Influence of water table and nitrogen management on residual soil NO3− and denitrification rate under corn production in sandy loam soil in Quebec

Agriculture, Ecosystems and Environment 79 (2000) 187–197

Influence of water table and nitrogen management on residual soil NO3 − and denitrification rate under corn production in sandy loam soil in Quebec Abdirashid A. Elmi ∗ , C. Madramootoo, C. Hamel Department of Natural Resource Science, Macdonald Campus of McGill University, 21 111 Lakeshore Road, Ste-Anne-de-Bellevue, Que., Canada, H9X 3V9 Received 9 February 1999; received in revised form 23 September 1999; accepted 3 December 1999

Abstract Nitrate-N (NO3 − ) effluents from agricultural ecosystems contributing to the degradation of water quality has become a serious environmental problem. A field experiment was conducted in 1996 and 1997 at St. Emmanuel, Que., Canada, to investigate the combined effects of water table management (WTM) and N fertilization on soil NO3 − level and denitrification rates in the top soil layer (0–0.15 m). The field was planted to corn (Zea mays L.) in both years. Treatments consisted of a factorial combination of two water table treatments, free drainage (FD) at about 1.0 m and subirrigation (SI) at 0.6 m below the soil surface, and two N fertilizer rates, 200 kg ha−1 (N200 ) and 120 kg ha−1 (N120 ). SI reduced NO3 − concentration in the top soil layer by 42 and 16% in 1996 and 1997, respectively. Nitrate levels in soil were 50% lower in N120 plots in 1996, and 20% in 1997 compared to the N200 plots. Denitrification was higher in SI compared to FD, but not influenced by N rate. As a consequence, WTM practices have implications for both water quality and greenhouse gas emissions. Climatic conditions played a large role in regulating N dynamics in the soil. Due to drier and cooler conditions in 1997, denitrification rates were lower than in 1996, leaving higher residual NO3 − in the soil profile following corn harvest. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Denitrification rate; Emissions; Nitrate; Subirrigation; Water quality

1. Introduction Nitrogen (N) is a key element in plant nutrition. High yielding crops, such as corn (Zea mays L.), require large amounts of N fertilizer to ensure optimum yield. Corn has become a major crop in the province of Quebec because of its high potential productivity. ∗ Corresponding author. Tel.: +1-514-398-7759; fax: +1-514-398-7990. E-mail address: [email protected] (A.A. Elmi)

Liang et al. (1992) reported a maximum grain corn yield of 15.2 Mg ha−1 resulting from the best combinations of hybrid, population density, fertilizer rate and irrigation. In an attempt to reach such an optimal yield, high rates of N fertilizer are often applied. Consequently, significant quantities of nitrate (NO3 − ) may be lost via leaching and eventually reach groundwater (Prunty and Montgomery, 1991). In many agricultural areas in the US, NO3 − levels have already exceeded USEPA safety limit for drinking water, 10 mg l−1 (Hubbard and Sheridan, 1989). Similarly, in the

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province of Quebec Madramootoo et al. (1992) documented NO3 − concentrations as high as 40 mg l−1 in subsurface drain flow from a sandy loam field cropped to potato (Solanum tuberosum L.), a value far exceeding the present Canadian health standard of 10 mg l−1 . Nitrate levels higher than 10 mg l−1 are linked to cases of methemoglobinemia (also known as blue baby syndrome) which can ultimately result in the death of infants of up to 6 months (Gelberg et al., 1999). The amount of leachable NO3 − in the soil profile generally increases with fertilizer application rate (Angle et al., 1993; Errebhi et al., 1998). For example, Angle et al. (1993) measured 2.5 mg NO3 − kg−1 in plots which had never been amended with manure or fertilizer, while plots fertilized with 260 kg N ha−1 contained 8.7 mg NO3 − kg−1 . A significantly higher NO3 − concentration (25 mg NO3 − kg−1 ) was observed when corn was fertilized with excessive N (Angle et al., 1993). Thus, a major challenge, now, for agricultural scientists is to develop management strategies which will minimize the adverse impacts of N fertilizers on the environment and water resources, without concomitant reductions in crop yield. Water table management (WTM) systems, including controlled drainage and subirrigation (SI), have been identified as beneficial practices for reducing NO3 − content in groundwater by enhancing denitrification in the water saturated zone (Gilliam and Skaggs, 1986; Wright et al., 1992). Kalita and Kanwar (1993) and Madramootoo et al. (1993) found NO3 − concentrations in the unsaturated zone to be higher than in the saturated zone. Gilliam and Skaggs (1986) predicted a 32% decrease in NO3 − leaching losses due to controlled drainage. While reduction for the potential NO3 − contamination of surface and ground waters is a positive aspect of denitrification (Gilliam, 1994), emission of N2 O is a serious environmental concern. It contributes to the greenhouse effect and participates in the depletion of ozone (Mooney et al., 1987). In order to properly assess ecological impacts associated with N2 O, knowledge of the proportion of denitrification gases entering the atmosphere as N2 O relative to N2 is paramount. In laboratory experiments, Weier et al. (1993) and Maag and Vinther (1996) indicated that under wet soil conditions N2 rather than N2 O is the dominant end-product of denitrification. The integration of WTM into a N fertilization strategy could further reduce environmental degradation in

