Aggregate stability, an important property influencing a soil's response to erosive forces, is affected by freezing. The objectives of this laboratory study were to determine how constrainment, number of freeze-thaw cycles, and water content at freezing affect the aggregate stability of six continental USA soils differing in texture, mineralogy, and organic-matter content. Moist aggregates, after being frozen and thawed either zero, one, three, or five times, were vapor wetted to 0.30 kg kg-' and analyzed by wet sieving. Soils with clay contents of 17% or more and organic-matter contents >3% were the most stable after freezing. Aggregate stability for fine-and medium-textured soils generally decreased linearly with increasing water content at freezing. This linear decrease in stability was more rapid for constrained samples than for unconstrained samples. The stability of field-moist aggregates generally increased from zero to one or three freeze-thaw cycles. For at least one low-organic-matter soil, stability increased from one to three freeze-thaw cycles, but then decreased at five cycles. After thawing, aggregates at water contents of 0.15 kg kg ' or more that were constrained when frozen were always significantly less stable than aggregates that were unconstrained when frozen.
Recent research efforts have shown that soil erosion decreases soil productivity. Erosion-caused crop production decreases of 15-40% are commonly reported with some values over 50%. Furrow erosion on irrigated land in Idaho decreases topsoil depth on the upslope approximately 33% of the field area and may increase topsoil depth on the downslope 50-55%. Crop yields arc generally decreased where topsoil depths are decreased, but yields are not generally increased where topsoil depths are increased beyond a critical depth. Crops vary in their sensitivity to decreases in topsoil depth, but all crops studied exhibited lower yields on the eroded areas. Soil productivity potential of one area representing several million ha of furrow irrigated land was reduced at least 25% by furrow erosion over 80 irrigation seasons. Technology is not available to restore soil productivity potential to the level that would exist had there been no erosion except for returning topsoil to eroded areas. Research and technology applications arc needed to reduce or eliminate topsoil loss and redistribution by irrigation erosion.
Irrigation-induced erosion is a serious problem in the western USA where irrigation water quality can vary seasonally and geographically. We hypothesized that source-water electrical conductivity (EC) and sodium adsorption ratio (SAR = Na/[(Ca + Mg)/2]° 5, where concentrations are in millimoles of charge per liter) affect infiltration and sediment losses from irrigated furrows, and warrant specific consideration in irrigation-induced erosion models. On a fallow Portneuf silt loam (coarse-silty, mixed, mesic Durixerollic Calciorthid), tail-water sediment loss was measured from trafficked and nontrafficked furrows irrigated with waters of differing quality. Treatments were the four combinations of low or high EC (0.6 and 2 dS m ') and low or high SAR (0.7 and 12 [mmol, L l]"). Slope is 1%. Twelve irrigations were monitored. Each furrow received two irrigations. Main effects for water quality, traffic, and first vs. second irrigations were significant for total soil loss, mean sediment concentration, total outflow, net infiltration, and advance time. Average tail-water soil losses were 2.5 Mg ha -' from low EC/low SAR furrows, 4.5 Mg ha-' from low EC/ high SAR furrows, 3.0 Mg ha' from high EC/high SAR furrows; and 1.8 Mg ha-' from high EC/low SAR furrows. Elevating water EC decreased sediment concentration from 6.2 to 4.6 g L -', but increasing SAR increased sediment concentration from 6.2 to 8.7 g L-'. Net infiltration decreased 14% in high SAR compared with low SAR treatments. Soil loss increased 68% for second irrigations, and net infiltration fell 23% in trafficked furrows, but water-quality effects were the same. Water quality significantly influenced infiltration and erosion processes in irrigated furrows on Portneuf soils. O F THE ESTIMATED 250 MILLION HA irrigated worldwide, at least 60% is surface irrigated. Soil erosion from irrigation, especially furrow irrigation, contributes to nonpoint-source pollution (Hajek et al., 1990) and is a serious threat to crop productivity in many regions (Carter, 1993).Agricultural research has focused primarily on rainfallinduced soil erosion, with comparatively little attention to furrow-irrigation-induced erosion. A common assumption has been that erosion in rills is mechanistically equivalent to that in irrigated furrows. While shear produced by concentrated flow causes soil detachment and entrainment in both, there are several important differences: (i) rill phenomenon often includes an additional force, raindrop impact, which detaches and transports adjacent soil particles to the rill stream; (ii) a furrow stream initially advances over dry soil, resulting in rapid wetting and destabilization of dry, low-cohesion soil aggregates and increased furrow erosion losses (Kemper et al., 1985), whereas rill soils are prewetted by precipitation; (iii) downstream flow rates decrease in furrows as water infiltrates, but increase in rain-fed rills owing mainly to tributary inflow, hence, furrow flow rates and potential erosion losses are greater in the upper reaches of a...
C ROP SEQUENCE AND TILLAGE should be employed so that N mineralization is synchronized with subsequent crop N uptake. Improving N utilization reduces potential NO3 leaching into the groundwater from agricultural fields. The amount of NO 3 leaching out of the root zone is a function of the NO3 concentration in the root zone and the water flux through this zone.Large amounts of N are mineralized after alfalfa is killed, especially the first year. Fox and Piekielek (1988) reported that 70, 20, and 10 % of the contribution from alfalfa was available the first, second, and third years, respectively. Varco et al. (1991) measured 31.5 mg kg -1 extractable NO3-N in the upper 0.20 m of soil 28 d after alfalfa was killed. Calculations by Peterson and Russelle (1991) for the Corn Belt demonstrated that the N fertilizer applied to corn could be reduced by 8% if growers would properly credit the extra N mineralized following alfalfa. In south-central Idaho, 2 yr of bean are commonly grown following alfalfa, which means a legume following another legume that does not need fertilizer N. Robbins and Carter (1980) determined that 85 to 95 kg NO 3-N ha-1 yr I moved below the root zone when bean followed Abbreviations: BT-BT, conventional-tilled bean grown in 1990 and 1991; CNT-WNT, no-till silage corn grown in 1990, and no-till winter wheat grown in 1990-1991; CT-WT, conventional-tilled silage grown in 1990 and conventional-tilled winter wheat grown in 1990-1991.
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