Nitrate leaching in the semiarid prairie: Effect of cropping frequency, crop type, and fertilizer after 37 years

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1 Nitrate leaching in the semiarid prairie: Effect of cropping frequency, crop type, and fertilizer after 37 years C. A. Campbell 1, F. Selles 2, R. P. Zentner 2, R. De Jong 1, R. Lemke 2, and C. Hamel 2 1 Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Central Experimental Farm, Ottawa, Ontario, Canada K1A 0C6; 2 Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2. Received 4 February 2005, acepted 6 March Campbell, C. A., Selles, F., Zentner, R. P., De Jong, R., Lemke, R. and Hamel, C Nitrate leaching in the semiarid prairie: Effect of cropping frequency, crop type, and fertilizer after 37 years. Can. J. Soil Sci. 86: A high concentration in drinking water can be a health hazard. Further, leached N represents an economic loss to the producer. Although leaching to ground water would be minimal on the semiarid prairies, leaching can occur especially where frequent summer fallowing is practiced. We used a crop rotation study, initiated in 1967 on a medium-textured Orthic Brown Chernozem, at Swift Current, Saskatchewan, to determine the influences of fallow frequency, crop types, and fertilizer on nitrate leaching after 37 yr. Nitrate distribution was measured to 4.5 m depth by 0.3-m increments, in 10 cropping systems in fall We deduced that some leached beyond the rooting depth (1.2 m) of spring wheat (Triticum aetivium L.), especially under a fallow-wheat rotation receiving N and P fertilizer. The amounts of leached tended to be greater and to be located deeper in the soil profile as fallow frequency increased (e.g., fallow-wheat > fallow-wheat-wheat > continuous wheat, all receiving N and P fertilizer based on soil test). However, in this semiarid environment, leaching was not great, being highest under fallow-wheat (N + P) (180 kg N ha 1 leached in 37 yr). In fallow-containing systems inadequate fertilizer N or P resulted in reduced crop growth and N uptake leading to a tendency for greater leaching of the (about 145 kg N ha 1 in 37 yr) mineralized during the fallow period than when such a system received N and P based on soil tests (about 66 kg N ha 1 leached in 37 yr). In continuously cropped treatments there was little evidence of leaching. Replacing wheat grown on fallow with the shallow-rooted flax (Linum usitatissimum L.) in a fallowwheat-wheat (N + P) rotation resulted in greater leaching in the flax system (156 vs. 66 kg N ha 1 ) due to less N uptake by flax. In contrast, when the wheat grown on fallow was replaced with fall rye (Secale cereale L.) there was no leaching, perhaps because the fallow period was much shorter (12 mo compared with 20 mo for spring wheat) and also because the fall-seeded crop used soil in the fall and early spring reducing opportunities for leaching. Key words: Wheat, flax, fall rye, lentil, cropping frequency Campbell, C. A., Seles, F., Zentner, R. P., De Jong, R., Lemke, R. et Hamel, C Lixiviation des nitrates dans la région mi-aride des Prairies : incidence de la fréquence des cultures, du type de culture et des amendements après 37 ans. Can. J. Soil Sci. 86: Une forte concentration de dans l eau potable peut être nocive. La lixiviation de l azote constitue aussi une perte pour l agriculteur. Bien que la lixiviation de dans la nappe phréatique soit minime dans les prairies mi-arides, cette région n est pas à l abri du phénomène, en particulier là où la jachère d été est monnaie courante. Les auteurs ont profité d une étude sur l assolement amorcée sur un tchernoziom brun orthique à texture moyenne à Swift Current (Saskatchewan), en 1967, pour vérifier l incidence de la fréquence des jachères, du type de culture et des amendements sur la lixiviation des nitrates au bout de 37 ans. La distribution des nitrates a été établie jusqu à 4,5 m de profondeur par tranche de 0,3 m, pour 10 systèmes agricoles, à l automne Des résultats, les auteurs déduisent qu une certaine quantité de nitrate s infiltre effectivement au-delà de la zone des racines (1,2 m) du blé de printemps (Triticum aestivum L.), surtout dans l assolement jachère-blé bonifié avec des engrais N et P. La quantité de perdue a tendance à être plus importante et à se retrouver plus profondément dans le sol quand la fréquence des jachères augmente (à savoir jachère-blé > jachère-bléblé > monoculture du blé, avec amendement de N et de P en fonction de l analyse du sol). Dans des conditions mi-arides cependant, on n observe pas de lixiviation importante des nitrates, la plus forte lixiviation se retrouvant avec l assolement jachère-blé (N + P) (180 kg de N par hectare de perdu en 37 ans). Dans les systèmes avec jachère, une fertilisation inadéquate avec du N ou du P réduit la croissance des cultures et l absorption de N, ce qui a tendance à accroître davantage la lixiviation du (autour de 145 kg de N par hectare en 37 ans) minéralisé pendant la période de jachère, comparativement à la lixiviation relevée quand le sol est fertilisé en fonction de son analyse (environ 66 kg de N par hectare en 37 ans). La monoculture ne semble pas entraîner de lixiviation. Quand on remplace le blé par du lin (Linum usitatissimum L.), aux racines peu profondes, après la jachère dans l assolement jachère-blé-blé (N + P), on note une plus forte lixiviation du (156 c. 66 kg de N par hectare), le lin utilisant moins de N. En revanche, quand on remplace le blé par du seigle d automne (Secale cereale L.) après la jachère, il n y en a aucune, peut-être parce que la période de jachère est plus courte (12 mois contre 20 pour le blé de printemps) et parce que la culture semée à l automne utilise le nitrate à l automne et au printemps, ce qui en laisse moins pour la lixiviation. Mots clés: Blé, lin, seigle d automne, lentille, fréquence des cultures 701 Abbreviations: Cont W, continuous wheat rotation; F, fallow; Flx, flax; F-Rye, fall rye; Lent, lentil; W, wheat

