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SWEEP Report #51

Loss of Nitrogen by Microbial Denitrification, Nitrification, Surface
and Tile Runoff:  Relation to Tillage Method

Researchers: 
D. J. McKenney, Department of Chemistry and Biochemistry, University of Windsor, Ont., and C. F. Drury, Research Station, Agriculture Canada, Harrow, Ont.

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

View/Download Report [1064 KB pdf]

Associated SWEEP/LSP Research

 

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Completed: May, 1992

Key Words:

ridge tillage, no-till, moldboard plow, surface run-off, tile run-off, nitrate, nitrite, nitric oxide, nitrous oxide, nitrogen, corn, yield, Kentucky bluegrass

Executive Summary

Concerns over contamination of natural water systems and rising costs of production in the agriculture industry have led to increasing use of conservation tillage methods as a means of reducing these effects. Reduced tillage, however, also affects water movement, aeration and N-transformations in soils. A simplified N cycle diagram for soils is shown in Figure 1. The research described in this report was focused on several of these aspects as summarized below.

Nitrate Loss in Surface and Tile Runoff with Conservation and Conventional Tillage

Nitrate concentrations and total volume of surface and tile runoff from conventional (moldboard plow) tillage (CT), no-tillage (NT), ridge tillage (RT) all planted in continuous corn, and Kentucky bluegrass (BG) treatments, were measured over three years, 1989 to 1991. All corn tillage treatments received a total of 178.6 kg/ha N annually over the growing season. The volume of water drained through the tiles in the corn tillage systems always exceeded the volume in surface runoff, typically by factors of 2 to 4. Tile outflow was greatest from the CT treatments, least from BG and approximately equal from RT and NT treatments in 1989 and 1990. Concentrations of NO3- in tile water from CT, RT and NT treatments exceeded the maximum recommended safe limit of 10 mg/l N in 79% of runoff events with flow-weighted concentrations between 12 and 17 mg/l N in 1989 and 1990. Flow-weighted NO3- concentrations were only 1.2 and 2.6 mg/l N from BG in 1989 and 1990 respectively. The total NO3- lost in tile water in 1989 was 18, 14, 14 and 1 kg/ha N from the CT, RT, NT and BG treatments, respectively, whereas in 1990 there was 29, 20, 20 and 3 kg/ha N lost from the CT, RT, NT and BG treatments. Nitrate losses in surface runoff were lower than tile runoff with maximums of 2.6 kg/ha N for the RT and NT treatments in 1989 and 5.5 kg/ha N for the RT treatment in 1990. In 1989 and 1990, both RT and NT treatments had greater yields and N-uptake than the CT treatment. A serious drought in 1991 limited corn yield, N uptake and NO3- loss.

Background and Potential Denitrification Variation with Depth Under Conservation and Conventional Tillage

Background and potential denitrification was measured in Brookston clay soil at depths to 0.9 m under several conservation tillage plots and a conventional moldboard plow tillage plot. The objective was to identify possible effects of the various tillage treatments on denitrification and on NO3- loss. Background denitrification (NO + N2O) was estimated in intact cores in 1989 and 1990 using a gas flow-through system which permitted net NO and N2O production rates to be measured. Rates decreased with increasing depth under all tillage treatments but no correlation was evident between background denitrification and tillage, perhaps due to the large spatial variability observed. Potential denitrification estimates were made using excess added NO3- and the C2H2 blockage method (Yoshinara et al., 1977). Estimates were obtained in 5 cm increments to a depth of 0.9 m. In all cases the top 5 cm increment showed highest potential for denitrification. Among the tillage treatments the highest potential was with the no-till Kentucky bluegrass plot and intermediate potentials in ridge till and no-till corn plots and lowest in the conventional till plot. Carbon dioxide production, NO3- and NH4+ levels followed the same pattern with depth. The relatively low denitrification potential below the uppermost soil layer suggests that in these plots denitrification would not be expected to greatly ameliorate NO3- loss by leaching.

Nitric Oxide and Nitrous Oxide Production: Water and Oxygen Effects

This study was designed to determine the effects of water and O2 on the speciation of denitrification gases (NO and N2O). Nitric oxide was found to be the principal end product from soil incubated under low moisture conditions, whereas the relative amount of N2O increased under wetter moisture regimes. The total amount of NO plus N2O produced increased with increasing water content for the Brookston clay loam whereas it peaked at 15% water content with the Fox sandy loam. The decrease in NO plus N2O at higher water contents was probably the result of the subsequent reduction of N2O to N2 in the Fox sandy loam soil. The residence time of the denitrification gases in the soil increased with increasing water content, hence facilitating the subsequent conversions of NO to N2O and N2. The thickness of the water film surrounding the microbes affected both the diffusion of O2 through the water and into the microbes as well as the diffusion of denitrification gases (NO, N2O, and N2) from the microbes into the atmosphere. In the sandy loam soil, O2 content and soil water affected both the amount and species of evolved denitrification gases. Oxygen was more effective in decreasing NO production at lower than at higher water contents.