crop production systems. Knowledge of interactions between WTM and N fertilizer is essential for the development of best management practices. The objectives of this study were to investigate the combined impacts of water table depths and N fertilization rate on (1) the quantity of potentially leachable nitrates in the upper soil layer, and (2) the relationship between denitrification rate and the reduction of NO3 − concentrations in the soil profile of a corn field.

2. Materials and methods 2.1. Field description and experimental design A field experiment was conducted during the 1996 and 1997 growing seasons near Coteau du Lac, Que. Most of the soil above the bedrock is of sedimentary origin. The soil is a Soulanges fine sandy loam (fine silty, mixed, nonacid, grigid Humaquept; FAO, Glesol) overlying a sandy clay loam in the mid layer (0.25–0.3 m) and finally a clay parent material (0.5–1 m). Surface topography was generally flat with an average slope of less than 0.5% (Kaluli, 1996). The field was planted (17 May for 1996 and 23 May for 1997) with corn, Hybrid Pioneer 3905, at a planting density of 80 000 plants ha−1 (0.75 m between rows and 0.15 m within rows). Fertilizer (diammonium phosphate, 18-46-0) was broadcast at the time of seeding at a rate of 150 kg ha−1 . Potassium (Muriate of potash, 0-0-60) was also applied at 90 kg ha−1 . To reach N fertilizer treatment level, ammonium nitrate (34-0-0) was surface applied after planting. Weeds were controlled with atrazine, dicamba, bromoxynil, and metolachlor. Details of N and herbicide applications are summarized in Table 1. Treatments consisted of a combination of two water management treatments, free drainage (FD) at about 1.0 m and SI at 0.6 m below the soil surface, and two N fertilizer rates, 200 kg N ha−1 (N200 ) and 120 kg N ha−1 (N120 ). A factorial arrangement of treatments was used in a randomized complete block design. There were three blocks, 120 m wide and 75 m long, and eight plots (15 m wide×75 m length) in each block. Blocks were arranged from east to west, and separated from each other by a 30 m wide strip of undrained land (Fig. 1). To minimize seepage and chemical flow between plots, 1.5 m deep plastic

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Table 1 Timing, rate, and chemical form of N applications and weed managements strategy Operation

N200 : N200 : N120 : N120 :

first application second applicationa first application second application

Herbicide applications

1996

1997

Date

Amount

23 20 23 20

23 177 23 97

May June May June

23 May

b

(kg ha−1 )

Date

Amount (kg ha−1 )

23 18 23 18

23 177 23 97

May June May June

25 June

c

a

Ammonium nitrate was applied to reach the treatment N fertilizer level in every second application. Atrazine at 1.5 kg active ingredient (ai) ha−1 , metolachlor at 1.92 kg ai ha−1 , and Dicamba at 0.31 kg ai ha−1 . c Bromoxynil at 0.31 kg ai ha−1 . b

curtains were installed between plots. Each plot with water table control at 0.6 m had two buffer plots on either side also with water table control at 0.6 m (Fig. 1). The purpose of the buffer plots was to help maintain the water table constant. In the middle of each plot, 75 mm diameter subsurface drain pipes were installed, at a depth of 1.0 m, with a slope of 0.3%. A building was located between Blocks A and B, and between Blocks B and C. All drains entered one of the two buildings. Tipping buckets were located at the outlet of each subsurface drain to monitor drain discharge. Piezometers were installed in duplicate in the

