Tillage and Water Content Effects on Surface Soil Hydraulic Properties and Shortwave Albedo

The modification of surface soil structure affects soil-water processes important in crop production and soil conservation. Effects of pretillage soil water content (PTSW) with multiple-pass tillage operations were determined on soil hydraulic properties and shortwave albedo on freshly tilled soil. Objectives included determination of whether PTSW can be used to improve management of seedbed hydraulic properties and whether tillage-induced soil surface conditions significantly affect albedo. Of the three tillage sequences (minimum, intermediate, and excess tillage) applied to a Templeton silt loam soil (mixed, mesic Udic Ustochrept), the excess tillage sequence resulted in the lowest macropore volume, mainly through a decrease in the volume of aeration pores (pores >300-ju.m diam.). The slope of the water characteristic between —1.0 and —1500 kPa matric potential was unaffected by tillage treatments. The excess tillage sequence resulted in a mean hydraulic conductivity (matric potential = —0.4 kPa ) of 11.1 cm h~', compared with 14.9 cm h~' following minimum tillage. Different PTSW did not cause significant differences in hydraulic properties of freshly tilled soil. The range of tillage-induced soil structures produced did affect shortwave albedo, but to a smaller extent than shown in some previous studies. Bare soil shortwave albedo variation with zenith angle appears soil specific; here a small decrease in albedo was observed as zenith angle increased. In this soil, with commonly practiced tillage sequences and a range of PTSW representative of field conditions, information about PTSW will not help manipulation of hydraulic properties and shortwave albedo in the fresh seedbed. F sown to cereal crops on the shallow, recent soils of New Zealand's Canterbury plains are often susceptible to wind erosion during the period from tillage until crop establishment. Evaporative water loss during this period may significantly affect the water available for crop growth in this environment. Erodibility is affected by the physical condition of the surface soil and by surface soil water content. Therefore, in assessing tillage practices for this environment, soil-water relations in the resulting seedbed are of importance. H.P. Cresswell, CSIRO Division of Soils, GPO Box 639, Canberra, ACT 2601, Australia; D.J. Painter, Dep. of Natural Resources Engineering, and K.C. Cameron, Dep. of Soil Science, P.O. Box 84, Lincoln Univ., Canterbury, New Zealand. Contribution from the Dep. of Natural Resources Engineering, Lincoln Univ. Received 2 Mar. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:816-824 (1993). The soil conditions resulting from tillage influence soil water storage and the loss of soil water through the process of evaporation. The rate of evaporation is determined by either the evaporative capacity of the atmospheric environment or by the supply of water to the evaporating sites, whichever is limiting. Surface soil structure has been shown to affect the proportion of incident radiation absorbed by the soil (Coulson and Reynolds, 1971; Idso et al., 1975; Jackson et al., 1990; Irons et al., 1992). Tillage affects soil hydraulic properties and consequently infiltration and water movement to evaporation sites. Thus, the modification of surface soil structure can influence evaporation by affecting both the available energy supply and water supply for evaporation. Mechanistic soil water and energy balance simulation models (e.g., Van Bavel and Hillel, 1976) provide useful tools with which to assess changes in the soil water balance resulting from changes in soil hydraulic and physical properties (e.g., Cresswell et al., 1992). To apply models of this type, there is a need to determine the effects of management practices on these soil properties. So, to properly evaluate the suitability of different tillage systems for particular soil and climatic environments, the following must be determined: (i) the soil structure produced by particular tillage systems at various soil water contents, and (ii) the influence of the soil structure produced on soilplant—water processes in a given climate. The former is the emphasis of this study. An earlier study (Cresswell, 1990; Cresswell et al., 1991) considered the effects of tillage management on surface soil physical properties that directly affect the process of sediment transport by wind; in fhis study we considered tillage effects on properties that influence erosion by affecting soil water relations of the seedbed (i.e., soil hydraulic properties and shortwave albedo). There is little quantitative information in the literature assessing the effects of multiple-pass tillage operations and PTSW on the soil hydraulic properties important in the description of soil water movement. Abbreviations: PTSW, pretillage soil water content; MAJED, mean absolute elevation difference; LD, limiting elevation difference; LS, limiting slope index; ANOVA, analysis of variance; ESD, equivalent spherical pore diameter; DNMRT, Duncan's new multiple-range test. CRESSWELL ET AL.: TILLAGE AND WATER CONTENT EFFECTS 817 Cresswell et al. (1991) found that multiple-pass tillage operations and PTSW interacted in producing significant effects on the aggregate size distribution of a fresh seedbed. They observed that the avoidance of excessive tillage reduced the likelihood of significant effects of PTSW on aggregate size distribution. This study was to determine if the influence of tillage and water content on soil physical properties observed by Cresswell et al. (1991) translate to large effects on the soil water characteristic and on hydraulic conductivity of freshly tilled soil. This will indicate whether pretillage soil water content in this cropping system can be used in manipulating seedbed hydraulic properties. We will also consider whether the range of soil surface conditions produced by tillage systems commonly used in New Zealand produce significant variation in shortwave albedo and whether this variation is large in comparison to albedo variation with surface soil water content. Although previous studies (Coulson and Reynolds, 1971; Idso et al., 1975; Jackson et al., 1990; Irons et al., 1992) have shown the dependence of albedo on soil surface roughness, the interaction of surface roughness with surface soil water content has received little attention. Albedo studies have often used surfaces much smoother than those used in commercial cereal cropping operations. We examined the magnitude of surface structure-induced variations in albedo and whether these variations were large enough to warrant albedo being a consideration when deciding on appropriate tillage operations. MATERIALS AND METHODS