In Iowa and many other Midwestern states, excess water is removed artificially through subsurface drainage systems. While these drainage systems are vital for crop production, nitrogen (N) added as manure or commercial fertilizer, or derived from soil organic matter, can be carried as nitrate-nitrogen (NO3-N) to downstream water bodies. A five-year, five-replication, field study was conducted in north-central Iowa with the objective to determine the influence of seasonal N application as ammonia or liquid swine manure on flow-weighted NO3-N concentrations and losses in subsurface drainage water and crop yields in a corn-soybean rotation. Four aqua-ammonia N treatments (150 or 225 lb-N/acre applied for corn in late fall or as an early season side-dress) and three manure treatments (200 lb-N/acre for corn in late fall or spring or 150 lb-N/acre in the fall for both corn and soybean) were imposed on subsurface drained, continuous-flow-monitored plots. Four-year average flow-weighted NO3-N concentrations measured in drainage water were ranked: spring aqua-ammonia 225 (23 ppm) = fall manure 150 every year (23 ppm) > fall aqua-ammonia 225 (19ppm) = spring manure 200 (18 ppm) = fall manure 200 (17 ppm) > spring aqua-ammonia 150 (15 ppm) = fall aqua-ammonia 150 (14 ppm). Corn yields were significantly greater (p=0.05) for the spring and fall manure-200 rates than for non-manure treatments. Soybean yields were significantly greater (p=0.05) for the treatments with a spring nitrogen application to the previous corn crop. Related: LPELC Manure Nutrient Management resources
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Why Study Sub-Surface Drainage and Manure Application?
Subsurface agricultural drainage has allowed for enhanced crop production in many areas of the world including the upper Midwest, United States. However, the presence of nitrate-nitrogen (nitrate-N) in subsurface tile drainage water is a topic of intense scrutiny due to several water quality issues. With the growing concern for the health of the Gulf of Mexico and local water quality concerns, there is a need to understand how recommended nitrogen management practices, such as through nitrogen rate and timing, impact nitrate-N concentrations from subsurface drainage systems. The objective of this presentation is to summarize results of studies from Iowa that have documented the impact of nitrogen application rate and timing on tile drainage nitrate loss.
What Did We Do?
The field experimental site was located near Gilmore City in Pocahontas County, IA. In the fall of 1999, seven treatments were initiated on 35 plots at the site to determine the effect of N source, rate, and application timing on crop yield and subsurface drainage water quality in a corn and soybean (CS) rotation. Two fertilizer N rates (168 or 252 kg ha-1) applied in the spring or fall and liquid swine manure (LSM) applied in spring or fall (218 kg ha-1) for corn production, and fall applied LSM for both crops in a CS rotation (168 kg ha-1) were randomly distributed in five blocks. Flow-weighted drainage samples were collected and volume measurements recorded for each plot through sampling/monitoring systems during drainage seasons in 2001-2004.
What Have We Learned?
This multi-year experiment demonstrated that rate and to a lesser extent timing affect concentration and losses and even at constant rates, these can be highly variable depending on precipitation patterns, N mineralization/denitrification processes and crop utilization in a given season. As expected, as nitrogen application rate to corn increases, the nitrate-N concentrations in subsurface tile drainage water increase. This highlights the need for appropriate nitrogen application to corn and to avoid over application. However, it is important to note that even when recommended nitrogen application rates are used, nitrate-N concentrations in subsurface drainage are still elevated and may exceed the EPA drinking water standard for nitrate-N of 10 mg L-1. Relative to timing of nitrogen application, i.e. moving from fall to spring application, our studies showed little to moderate potential to decrease nitrate-N concentrations. Likely the largest factor when looking at the effect from fertilizer application timing is when precipitation and associated nitrate-N loss occurs. Although timing of nitrogen application is important, perhaps the most important factor is to apply the correct amount of N. Manure treatments out yielded commercial N in all years. No significant differences in corn yield for any year were noted between application timing. Soybean yields were affected by N timing and less so by application rate.
Other management practices need to be explored for their potentials in reducing nitrate loads from tile drained systems. Promising practices include drainage management, alternative cropping systems and edge-of-field practices.
Matthew Helmers, Associate Professor, Department of Agricultural & Biosystems Engineering, Iowa State University, firstname.lastname@example.org
Xiaobo Zhou, Assistant Scientist, Department of Agricultural & Biosystems Engineering, Iowa State Univeristy
Carl Pederson, Agricultural Specialist, Department of Agricultural & Biosystems Engineering, Iowa State University
Lawlor, P.A., A.J. Helmers, J.L. Baker, S.W. Melvin, and D.W. Lemke. 2011. Comparison of liquid swine manure and ammonia nitrogen dynamics for a corn-soybean crop system. Trans. ASABE 54(5): 1575-1588.
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Funding for this project was provided by the Iowa Department of Agriculture and Land Stewardship through the Agricultural Water Management fund.
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