middle of each treatment and buffer plots, to a depth of 1.5 m. The piezometers were capped to prevent rainfall from entering. A graduated rod with a water sensor was used to monitor the water table levels during both growing seasons. Soil temperature at a depth of 0–0.15 m was measured using a water-resistant probe (Hanna instrument, HI9024/HI9025). 2.2. Sampling procedure and analysis SI was initiated in mid July for both years, after all other field operations were completed. In 1996, soil

Fig. 1. Schematic representation of the field and treatment arrangements.

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sampling began on 15 July in conjunction with SI. In 1997, however, the soil sampling procedure was slightly modified and started immediately after planting. Denitrification rates were measured bi-weekly during the two growing seasons. On each sampling date, aluminum cylinders (50 mm id × 150 mm long) were used to collect undisturbed soil cores in duplicate from randomly selected locations in the center between the two middle rows of each plot. The cylinders used were perforated along the sides (horizontal and vertical) at 50 mm intervals to enhance acetylene gas diffusion. Samples were placed in 2 l plastic jars fitted with rubber stoppers for gas sampling with 5% of acetylene (C2 H2 ) to block further transformation of N2 O to N2 , allowing measurement of total denitrification as accumulated N2 O and also inhibit nitrification process (Yoshinari et al., 1977). To represent field conditions, samples were incubated outdoors overnight. The concentration of N2 O produced through denitrification was determined following the procedure of Liang and Mackenzie (1997). Briefly, before gas sampling, the air in the plastic jars was thoroughly mixed by inserting a syringe and pumping several times. About 4 ml of gas were removed from the jars and injected into a gas chromatograph [GC, (5870 series II Hewlett Packard)] equipped with a 63 Ni electron capture detector (ECD) to measure the concentration of N2 O. Values for N2 O emissions by denitrification were calculated on a per area basis (g N ha−1 ). In 1997, there was a problem with the GC and the gas samples could not be analyzed immediately after the incubation period. Therefore, 7 ml of head space gas were withdrawn from incubating jars after the 24 h of incubation

period and stored in vacuutainers (Vacuutainers Brand, Beckon Dickson company, Rutherford, NJ). Standards of N2 O in N2 were also transferred to vacuutainers at that time and they were used for calibration of the analysis of N2 O at each sampling date. After denitrification measurement, soil cores were dried at 65◦ C for 3 days and the soil then ground. Soil moisture content to depth of 0.15 m was determined gravimetrically. To monitor NO3 − levels in the soil, triplicate soil samples were collected up to a 0.20 m depth on the same sampling dates as for denitrification. The soil samples were thoroughly mixed, then 10 g moist subsample was taken and shaken with 100 ml of 1M KCl for 60 min. The extracted solution was filtered through Whatman #5 filter papers, and then frozen until analysis. NO3 − and NH4 + were determined colorimetrically using an autoanalyzer (Quickchem, Milwaukee, WI) and then converted into kg ha−1 using bulk densities from respective soil samples. Significance of main treatment effects on NO3 − and denitrification rates in the soil and their interactions were investigated using General Linear Models (GLM) procedure of the Statistical Analysis System (SAS Institute, Version 6.12).

3. Results and discussion 3.1. Climatic data Monthly rainfall for the 1996 and 1997 growing seasons (May–October), and the long term (30 year) averages (1961–1990) are shown in Table 2. During the 1996 growing season, total amount of rainfall was

Table 2 Monthly precipitation (mm) in the growing seasons of 1996 and 1997 as compared to long term (1961–1990) averages measured at Côteaudu-Lac weather station Month

May June July August September October Total

1961–1991

1996

1997

Rain (mm)

Rain (mm)

Deviation (%)

76.3 90.1 94.6 93.9 90.6 76.7

103.8 81.8 133.9 40.8 140.6 66

36 −9.2 41.5 −56.6 55.2 −13.9

64.8 98 97 86.3 81.4 41.4

−15.1 8.8 2.5 −8.1 −10.2 −46

522.2

566.9

8.6

468.9

−10.2

Rain (mm)

