Stewart W. Melvin

Seasonal Changes in Flow and Nitrate-N Loss from Subsurface Drains

Subsurface drainage from thirty-six, 0.4-ha plots was monitored for three years (1990 to 1992) from chisel plow, moldboard plow, ridge till, and no-till systems with continuous corn and corn-soybean rotations. Data were analyzed in four seasonal stages to determine variations in drain flows and nitrate-N contents in drain effluent. The hypothesis of this study was that differences among tillage systems would change during the monitoring season as rainfall patterns varied and as plots were fertilized and cultivated. Forty-five to 85% of the annual nitrate-N loss through subsurface drainage occurred in the spring and fall when crops were not actively growing. These losses, however, were not significantly different among tillage systems. Relative changes in drain flows and nitrate-N concentrations before and after summer cultivation were similar among the four tillage systems even though no-till and ridge till systems were undisturbed before this time. Nitrate-N losses or concentrations did not increase during the stage following fertilizer application. No-till plots had significantly higher subsurface drain flow than moldboard plow plots only under continuous corn, possibly an effect of reduced yields from long-term no-till continuous corn. Nitrate-N concentrations in drain effluent from moldboard and chisel plow systems, however, were significantly greater than concentrations from no-till and ridge till systems for all crop rotations. Lower nitrate-N concentrations from no-till and ridge till systems may have resulted from greater bypass flow, denitrification, and immobilization under nonplowed systems.

Nitrogen Application Rate Effect on Nitrate-Nitrogen Concentration and Loss in Subsurface Drainage for a Corn-Soybean Rotation

Excess precipitation in Midwest agricultural production areas is often removed artificially via subsurface drainage systems that intercept and divert it to surface waters. Nitrogen (N), either applied as fertilizer or manure or derived from soil organic matter, can be carried as nitrate with the excess water in quantities that may have deleterious effects downstream. A field study was initiated in 1989 in Pocahontas County, Iowa, on 0.05 ha plots of glacially derived clay loams. The objective of this three-phase study was to determine the effect of N application rate on NO3-N concentration and loss in a corn-soybean rotation over a wide range of weather conditions. Nitrogen-rate treatment phases with five seasons each (six for phase II) were imposed on subsurface-drained, continuous-flow-monitored plots over a 16-year period. Phase I N rates ranged from 0 to 168 kg N ha-1 in 56 kg N ha-1 increments. Separate plots were used for each crop in phase I, and significant NO3-N concentration differences were not observed between corn or soybean plots; this led to combining both crops in a split-plot configuration for phases II and III to study system effects. Phase II N rates ranged from 45 to 179 kg N ha-1 in 45 kg N ha-1 increments. Phase III was limited to two rates, 168 and 252 kg N ha-1. Average yearly flow-weighted NO3-N concentrations ranged from 3.9 mg L-1 (45 kg N ha-1, 1995) to 28.7 mg L-1 (252 kg N ha-1, 2001). Average flow-weighted NO3-N concentrations (in mg L-1) ranked by N rate were: 23.4 (252), 13.2 (179), 15.5 (168), 11.9 (134), 11.7 (112), 8.1 (90), 9.5 (56), 5.7 (45), and 8.9 (0). Losses were precipitation dependent and were reflective of individual seasons and rates imposed. Average flow-weighted NO3-N losses (kg ha-1) ranked by N rate and by phase were: 58 (168), 68 (112), 48 (56), 50 (no N) for phase I; 8 (179), 15 (134), 19 (90), 7 (45) for phase II; and 49 (252), 32 (168) for phase III. Results indicate that concentrations generally increased with rate; the effect on losses was variable due to disparity in drainage volumes among years. Corn yield during all periods showed a strong correlation between N rate and yield. As N rate increased, yield increased. It should be noted that at least 50% of the years showed limited yield response to N application above the next to the highest rates. To achieve average NO3-N concentrations less than 10 mg L-1 (USEPA drinking water standard) in subsurface drainage at this site, N application rates would need to be less than 112 kg N ha-1. Rates currently recommended for this area range from 112 to 168 kg N ha-1. Results from this study have significant implications for N fertilizer management and subsurface drainage NO3-N loss to surface waters in the state, the Mississippi River, and the Gulf of Mexico.

