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AbstractRunoff and soil loss data were collected from seven cropping and tillage treatments over a 12-year period (1983–1994) from 28 (3.2 m wide by 27.4 m long) natural rainfall erosion plots located on a silt loam soil (Udollic Ochraqualf)near Kingdom City, MO. The treatments were continuous corn and soybean cropping under conventional, chisel, and no-till tillage methods, and continuous cultivated fallow. Although cropping slightly influenced runoff and soil loss, the differences were not statistically significant (p ≤ 0.05). When averaged over tillage, mean annual runoff and soil loss from soybean were 3 and 12% higher than those from corn, respectively. Cropping effect on runoff was only significant (p ≤ 0.05) during the residue (P4) cropstage period. Most of the soil loss (approx. 80% of the annual loss) occurred during the rough fallow (F) and seedbed (SB) periods. For these periods, cropping had no significant effect (p ≤ 0.05) on soil loss. Mean annual runoff and soil loss from continuous fallow were substantially greater compared to those from corn or soybean. Tillage, particularly no-till, had significant effects (p ≤ 0.05) on runoff and soil loss. When averaged over crop, no-till increased mean annual runoff by 14 and 20% compared to conventional and chisel, respectively. On the other hand, chisel decreased runoff by 5% compared to conventional. Soil loss from no-till method was 7 times lower than conventional and 5 times lower than chisel. Chisel lowered soil loss by 31% compared to conventional. The effects of tillage on runoff and soil loss were substantially greater during the F and SB cropstage periods. Overall, the study showed that (1) cropping had little effect on runoff and soil loss, and (2) no-till significantly increased runoff and substantially reduced soil loss when compared to the conventional method.

Cornstalk (Zea maysL.) residue can be used for many competing purposes including energy production and erosion control. This study was conducted to determine the rate of cornstalk decomposition and N and P release on a till‐planted watershed in the Corn Belt. Samples of cornstalk residue were periodically collected from 12 sites distributed over the watershed and analyzed for weight loss and plant nutrient content. After 12 months, 79% of the residue had decomposed. About two‐thirds of the decomposition occurred from November to April when soil temperatures were unfavorable for high microbial activity. Nitrogen and P were released from the residue during the early stages of decomposition. After 12 months, 60% of the N and 74% of the P had been lost. The percentages of N and P in the residue rapidly increased during the last 3 months of the study when soil moisture and soil temperature were favorable for optimum activity. The C/N ratio declined from 48 to 23. Narrowing of the ratio was caused by a proportionately greater decrease in C content than in N content.

Higher rates of soil loss have been observed for soybean [Glycine max(L.) Merr.] cropping than for corn (Zea maysL.) cropping. The objective of this study was to determine whether 4 yr of continuous soybean and continuous corn cropping had altered the size and stability of soil aggregates within the tillage zone, which could affect soil seal formation and erodibility. Samples for analyses were obtained in June and October of 1980 from the Monona (fine‐silty, mixed, mesic, Typic Hapludolls) and Clarion (fine‐loamy, mixed, mesic, Typic Hapludolls) soils in Iowa. The mean‐weight diameter of dry‐sieved aggregates was significantly (p<0.05) lower for soybeans than corn in October, but the values were similar in June. The mean‐weight diameter of wet‐sieved aggregates was lower for soybeans than corn in both June and October, but the differences were not statistically significant. The mass of clay released from the bulk soil and two macroaggregate size fractions with laboratory shaking was slightly, but significantly (p<0.10), higher for corn than soybeans.

