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SWEEP Report #60

The Effect of Conservation Tillage Practices on
the Losses of Phosphorus and Herbicides in
Surface and Subsurface Drainage Waters

Researchers: 
J. Gaynor and D. Bissonnette, Southwestern Ontario Agricultural Research Corporation (SWOARC), Harrow, Ont.

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

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Associated SWEEP/LSP Research

 

 

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Completed: May, 1992

Key Words:

conservation tillage, phosphorus, herbicides losses, surface water, subsurface water, soil erosion, no-till, ridge tillage, phosphorus transport, poorly drained, Brookston Clay Loam soil, zero-till, atrazine, metolachlor

Executive Summary

Part A - Soil Erosion, Phosphorus Losses and Corn Yield

Deterioration of agricultural land by soil erosion and compaction has encouraged the use of conservation tillage with residue management in southwestern Ontario. Many types of conservation tillage systems have been developed but all are characterized by reduced tillage with over 30% crop residue on the soil surface. Zero and ridge tillage are examples of two conservation tillage practices which leave sufficient crop residue to reduce water flow velocities, increase water infiltration and produce less soil compaction. Conservation tillage has been shown to be effective in reducing soil loss by fluvial transport and in some cases increasing crop yield.

Phosphorus (P) in subsurface, surface and lake waters has been linked with algal blooms which impairs water use and can lead to shifts in aquatic species and populations. Phosphorus in subsurface and surface runoff from agricultural activities has been associated with sediment-phase transport. Thus, common belief held that a reduction in sediment transport would result in reduced phosphorus transport.

Southwestern Ontario is characterized as ideally suited for agricultural production because of its proximity to large U.S. and Canadian markets, adequate and well distributed rainfall, fertile soils and favorable climate for crop production. Because of its proximity to large populations and the largest freshwater resource in North America, past and current agricultural practices have been intensively investigated with respect to yield and to a lesser extent on impairment of water quality. A study was begun in 1984 to identify sediment and phosphorus transport from a poorly drained, Brookston clay loam soil, cropped to corn under three tillage practices. Water quality and yield from a conventional (fall plow, spring disk and midseason cultivation) tillage treatment, was compared with that from a zero tillage (no tillage other than that associated with seeding and fertilizer application) and ridge tillage (ridges reformed in mid-season) treatment. Research results for the contract period January, 1988 to September 30, 1990 are reported herein.

Rainfall was measured with a rain gauge at the site to determine input. Surface and subsurface runoff for each runoff event was quantified from depth of water over a weir. Water samples were collected manually from each treatment throughout the hydrograph for sediment and orthophosphate analysis. Total soluble and sediment P were also determined over the three year period but an error in analysis invalidated the results for the 1988 and 1989 samples. These phosphorus forms were determined for the 1990 season. The sediment and orthophosphate concentrations were averaged within an event and event transport or loss was calculated from the product of average concentration of the component of interest and runoff volume. Annual losses were computed as the sum of event losses for the year. Surface and subsurface amounts were expressed as percent of total loss. The year was divided into three seasons, before planting (or fertilizer application, January 1 to planting), growing season (planting to September 30) and after harvest (October 1 to December 31). The seasonal losses were reported as a percent of total loss.

A wide range in climatic conditions from dry to wet growing season occurred over the duration of the study which affected grain yield, erosion and nutrient transport. Grain yield was similar within years from each of the tillage treatments. In 1988 (a dry year) and 1989 (a wet year) yields were low averaging 6 t/ha. In 1990 (a normal year) grain yield averaged 9 t/ha with slightly greater yield from the conservation tillage treatments than from conventional tillage. Conservation tillage has been reported to increase yield on well drained soil because of moisture conservation from crop residue but yield effects on poorly drained soils have been inconsistent and may be decreased. Excessive soil moisture during crop development as occurred in 1989, has been reported to reduce grain yield on Brookston soil.

Conservation tillage had a variable effect on sediment transport. In two of the three years, sediment transport was reduced by the conservation tillage treatments compared to that from conventional tillage. No tillage effects on sediment transport were observed in one year (1990) probably because no major runoff producing events occurred in the spring when soil erosion was most susceptible and sampling was terminated in September, 1990. Soil loss averaged over three years was 53011 kg/ha from ridge tillage, 39114 kg/ha from zero tillage and 89714 kg/ha from conventional tillage. Zero tillage was more effective at reducing erosion than ridge tillage over the three years. A greater proportion of sediment transport occurred in surface runoff from the conservation tillage treatments whereas subsurface transport accounted for a greater proportion of sediment loss from conventional tillage. It is possible that much of the sediment transported through subsurface drainage from conventional tillage originated from preferential surface flow through cracks developed when soil moisture content was low.