2 702 CANADIAN JOURNAL OF SOIL SCIENCE High nitrate content in drinking water can be a health hazard, causing methaemoglobinaemia (Reynolds et al. 1995; Chambers et al. 2001). Further, leached N represents an economic loss to the producers. Although large areas of the Canadian prairies are classified as semiarid, nitrate leaching can still take place, especially in the more humid regions, such as in the Black and Grey Luvisolic soil zones (Cowell and Doyle 1993; Campbell et al. 1975, 1984, 1995). This is particularly true when summer fallow is a component of the crop rotation because it enhances storage of water, which primes the soil for leakage, and it increases N mineralization and nitrification which, without plant uptake of N, will encourage nitrate leaching during wet early springs (Campbell et al. 1984, 1995; Bauder et al. 1993). When fertilizers are applied based on soil test criteria, nitrate leaching is minimized, especially if continuous cropping is also practiced (Campbell et al. 1984, 1994). In contrast, over-fertilization and under-fertilization can sometimes lead to significant nitrate leaching (Campbell et al. 1993, 1995). Legumes can also promote nitrate leaching because they increase the N-supplying power of soils (Campbell et al. 1991) and, when plowed down as green manure the mineralized/nitrified N can leach, especially if summer fallowing is a component of the rotation (Campbell et al. 1994). Other crops can also influence leaching. For example, perennial, deep-rooted grasses will minimize leaching to the subsoil (Campbell et al. 1975, 2006; Izauralde et al. 1995). In contrast, slow growing and shallow-rooted annual crops such as flax, do not produce much biomass, thus they use much less N than cereals and, consequently, they leave more -N and water in the soil than spring wheat (Campbell and Zentner 1996). Fall-seeded crops, such as fall rye and winter wheat, because they grow in September and October and start growth again in early spring (April) even before spring crops are seeded, can remove and some stored water from the soil thereby reducing opportunities for to leach into the subsoil (Campbell et al. 1984). The objective of this study was to determine the influence of cropping frequency, fertilizer application, and type of crop on leached in a medium-textured soil in semiarid southwestern Saskatchewan after 37 yr ( ). MATERIALS AND METHODS Experimental Design and Crop Management Details of the design and management of this old rotation experiment at Swift Current have been reported previously (Campbell et al. 1983; Zentner and Campbell 1988; Campbell and Zentner 1993; Campbell et al. 2004); therefore, only a review of information pertinent to this leaching study is presented. The experiment was established in 1967 on the South Farm of the Semiarid Prairie Agricultural Research Centre of Agriculture and Agri-Food Canada, at Swift Current, Saskatchewan (latitude 50.3 N, longitude W, and elevation 883 m). The land was slightly sloping (< 3%) and had been cropped previously to a fallow-spring wheat (F-W) rotation receiving minimal fertilizer since The soil is a Swinton loam (Ayres et al. 1985), an Orthic Brown Chernozem (Canada Soil Survey Committee, Subcommittee on Soil Classification 1978). The ph (water paste) of the top 15 cm of soil was 6.5 and the water table is located beyond 10 m. In 1967, twelve crop rotation-fertility treatments were established on 81, 0.04-ha plots in a randomized completeblock design with three replicates. Over time, four of the rotations were consolidated into two rotations resulting in a total of 10 treatments (Table 1). The treatments were: fallow-wheat that received recommended rates of N and P fertilizer (F-W, N + P); three F-W-W treatments fertilized with N + P, N only, and P only; two 3-yr mixed rotations that received N + P, i.e., F-flax-W (F-Flx-W) and F-fall rye-w (F-Rye-W); two continuous wheat rotations (Cont W), that received either N + P or only P. In addition, two extra Cont W (N + P) systems intended to be used as flexible (if-type) rotations were converted in 1978 to a wheat-grain lentil (Lens culinaris Medikus) (W-Lent, N + P) rotation. Further, in 1985 we established a 6-yr F-W- W-W-W-W (N + P) rotation from an oat (Avena sativa L.) (hay)-w-w (N + P) and a Flx-W-W (N + P) system that had been in place for the previous 18 yr. The F-Rye-W rotation was changed to chemical fallow-winter wheat-winter wheat (CF-WW-WW) from 1985 to 1992 and then to CF-Rye-W thereafter (winter wheat was too susceptible to winter kill). All phases of each rotation were present every year and each rotation was cycled on its assigned plots. Seedbed preparation, herbicide application, seeding, harvesting, and tillage operations were reported previously (Campbell et al. 1983, 1992; Zentner and Campbell 1988). Wheat and flax crops were generally seeded in mid-may, and fall rye and winter wheat in early September at the recommended rates of 67, 31, 63 and 67 kg ha 1, respectively. Recommended cultivars were planted each year, but cultivars were changed as new ones became available (Campbell et al. 2004). Commercial farm equipment was used to perform all cultural and tillage operations. Weed control was achieved by a combination of mechanical tillage and herbicides using recommended methods and rates (University of Saskatchewan 1975). In the fall, after harvest, 2, 4-D was applied to all plots to control winter annual weeds. On average, summer fallow plots received about four shallow tillage operations. In-crop weed control generally involved the use of bromozynil plus MCPA (1:1) and tralkoxydim. Fertilizer N, as NH 4 was applied (broadcast in spring) based on levels of soil -N (0- to 0.6-m depth) measured in individual plots in the previous fall. From 1967 to 1989 we used N rates recommended by the soil testing laboratory of the University of Saskatchewan (Saskatchewan Agriculture 1985) with rates of N applied to bring the total mineral N (soil test + fertilizer N) level to 65 kg ha 1 for cereals and 45 kg ha 1 for flax (assumes normal moisture conditions). In 1990, the soil testing laboratory recommendations for N were increased to 90 kg ha 1 of total N for cereals grown on fallow, 73 kg ha 1 for cereals grown on stubble, and 62 kg ha 1 for flax grown on fallow. Phosphorus fertilizer (monoammonium phosphate) was applied with the seed in accordance with the general recom-