Greater soil losses are generally thought to occur with soybean (Glycine max. [L.] Merr.) cropping than with corn (Zea maysL.) cropping. However, runoff and soil losses from corn and soybean cropping systems have been measured simultaneously in only a few studies. The objectives of our study were to evaluate differences in soil and water losses between continuous corn and continuous soybean cropping for conventional, field cultivation, and no‐till methods of tillage; and to evaluate differences between measured cropping and management (C) factors and those in Agricultural Handbook 537, USDA that are currently used in soil conservation planning. These objectives were accomplished by analyzing soil and water loss data from a 7‐yr study conducted on a claypan soil in central Missouri. Cropping differences were evaluated for five seasonal periods based upon cultural operation dates and estimated amounts of canopy cover. Average annual soil loss from soybeans was significantly higher (p<0.01) than that of corn for the conventional and no‐till methods. Seasonal periods having the greatest cropping differences in soil loss were period F (rough fallow), period 12 (30 to 60 d after planting) and period 4 (fall harvest to spring tillage). Annual Cfactors for soybeans were about two times those of corn for all tillage methods. Measured Cfactors for all tillage methods were consistently lower than those presented in Agricultural Handbook 537, USDA.

Raindrops falling through still air drift randomly causing difficulty in collecting uniform soil splash samples from single‐drop detachment studies. We developed a simple electronic discriminator to eliminate this problem that can be used easily with any single drop rainfall tower. The main detector of the discriminator consists of two optical light source‐sensors positioned at right angles in a horizontal plane. These sensors detect and select for use only those drops falling within a 6.4 mm square target window. Labor and materials for constructing the discriminator cost about $400. Two experiments were conducted to test the performance of the discriminator. One study analyzed drop masses collected both with and without use of the discriminator, and compared these quantities with drops collected directly from the drop former. No significant difference in mass between drops collected with the device and those collected directly from the drop former was found. A significant (P<0.01) decrease in drop mass was found without use of the device. The discriminator also decreased drop mass variance about six times. The second experiment measured soil splash from uniformly packed cores of Norbourne fine‐sandy loam (Typic Argiudolls) packed at an average bulk density of 1.35 Mg m−3and equilibrated to −2 kPa soil water potential. Results show an average splash increase of about 46% (4.2 mg) with use of this device versus without it. Soil splash variance was also reduced by 19%, further confirming that use of the discriminator significantly reduces alterations in drop characteristics, thus insuring against biasing of raindrop kinetic energy versus soil splash relationships.

A rainfall simulator study was conducted on a Sidell silt loam (Typic Arguidolls, fine‐silty, mixed, mesic) in northwest‐central Indiana to evaluate the effectiveness of different lengths and percentage covers of cornstalk residue strips in reducing total nitrogen and available phosphorus discharges associated with the sediment. A 2.7‐m long residue strip with 50% surface cover reduced nutrient discharges by about 70% when the nutrient loads entering and leaving the residue strip were compared. Reductions in sediment and nutrient discharges with increasing length and percentage cover of the residue strips were almost proportional. The sediment was separated by sieving and gravity sedimentation into 10 size fractions ranging from > 2 to <0.002 mm in diam. About 50% of the sediment entering the residue strips was composed of particles >0.05 mm. The 2.7‐m long residue strip with 50% surface cover filtered out most of the particles >0.05 mm. As a result, 85% of the sediment leaving this residue strip was in the <0.035‐mm size fractions. Nutrient concentrations of the fractions >0.21 mm entering the residue strips were higher than those concentrations of the 0.05‐ to 0.01‐mm fractions entering the strips. Nutrient concentrations of the fractions <0.21 mm and >0.01 mm increased as the sediment moved through the residue strips, with the effect being related to residue length and percentage cover. Residue reduced the transport capacity of runoff below its sediment load, which caused the denser particies within these size fractions to be deposited. The less dense particles that were not deposited were composed of a greater proportion of small silt and clay primary particles, which increased the nutrient concentrations.