Implications

Although conservation tillage (RT and NT) decreased the concentration and total loss of NO3- the concentrations generally exceeded the 10 mg/l N recommended limit. Further reduction could perhaps be achieved through use of cover crops, intercrops or controlled drainage systems. In conservation tillage systems, particularly RT, losses of N by denitrification would likely be reduced if nitrogen fertilizer was added below the surface 5 cm zone. Since tillage influences water infiltration and storage and the aeration status of the soil the overall denitrification rate is affected as well as the ratio of NO:N2O. Thus tillage indirectly affects the environmental impacts of these important atmospheric trace gases, and also contributes to the observed large spatial variability in flux of these gases from soils. Greater levels of NO evolve from drier soils and relatively greater levels of N2O from wetter soils. Under all tillage systems both background and potential denitrification decreased markedly with depth reflecting low populations below the surface layer. Hence in the plots used in this study, denitrification would not be expected to greatly ameliorate loss of NO3- by leaching.

 

Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)

The report covered three separate experiments all related to the occurrence of nitrogen losses from soil under a variety of tillage systems.

The first experiment compared the affected volume, concentration, and total amount of NO3- lost from surface and tile run-off, under ridge till, no-till, and moldboard plow systems planted to corn, compared to a Kentucky bluegrass treatment. As the amount of nitrogen utilized by plants will reduce the nitrogen available to be leached, grain corn yields and nitrogen uptake of the three tillage plots were compared.

The plots were centred above tile placed at a depth of 0.6 m. Both surface and tile water were collected from natural rainfall events. Rainfall for 1989 was below average, for 1990 above average and in 1991 there were drought conditions. The results of the study showed that the greatest movement of water for all of the corn tillage systems was through the tile rather than surface run-off. The corn tillage system that had the greatest volume of tile flow was the moldboard plow system.

Grain corn yield and tissue nitrogen under the ridge till and the no-till plots was higher than the moldboard plow system. The higher nitrogen uptake in the conservation plots over the moldboard plow plots was similar to the reduction in the amount of NO3- lost by leaching. While the concentration of NO3- from the tile was less under the conservation tillage it was still above drinking water standards. Bluegrass sod was the only treatment that consistently had both surface and tile run-off waters within drinking water standards.

The objective of the second experiment was to identify the effects of various tillage methods on the denitrification process with depth. The treatments consisted of bluegrass, and continuous corn under ridge tillage, no-till and moldboard plow as in the first experiment. Soil cores were taken in October 1989 and July 1990 and background levels of nitric oxide and nitrous oxide were determined by using the continuous flow technique. Potential denitrification estimates were determined by using the acetylene inhibition technique.

Background denitrification rates were about the same level in 1989 and 1990 and decreased with depth under all tillage systems. No relationship between tillage systems and production rates was demonstrated between the two years. Large variations in rates persisted within and among sampling sites. Both background levels and potential denitrification decreased with depth.

The objective of the third experiment was to study the physical effects of water on NO and N2O production under conditions favouring denitrification. The combined effect of O2 and soil water content on NO production was also studied.

Comments:

At times it was difficult to determine if the information provided in the report was the results of the study or part of a literature review. The differences in weather conditions that occurred between the three years provides some interesting information. However, further trials should be conducted to determine if the results obtained for each of the three years is an accurate indication of surface and tile run-off as well as corn yield for each system. No explanation was given as to why in 1989 there was a greater loss of NO3- per event than 1990.

Study 2 only looked at potential denitrification rates under laboratory conditions rather than actual rates under field conditions. This was a lab study not a field study and further examination is needed to determine if the results are transferable to field conditions. The length of the study should have been longer than 2 years before making estimates on the potential for denitrification.

Continuous corn is known to degrade both surface and subsurface soil structure over time, which could affect the results. Monoculture corn is not used as extensively as it once was on Ontario farms. Mulch tillage, the most popular conservation tillage system for lowland clayey soils was not included in this study. The no-till system used in the study, the planter shoe tillage, is not common in Ontario. With this in mind another system of no-till, one more representative of what is used in Ontario, should have been studied.

Nail-like spikes extending above each statistical box are not explained. It could be assumed that these refer to the Standard Deviation of Error, however it would be preferable if this were explained.

Carbon levels affect denitrification, however the organic matter levels in the various zones within the profile are not reported.

Associated SWEEP/LSP Research:

  • SWEEP Report #17 - Effect of Ammonia on Soil Properties and Relevance to Soil and Water Quality

  • SWEEP Report #28 - The Effect of Split Applications of Nitrogen on Corn Yield Under Ridge and No-Till Conditions

  • SWEEP Report #35 - Nutrient Distribution and Stratification Resulting from Conservation Farming

  • SWEEP Report #37 - Effects of Tillage on the Quality and Quantity of Surface and Subsurface Drainage Water: Uplands

  • LSP7017 - Cropping and Soil Management Effects on the Dynamics of Crop Residue Derived-N on the Coarse Textured Soils in Southern Ontario

  • LSP7018 - Nitrogen Research with Corn Using Conservation Tillage

  • LSP7020 - Nitrogen Conserving Farm Systems

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

(A) Research on the nitrogen cycle is needed but not in field studies.

 

 

 

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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 03:56:20 PM