3 CAMPBELL ET AL. CROPPING FREQUENCY, CROP TYPE AND FERTILIZER ON NITRATE LEACHED 703 Table 1. Bulk densities in deep cores taken in fall 2003 Bulk density (Mg m 3 ) in depths (m) Rotation Rotation y x F-(W)-W (+ P) F-(W)-W (N + P) F-Flx-(W) (N + P) F-Rye-(W) (N + P) F-(W)-W (+ N) Cont W (N + P) F-(W) (N + P) Cont W (+ P) F-(W)-W-W-W-W (N + P) z (W)-Lent (N + P) z Mean z Rotation no. 13 was constructed in 1985 from rotations 6 and 7. Rotation 6 was oat (hay)-w-w (N + P) and rotation 7 was Flx-W-W (N + P) from 1967 to Rotation 19 was constructed in 1979 from rotations 9 and 10, two Cont W (N + P) rotations that were intended to be flex cropped. y Rotation phase shown in parentheses is the one sampled in fall x The values for the 0- to 0.15-m depth are the means for the 0- to m and to 0.15-m depths. Values for m averaged 1.13 and for m, averaged 1.33 Mg m 3. mendations for the area and crop (University of Saskatchewan 1975). On average, all cropped treatments designated to receive P received 9 10 kg P ha 1 yr 1. Wheat grown on fallow received about 8 kg N ha 1 yr 1 up to 1991 and, since then, about 41 kg N ha 1 yr 1 partly due to the more favourable growing season weather conditions during the last 11 yr and partly to the new fertilizer guidelines since On average, wheat grown on wheat stubble received about kg N ha 1 yr 1 in the first 18 yr, and about 50 kg N ha 1 yr 1 in the last 11 yr. In 1980 and 1982, N was inadvertently applied to the Cont W (+ P) treatment at rates of 70 and 40 kg N ha 1, respectively. Wheat, fall rye, flax and lentil were harvested at the fullripe stage. Yield determinations were made by cutting a swath 5 m wide and 40 m long through the middle of all cropped plots and the grain harvested with a conventional combine. Small areas (2.32 m 2 ) were hand harvested in each plot to determine N concentrations in the grain and straw. After air drying the hand samples to a moisture content of about 6%, they were ground and analysed for N concentration (Starr and Smith 1978). The N content of the grain and straw were corrected to a constant 13.5% moisture basis. The straw was distributed on the plots by a paddle-type spreader attachment on the combine. Straw was not soil-incorporated until the following spring or early summer. Soil Sampling Soil samples were taken each year (0- to 0.15-, to 0.3-, 0.3- to 0.6-, 0.6- to 0.9- and 0.9- to 1.2- m depths) with a hydraulic soil corer in spring prior to commencement of field operations, immediately after harvest, and in mid-october just before freeze up. Two cores were taken per plot and these cores combined by depth, sub-sampled for gravimetric soil water determination, and the remaining soil air-dried, ground and analysed for -N and bicarbonate-extractable (Olsen) P (Hamm et al. 1970). The bulk densities (mean for all plots) used to convert soil water, and -N and bicarbonate-p concentrations to volumetric units for these samples were 1.23, 1.35, 1.29, 1.51 and 1.64 Mg m 3, for the five depths, respectively. However, we do not show these -N data because they do not reflect leaching, the subject of this paper. In the fall of 2003, soil cores were taken to assess leaching by sampling to a depth of 2.4 m (0- to 0.15-, to 0.3- and 0.3- m increments thereafter) in the wheat phase of each rotation (Table 1) using a heavy-duty hydraulic soil sampler. These samples were used to determine bulk densities (Table 1) and soil -N (Hamm et al. 1970). Preliminary assessment of the year 2003 distribution with depth data indicated a need to sample deeper than 2.4 m. In May 2004 we resampled all plots with an auger sampler; soil from the 0- to 2.4-m depth was discarded, and samples were taken at 0.3-m depth intervals from 2.4- to 4.5-m depth. The soil removed with the auger in each increment was mixed, a subsample was taken for moisture determination and the remaining sample was air-dried, ground and analysed for -N. To convert -N concentrations to mass in the 2.4- to 4.5-m segment, we assumed the bulk density for this segment to be 1.71 Mg m 3, the mean for the 2.1- to 2.4- m depths in all plots sampled the previous fall (Table 1). Because we only sampled the soil to a depth of 1.2 m at the start of the experiment in 1967, we assumed that the -N below 1.2 m in 1967 in all treatments could be represented by the distribution of -N in Cont W (+ P) measured in the fall 2003 and spring We reasoned that no -N would have leached beyond 1.2 m depth in this treatment during the study period since very little N was applied (8 kg N ha 1 yr 1 ) and there was no fallow period to produce mineral N nor to increase water storage in the soil. We also assumed that nitrate losses from the 1.2- to 4.5-m depth through all processes such as denitrification and leaching would have been negligible in the Cont W (+ P) treatment over the 37 yr. Consequently, we calculated leached as the differences between the content in the 1.2- to 4.5-m depths in 2003 for a particular treatment compared with the content in the same depth under Cont W (+ P) in 2003.