Generally, dissolved (ortho) phosphorus transport was higher from zero tillage than from ridge or conventional tillage. Dissolved phosphorus transport was lowest from conventional tillage. Annual dissolved phosphorus transport ranged from 2.7 to 10.4% of that applied from 1988 to 1990. Subsurface drainage accounted for greater than half of the orthophosphate loss from the conservation tillage treatments in two (1988 and 1990) of the three years. In 1989, surface runoff loss exceeded subsurface runoff loss of orthophosphate from zero tillage. Over 74% of orthophosphate transport from conventional tillage was from subsurface drainage in two of the three years. The seasonal loss pattern for dissolved phosphorus and water runoff were similar. The cumulative, three year sum of dissolved phosphorus transport was 74 to 174% higher from the conservation tillage treatments than from conventional tillage (166545 g/ha). Orthophosphate transport was 57% higher from zero than ridge tillage (2895751 g/ha).

Preliminary studies indicated that total soluble P and total phosphorus (sum of sediment P and soluble P) transport was higher from the conservation tillage treatments than from conventional tillage. The proportion of soluble P transported as orthophosphate was less from ridge (47%) and conventional tillage (56%) than zero tillage (80%). Sediment phosphorus loss was similar between ridge and conventional tillage (19914 and 19743 g/ha, respectively) but higher from zero tillage (26633 g/ha). Thus, much (84 to 93%) of the phosphorus transported from the conservation tillage treatments and conventional tillage occurred in the dissolved form. Other studies have shown that up to 98% of phosphorus transport may be associated with the sediment phase but that leachate from crop residue can account for considerable soluble phosphorus transported from conservation tillage treatments. Ridge reforming in ridge tillage may incorporate some of the crop residue with soil allowing for greater residue decomposition which could account for the greater release of soluble P from ridge compared to zero tillage. Sediment P and orthophosphate transport was higher from zero tillage than from ridge tillage because of P enrichment of runoff, but total P (the sum of all P forms) loss was higher from ridge tillage because of the greater proportion of other soluble P forms. Total phosphorus transport, assuming all P originated from fertilizer ranged from 70.3% from conventional to 157% from ridge tillage. Since total P forms were only determined from January to September, 1990, further research is required to substantiate these results. Tillage effects in this year were not statistically significant because of the high variability.

Phosphorus and sediment transport was also measured from a sod treatment in 1990. Results are not directly comparable to the corn treatments because sod received twice the amount of phosphorus as corn. However, several interesting inferences with the corn treatment can be made. Water runoff from the sod treatment was less than that from the corn treatment and ranged from 14 to 28% of the rainfall. Sediment transport was reduced to 129 kg/ha which was 30 to 37% of that from the conservation tillage treatments. Over 90% of the sediment loss occurred through subsurface discharge compared to 42 to 68% from corn culture. Dissolved orthophosphorus transport from the sod (6.5% of applied) was similar to that from ridge tillage (6.4%). The percentage of total phosphorus loss (141%) from the sod treatment was also similar to that from ridge (157%) tillage. As noted for ridge tillage, much of the total phosphorus transported from sod was in the soluble form.

This work indicates that for a poorly drained, Brookston clay loam soil, conservation tillage effectively reduced soil erosion but not phosphorus transport. Transport of soluble forms of phosphorus were higher from both conservation tillage treatments than from the conventional tillage treatment indicating that these tillage practices may result in increased phosphorus bioavailability and reduced water quality. Further research will be needed to confirm the result that total phosphorus transport is also increased with conservation tillage. Thus, if the Great Lakes water quality objective with respect to phosphorus is to be met, alternative or modified conservation tillage systems will have to be developed. The introduction of crops which have a low fertility requirement may result in lower recommended rates of fertilizers and hence improved water quality. Research at Agriculture Canada, Harrow is continuing to study alternative crop and soil management practices that affect water quality (herbicides and nitrate) but resources are not available to include phosphorus. The approach is to develop a management system, whereby through water table control, a favorable environment for efficient use of nutrients applied for crop growth will occur.