Deviation (%)

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8.6% greater than the long term average. However, the month of August was exceptionally dry, with 56% less precipitation than average, whereas July and September were both very wet, with rainfalls of 41 and 55% above average, respectively (Table 2) increasing the risk of NO3 − leaching. In comparison, 1997 rainfall differed from that of 1996 in two main respects. First, deviation of monthly rainfall from the long term (30 years) average was generally smaller, and therefore rainfall distribution was relatively more uniform. Secondly, in spite of rainfall in June and July being slightly above normal (Table 2), the total 1997 growing season rainfall was 10.2% below average. Variations in the amount and distribution of rainfall have a strong influence on water table fluctuations, and consequently on soil moisture levels and N dynamics in the soil profile. Soil temperature was on average higher in 1996 than 1997 (Fig. 2). During the experimental period, average soil temperature in 1996 was 20.5◦ C, whereas it was 17.9◦ C in 1997. Soil temperature directly influences microbial activity in the soil, which is responsible for denitrification (Bergstrom and Beauchamp, 1993; Granli and Bøckman, 1994; MacKenzie et al., 1997). 3.2. Water table depth (WTD) Water table level fluctuated throughout both growing seasons, responding primarily to rainfall events.

191

For example, in 1996, the shallowest WTD was observed on 15 July. In that month, high amounts of rainfall (Table 2) saturated the soil resulting in rise of water table. In contrast, WTD dropped significantly on 22 August and 3 September compared to previous sampling dates (Fig. 3). This decrease had two main reasons. First, in spite of the fact that 1996 was a wet year, August was extremely dry with rainfall 56% below the long term monthly average. Second, although September was very wet, 55% above average, only 1.4 mm of rainfall occurred between the 22 August and 3 September sampling dates. As a result, WTD in SI plots were as deep as in FD plots (Fig. 3). Overall, as shown in Fig. 3, average WTDs in SI plots were deeper in the drier season of 1997 (0.8 m) than 1996 (0.7 m). 3.3. Water table and soil NO3 − Freely draining plots had higher soil NO3 − levels than subirrigated plots. This trend was consistent over the two seasons of this study except for 11 July 1997 (Fig. 4). Due to the large experimental error (Fig. 4a), 22 August was the only sampling date when the effect of the water table was not statistically significant (p<0.05) during the 1996 growing season. Comparatively, the effect of water table on soil NO3 − in the 1997 growing season was statistically variable. Nonetheless, trends appear to suggest treatment effect

Fig. 2. Mean soil temperature (◦ C) at 0–0.20 m depth at the time of soil sampling.

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Fig. 3. Mean water table depth (m) fluctuations in (a) 1996 and (b) 1997 as influenced by free drainage (FD ) and subirrigation (SI) treatments.

(Fig. 4b). Overall NO3 − concentrations were reduced by 42 and 16% in 1996 and 1997, respectively, in the SI treatment compared to the FD treatment. This is probably due to the shallower water table enhancing denitrification in SI plots. This is an indication that maintaining a shallow water table depth could be a useful tool in reducing NO3 − pollution to the groundwater. Similarly, Fogiel and Belcher (1991) found that controlled drainage/subirrigation reduced NO3 − loading through drainage by 25–59% over a 2-year period compared with conventional drainage. Gilliam and Skaggs (1986) predicted a 32% decrease in NO3 −

losses due to controlled drainage. It should, however, be pointed out that the decrease in NO3 − concentrations under controlled/subirrigation plots may be accompanied by an increase in N2 O production. On the other hand, Kliewer and Gilliam (1995) estimated that N2 O accounted for only 2% of the measured denitrification potential for each water table treatment. They concluded that water table level had no impact on percentage of total denitrification evolving as N2 O to the atmosphere. This and similar findings appear to suggest that the ecological impact of N2 O produced during the denitrification process may not

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193

Fig. 4. Soil NO3 − concentration differences between free drainage and subirrigated plots during (a) 1996 and (b) 1997 growing seasons. Vertical bars represent standard errors.

be as serious as was previously thought. To confirm this under natural conditions, field trials are needed to quantify the proportion of N2 O:N2 ratio evolution. 3.4. Nitrogen rate effect on soil NO3 − level and denitrification rate The effect of N fertilization rate on the level of NO3 − in the soil was evident (Fig. 5). Because of the higher rate of N fertilizer, soil NO3 − concentrations were higher in the N200 treatment than N120 during both growing seasons; the 11 and 26 June sampling dates in 1997 being the only exceptions (Fig. 5).

