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Journal of Environmental Quality 32:607-612 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Plant and Environment Interactions

Residual Soil Nitrate after Potato Harvest

Gilles Bélanger*,a, Noura Ziadia, John R. Walshb, John E. Richardsc and Paul H. Milburnd

a Agriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, 2560 Hochelaga Boulevard, Sainte-Foy, Québec, Canada G1V 2J3
b McCain Foods Limited, 317 Main Street, Florenceville, New Brunswick, Canada E7L 3G6
c Agriculture and Agri-Food Canada, Atlantic Cool Climate Crops Research Centre, Brookfield Road, P.O. Box 39088, St-John's, Newfoundland, Canada A1E 5Y7
d Agriculture and Agri-Food Canada, Potato Research Centre, P.O. Box 20280, Fredericton, New Brunswick, Canada E3B 4Z7

* Corresponding author (Belangergf{at}agr.gc.ca)

Received for publication April 12, 2002.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrogen loss by leaching is a major problem, particularly with crops requiring large amounts of N fertilizer. We evaluated the effect of N fertilization and irrigation on residual soil nitrate following potato (Solanum tuberosum L.) harvests in the upper St-John River valley of New Brunswick, Canada. Soil nitrate contents were measured to a 0.90-m depth in three treatments of N fertilization (0, 100, and 250 kg N ha-1) at two on-farm sites in 1995, and in four treatments of N fertilization (0, 50, 100, and 250 kg N ha-1) at four sites for each of two years (1996 and 1997) with and without supplemental irrigation. Residual soil NO3–N content increased from 33 kg NO3–N ha-1 in the unfertilized check plots to 160 kg NO3–N ha-1 when 250 kg N ha-1 was applied. Across N treatments, residual soil NO3–N contents ranged from 30 to 105 kg NO3–N ha-1 with irrigation and from 30 to 202 kg NO3–N ha-1 without irrigation. Residual soil NO3–N content within the surface 0.30 m was related (R2 = 0.94) to the NO3–N content to a 0.90-m depth. Estimates of residual soil NO3–N content at the economically optimum nitrogen fertilizer application (Nop) ranged from 46 to 99 kg NO3–N ha-1 under irrigated conditions and from 62 to 260 kg NO3–N ha-1 under nonirrigated conditions, and were lower than the soil NO3–N content measured with 250 kg N ha-1. We conclude that residual soil NO3–N after harvest can be maintained at a reasonable level (<70 kg NO3–N ha-1 ) when N fertilization is based on the economically optimum N application.

Abbreviations: Nop, economically optimum nitrogen fertilizer application


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
EFFICIENT USE OF N fertilizer is essential to increase the economic return of potatoes and minimize potentially negative effects on water and air quality. Nitrogen fertilizer applications that exceed crop requirements for maximum corn (Zea mays L.) yield can result in an accumulation of soil NO3–N, which is susceptible to leaching, denitrification, or volatilization (Roth and Fox, 1990).

Nitrate leaching is largely influenced by the amount of mineral N within the topsoil at the end of the growing season (Steenvoorden, 1989; Roth and Fox, 1990). The amount of nitrate leached beyond a depth of 1 m is directly related to the quantity of mineral N within the soil to a 1-m depth (Onken et al., 1985). Olsen et al. (1970) report a greater leaching of nitrate between fall and spring sampling than during the growing season in a silt loam soil. Therefore, the amount of residual mineral N within the root zone should be as small as possible at the end of the growing season. This can be achieved by combining carefully timed, appropriate fertilizer N applications with an optimal water supply to the crop (Steenvoorden, 1989).

The amount and distribution of nitrate within the soil profile at the end of the growing season is greatly affected by N fertilizer applications (Isfan et al., 1995) and by the volume and method of supplemental irrigation (Olsen et al., 1970; Campbell et al., 1983). Magdoff (1991) found that 40 to 150 kg NO3–N ha-1 remained in the soil following a corn crop, even without excess N fertilization. Roth and Fox (1990) also found 41 to 138 kg NO3–N ha-1 to a depth of 1.2 m in the fall following a corn crop fertilized with an economic N application.

