Peter A. Lawlor

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.

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.

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.

Water Balance Investigation of Drainage Water Management in Non-Weighing Lysimeters

Artificial subsurface drainage systems are often used throughout the upper Midwest to remove excess precipitation and improve crop production. However, these drainage systems export nitrate-nitrogen (NO3-N) to downstream water resources. Management practices are needed to reduce this export of NO3-N with subsurface drainage water. One such practice being considered is the use of drainage water management where subsurface water is held in the soil profile during portions of the year. Previous research has shown that drainage water management has potential to reduce subsurface drainage volume but there is still a need to understand the performance of the practice and the pathways of water flow under varying conditions. The objectives of this study, therefore, were to quantify the pathways of water movement for conventional or free drainage (FD) and drainage water management (DWM) during the growing season. In this study, six non-weighing lysimeters (0.92 × 2.30 m) with a depth of 120 cm were monitored over a 3-yr period under natural and simulated rainfall conditions. The objectives were performed to measure the effects of drainage water management (DWM) on surface runoff, subsurface drainage, and crop yield. The in-season data from natural rainfall conditions showed that DWM reduced subsurface drainage by approximately 14%. The simulated rainfall data showed that DWM increased surface runoff by 54% when the water table was established at 90 cm below the soil surface, and by 87% when the water table was established at 60 cm below the soil surface. Overall DWM was found to have the potential to reduce subsurface drainage but there is the potential that at least a portion of this reduction may be reflected in an increase in surface runoff.

Nitrification Inhibitor and Nitrogen Application Timing Effects on Yields and Nitrate-Nitrogen Concentrations in Subsurface Drainage from a Corn-Soybean Rotation

Excess precipitation in Iowa and many other agricultural production areas is removed artificially via subsurface drainage systems that intercept and usually divert it to surface waters. Nitrogen, either applied as fertilizer or manure and derived from soil organic matter, can be carried as nitrate with the excess water in quantities that can cause deleterious effects downstream. A four-year, five-replication, field study was initiated in the fall of 1999 in Pocahontas County, Iowa on 0.05 ha plots that are predominantly Nicollet, Webster, and Canisteo clay loams with 3-5% organic matter. The objective was to determine the influence of seasonal N application and the use of nitrapyrin [inhibitor; 2-chloro-6 (trichloromethyl) pyridine] on flow-weighted nitratenitrogen concentrations and yields in a corn-soybean rotation, combined on single plots. Six aqua-ammonia nitrogen treatments (168 and 252 kg/ha at planting and in late fall, and 168 kg/ha at planting and late fall with nitrapyrin) were imposed on subsurface drained, continuous-flow-monitored plots. Combined fall 1999 and spring 2000 precipitation was 42% of normal average. Subsequently, normal precipitation was recorded for both fall and spring periods (after fall application, and before spring application) until spring and fall 2002 (51% and 73% of normal, respectively). Spring 2003 precipitation was again only 51% of normal average. Four-year average, flowweighted nitrate-nitrogen concentrations ranked in highest to lowest order: spring- 252(22.9 mg/L;a) > fall-252(18.1 mg/L;b) > spring-168 w/inhibitor(17.7 mg/L;bc) > fall- 168 w/inhibitor(16.0 mg/L;bcd) > spring-168(14.8 mg/L;cd) > fall-168(14.2 mg/L;cd). Spring application plots had significantly greater soybean yield the following season compared to fall applications. Greatest corn yields were observed for the spring-252 and fall-168 rates, but were only significantly different than the spring-168 rate for yield. Therefore, under slightly dry to normal precipitation conditions, corn yields and nitrate-nitrogen concentrations in subsurface drainage were not significantly different between seasonal timing or inhibitor use treatments at the 168 kg/ha nitrogen rate.