These observations suggest that NO3 − concentrations would be expected to increase with N inputs regardless of whether WTM is present or not. It is well documented that NO3 − accumulation in the soil profile and subsequent potential for leaching losses increases with increasing N application (Roth and Fox, 1990; Angle et al., 1993; Drury et al., 1996). It is interesting to note that N fertilizer rate had little (Table 3) or no significant effect (Table 4) on the total denitrification rate. This was unexpected because denitrification rates would be expected to be higher in plots receiving higher N application rates (MacKenzie et al., 1997; Ellis et al., 1998; Henault et al., 1998).

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Fig. 5. Soil NO3 − concentration differences between 120 and 200 kg ha−1 of N fertilizer rate in (a) 1996 and (b) 1997 growing seasons. Vertical bars are standard errors.

One plausible explanation could be that mineral N content was not the main limiting factor of soil N2 O emission (Henault et al., 1998). One important feature that must be emphasized when comparing N200 and N120 treatments in 1997 is the higher NO3 − in the latter treatment on 11 and 26 June sampling dates. These high NO3 − concentrations were unexpected, and may be erroneous values. It may be due to NO3 − influx from recharge areas or seepage through confining plots. The higher cumulative soil NO3 − in 1997 than 1996 (34.8 and 9.58 kg ha−1 , respectively), may be due to the relatively dry con-

ditions in 1997 during which denitrification was not enhanced. This speculative relationship is consistent with the lower denitrification rate in 1997 than 1996 (Tables 3 and 4). 3.5. Denitrification and water table management WTD showed a strong influence on denitrification. There was no significant interaction between any of the treatment factors, therefore, main effects were examined independently. In both growing seasons, with the exception of the 22 August 1996 sampling date (very

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195

Table 3 Denitrification rates (g N ha−1 per day) as influenced by water table depth and N fertilization rate and analysis of variance during 1996 growing season Treatments

Sampling dates 15 July

06 August

22 August

03 September

20 September

05 October

FDa SIb N120 c N200 d

184.7 225 124 285

31.36 112.95 62.5 82

27.3 21.8 15.74 33.38

5.5 30.51 20.75 15.27

14.32 25.78 26.95 13.3

4.64 15.3 9.47 10.47

Mean

204

72.25

24.56

18

20.2

10

ns ns ns

ns ns

Summary of analysis of variance WTMe N-rate WTM × N

nsf

∗∗

ns

ns ns

∗∗

∗

ns ns ns

∗∗

ns

∗

a

Free drainage. b Controlled/subirrigation. c 120 kg N ha−1 . d 200 kg N ha−1 . e Water table management. f ns=not significant at 5%. ∗ ,∗∗ =statistically significant at 5 and 1% probability level, respectively.

dry month), denitrification rates were always higher in SI than FD treatments (Tables 3 and 4). Higher denitrification losses were associated with higher moisture content in SI treatment plots as compared to FD treat-

ment (data not shown). Increases in N2 O evolution rates in the June–July period (Tables 3 and 4) were enhanced by N fertilizer application combined with warming soils. Similar to our finding, Christensen and

Table 4 Denitrification rates (g N ha−1 per day) as influenced by water table depth and N fertilization rate and analysis of variance during 1997 growing season Treatments

Sampling days 28 May

11 June

26 June

11 July

23 July

06 August

18 August

03 September

17 September

03 October

Fda SIb N120 c N200 d

nig ni 36.76 25.36

ni ni 150 64

ni ni 143 140

36.1 38.1 34.83 39.45

6.7 14.15 11.45 9.36

6.77 7.2 9.1 4.87

1.1 8.78 7 2.87

4.84 12 10.62 6.2

7.84 20 17.8 10.64

4.93 11.15 9.81 6.27

Mean

31.54

107.2

141.5

37.14

10.41

7

4.94

8.42

14.2

8.02

ni ns na

nsh ns ns

ns ns ns

∗∗

∗∗

∗∗

∗

ns ns

ns ns

ns ns

ns ns

ns ns

Summary of analysis of variance WTMe N-rate WTM × N a

ni ns naf

ni ∗

na

∗

Free drainage. b Controlled/subirrigation. c 120 kg N ha−1 . d 200 kg N ha−1 . e Water table management. f na=not applicable. g ni=not initiated. h ns=not significant. ∗ ,∗∗ =significant at 5 and 1% probability level, respectively.