Studies on potatoes in Maine, Ontario, and New York show that about 40 to 60% of the N applied at planting is recovered in the tubers (Bouldin and Selleck, 1977; Cameron et al., 1978). These studies also report high NO3–N concentrations in shallow ground water beneath the potato field or within the soil profile beneath the root zone, suggesting that leaching of applied N had occurred. In Eastern Canada, however, little information is available on residual soil NO3–N after potato harvest. In a previous paper, we defined the economically optimum nitrogen application (Nop) that can be used to manage more efficient N fertilization (Bélanger et al., 2000b). To our knowledge, there are no reports on the consequences of fertilizing potatoes using the concept of Nop and residual soil NO3–N after harvest. We hypothesized that residual soil NO3–N after harvest can be maintained at a reasonable level (<70 kg NO3–N ha-1 ) when N fertilization is based on Nop. The objectives of this study were to (i) quantify the effect of N fertilization and irrigation on residual soil NO3–N content in the fall following potato harvest in eastern Canada and (ii) determine if residual soil NO3–N after harvest can be maintained at a reasonable level when N fertilization is based on Nop.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Experiments
Measurements of residual soil NO3–N were conducted at two sites in 1995, and four sites in 1996 and 1997 (Table 1) as part of a larger study conducted at four on-farm sites in each of three years (1995 to 1997) in the upper St-John River valley of New Brunswick, Canada. The sites of the large study are referred to as S1 to S4 in 1995, S5 to S8 in 1996, and S9 to S12 in 1997 (Bélanger et al., 2000a). Nitrogen as ammonium nitrate was applied at planting at 0, 50, 100, 150, 200, and 250 kg N ha-1 on two potato (Solanum tuberosum L.) cultivars (Shepody and Russet Burbank). The N applications are referred to as N0 to N250. At each site, the experiment consisted of two large blocks (irrigated and nonirrigated). Within each block, a split-plot arrangement of the experimental treatments was used with cultivars as main plots and N fertilization applications as subplots with four replicates. Individual plots consisted of six rows each 7.6 m in length. At harvest, the middle two rows of each plot were used to determine total and marketable yields.


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  		Table 1. Soil characteristics, water input, and previous crop at each site.{dagger}

 
Irrigation applications were scheduled with the Wisdom computer software program (Curwen and Massie, 1984). This program uses a water budget approach to schedule irrigation. Crop water-use estimates to monitor soil-moisture levels were based on potential evapotranspiration. The potential evapotranspiration (PET) was calculated with a modified Priestly–Taylor equation (Jones, 1992) and was adjusted for canopy cover. This calculation required daily net solar radiation and mean daily air-temperature data. The soil water-holding capacity (WHC) was characterized for a 30-cm rooting depth by measuring the moisture differential between 0.01 (0.03 in 1995) and 1.5 MPa with a pressure plate extractor apparatus. Water was applied when soil moisture reserves fell to 65% of the soil water-holding capacity. The allowable soil moisture depletion [WHC - (0.65 x WHC)] among sites ranged from 16 to 33 mm. The application rate was adjusted to compensate for expected PET. Some irrigation applications were less than the amount required to restore theoretical moisture deficits when rainfall was expected. Irrigation was applied at 0.68 cm h-1 with a portable overhead irrigation system operated at a nominal pressure of 0.34 MPa at the sprinkler heads. Sprinklers were installed on an 18- x 18-m grid. The irrigation block extended about 10 m beyond the plot boundaries in all directions.

Shortly after harvest (early October; Bélanger et al., 2000a), soil samples were taken to a 0.90-m depth in 0.15-m increments. A single core per plot was taken with a 4-cm-diameter soil probe. Soil was sampled only on cultivar Shepody plots at two sites (S2 and S4) for three N treatments in 1995 (N0, N100, and N250) and for four N treatments at all sites in 1996 and 1997 (N0, N50, N100, and N250). Inorganic N was extracted with a 10:1 ratio of 2 M KCl to air-dry soil. The extract was filtered and NO3–N was determined with a colorimetric hydrazine-reduction method (Tel and Heseltine, 1990).