4 704 CANADIAN JOURNAL OF SOIL SCIENCE Table 2. Significant leaching z (P 0.10) of - N y since 1967 as determined by contrast vs. Cont W (+ P) F- (W) y -W F-(W)-W F-Flx-(W) CF-Rye-(W) F-(W)-W Cont W F-(W) F-(W)-W- (W)-Lent Depth (m) (+ P) (N + P) (N + P) (N + P) (+ N) (N + P) (N + P) W-W-W (N + P) (N + P) -N (kg ha 1 ) z -N was assumed to be leached if values for a treatment in 2003 were significantly greater than those for the ContW (+ P) in 2003 because the latter values were assumed to be unchanged in the 1.2- to 4.5-m depth since y Zero values indicate no significant change; positive values indicate significant increase (P 0.10) (i.e., leaching). Table 3. Nitrate distribution with depth in wheat phase of crop rotations in old rotation study at Swift Current in fall 2003 z F- (W) y -W F-(W)-W F-Flx-(W) CF-Rye-(W) F-(W)-W Cont W F-(W) Cont W F-(W)-W- (W)-Lent Depth (m) (+ P) (N + P) (N + P) (N + P) (+ N) (N + P) (N + P) (+ P) W-W-W (N + P) (N + P) Root Zone -N (kg ha 1 ) (10) y 18 (7) 18 (9) 22 (3) 20 (8) 20 (2) 23 (9) 11 (3) 23 (5) 27 (12) (8) 3 (1) 4 (2) 2 (1) 9 (6) 3 (1) 4 (1) 3 (1) 4 (1) 6 (5) (35) 7 (1) 5 (2) 3 (1) 24 (19) 5 (2) 7 (1) 4 (1) 10 (1) 35 (46) (52) 21 (7) 20 (8) 6 (4) 40 (26) 20 (7) 18 (2) 7 (2) 26 (9) 72 (90) (45) 26 (7) 58 (12) 9 (4) 39 (22) 42 (27) 22 (5) 19 (6) 25 (5) 46 (41) (59) 35 (5) 68 (9) 13 (4) 58 (26) 47 (18) 25 (5) 30 (7) 35 (7) 44 (38) (34) 46 (14) 73 (13) 26 (11) 79 (24) 43 (5) 47 (15) 40 (20) 41 (12) 38 (23) (33) 57 (22) 65 (20) 36 (20) 85 (38) 39 (4) 69 (17) 45 (17) 52 (21) 33 (15) (27) 45 (36) 33 (1) 25 (16) 42 (22) 26 (11) 94 (58) 22 (3) 29 (12) 33 (13) (35) 36 (30) 25 (3) 30 (13) 30 (12) 23 (10) 61 (40) 19 (3) 23 (10) 26 (7) (17) 13 (12) 20 (19) 13 (12) 10 (2) 12 (10) 27 (23) 12 (3) 7 (3) 24 (12) (10) 10 (9) 12 (10) 15 (17) 8 (1) 10 (8) 19 (21) 12 (7) 6 (2) 19 (11) (14) 8 (7) 9 (5) 13 (16) 6 (1) 9 (7) 14 (16) 9 (3) 5 (3) 14 (9) (16) 6 (5) 7 (5) 14 (18) 6 (1) 8 (7) 13 (16) 7 (2) 5 (2) 12 (7) (11) 5 (5) 7 (4) 8 (9) 5 (1) 9 (5) 10 (13) 6 (2) 5 (2) 11 (8) m Std. error of mean = m Std. error of mean = 27 z Samples for m sampled with hydraulic soil corer in fall 2003; depths beyond 2.4 m sampled by auguring in spring Beyond 2.4 m, bulk densities were assumed to be the same as for average m depth of all treatments. y Values in parentheses are standard deviations. Precipitation and air temperature were measured daily at a meteorological station located 0.5 km west of the test site. Statistical Analysis The bulk density and -N distribution with depth, sampled in the selected crop rotation phases in the sampling, were analysed as a split plot with rotations as main plots and depth segments as subplots. Analysis of variance was conducted on total -N in the 0- to 1.2- m and in the 1.2- to 4.5-m depths and standard errors for each analysis were calculated. We converted -N concentration measured in the 0- to 1.2- m depth in all rotation phases in spring 1967 to a mass basis using the bulk densities measured for deep leaching assessment in fall The -N in spring 1967 was analysed as a split plot with rotation as main plot and depth as subplot. We also conducted analysis of variance on the total -N in the 0- to 1.2- m depth. The -N concentrations in the 0- to and to 0.3- m depths sampled in 1967 and 2003 were converted to a mass basis and summed to give 0- to 0.3- m values. To determine if rotations influence the amount of -N leached below the 1.2-m depth, we conducted an analysis of

5 CAMPBELL ET AL. CROPPING FREQUENCY, CROP TYPE AND FERTILIZER ON NITRATE LEACHED 705 Fig. 1. Effect of N fertilizer on leached in a Cont W system over 37 yr. Table 4. Nitrogen applied to, and harvested by rotations ( ) N applied z N applied N harvested z N harvested Treatment Fertilizer (kg ha 1 yr 1 ) (kg ha 1 rotation 1 ) y (kg ha 1 yr 1 ) ( ) y F- (W) N + P F-(W)-W N + P F-W-(W) N + P F-(W)-W + P 5 55 F-W-(W) + P F-(W)-W + N F-W-(W) + N F-(Flx)-W N + P 8 36 F-Flx-(W) N + P F-(Rye)-W N + P F-Rye-(W) N + P (F)-W-W-W-W-W x N + P F-(W)-W-W-W-W N + P F-W-(W)-W-W-W N + P F-W-W-(W)-W-W N + P F-W-W-W-(W)-W N + P F-W-W-W-W-(W) N + P Cont W N + P Cont W + P (W)-Lent w N + P W-(Lent) N + P z Values are for rotation phase shown in parentheses. y For F-W = kg ha 1 yr 1 37/2; for F-W-W = kg ha 1 yr 1 for sum of cropped phases 37/3; for Cont W =kg ha 1 yr 1 37, etc. x This 6-yr rotation was constructed in 1985 from two 3-yr continuous cropping-type rotations (N + P). The fallow phase received an average of 15 kg N ha 1 yr 1 when it was being cropped in the first 18 yr. w This W-Lent rotation was constructed in 1979 from two Cont W rotations (N + P). The 12-yr mean values from the Cont W phase was 28.5 kg ha 1 and the 24-yr mean for (W) -Lent was 31 and for W-(Lent) 16 kg ha 1.

