Reid D. Christianson

Nitrate-Nitrogen Losses through Subsurface Drainage under Various Agricultural Land Covers

Nitrate-nitrogen (NO₃-N) loading to surface water bodies from subsurface drainage is an environmental concern in the midwestern United States. The objective of this study was to investigate the effect of various land covers on NO₃-N loss through subsurface drainage. Land-cover treatments included (i) conventional corn ( L.) (C) and soybean [ (L.) Merr.] (S); (ii) winter rye ( L.) cover crop before corn (rC) and before soybean (rS); (iii) kura clover ( M. Bieb.) as a living mulch for corn (kC); and (iv) perennial forage of orchardgrass ( L.) mixed with clovers (PF). In spring, total N uptake by aboveground biomass of rye in rC, rye in rS, kura clover in kC, and grasses in PF were 14.2, 31.8, 87.0, and 46.3 kg N ha, respectively. Effect of land covers on subsurface drainage was not significant. The NO₃-N loss was significantly lower for kC and PF than C and S treatments (p < 0.05); rye cover crop did not reduce NO₃-N loss, but NO₃-N concentration was significantly reduced in rC during March to June and in rS during July to November (p < 0.05). Moreover, the increase of soil NO₃-N from early to late spring in rS was significantly lower than the S treatment (p < 0.05). This study suggests that kC and PF are effective in reducing NO₃-N loss, but these systems could lead to concerns relative to grain yield loss and change in farming practices. Management strategies for kC need further study to achieve reasonable corn yield. The effectiveness of rye cover crop on NO-N loss reduction needs further investigation under conditions of different N rates, wider weather patterns, and fall tillage.

Water table, drainage, and yield response to drainage water management in southeast Iowa

Subsurface drainage is an important practice for optimizing crop production, but it accelerates nitrate-nitrogen (NO3-N) loss to downstream water bodies. As a result, there is a need for practices that can maintain crop production while decreasing subsurface drainage volume and NO3-N export. The objectives of this work were to evaluate the impact of drainage water management through controlled drainage, shallow drainage, conventional drainage, and no drainage on subsurface drainage volumes, water table depths, crop yields, and NO3-N export. This research was conducted at the Iowa State University Southeast Research Farm and consisted of four management schemes with two replicates for a total of eight plots. Plots consisted of a corn (Zea mays L.)–soybean (Glycine max L. Merr.) rotation with half of the plot planted in corn and half planted in soybeans each year. Findings from four years show that undrained plots had a high occurrence of elevated water tables. Controlled and shallow plots had elevated water tables in the early spring and early fall in accordance with the rainfall and management protocols for controlled drainage. Water table response was quick, with drawdown to tile depth within one to two days after significant rain events. During the period of the study, drainage water management through controlled drainage or shallow drainage reduced overall drainage volume by 37% and 46%, respectively. Average annual NO3-N loss for the study period was reduced by 36% and 29% for controlled drainage and shallow drainage, respectively. Over the four-year period, corn yields in the controlled plots were significantly lower than conventional drainage; however, yields were not statistically different from shallow drained plots. There was no statistically significant difference between drained plots in terms of soybean yield for the study period. Undrained plots, however, had significantly lower yields for corn when compared with shallow and conventional treatments and for soybeans when compared to all treatments. This study highlighted the potential for use of drainage water management practices in reducing subsurface drainage volume and downstream NO3-N loss. In addition, the study highlighted the overall importance of drainage on maintaining crop yields.

Technical Note: Hydraulic Property Determination of Denitrifying Bioreactor Fill Media

Denitrification bioreactors are one of the newest options for nitrate removal in agricultural drainage waters. Optimization of denitrification bioreactor design requires the ability to identify concrete values for the hydraulic properties of bioreactor fill media. Hydraulic properties, chiefly saturated hydraulic conductivity but also porosity and particle size, are not known for many types of possible bioreactor media though they have a significant impact upon bioreactor design and performance. This work was undertaken to more fully quantify the hydraulic properties of the major type of fill media used in Iowa denitrification bioreactors through a series of porosity, hydraulic conductivity, and particle size analysis tests. In addition, a particle size analysis was performed for two types of woodchips and one type of wood shreds in order to quantify and highlight the differences between what is commonly referred to as wood fill. Saturated hydraulic conductivity was determined for blends of woodchips, corn cobs, and pea gravel. For one of the most common types of woodchips used in bioreactors, the porosity varied from 66% to 78% depending on packing density and the average saturated hydraulic conductivity was 9.5 cm/s. It was found that additions of pea gravel significantly increased the hydraulic conductivity of woodchips though additions of corn cobs did not. Regardless of the fill mixture used, it is vital to design the bioreactor using the hydraulic properties for that specific media.