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Tiedje (1990), Beauchamp et al. (1996) and Fan et al. (1997) concluded that these peaks were due to warming of saturated soils, and enhanced microbial activity. Denitrification rates during both growing seasons appeared to be regulated largely by climatic factors such as soil temperature, and amount and distribution of rainfall. In relatively dry periods, for example in the 1997 growing season, water table dropped sharply and denitrification N loss was not promoted resulting in NO3 − accumulation in the soil profile. In rainy periods, on the other hand, the water table rose and denitrification losses were promoted. In 1996, for example, the highest average denitrification flux was measured on the 11 July sampling date (Table 3). Heavy rainfall occurring at the beginning of this month saturated the soil and caused the rise of the WTD to about 0.45 m below the soil surface (Fig. 3). In 1997, denitrification peaked on the 26 June sampling date (Table 4). Four days before this sampling (22 June), the highest amount of daily rainfall during that season was measured (daily rainfall data not shown) creating an anaerobic environment favourable for denitrification process.

3.6. Seasonal variability and denitrification Consistently higher denitrification losses were measured in 1996 in all treatments, compared to 1997. Several environmental and field management practices may be responsible for the large differences in seasonal denitrification. This seasonal differences may be attributable to soil temperatures which were generally higher in 1996 than 1997 (Fig. 2). Temperature is considered as one of the most influential factors on the magnitude of denitrification (Granli and Bøckmam, 1994). Bergstrom and Beauchamp (1993) and Liang and Mackenzie (1997) asserted that lower temperatures can result in a reduction in the denitrification rate. Rainfall events in May and July 1996 (36 and 41.5% above average, respectively), following the first and second N fertilizer applications, respectively, might have increased moisture content beyond the saturation limit, and hence enhanced denitrification. Additionally, considerable amount of NO3 − might have been leached from the surface layer to deeper depths, leaving less NO3 − in the soil surface.

As shown in Table 1, herbicide application in 1997 was somewhat later. As a result, tremendous weed growth was observed in all plots. It is therefore reasonable to assume that significant amount of NO3 − which would have been lost as denitrification might have been taken up by weeds. This, however, contradicts the larger amount of NO3 − remaining in the soil in 1997 than 1996 (34.8 and 9.58 kg ha−1 , respectively). Therefore, another plausible explanation could be the enrichment of NO3 − through mineralization from the previous hot and wet growing season. Nitrogen mineralization is an important source of NO3 − and can supply from 30 to 100% of N nutritional needs and increases with precipitation (Douglas et al., 1998). Since denitrification measurements in 1996 started mid July, it is likely that denitrification peak was not captured. Therefore, one should be cautious when comparing the two seasons. Similarly, since measurements of denitrification rate were carried out only during the growing season, cumulative denitrification losses reported in Tables 3 and 4 should not be assumed as being annual losses. If annual losses were to be estimated, additional sampling must be continuing during spring thaw when denitrification may be vigorous (Christensen and Tiedje, 1990; Ellis et al., 1998; Henault et al., 1998).

4. Conclusions The results of this study demonstrate that WTM practices provide water quality benefits by enhancing NO3 − removal through denitrification and therefore reducing the amount of residual NO3 − in the soil profile. Concentrations of soil NO3 − were greater in plots receiving a high rate of N fertilizer. The soil NO3 − concentrations under WTM were reduced by 42 and 16% during the 1996 and 1997 growing seasons, respectively. Denitrification was higher in SI plots than FD plots, but it was not significantly influenced by N application rate. However, WTM strategies that enhance evolution of N2 O may result in a trade-off between increased emissions of greenhouse gases and improved water quality. Further research under a wide range of management practices and environmental conditions is required to help understand specific effects of WTM on the ratio of N2 O:N2 evolution.

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