Part B - Herbicides

Conservation tillage with residue management has been proposed as the most effective means to reduce soil deterioration in southwestern Ontario. Residue management with conservation tillage has reduced soil erosion, increased organic matter content and reduced soil compaction. In some situations, impairment of water quality by pesticide contamination has been reduced by conservation tillage treatments. However, increased impairment of water quality has also been observed. A three year study was conducted on a poorly drained, low slope Brookston clay loam soil to investigate the effect of conventional tillage (fall plow, spring disk and mid season cultivation), zero tillage (direct planting in previous crop residue) and ridge tillage (planting in 15 to 20 cm high ridges, reformed in midseason) on surface and subsurface transport of atrazine and metolachlor applied preemergence to corn. Each runoff producing event was monitored to calculate the proportion of rainfall originating as runoff and water samples from the runoff events were collected for determination of herbicide concentration. Herbicide transport in the runoff was then computed for each event from the concentration profiles and runoff volumes. Residues of atrazine and its major dissipation product, des-ethyl atrazine, and metolachlor were determined in soil from the three tillage practices throughout the growing season and related to herbicide transport in the aqueous phase.

The proportion of rainfall originating as runoff was independent of tillage but dependent upon rainfall intensity, duration and antecedent soil moisture content. Less runoff (23% of rainfall) occurred in 1988 which was a dry year, compared to 28% in 1989, a normal year, and 36% in 1990, a wet year. Subsurface runoff exceeded surface runoff in all treatments. A greater proportion of the runoff occurred from the surface of the conservation tillage treatments compared to conventional tillage. The greater proportion of runoff from the surface of the conservation tillage treatments could impact on water quality since herbicide concentrations are higher in surface than subsurface runoff water.

Runoff producing events which occurred soon after herbicide application, transported the largest amount of herbicide. Where surface and subsurface runoff events occurred, herbicide concentration was highest in the surface runoff water and the mean concentrations were higher from the conservation tillage treatments than from conventional tillage. Herbicide concentrations of the runoff water were higher for metolachlor than for atrazine. However, because of the longer soil persistence of atrazine and inclusion of the primary metabolite, des-ethyl atrazine, in the amounts, less metolachlor was lost in the runoff water than atrazine. Tillage had no effect on transport quantities of atrazine and its metabolite or metolachlor. Average three year transport quantities for triazine (sum of atrazine and des-ethyl atrazine) were 581 g/ha from ridge, 363 g/ha from zero and 6213 g/ha from conventional tillage representing 3 to 4% of total atrazine applied from 1988 to 1990. Up to 25% of the total triazine transported from the tillage treatments was the dealkylated (des-ethyl atrazine) product. Corresponding average transport quantities for metolachlor were 463 g/ha from ridge, 481 g/ha from zero and 536 g/ha from conventional tillage representing 2% of that applied. Atrazine and metolachlor on crop residue were readily leached into soil by rainfall received soon after application.

Minor changes in soil persistence of atrazine and metolachlor were related to tillage. Atrazine and its dissipation product persisted longer than metolachlor in two of three years and in some years both herbicides showed greater persistence on ridge tops than in the valley of ridge tillage. Soil persistence of the herbicides was longer in conventional than zero tillage.

In conclusion, tillage and crop residue did not influence herbicide transport in surface and subsurface runoff from a level plane, poorly drained soil. The proportion of rainfall appearing as runoff was similar among tillage practices therefore the lack of a herbicide response to tillage was not unexpected. The quantity of herbicide transported from the three tillage treatments was less than 8% of that applied in the worst year while the three year cumulative loss amounted to less than 4% of that applied. Transport quantities of herbicide were more related to incidence of runoff producing events to herbicide application, rainfall intensity and duration, and antecedent soil moisture content. Tillage practices which alter the hydrologic response of watersheds will impact more on herbicide transport losses than tillage alone. Thus, tillage combined with other cultural practices such as band application of herbicide over the seed row, intercropping with a grass or legume forage, or adopting post emergence weed control strategies with low persistence herbicides should be investigated to determine their effect on impairment of runoff.

 

Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)
Part A: A three year study was conducted to identify sediment and phosphorus transport from a poorly drained, Brookston clay loam soil, cropped to corn under three tillage practices: conventional, no-till and ridge tillage. Water quality and corn yields were measured for all plots and compared. Rainfall was measured and surface and subsurface runoff was quantified for each rainfall event. Water samples were taken and analyzed for sediment and orthophosphate.