Nitrogen Application Rate Effects on Corn Yield and Nitrate-Nitrogen Concentration and Loss in Subsurface Drainage

Excess precipitation in Iowa and many other agricultural production areas is removed artificially via subsurface drainage systems that intercept and usually divert it to surface waters. Nitrogen (N), either applied as fertilizer or manure or derived from soil organic matter, can be carried as nitrate (NO3) with the excess water in quantities that can cause deleterious effects downstream. Over a 16-year period, three N-rate treatment phases with five seasons (six for Phase II) each were imposed on conventionally tilled, subsurface drained, continuous-flowmonitored plots. The field study was initiated in the spring of 1989 in Pocahontas County, Iowa on 0.05-ha plots that are predominantly Nicollet, Webster, and Canisteo clay loams with 3-5% organic matter. The objective was to determine the influence of N fertilizer rates on flowweighted NO3-N concentration and loss along with yield in a corn-soybean rotation, over a wide range of weather conditions. Phase I N rates ranged from 0-168 kg N ha-1 in 56 kg N ha-1 increments. Although separate plots were used for each crop in Phase I, significant nitrate N concentration differences were not observed, at comparable rates, between corn or soybean plots; this lead to combining both crops in a split plot configuration for Phases II and III. Phase II N rates ranged from 45-179 kg N ha-1 in 45 kg N ha-1 increments. Phase III data were limited to two N rates, 168 or 252 kg N ha-1. Average yearly flow-weighted NO3-N concentrations (rate) ranged from 3.9 (45 kg N ha-1 in 1995) to 28.7 mg L-1 (252 kg N ha-1, in 2001). Average, flow-weighted NO3-N concentrations ranked in highest to lowest order for all rates (in mg L-1): 252 (23.4a) > 168 (15.5b) >179 (13.2b) > 112 (13.1b) >134 (11.9bc) > 56 (11.1bc) > 0 (9.9cd) >90 (8.1cd) > 45 (5.7cd). Losses were very precipitation dependent and were reflective of individual seasons and treatments imposed. Highest losses (88 kg N ha-1) were recorded in 1991, a high-flow year preceded by below normal precipitation, for the 112 kg N ha-1 rate. Loss was highly variable from year to year depending on drainage patterns. Corn yield ranking for all treatments in highest to lowest order (in kg ha-1): 252 (9313a) > 168 (8657b) > 112 (8211c) > 179 (7964c) >134 (7610c) > 90 (7164c) > 56 (6572d) >45 (6230d) > 0 (5078e). At commonly applied N rates between 168-179 kg N ha-1, average NO3-N concentrations in subsurface drainage were observed to be 13-15 mg L-1 and in an average drainage year (263mm) have approximately 32-38 kg ha-1 NO3-N lost to subsurface drains. Results from this study may have significant implications for fertilizer N management and subsurface drainage NO3-N loss to surface waters in the state, the Mississippi River and the Gulf of Mexico.

Impact of Fertilizer Application Timing on Drainage Nitrate Levels

Nitrate loss from drainage systems in Iowa and other upper Midwestern states is a concern relative to local water supplies as well as the hypoxic zone in the Gulf of Mexico. As a result, there is a need to quantify how various nitrogen management practices impact nitrate loss. One practice that is commonly mentioned as a potential strategy to reduce nitrate loss is to vary fertilizer application timing and specifically apply nitrogen as close to when the growing crop needs it as possible. At a site in Gilmore City, Iowa, a number of fertilizer timing and rate schemes within a corn soybean rotation were used to study the impacts on nitrate leaching. Timing schemes include nitrogen application in the fall and an early season sidedress in the spring with each scheme having four replicates for both corn and soybeans. Fertilizer application rates investigated are 84 and 140 kg/ha (75 and 125 lb/ac) in the fall and 84 and 140 kg/ha (75 and 125 lb/ac) in the spring. The timing and rates have been practiced since 2005 with contrasting weather conditions each year. Overall, an annual basis there was not significant differences in nitrate concentrations or loss exiting the drainage system between the application rates or between the fall and spring application. In addition, there was not a yield penalty to the corn crop when fertilizer as applied in the fall versus the spring.