The Effect of Shallow Subsurface Drains on Nitrate-N and Orthophosphorus Losses from Drained Agricultural Lands
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Date
2006-05-10
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Abstract
Field and modeling studies were conducted to determine the effect of subsurface drain depth on nitrate and orthophosphorus losses in drainage effluent. The field research was conducted on a site located near Plymouth, NC at the Tidewater Research Station. The soil at the site is a Cape Fear loam. Two drainage systems were installed on a 2.5 ha pasture which was used for grazing beef cattle. The pasture was under a Bermuda-Rye rotation during the study period. Each drain plot consisted of five- 125 m drain lines 10 cm in diameter. The average depth for plot 1 was 1.44 m at 25 m spacing. Plot 2 had an average depth of 0.70 m at 12.5 m spacing.
Precipitation, water table depths, and subsurface drainage rates were measured during the 31-month study period from October 1, 2002 through April 30, 2005. Subsurface drainage and shallow groundwater were sampled and analyzed to determine nitrate and orthophosphorus concentrations. Outflows from the shallow drains were 17.1 % less than from the deep drains for the 31-month period. Rainfall was average to well above average for the majority of the study period. An analysis of variance (ANOVA) showed that cumulative drainage volumes from the shallow drains were significantly less from that of the deep drains in only 2004 and 2005. High water tables in both plots resulted from the average to well above average precipitation during the study.
Average nitrate concentrations in the groundwater at 90-120 cm, 150-180 cm and 210-240 cm were significantly lower for the shallower drains than for the deeper drains. The average concentrations in the drainage water for nitrate were not significantly different between the treatments. Plot 1 averaged 6.1 mg⁄L and Plot 2 averaged 6.6 mg⁄L. Nitrate export by the shallow drains was 9.8 % less than from the deeper drains. Exports from the two plots were not significantly different when the average cumulative values for each year, or partial year were compared using analysis of variance (ANOVA). However, when export values were analyzed by month (under similar weather patterns) and then compared with a paired two-sample t-test for means, the shallow system did significantly reduce nitrate export between the two systems.
Orthophosphorus concentrations in the groundwater at 90-120 cm, 150-180 cm and 210-240 cm were significantly higher for the shallower drains than for the deeper drains. The average concentrations in the drainage water for orthophosphorus were significantly higher for the shallow plots. Plot 2 averaged 0.43 mg⁄L and Plot 1 averaged 0.24 mg⁄L. Orthophosphorus export was increased during the course of the study by the shallow system by 103 %. The export was significantly different when the average cumulative values for each plot were compared using ANOVA and when monthly values were compared using a paired two-sample t-test.
Long-term (72-year) simulations using DRAINMOD predicted that yearly drainage volumes from the shallow plot were significantly less than from the deep plot at the 10% level. The average yearly reduction in drainage volume was approximately 24 %.
The simulated predicted average yearly reduction in nitrate losses was 22 %. Analysis of variance showed that Plot 2 exported significantly less nitrate than Plot 1 at the 10 % level with a p-value of <0.0001 for the 72 years of simulated data. On a cumulative basis, the shallow system reduced predicted nitrate loss by approximately 22 %.
The average yearly predicted losses of orthophosphorus from the deep system was 1.0 kg/ha while from the shallow system it was 1.5 kg/ha, an average increase of 0.5 kg⁄ha. This implies an average yearly increase in orthophosphorus losses of 50 %. The ANOVA analysis showed that Plot 2 exported significantly more OP than Plot 1 at the 10% level over the 72 years simulated. On a cumulative basis for the 72-year simulation period, the shallow system increased orthophosphorus export by 40.3 %. The long-term simulations show that the shallow system will export on average between 40 to 50 % more orthophosphorus than the deeper system.
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Keywords
swine waste, nitrate, phosphorus, irrigation, subsurface, drainage
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Degree
MS
Discipline
Biological and Agricultural Engineering