Data Analysis
Analyses of variance across sites were calculated for soil NO3–N content within the 0- to 0.90-m soil layer, and for the proportion of soil NO3–N contained within the 0- to 0.30- and 0.75- to 0.90-m layers. Because irrigation treatments were not replicated, we could not statistically analyze the effect of irrigation for each site. We therefore considered sites as random effects, and their interaction with irrigation was used to test the effect of irrigation. An analysis of variance for each individual site was used to test the effect of N fertilization. A linear parallel curve analysis with grouped data (FIT routine of Genstat) was used to determine if the relationship between residual soil NO3–N content at 0 to 0.90 m and 0 to 0.30 m differed for irrigated and nonirrigated conditions. Statistical analyses were performed with Genstat (Genstat 5 Committee, 1993) and statistical significance was defined at P < 0.05.

The residual soil NO3–N content at Nop was estimated from the relationship between residual soil NO3–N content within a 0.90-m depth and applied N at each site (Roth and Fox, 1990). The relationship between soil NO3–N content and applied N was determined at each experimental site with either an exponential model [Y = a exp(bX)] or a linear model (Y = cX + d), where Y is soil NO3–N content at 0 to 0.90 m (kg ha-1), X is the N application (kg ha-1), and a, b, c, and d are estimated parameters. The choice of the model was based on the general shape of the data points and the coefficients of determination (R2). Linear and nonlinear regressions were calculated by Excel (Microsoft Corporation, 1997). The economically optimum fertilizer application (Nop) for marketable yield was predicted with a polynominal regression model with a quadradic term as reported by Bélanger et al. (2000b). The Nop (kg N ha-1) is defined as the N application where one dollar of additional N fertilizer returns one additional dollar of potatoes, and it describes the minimum rate of N application required to maximize economic return (Colwell, 1994). This analysis assumes that fertilizer N costs are the only variable costs and that all other costs are fixed. The Nop was calculated by setting the first derivative of the N response curve equal to the ratio between the cost of fertilizer and the price of potatoes. The ratio of the cost of N fertilizer to the price of potatoes was 0.006 (Bélanger et al., 2000b).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Residual soil NO3–N within a 0.90-m depth, averaged across all N applications under irrigated and nonirrigated conditions, ranged from 30 kg NO3–N ha-1 at S8 to 136 kg NO3–N ha-1 at S4. This large variation suggests that field history, and physical and chemical soil properties, affect residual NO3–N content (Roth and Fox, 1990). Site S4 also had the highest soil (0–0.30 m) NO3–N content in spring (107 kg NO3–N ha-1; Table 1), while the S8 site had the lowest soil NO3–N content in spring before planting (7 kg NO3–N ha-1; Table 1). There was, however, no significant relationship between the soil NO3–N content within a 0.30-m depth in spring and the residual soil NO3–N content within a 0.90-m depth in the fall for the unfertilized check plots (data not shown).

Effect of Nitrogen Fertilizer Applications and Irrigation
Applied N fertilizer significantly affected residual soil NO3–N content within the 0- to 0.90-m soil layer (Table 2). Residual soil NO3–N in the fall increased significantly with increasing applied N across sites (Table 3) and at each individual site with and without irrigation (Table 4). Soil NO3–N content over the entire profile (0–0.90 m), and averaged across all sites, was 33 kg NO3–N ha-1 in the check plots and 152 kg NO3–N ha-1 with N250. Olsen et al. (1970) and Roth and Fox (1990) for corn, and Bock and Hergert (1991) for potato crops also found that NO3–N remaining in the soil after harvest increased with increasing N applications. Jokela and Randall (1989) found 66 to 160 kg NO3–N ha-1 within a 1.2-m depth after silage corn. Magdoff and Amadon (1980) also found 40 to 60 kg ha-1 of NO3–N present within a 1.2-m depth in the fall following fertilized silage corn.


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  		Table 2. Analysis of variance across sites for residual soil nitrate (NO3–N) content within the 0- to 0.90-m soil layer, the proportion of NO3–N content within the 0- to 0.30-m layer, and the proportion of NO3–N content within the 0.75- to 0.90-m layer.