6 706 CANADIAN JOURNAL OF SOIL SCIENCE variance and contrasted the -N in the 1.2- to 4.5-m depth under the Cont W (+ P) treatment in 2003 versus - N in the same depth for each of the other treatments. RESULTS AND DISCUSSION Based on our assumption that the -N content and distribution in the 1.2- to 4.5-m depth of the Cont W(+ P) treatment in 2003/2004 has remained constant since 1967, then only F-W(N + P) showed a significant (P 0.10) quantity of -N leached in the 1.2- to 4.5-m depth (Table 2). However, there were at least three other treatments that showed evidence that some leaching had occurred. These were F-W-W(+ N), F-W-W(+ P) and F-Flx-W(N + P). These three treatments and F-W(N + P) had a total of 147, 144, 156 and 180 kg -N ha 1 more thant Cont W(+P), respectively, in the 1.2- to 4.5 m depth in 2003 (Table 3). Effect of Fertilizer Although some treatment effects were not statistically significant (P 0.10), several showed meaningful tendencies. For example, the effect of applying N to continuous wheat [i.e., Cont W (N + P)] tended to increase -N in the subsoil by about 47 kg ha 1 ( ), mainly in the 1.2- to 1.8-m depth (Fig. 1; Table 3). This leached N from the Cont W (N + P) treatment might reflect the 1073 kg ha 1 more fertilizer N applied to it than to Cont W (+ P) over the 37-yr period (Table 4). Although there was about 518 kg ha 1 more N harvested in Cont W (N + P) than in Cont W (+ P) (Table 4), there was greater likelihood of extra mineral N being left in the soil under the N + P treatment and this would be susceptible to leaching when drought years (with low N uptake) were followed by wet post-harvest periods (Campbell et al. 1992; Randall et al. 1997). For example, years such as 1968/1969, 1969/1970, 1972/1973, 1973/1974, 1978/1979 and 1984/1985 (Table 5) might fit this hypothesis. The peak in -N accumulation at m under the Cont W (+ P) treatment likely reflects a residual deposit from past management of the experimental area as F-W prior to initiation of this experiment. In the F-W-W system, withholding of N or P fertilizer tended to result in greater -N leaching beyond the root zone than when N and P were applied based on soil test (Fig. 2 and Tables 2 and 3). The two less well-fertilized systems had about 80 kg ha 1 more -N leached than F-W-W (N + P) over the 37 yr. All three systems allowed -N to leach to a depth of about 2.4 m (Table 2 and Fig. 2). These results support the earlier findings of Campbell et al. (1993) who showed that -N leaching does not only occur when excess N is applied but may also occur when too little N is applied. What was surprising in this study is that even when no N was applied (only P) -N leaching can still occur in a fallow-based system. This is because most of the N being leached is derived from N mineralization, especially during the fallow year (data not shown), and because of the absence of N uptake by a crop. About kg ha 1 more N was taken up (Campbell et al. 2004) and harvested (Table 4) in the F-W-W (N + P) system than in the treatments receiving only N or P. Table 5. Precipitation received during and after the growing season of spring-seeded crops ( ) Period precipitation (mm) Year May 01-Aug. 31 Sep. 01-Apr. 30 Total Avg yr Avg Effect of Cropping Frequency The amount of -N leached under the F-W (N + P) rotation (180 kg ha 1 ) was almost three times that leached under F-W-W (N + P) (65 kg ha 1 ) and four times that leached under Cont W (N + P) (47 kg ha 1 ) (Table 3). This tendency in leaching was not directly related to fertilizer N applied (Cont W >F-W-W> F-W), nor to N removed in the grain (Cont W > F-W-W> F-W) (Table 4); it was directly proportional to N mineralized (F-W > F-W-W> Cont W) (Campbell et al. 1992), and presumably to the greater water stored in these three systems (F-W > F-W-W > Cont W) (Campbell et al. 1987). Comparisons with the Cont W (+ P) system suggest that -N was leached deepest under F-W with a sharp peak at m; the next deepest, but broader peak was under F-W-W ( m), and the shallowest ( m) and most dispersed peak was found under Cont W (Fig. 3). There was little difference in leached under the 6-yr F-W-W-W-W-W (N + P) rotation compared with the Cont W (N + P) (Tables 2 and 3), but we must remem-