Beyond the nutrient strategies: Common ground to accelerate agricultural water quality improvement in the upper Midwest

Nutrients in drainage waters from the Upper Mississippi River Basin states have been a well-documented contributor to the Gulf of Mexico hypoxic zone for decades, and in response, twelve states have developed strategies to address this issue, with Iowa, Minnesota, and Illinois performing rigorous science assessments which estimated nitrogen and phosphorus reduction effectiveness for numerous agricultural non-point source conservation practices. The practices identified in these strategies were compared to identify areas of consensus and discord on nutrient load reduction potentials. Additionally, each practice was assessed for (1) the suitability to stack or be layered with other practices (stackability), (2) the ability to track implementation within a state or regionally (trackability), and (3) the level of production system change required to implement the practice. Overall, there was general consensus among the state strategies in the nutrient load reduction effectiveness of most practices with the exception of cover crops (10%-31% nitrogen reduction) and bioreactors (13%-43% nitrogen reduction). The most effective water quality-improvement practices (i.e., land-use change practices) required relatively more production system changes to agronomic management and were the most trackable (scores: 5, 1-5 scale), although they were also less stackable with other practices (scores: 1 to 1.8; 1-5 scale) and were the least cost effective on a unit area basis (generally

Curve Number Estimation Accuracy on Disturbed and Undisturbed Soils

15 to

Modeling Field-Scale Bioretention Cells with Heterogeneous Infiltration Media

964 per ha). The most cost effective practices tended to be highly stackable (e.g., nitrogen management: (-)

Water Table Response to Drainage Water Management in Southeast Iowa

49 per ha and stackability of 4.7), which indicated that stacking a variety of practices may be the most cost effective use of conservation dollars. The practices that were most difficult to track had relatively lower nitrogen loss reduction effectiveness, but these practices were less costly to implement and required relatively less production system change to agronomic management, two factors of importance to many producers.

Challenges in Implementation

AbstractEstimates of the USDA-NRCS runoff curve number (CN) are generally based on a soil map and observed land cover. Because the CN method is increasingly applied to disturbed and urbanized land, the objective of this work was to collect effective saturated hydraulic conductivities, Ksat, and sorptivities, So, from a range of land use types, use the results to estimate a CN, and compare these CNs with CN estimates made from soil survey information and corresponding land cover. A total of 331 double ring infiltration tests were conducted over the 15 sites. Based on land use and site history, the test sites were classified into categories of engineered, urban altered, rural altered, rural unaltered, and prairie. Measured Ksat values were skewed so the medians of these data were a better predictor of central tendency. The prairie and rural unaltered median Ksat values were closer to soil map estimates than the other categories (between 0.0 and 91% different from soil survey). Two empirical methods develope...

Development of a Bioretention Cell Model and Evaluation of Input Specificity on Model Accuracy

To address increasing stormwater management concerns in metropolitan and suburban areas, bioretention systems are a mitigation technology that helps address both water quantity and quality. However, it is critical for stormwater managers and engineers to model the hydraulic performance of these systems before investing in the infrastructure. This means an appropriate model must be selected to efficiently reflect important aspects of the given site and design. This work investigated the ability of a one-dimensional model to simulate water movement through a heterogeneous bioretention cell. For model validation, two full-size bioretention cells in Grove, Oklahoma, were flooded a total of three times, and parameters related to overflow, drainage and relative change in soil moisture were measured. Observed values were compared to predictions using an uncalibrated model previously developed and run with area-weighted soil parameters (“original model”). In addition, observations were compared to an uncalibrated “revised model,” which allowed modeling of distinct infiltration media. The revised model allowed for two separate soil types in the horizontal plane and simulated maximum subsurface drainage flow rate (23.6% to 33.7% from observed), volume (7.9% to 38.9% from observed), and timing (14.7% to 92.5% from observed) better than the original model, but the original model generally simulated overflow volume (12.2% to 77.1% from observed) and peak overflow rate (3.6% to 9.6% from observed) more closely. It was concluded that the revised model was more appropriate for modeling heterogeneous systems when concerns exist about timing of hydrographs and all underdrain parameters.

Impact of Fertilizer Application Timing on Drainage Nitrate Levels

A key component in managing subsurface drainage is controlling water table depth to limit excess drainage off site. The objectives of this work were to evaluate the impact of drainage water management through controlled drainage and shallow drainage on subsurface drainage volumes, water table depths, and crop yields. This research was conducted at the Iowa State University Southeast Research Farm and consisted of four paired management schemes for a total of eight plots. Plots consisted of a corn-soybean rotation with half of the plot planted in corn and half planted in soybeans each year. Preliminary findings for three years show undrained plots had a high occurrence of elevated water tables. Controlled and shallow plots had elevated water tables in the early spring and early fall in accordance with the rainfall and management protocols for controlled drainage. Water table response was quick with drawdown to tile depth within 2 to 3 days after significant rain events. Total annual drainage from the shallow and controlled plots was approximately equal and ranged from 20 to 40% of rainfall, while the conventional plots typically drained greater than 40% of the rainfall. There was no statistically significant difference between drained plots in terms of corn and soybean yield for the study period. Undrained plots, however, had slightly lower yields for both corn and soybeans. Overall, during the period of the study drainage water management through controlled drainage or shallow drainage reduced overall drainage volume while maintaining crop yield.