Most mathematical models that are being developed to predict sediment discharges from farm fields, watersheds, and basins consider the detachment, transport, and deposition of soil aggregates. This study was conducted to determine the effect of three tillage treatments and two cornstalk residue levels on the size distribution and N and P concentrations of eroded soil aggregates. Measurements were made by applying simulated rainfall to 3.7 by 10.7 m plots located on a silt loam soil in north central Indiana. Over 60% of the sediment from moldboard‐plowed and disked‐only plots was composed of aggregates > 0.05 mm, whereas about 30% of the sediment from chisel‐plowed plots was composed of aggregates > 0.05 mm. The addition of cornstalk residue to give 20% surface cover on the moldboard‐plowed and disked‐only plots decreased the percentage of >0.05‐mm aggregates by about 12%. Nitrogen and P concentrations of the 2‐ to 1‐, 1‐ to 0.5‐, and 0.5‐ to 0.21‐mm aggregates were generally 10% lower than the N and P concentrations of the aggregates <0.05 mm. The highest N and P concentrations were associated with the 0.21‐ to 0.05‐mm aggregates. However, this size fraction represented a small percentage of the eroded particles. Nitrogen and P concentrations of aggregates eroded from the chisel‐plowed plots were about 40% higher than the N and P concentrations of the aggregates eroded from the moldboard‐plowed plots. Differences in N and P concentrations reflected differences in the transport capacity of the runoff. As increased residue cover reduced runoff velocity, only those aggregates of lower densities and sand contents were transported. As a result, aggregates eroded from chisel‐plowed plots contained a greater proportion of silt and clay which was reflected in their higher N and P concentrations than did aggregates eroded from moldboard‐plowed plots. Nitrogen and P concentrations of uneroded aggregates separated by wet sieving were determined and compared with the N and P concentrations of the eroded aggregates. Enrichment ratios were > 1.0, which indicated that nutrient enrichment occurred for all size fractions of eroded aggregates.

Seasonal losses of nitrogen and phosphorus in surface runoff were determined for a 7‐year period from three corn‐cropped watersheds in southwestern Iowa. Three seasonal periods were defined: fertilizer, seedbed, and establishment period from April through June (P1); reproduction and maturation period from July through November (P2); and residue period from December through March (P3). Most of the average annual total N and P losses were associated with the sediment portion of runoff and occurred during P1. The extreme susceptibility of the loess soils to erosion during P1 must be taken into account when designing conservation practices to control plant nutrient losses. Seasonal discharges of runoff, sediment, and nutrients were much lower from a level‐terraced watershed than from two contour‐farmed watersheds, which demonstrates the benefit of terracing in resource conservation. Average annual soluble N and P losses were quite low and never exceeded 1% of the annual fertilizer application. These losses were the highest during P3 from the contour‐farmed watersheds. Water and sediment weighted nutrient concentrations were the highest from the contour‐farmed watersheds during P3, when residues covered the soil surface. Leaching of soluble nutrients from the residue and the greater selectivity of the soil erosion process for the finer fractions of the soil (i.e., clay and organic material) during snowmelt runoff are probably the factors responsible.

Mathematical models are needed to better estimate effects of alternative management methods on the amounts of bioavailable P lost in surface runoff over a variety of soil, topographic, and climatic conditions. In this study, methods for estimating concentrations of soil‐derived labile (dissolved plus adsorbed) and dissolved P in runoff are proposed and tested. The methods are based on the assumption that most labile P contributed by soil to runoff is derived from soil particles that are detached and transported with runoff water. Effects of erosive selectivity are accounted for by dividing soil and sediment into five undispersed size fractions. Isotherms relating adsorbed labile and dissolved P levels were developed for each from soil analyses. Labile P concentrations in runoff were estimated by summing the product of labile P concentrations prior to entrainment in runoff and measured sediment concentrations for each of the size fractions. Dissolved P concentrations were estimated by distributing labile P between dissolved and adsorbed forms using a mass balance approach. Methods of estimation were tested with runoff from 14 natural rainstorms occurring on 10 fallow plots having soil P levels ranging from about 1.3 to 3.2 mmol kg−1as labile P. Linear regressions of observed on estimated values of both labile and dissolved P were significant. For most events, slopes and intercepts of regression lines were not significantly different from one and zero, respectively, indicating good absolute agreement. Results indicate that soil analyses in combination with models for predicting the amount and size distribution of eroded soil form a basis for estimating movement of bioavailable P forms in runoff. However, additional studies are needed to determine the range of soil types for which the assumptions inherent in the development of the approach are valid.