7 CAMPBELL ET AL. CROPPING FREQUENCY, CROP TYPE AND FERTILIZER ON NITRATE LEACHED 707 Fig. 2. Effect of N and P fertilizer on leached in a F-W-W system over 37 yr. Fig. 3. Effect of cropping frequency on leached over 37 yr. ber that this 6-yr system was continuously cropped in the first 18 yr. The results shown in Fig. 3 were in accord with our expectations based on a model analysis of the water and disposition in the F-W (N + P) and Cont W (N + P) rotations during the first 24 yr of the experiment (Akinremi et al. 1993). Using a modified version of the LEACHM model (Akinremi et al. 2005), Akinremi et al. (1993) estimated that 121 kg -N ha 1 was lost through leaching in F-W (N + P) in 24 yr. Most of this leaching occurred in the fallow phase, especially when this phase coincided with a

8 708 CANADIAN JOURNAL OF SOIL SCIENCE Fig. 4. Effect of type of crop on leached in a F-crop-crop system over 37 yr. year of high rainfall. Only a small amount of -N was predicted lost under Cont W (N + P) over the 24 yr. Effect of Crop Type Under the F-Flx-W (N + P) system about 90 kg ha 1 more -N was leached than under F-W-W (N + P) (Fig. 4 and Tables 2 and 3). This supports results we obtained previously in this experiment (Campbell and Zentner 1996). The greater -N lost under the flax system is partly because of its shallower rooting pattern than for wheat, and partly because flax produces less plant biomass and the rotation system removes less N in the grain (999 vs kg ha 1 ), even though it also receives less fertilizer N as well (493 vs. 641 kg ha 1 ) (Table 4). The deficit between N harvested minus N added as fertilizer for F-Flx-W (N + P) was 506 kg ha 1 while for F-W-W (N + P) it was 629 kg ha 1. This suggests that more net N is being harvested from F-W-W (N + P) than from F-Flx-W (N + P) (about 120 kg ha 1 ) thereby leaving less N to be leached under the F-W-W system (assuming N mineralized is about equal under both systems). When the wheat crop grown on fallow in this 3-yr rotation was replaced by a fall-seeded crop such as fall rye, there was no -N leached (Tables 2 and 3). These results support our earlier findings (Campbell et al. 1984) for this experiment. Compared with the F-W-W (N + P) system, less (148 kg ha 1 ) net N is being harvested with CF-Rye-W (N + P) (Table 4) suggesting that potentially more N leaching could occur in the latter system. However, this was not the case, probably because the length of fallow period (i.e., 12 mo for rye; 20 mo for spring wheat) and the timeliness of N uptake by the fallseeded crop played important roles. Because of the shorter fallow period less water is stored in the second fall, thereby reducing fall N mineralization and overwinter drainage beyond 1.2 m. Further, because the rye grows in September to October and during the following April and early May when spring-seeded crops are not growing, it takes up excess and uses excess stored water, thereby reducing opportunities for leaching in wet springs. Similar results have been reported in Atlantic Canada where fall-seeded catch crops such as winter wheat and winter rye have been shown to be effective in reducing the -N left in the soil following potato production (Reynolds et al. 1995). The W-Lent (N + P) rotation was initiated on Cont W (N + P) plots in Since that change, the N-supplying capacity of this soil has been gradually increasing (Campbell et al. 1992) and the fertilizer N requirements have been decreasing relative to Cont W (N + P) (Table 4). The N harvested in grain from these two rotations has been similar (Table 4); thus, the harvested N has exceeded N added as fertilizer by much more under W-Lent (N + P) than under Cont W (N + P) (i.e, 592 vs. 185 kg N ha 1 ). This implies that unless there has been considerably more N mineralized in W-Lent (N + P) than in Cont W (N + P) in the last 24 yr, there should be less -N available for leaching under W-Lent (N + P) than under Cont W (N + P). Based on the distributions of -N below 1.2-m depth (Fig. 5), there is no significant difference in the level of -N leached between these two systems (Tables 2 and 3), but as found in 1990 (Campbell et al. 1992), substantially more - N is located in the 0.6- to 1.2- m depth under W-Lent (N + P) (Fig. 5, and Table 3). The extra -N in the 0.6- to 1.2-m depth under W-Lent (N + P) likely reflects increased N mineralization associated with greater N-supplying capacity of the soil in this treatment during the last yr, when growing