 

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  		Table 3. Residual soil NO3–N content within the 0- to 0.90-m soil layer and proportions of NO3–N content within the 0- to 0.30-m and 0.75- to 0.90-m layers.

 

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  		Table 4. Residual soil NO3–N content within the 0- to 0.90-m soil layer as affected by N fertilizer applications and irrigation.

 
Irrigation significantly reduced residual soil NO3–N content within the 0- to 0.90-m layer (Tables 2 and 3). When all sites were combined, the residual soil NO3–N content (0–0.90 m) ranged from 29 to 123 kg NO3–N ha-1 under irrigated conditions and from 35 to 182 kg NO3–N ha-1 under nonirrigated conditions (Table 3). The decrease in soil NO3–N accumulation with supplemental irrigation varied also with applied N as shown by the significant irrigation x N interaction (Table 2). When all sites were combined, irrigation decreased residual soil NO3–N content within a 0.90-m depth by 6 kg NO3–N ha-1 for check plots and by 58 kg NO3–N ha-1 for the N250 application (Table 3). For three (S2, S7, and S8) of the nine sites, however, the residual soil NO3–N content with the N250 application was numerically greater with irrigation than without irrigation (Table 3). It is also interesting to note that the nitrate concentration of tubers at harvest was higher under nonirrigated conditions than under irrigated conditions, particularly under high N application (Bélanger et al., 2002).

This decrease in residual soil NO3–N under irrigation at most sites may be explained, in part, by a greater N uptake by potatoes as reported by Bélanger et al. (2001a), but also by greater N leaching during the growing season, or by denitrification. Campbell et al. (1983) attributes the lower NO3–N in the subsoil during wet years to the higher N uptake by plants. The proportion of NO3–N within the 0.75- to 0.90-m layer was significantly greater with irrigation (11.7%) than without irrigation (9.1%) (Tables 2 and 3); this may indicate greater leaching during the growing season when irrigation was used. Liang et al. (1991) attributes the decrease in soil NO3–N with irrigation for corn production to denitrification in a sandy clay loam. Denitrification was not measured in our study but the clay and silt contents of the soils indicate that denitrification may have occurred. Further research is required to assess the effect of irrigation on nitrate leaching and denitrification during the growing season.

Saffigna and Keeney (1977) report that NO3–N leaching was reduced from 200 to 120 kg ha-1, and the average NO3–N concentration of the drainage leachate decreased from 23 to 16 mg L-1, when total N and irrigation water were reduced and total N was split into fractional amounts that were applied several times during the growing season. The study of Saffigna and Keeney (1977) also suggests that proper irrigation management should limit irrigation to less than 100% of water-holding capacity to minimize nitrate leaching. When soil is saturated, each additional 2.5 cm of irrigation (or rainfall) can move nitrate 15 to 20 cm d-1 in sandy soils. Also, studies using lysimeters show that most leaching occurs during the late fall and early spring when evapotranspiration is low (Bergström, 1987).

When averaged across all treatments and sites, the soil NO3–N within the surface 0.30 m in the fall as a proportion of the total within the 0.90-m depth was not affected by irrigation and averaged 59% (Tables 2 and 3). The fraction of N in the surface 0.30 m increased with increasing N rate under both irrigated and nonirrigated conditions (Table 3). Residual soil NO3–N content within the surface 0.30 m was highly correlated with the NO3–N content within a 0.90-m depth under irrigated and nonirrigated conditions (R2 = 0.95; Fig. 1) . This result agrees with that of Roth and Fox (1990). A statistical difference between the two regression slopes (Fig. 1) indicates that the relationship between NO3–N content within a 0.30- and 0.90-m depth depends on supplemental water additions. This relationship between residual soil NO3–N content at 0 to 0.30 m and 0 to 0.90 m suggests that soil NO3–N concentration within the surface 0.30 m measured can be used to identify fields that have potentially polluting soil NO3–N contents during late fall and early spring. It is well known that high levels of soil NO3–N in the fall increase the possibility of leaching. Onken et al. (1985) also reports highly significant relationships between NO3–N concentration in the upper layers and those within the lower layers for irrigated corn production. In a study with irrigated sorghum [Sorghum bicolor (L.) Moench] on a clay loam, Onken and Sunderman (1972) also found that surface NO3–N was related to total NO3–N within the soil profile and the percentage of total NO3–N. Nitrogen found at given depths was relatively constant over years, locations, and fertilizer applications.



