-, and 150-mm water tension. At each infiltration site, an area approximately 25 to 30