9 CAMPBELL ET AL. CROPPING FREQUENCY, CROP TYPE AND FERTILIZER ON NITRATE LEACHED 709 Fig. 5. Effect of a pulse crop on leached in a continuous cropping system over 37 yr. season preciptitation was generally above average (Table 5) and crop production, and thus water use, was also above average (Campbell et al. 2004). Consequently, excess soil water to facilitate leaching would be minimal under continuously cropped systems during this period. These results further imply that a soil test for N based on in the top 0.6-m depth, as used in Saskatchewan, though adequate for cereals, is inappropriate for legumes. In the New Rotation experiment being conducted at Swift Current since 1987, Campbell et al. (2006) found that no -N leaching has occurred under Cont W (N + P) during the last 17 yr even though this was a period with generally above-average precipitation (Table 5). Whether or not leaching occurs depends on whether the precipitation occurs during the growing season when plants will use water and N rapidly (e.g., 1990s, Table 5) or if it occurs after harvest until early spring, when there is little active growth and excess water and can readily accumulate (e.g., , Table 5). Under continuously cropped systems good growing season precipitation is less likely to result in leaching because growth and N use will be high leaving little for leaching. However, under fallow-crop systems, good growing season precipitation during the fallow period will promote N mineralization and soil water storage and thus facilitate leaching, especially if the following fall to spring period is also wet. CONCLUSIONS A 37-yr crop rotation study that used conventional tillage management in the semiarid Canadian prairies showed that the main factor influencing leaching was crop or fallow frequency; rate of fertilizer and type of crop were also important. The amount of leached tended to be greater and deeper as fallow frequency increased. In fallow-containing systems, use of proper fertilization (soil test dictated) is imperative if leaching is to be minimized; inadequate fertilizer N or P resulted in reduced crop growth and N uptake, and tended to increase leaching of the mineralized in the fallow period. In continuously cropped systems, leaching was directly proportional to N input. There was no difference in content below the rooting depth of W- Lent (N + P) and Cont W (N + P); however, the latter treatment tended to have slightly more leached than Cont W (+ P). Further, the extra N input via N fixation under W-Lent (N + P) resulted in greater content in the lower segment of the rooting depth under W-Lent (N + P) than under Cont W (N + P). Replacing spring wheat grown on fallow with the shallow-rooted, less-robust crop like flax in a F-crop-crop (N + P) rotation resulted in less N uptake by the flax-containing system and thus, greater leaching. In contrast, replacing spring wheat grown on fallow with a fall-seeded cereal such as fall rye resulted in little or no leaching, partly because the fallow period is only 12 mo for fall rye compared with 20 mo for spring wheat (less mineralized, less water stored) and partly due to the rye using -N in September-October and the following April before much of it can be leached. ACKNOWLEDGEMENTS The authors acknowledge Barry Blomert, Rod Ljunggren, Darrel Hahn, Gary Winkleman, Arnie Ens, Doug Judiesch, Don Sluth, Keith Hanson, Ken Deobold, Evan Powell and Jodiene Cooke for technical assistance. Akinremi, O. O., Campbell C. A., Jame, Y. W., Zentner, R. P. and Chang, C Simulating nitrogen dynamics and nitrate leaching using LEACHM model. Pub. No. 379M0083. Research Branch, Research Station, Agriculture Canada, Swift Current, SK. 89 pp.