Manure Incorporation Equipment Effects on Odor, Residue Cover, and Crop Yield

Land application of manure may produce unacceptable odors. Field experiments in undisturbed (no-till) soybean and corn residue were conducted to evaluate six liquid swine manure application/incorporation methods. The methods were injection with a commercial (1) chisel or (2) sweep, (3) incorporation with tandem disk harrow after broadcast application, (4) broadcast application with no incorporation, (5) injection with a narrow-profile knife, and (6) surface application behind row cleaners. The row cleaner and all injection treatments used spoke-covering wheels. Air samples over the soil surface were obtained immediately following and one day after manure application, and odor level was measured by olfactometry (i.e., the amount of air dilutions to reach odor threshold). Residue cover and yield were also measured. Incorporation techniques typically reduced odor level by a factor of three to ten as compared with a broadcast application. One day after application, odor was greatly reduced and often indistinguishable from that of untreated soil (no manure application). Residue cover differences among application methods were more pronounced in soybean residue. Application by the narrow-profile knife, row cleaner, and chisel maintained soybean residue cover better than other incorporation methods yet limited odor similar to these methods. Although cover was reduced over winter, greater soybean residue cover remained after planting with fall than with spring manure applications. Differences in odor level and residue cover among methods were less in corn than soybean residue. All incorporation techniques reduced odor levels, and chisel incorporation maintained corn residue cover after planting similar to broadcast application. For both crops, broadcast application maintained the greatest residue cover but had the highest odor level. Incorporation of manure generally reduced odor, reduced residue cover, increased corn yield, and did not affect soybean yield.

Tractive device effects on soil physical properties

Abstract Soil compaction can alter the root environment positively or negatively, depending on soil physical properties and weather conditions. Track-type tractors have the potential for causing less soil compaction because the tracks usually have a greater surface area than wheels for tractors with equivalent power ratings. The objective of this study was to determine the effect of tracked and wheeled tractors on soil physical properties related to the plant rooting environment. Plots were established on a Chequest silty clay loam (Typic Haplaquoll) in Lee County, IA in 1984. Plots were moldboard plowed to a depth of 200 mm and disked to a depth of 125 mm prior to trafficking, and field cultivated to a depth of 100 mm after trafficking. Soil cores (75 mm in diameter) were taken in August 1987 at 50–125 mm and 125–200 mm depths from untrafficked areas and areas trafficked by a steel-tracked crawler, rubber-tracked crawler, two wheel drive tractor, and four wheel drive tractor. Measurements included bulk density, saturated conductivity, air-filled porosity, and pore-size distribution. The untrafficked soil cores had lower bulk densities than trafficked cores at both depths, but lower bulk density was associated with lower ground pressure only for the 50–125 mm depth. Air-filled porosity at field capacity (1.0 m tension) followed the same trends as bulk density. The greater porosity of the untrafficked cores, compared with the trafficked cores, at both depths was owing to the presence of pores with greater than 30 μm equivalent pore diameters. Among trafficked treatments, cores from the upper depth of the steel-tracked crawler plots had greater porosity for 60–300 μm pore diameters compared with cores from the wheeled tractor plots. Generally, the wheeled tractors (with ground pressures of about 125 kPa) created a more compacted condition in the soil than the track-type tractors (with ground pressures of 30–40 kPa) at the upper depth, but physical property differences in trafficked cores below the 125 mm depth were minimal.

Subsurface Drainage in Iowa and the Water Quality Benefits and Problem

It is estimated that there are approximately 3.6 million ha of land with artificial subsurface drainage in Iowa, with 2.4 million ha of that within the 3000 organized drainage districts (total land area of the state is 14.6 million ha). This drainage has made otherwise wet soils very productive. Much of this drainage was installed early last century and is reaching the end of its service life. One challenge will be the repair/replacement of these drainage systems. Because subsurface drainage “short circuits” some infiltrating water back to surface water resources, there is also a water quality challenge. Research has shown that during rainfall-runoff events, the presence of artificial subsurface drainage generally delays and reduces the volume of surface runoff. Therefore, total losses of sediment, phosphorus, ammonium-nitrogen, pesticides, and micro-organisms are decreased with subsurface drainage. However, nitrate-nitrogen leaching is increased with subsurface drainage water, and has been implicated as a major factor relative to hypoxia in the Gulf of Mexico. Research has identified several factors relative to soils, weather, and management (cropping, tillage, chemical application practices, and drainage parameters) that influence the nitrate-nitrogen leaching problem. This will be discussed along with implications for possible changes in the drainage systems and land management that may be needed to sustain production while reducing nitrate-nitrogen losses.