The results indicated that conservation tillage had a variable effect on sediment transport. In 2 of the 3 years, sediment transport was reduced by the conservation tillage treatments compared to that from conventional tillage. Zero-tillage was more effective at reducing erosion than ridge tillage over the three years. A greater proportion of sediment transport occurred in surface runoff from the conservation tillage treatments whereas subsurface transport accounted for a greater proportion of sediment loss from conventional tillage.

Dissolved phosphorus transport was higher from zero tillage than from ridge or conventional tillage. In 2 out of 3 years, subsurface drainage accounted for greater than half and 74% of the orthophosphate loss from conservation tillage and conventional tillage treatments respectively. Sediment phosphorus loss was similar between ridge and conventional tillage but higher from zero till. Ridge reforming in ridge tillage may incorporate some of the crop residue with soil allowing for greater residue decomposition which could account for the greater release of soluble P from ridge compared to zero tillage.

Transport of soluble P was higher in no-till and ridge till than conventional tillage. This work concludes that for a poorly drained, Brookston clay loam soil, conservation tillage effectively reduced soil erosion but not phosphorus transport.

Part B: A three year study conducted on a poorly drained, low slope Brookston clay loam soil to investigate the effect of conventional tillage, zero tillage and ridge tillage on surface and subsurface transport of atrazine and metolachlor applied preemergence to corn. Each runoff producing event was monitored to calculate the proportion of rainfall that left as runoff (which was collected to determine herbicide concentration).

The proportion of rainfall that ran off the plots was independent of tillage but dependent upon rainfall intensity, duration and soil moisture content. Subsurface runoff exceeded surface runoff in all treatments. A greater proportion of the runoff occurred from the surface of the conservation tillage treatments compared to conventional tillage. The greater proportion of runoff from the surface of the conservation tillage treatments could impact on water quality since herbicide concentrations are higher in surface than subsurface runoff water.

Runoff producing events which occurred soon after herbicide application, transported the largest amount of herbicide. Herbicide concentration was higher in the surface runoff water compared to subsurface runoff and mean concentrations were higher from conservation tillage than from conventional tillage treatments. Tillage had no effect on transport quantities of atrazine or metolachlor. Atrazine and metolachlor on crop residue were readily leached into soil by rainfall received soon after application. Herbicides showed greater persistence on ridge tops than in the valleys in ridge tillage. Soil persistence of the herbicides was longer in conventional than zero tillage.

Tillage and crop residue did not influence herbicide transport in surface and subsurface runoff on poorly drained soil.

Comments:

This is an excellent study that has raised some questions about phosphorus movement in conservation tillage systems on a poorly drained soil. The study showed that the herbicide movement of atrazine and metolachlor were not affected by tillage systems and crop residue.

Associated SWEEP/LSP Research:

  • SWEEP Report #14 - The Effect of Terraces on Phosphorus Movement
  • SWEEP Report #18 - Effects of Management on Soil Hydraulic Properties
  • SWEEP Report #23 - Processes Involved in Mobilization of Phosphorus in Different Farming Systems in Southwestern Ontario: Nutrient Levels in Plant Tissues and Soils
  • SWEEP Report #37 - Effects of Tillage on the Quality and Quantity of Surface and Subsurface Drainage Water: Uplands
  • SWEEP Report #55 - Soil Loss by Tillage Erosion: The Effects of Tillage Implement, Slope Gradient, and Tillage Direction on Soil Translocation by Tillage
  • SWEEP Report #53 - Phosphorus Movement in Soil as a Function of Phosphorus Solubility and Reactivity
  • SWEEP Report #45 - Management of Farm Field Variability III. Effect of Tillage Systems on Soil and Phosphorus Loss
  • SWEEP Report #60a - Appendix - The Effect of Conservation Tillage Practices on the Losses of Phosphorus and Herbicides in Surface and Subsurface Drainage Waters

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

(A) There is a need to determine the source of soluble phosphorous in the conservation tillage systems (crop residue, surface soil) and further study the effect on total phosphorus losses in poorly drained soils. This study should be extended to include different soil types and topographic profiles to see how they affect phosphorus and herbicide movement. Another study should be initiated to see how different tillage systems affect hydrologic response of watersheds and how this will impact herbicide transport and loss.

 

 

 

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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 04:21:49 PM