10 710 CANADIAN JOURNAL OF SOIL SCIENCE Akinremi, O. O., Jame Y. W., Campbell, C. A., Zentner, R. P., Chang, C. and de Jong, R Evaluation of LEACHMN under dryland conditions. I. Simulation of water and solute transport. Can. J. Soil Sci. 85: Ayers, K. W., Acton, D. F. and Ellis, J. G The soils of the Swift Current map area 72J Saskatchewan. Saskatchewan Institute of Pedology. Publ. 86. Extension Division, University of Saskatchewan, Saskatoon, SK. Extension Publ Bauder, J. W., Sinclair, K. N. and Lund, R. E Physiographic and land use characteristics associated with nitratenitrogen in Montana groundwater. J. Environ, Qual, 22: Campbell, C. A., DeJong, R. and Zentner, R. P Effect of cropping, summer fallow, and fertilizer nitrogen on nitrate-nitrogen lost by leaching on a Brown Chernozemic Loam. Can. J. Soil Sci. 64: Campbell, C. A., Lafond, G. P., Leyshon, A. J. C., Zentner, R. P. and Janzen, H. H Effect of cropping practices on the initial potential rate of N mineralization in a thin Black Chernozem. Can J. Soil Sci. 71: Campbell, C. A., Lafond, G. P., Zentner, R. P. and Jame, Y. W Nitrate leaching in a udic haploboroll as influenced by fertilization and legumes. J. Environ. Qual. 23: Campbell, C. A., Myers, R. J. K. and Curtin, D Managing nitrogen for sustainable crop production. Fert. Res. 42: Campbell, C. A., Nicholaichuk, W. and Warder, F. G Effect of a wheat-summerfallow rotation on subsoil nitrate. Can. J. Soil Sci. 55: Campbell, C. A., Read, D. W. L., Zentner, R. P., Leyshon, A. J. and Ferguson, W. S First 12 years of a long-term crop rotation study in southwestern Saskatchewan yield and quality of grain. Can. J. Plant Sci. 63: Campbell, C. A., Selles, F., De Jong, R., Zentner, R. P., Hamel, C., Lemke, R., Jefferson, P. G. and McConkey, B. G Effect of crop rotations on leached over 17 years in a medium-textured Brown Chernozem. Can. J. Soil Sci. 86: Campbell, C. A. and Zentner, R. P Soil organic matter as influenced by crop rotations and fertilization in an aridic haploboroll. Soil Sci. Soc. Am. 57: Campbell, C. A. and Zentner, R. P Disposition of nitrogen in the soil-plant system for flax and spring wheat-containing rotations in the Brown soil zone. Can. J. Plant Sci. 76: Campbell, C. A., Zentner, R. P., Selles, F. and Akinremi, O. O Nitrate leaching as influenced by fertilization in the Brown soil zone. Can. J. Soil Sci. 73: Campbell, C. A., Zentner, R. P., Selles, F., Biederbeck, V. O. and Leyshon, A. J Comparative effects of grain lentilwheat and monoculture wheat on crop production, N recovery and N fertility in a Brown Chernozem. Can. J. Plant Sci. 72: Campbell, C. A., Zentner, R. P., Selles, F., Biederbeck, V. O., McConkey, B. G., Lemke, R. and Gan, Y. T Cropping frequency effects on yields of grain, straw, plant N, N balance and annual production of spring wheat in the semiarid prairie. Can. J. Plant Sci. 84: Campbell, C. A., Zentner, R. P. and Steppuhn, H Effect of crop rotations and fertilizers on moisture conserved and moisture use by spring wheat in southwestern Saskatchewan. Can. J. Soil Sci. 67: Chambers, P. A., Guy, M., Roberts, E. S., Charlton, M. N., Kent, R., Gagnon, C., Grove, G. and Foster, N Nutrients and their impact on the Canadian environment. Agriculture and Agri-Food Canada, Environment Canda, Fisheries and Oceans Canada, Health Canada and Natural Resources Canada. 241 pp. Canada Soil Survey Committee, Subcommittee on Soil Classification The Canadian system of soil classification. Publ Canada Department of Agriculture, Ottawa, ON. Cowell, L. E. and Doyle, P. J Nitrogen use efficiency. Pages in D. A. Rennie, C. A. Campbell, and T. L. Roberts, eds. Impact of micronutrients on crop responses and environmental sustainability on the Canadian prairies a review. Canadian Society of Soil Science, Ottawa, ON. Hamm, J. W., Radford, F. G. and Halstead, E. H The simultaneous determination of nitrogen, phosphorus and potassium in sodium bicarbonate extracts of soils. Pages in Technicon International Congress, Advances in automatic analysis. Vol.2. Industrial analysis, Futura Publ. Co., Mt. Kisco, NY. Izaurralde, R. C., Feng, Y., Robertson, J. A., McGill, W. B., Juma, N. G. and Olson, B. M Long-term influence of cropping systems, tillage methods, and N sources on nitrate leaching. Can. J. Soil Sci. 75: Randall, G. W., Iragavarapu, T. K. and Bock, B. R Nitrogen application methods and timing for corn after soybean in a ridge-tillage system. J. Prod. Agric. 10: Reynolds, W. D., Campbell, C. A., Chang, C., Cho, C. M., Ewanek, J. H., Kachanoski, R. G., MacLeod, J. A., Milburn, P. H., Simard, R. R., Webster, G. R. B. and Zebarth, B. J Agrochemical entry into groundwater. Pages in D. F. Acton and L. J. Gergorich, eds. The health of our soils toward sustainable agriculture in Canada. Centre for Land and Biological Resources Research, Research Branch, Agriculture and Agri-Food Canada, Ottawa, ON. Saskatchewan Agriculture Saskatchewan fertilizer practices. Agdex 541, Publ. No. M , Plant Industry Branch, Saskatchewan Agriculture, Regina, SK. 12 pp. University of Saskatchewan Guide to farm practice in Saskatchewan 75. University of Saskatchewan. Extension Division, Saskatoon, SK. 176 pp. Starr, C. and Smith, D. B A semi-micro dry block and automated analyzer technique suitable for determining protein nitrogen in plant material. J. Agric. Sci. 91: Zentner, R. P. and Campbell, C. A First 18 years of a longterm crop rotation in southwestern Saskatchewan yields, grain protein, and economic performance. Can. J. Plant Sci. 68: 1 21.