Fall Contour Ripping Increases Water Infiltration into Frozen Soil

Crop residue management to trap snow and soil management to improve water infiltration into frozen soil might reduce spring runoff and increase soil water storage. We hypothesized that soil macropores created by tillage would improve water infiltration when the soil was frozen. This hypothesis was tested by ripping a Dooley sandy loam (fine-loamy, mixed Typic Argiboroll) in the fall of the year and then measuring water infiltration when the soil was frozen. A single subsoiling shank was used to rip soil to a depth of 0.3 m at 6-m contour intervals. Ripping was compared with no ripping using a randomized experimental design having three replications. Studies were conducted during 4 yr near Culbertson, MT, on plots seeded annually to spring wheat (Triticum aestivum L.). Soil water was measured with neutron attenuation and gravimetric methods. We used a constant-head (100 mm) method to measure water infiltration into frozen soil and a rainfall simulator for unfrozen soil. Final infiltration rate on frozen, ripped soil averaged 16 vs. 2 mm h ' without ripping. Final unfrozen infiltration rate in spring was 34 mm h-' with ripping vs. 15 mm h ' without ripping. Average spring water content of the top 1.2 m of soil, to a distance 1.5 m downslope from a rip, was 32 mm greater with ripping than without ripping at comparable slope positions. There were no wheat yield differences between treatments. Contour ripping can decrease water runoff, and seems best suited where spring runoff and soil erosion caused by heavy winter snows is a problem. S OIL FREEZING AND THAWING affect large agricultural areas, as well as range and forest land. It is estimated that during some portion of the year, 50% of the earth's land mass may be frozen (Sharratt et al., 1997). Formanek et al. (1990) estimated that freezing and thawing impact nearly 1.2 million km' of crop land, 1.3 million km2 of forest land, and 1.8 million km 2 of grazing land in the USA. Some of the most productive agricultural soils in the USA are affected by periodic freezing and thawing. Therefore, from an agricultural perspective, defining interactions of freezing and thawing on the productivity and sustainability of the soil resource is of high importance. Considerable research has been conducted to identify problems and possible solutions to runoff and erosion hazards caused by freezing and thawing of soil. HowJ.L. Pikul, Jr., USDA-ARS, Northern Grain Insects Res. Lab., 2923 Medary Ave., Brookings, SD 57006; and J.K. Aase, USDA-ARS, 3793 North 3600 East, Kimberly, ID 83341. Received 19 Sept. 1997. *Corresponding author ([email protected] ). Published in Soil Sci. Soc. Am. J. 62:1017-1024 (1998). ever, implementing solutions on a field scale can be difficult. Typically, soil erosion control efforts have been through tillage and residue management systems that maintain adequate surface roughness and suitable amounts of crop residue on the surface. For example, Allmaras et al. (1979) showed that erosion control in eastern Oregon often requires combinations of tillage, residue management, contouring, and terracing. But, even the best management practices may fail to reduce water runoff and erosion when the soil is frozen or partially frozen (Saxton et al., 1981). Hydraulic conductivity of frozen soils is inherently low and there is a strong association of frozen soils with increased runoff, flood-producing runoff, soil erosion, and sediment production (Tigerman and Rosa, 1951; Storey, 1955; Johnson and McArthur, 1973). Frozen soil layers limit water infiltration and focus runoff into topographic depressions where large ephemeral ponds may form (Baker and Spaans, 1997). These depressionfocused recharge areas may represent a pollution source, especially where water tables are close to the surface (Derby and Knighton, 1997). Soil water content at the time of freezing is the primary factor affecting soil hydraulic conductivity and consequently water infiltration rates when the soil is frozen (Kane, 1980). Additionally, the freezing process causes water to flow from unfrozen soil to the freezing front. Pikul et al. (1989) showed typical patterns of water redistribution near the soil surface during diurnal freezing and thawing cycles. Water flow paths become increasingly tortuous as pore space fills with ice. When soils freeze in a dry condition, air-filled pores provide important preferential water flow paths through frozen soil and increase water infiltration. Macropores created by tillage can be especially important to water infiltration because these pores would normally be drained of water at the time of freezing (Gray et al., 1990; Zuzel and Pikul, 1987). On the Canadian prairies, where snowfall accounts for approximately 30% of the annual precipitation, Maule and Chanasyk (1990) reported that snowmelt recharge, measured from fall to post-melt in the spring, was 36% greater in fields that were chiseled in the fall compared with fields that were Abbreviations: CWI, cumulative water infiltration; FDD, freezing degree days; WIR, water infiltration rate. 1018 SOIL SCI. SOC. AM. J., VOL. 62, JULY-AUGUST 1998 not chiseled. On the other hand, Lal and Steppuhn (1980) reported in a literature review that tillage did not increase overwinter soil water storage on the Canadian prairies. Pikul et al. (1992) have shown that soil ripping can intercept and infiltrate meltwater through frozen soil and that spacing of soil rips can be estimated from historic precipitation patterns and permeability of unfrozen subsoil. These results (Pikul et al., 1992) were from experiments conducted on a loess silt loam, in eastern Oregon, where soils frequently freeze but at shallow depths compared with typical soil freezing patterns in the northern Great Plains. Combinations of stubble management for snow catch and contour ripping for water infiltration can reduce runoff and increase soil water storage. In a study in southern Saskatchewan, Canada spring wheat yielded more on plots managed for snow catch and water infiltration than on undisturbed stubble check plots (Gray et al., 1990). New harvest technologies like stripper headers (Wilkins et al., 1996) leave wheat fields with tall stubble for maximum snow catch. Various mechanical treatments such as ripping, pitting, and contour furrowing have been used on rangelands to create surface storage and increase water infiltration. Contour furrowing in eastern Montana on sloping land with low infiltration capacity improved precipitation-use efficiency and increased snow water accumulation by 60% and soil water recharge by 161% compared with natural range (Wight and Siddoway, 1972; Neff and Wight, 1977). To successfully grow a crop every year on the semiarid Great Plains, it is necessary to efficiently use growingseason precipitation and to conserve as much precipitation as possible between harvest and seeding. Poor soil water storage efficiency, enhanced soil erosion, and development of saline seeps are associated with summer fallow (Tanaka and Aase, 1987; Black and Bauer, 1988). Methods to increase snowmelt infiltration into frozen soil need to be considered as part of management plans in regions where soil water limits plant growth and where soil erosion from spring runoff may be a problem. Previously, we presented the concept of contour ripping as a tillage strategy to improve water infiltration into frozen soil. Three silt loam soils from Oregon and a sandy loam from Montana were tested (Pikul et al.,