Indicator Bacteria in Subsurface Drain Water Following Swine Manure Application

Appropriate manure application rates, timing, and methods are necessary to maximize nutrient utilization by plants from manure, while minimizing water resource pollution potential, including that of enteric organisms. A field study and a soil column study examined the response of indicator bacterial densities in subsurface drain water to different swine manure applications. The field study focused on the impacts of different manure management regimes on fecal coliform, fecal streptococcus, and Escherichia coli (E. coli) densities in subsurface tile drain water. Eight swine manure treatments were compared with a control treatment where commercial urea ammonium nitrate was applied. Manure treatments included fall injection, spring injection, and late winter broadcast at application rates of 168 kg N/ha and 336 kg N/ha. Results indicated that the highest incidence of significantly elevated bacterial levels occurred where manure had been broadcast in late winter at a rate of 336 kg N/ha.

COMPARISON OF SIMULATED (DRAINMOD) AND MEASURED TILE OUTFLOW AND WATER TABLE ELEVATIONS FROM TWO FIELD SITES IN IOWA

ABSTRACT Four years of field data on subsurface drain flows and water table elevations from two experimental sites in Iowa were used to compare the predicted values by DRAINMOD, a water management model. DRAINMOD simulations conducted for Nicollet silt loam and Kenyon loam soils of Iowa predicted water table elevations within an average deviation of 15 cm and 19 cm, respectively. The subsurface drain outflows predicted by DRAINMOD were within an average deviation of 0.065 cm/day.

Tillage Implement Operational Effects on Residue Cover

Crop residue cover protects soil from erosion caused by raindrop impact and runoff. Fall and spring season factorial field experiments indicated that operator-controlled adjustments of tool configuration, depth, and under certain conditions speed affected corn residue cover buried by a tandem disk harrow and chisel plow and soybean residue cover buried by a knife-type fertilizer applicator.

Temporal Subsurface Flow Patterns from Fifteen Years in North-Central Iowa

Subsurface drainage in the Upper Midwest is of importance to agricultural production. However, proper management of these systems through in-field management, drainage management, or edge of field practices is needed to limit negative environmental impacts particularly from nitrate-nitrogen leaching losses. One management practice being considered is drainage management where the outflow of subsurface drainage is managed to conserve water and decrease the overall outflow of subsurface drainage. To understand how and when drainage management may be utilized in the upper Midwest it is important to review long-term drainage data to understand the timing, duration, and volumes of subsurface drainage in these climates. An on-going drainage study from north-central Iowa allows for reviewing fifteen years of subsurface drainage which encompasses a range of climatic conditions. This information has been reviewed with the objective of understanding the timing, duration, and drainage volumes considering temporal drainage flow patterns. In particular, the monthly and seasonal flow patterns have been investigated using this long-term drainage record. On this site with a relatively narrow drain spacing of 7.6 m, drainage volume was approximately 40% of the precipitation. The time period from April through June had approximately 50% of the average annual precipitation and approximately 70% of the average annual drainage. In addition, the percent of annual drainage occurring after August 1 was only approximately 7%. The timing of subsurface flow in these areas specifically during the spring coincides with time of planting, crop germination, and early crop development has implications when considering drainage management practices and the effectiveness of these practices to limit flow and therefore nitrate-nitrogen leaching losses. To minimize outflow of drainage water, these drainage management systems would need to allow for adequate flexibility to ensure crop production while effectively managing subsurface drainage flow to potentially minimize the outflow of water.

Measurement of Leakage from Earthen Manure Structures in Iowa

Leakage from a representative sample of 28 earthen manure storage structures and lagoons (selected from 459 built in Iowa between 1 January 1987 and 31 December 1994) was determined using a water-balance approach. Forty-three percent (43%) of tested structures had leakage rates significantly (p < 0.05) lower than the regulatory limit of 1.6 mm/d (1/16 in/d) specified by the State of Iowa at the time the basins were constructed. Leakage from 53% of the structures was too close to the regulatory limit to be categorized as being significantly above or below it. One structure (4%) exhibited leakage significantly greater than the regulatory limit. Regression analysis indicates a slight, but statistically significant, decline in leakage rate with increasing structure age. Structures constructed in glacial till showed significantly lower leakage rates than those constructed in sand and gravel, colluvium, or loess. Comparison of slurry pits and lagoons showed no significant difference in leakage rate.