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  		Fig. 1. Relationship between residual soil NO3–N content at 0 to 0.90 and 0 to 0.30 m under irrigated and nonirrigated conditions. Data points are for all N fertilizer applications at 10 sites.

 
Relationship between Residual Soil Nitrate Nitrogen Content and Economically Optimum Nitrogen Fertilizer Application
Estimates of residual soil NO3–N at Nop for marketable yield varied considerably with sites under irrigated and nonirrigated conditions (Table 5). The range of these estimates, however, was much greater under nonirrigated (62–260 kg NO3–N ha-1) than under irrigated (46–99 kg NO3–N ha-1) conditions. Averaged across all sites, the residual soil NO3–N content for Nop was 66 kg ha-1 under irrigated conditions and 131 kg ha-1 under nonirrigated conditions (Table 5). This result is not surprising since irrigation decreases residual soil NO3–N content, as reported previously.


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  		Table 5. Estimated residual soil NO3–N content within the 0- to 0.90-m layer at the economically optimum fertilizer application (Nop) for marketable yield.

 
The possibility of high residual soil NO3–N and subsequent NO3–N losses increased when N fertilizer applications were above Nop (Fig. 2) . In general, for N applications greater than Nop, residual NO3–N was greater than for lesser additions, especially under nonirrigated conditions (Fig. 2). The residual soil NO3–N content for Nop (Table 5) was less than for N250 applications at all sites (Table 4). These results confirm that residual soil NO3–N content and the potential for N losses increase when N inputs exceed crop requirements. In addition, estimated residual soil NO3–N content at Nop under irrigated conditions rarely exceeded the norm of 70 kg NO3–N ha-1 established in Europe (Neeteson, 1995). A large residual soil NO3–N content at Nop (>150 kg NO3–N ha-1) was observed at three sites (S4, S10, and S11) under nonirrigated conditions (Table 5); no clear explanations for these could be found. For N250, however, NO3–N accumulation varied widely and exceeded 70 kg NO3–N ha-1 except at S8 (63 kg ha-1; Table 4) under irrigated conditions. Thus, values as high as 430, 334, and 221 kg NO3–N ha-1 were obtained within the 0- to 0.90-m profile when 250 kg N ha-1 was applied. In a corn study conducted in Pennsylvania, Roth and Fox (1990) report estimates of NO3–N accumulation at Nop of 115 kg NO3–N ha-1 in nonmanured experiments and 138 kg NO3–N ha-1 in manured experiments. Magdoff (1991) also reports that 40 to 150 kg NO3–N ha-1 remained in the soil following corn crop, even without excessive N applications. Our results suggest that even though fertilizer N can still accumulate in the soil at Nop, the possibility of high residual soil NO3–N after harvest is reduced when potatoes are fertilized according to their N requirements. The utilization of Nop to define crop requirements at a given site in a given year is discussed by Bélanger et al. (2001b).



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  		Fig. 2. Relationship between residual soil NO3–N content to a 0.90-m depth and the difference between N applied and the economically optimum nitrogen application (Nop) for marketable yield under irrigated and nonirrigated conditions. N0, N50, N100, and N250 refer to 0, 50, 100, and 250 kg N ha-1 before planting.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our results confirm that soil NO3–N content within the 0- to 0.90-m soil layer measured in the fall after potato harvest increases with increasing N applications. Residual soil NO3–N after harvest can be maintained at a reasonable level (<70 kg NO3–N ha-1 ) when N fertilization is based on the economically optimum N application. Irrigation reduced residual soil NO3–N content, primarily under high N application. Residual soil NO3–N content within the surface 0.30 m was highly correlated with total NO3–N to 0.90 m, indicating that the soil NO3–N concentration within the surface 0.30 m can help to identify fields with potentially polluting soil NO3–N content.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution no. 738 Agriculture and Agri-Food Canada.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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