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Dep. of Plant Sci., Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2
Corresponding author (m_entz{at}umanitoba.ca)
Received for publication July 14, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: AA-WWWW, wheat following 2 yr of alfalfa • AAAA-WW, wheat following 4 yr of alfalfa
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The movement of NO3–N through a soil profile is directly linked to the movement of water through the soil profile (Nielsen et al., 1982). Preferential flow of soil water down structural pathways in the soil profile can also be responsible for the rapid movement of applied N fertilizers (Coles and Trudgill, 1985). Although NO3–N movement is primarily downward, Boswell and Anderson (1964) indicated that upward movement of NO3–N might also occur when evaporation exceeds precipitation. Leaching of NO3–N is often greatest in wet climates and irrigated cropping systems under intensive crop management where a significant proportion of water moves downward through the soil profile (Catchpoole, 1986). However, Campbell et al. (1984) stated that in spite of a net annual water deficit, NO3–N leaching beyond the root zone of annual crops occurs more readily than previously suspected under dry, subhumid cropping conditions such as those found in the prairie provinces of western Canada. Jung et al. (1989) noted that NO3–N leaching occurred mainly during periods of winter fallow or after incorporation of perennial legume residues into the soil.
The amount of NO3–N in the soil profile has been shown to vary considerably with the type of cropping system (Papendick et al., 1987; Weed and Kanwar, 1996). Diversified cropping systems tend to result in less NO3–N leaching and lower subsoil NO3–N concentrations than monoculture grain crops. For example, Varvel and Peterson (1990) found that rotating continuous corn (Zea mays L.) and grain sorghum (Sorghum bicolor L.) with soybean [Glycine max (L.) Merr.] or clover (Trifolium spp.) reduced both the requirement for inorganic N fertilizer and the amount of N available for leaching compared with monoculture corn. Cropping systems that include perennial plants are particularly effective in reducing subsoil NO3–N accumulation. For example, Olsen et al. (1970) observed that rotating corn with oat (Avena sativa L.) and bromegrass (Bromus inermis L.) followed by alfalfa significantly reduced the concentration of NO3–N in the soil profile compared with continuous corn rotations. Similar observations were made by Muir et al. (1976). Stewart et al. (1968) measured NO3–N concentrations of 292 kg ha-1 to a 6-m depth under a cultivated dryland system compared with 88 and 101 kg ha-1 under alfalfa and native grassland, respectively. Campbell et al. (1975) observed no appreciable NO3–N concentrations to a 15-m depth under natural prairie grass systems in Saskatchewan.
Nitrate extraction by alfalfa has received attention by several workers (e.g., Russelle et al., 1993); however, little detailed work has been conducted on temporal extraction patterns or post-termination NO3–N dynamics in regions with short growing seasons such as western Canada. Mathers et al. (1975) observed water and NO3–N extraction with alfalfa to a 180 and 360-cm depth in the 1st and 2nd yr, respectively. Huang et al. (1996) determined that alfalfa had limited capability in the 1st yr to remove NO3–N from below 120 cm in the soil profile. They estimated that switchgrass (Panicum virgatum L.) could effectively remove as much as 20 kg ha-1 NO3–N from below this depth, which was more than twice as much as alfalfa and 20 times that of corn. Peterson and Russelle (1991) cited reports that alfalfa roots can absorb nutrients and water from depths of 11 m; however Campbell et al. (1994) and Voorhees and Holt (1969) found that the depth of significant water and nutrient removal by alfalfa was approximately 2.5 m. Others (Blumenthal and Russelle, 1996; Schertz and Miller, 1972; Stewart et al., 1968) have also observed less NO3–N leaching and greater NO3–N extraction when alfalfa is grown. Izaurralde et al. (1995) found distinct NO3–N distribution peaks around the 2-m depth under a wheat–fallow rotation compared with a greater surface NO3–N distribution with a rotation of wheat, oat, barley, and alfalfa–bromegrass mixture. They suggested that the alfalfa–bromegrass component was recycling deep-leached NO3–N upward to shallower soil levels.
Peterson and Russelle (1991) pointed out that the use of alfalfa in cropping systems as a means of reducing or eliminating NO3–N pollution of ground water warrants care when rotating from alfalfa to successive crops in order to prevent excessive or poorly timed N mineralization, thereby minimizing potential leaching of NO3–N during noncropped periods. Quantities of NO3–N released following alfalfa termination range from approximately 100 to 250 kg N ha-1 (Hesterman et al., 1987; Mohr et al., 1999). In addition, because alfalfa results in greater soil hydraulic conductivity (Meek et al., 1989), the risk of NO3–N leaching is further increased (Bouma, 1991).
The synchrony between N release from alfalfa decomposition and the N requirements of subsequent crops greatly affects the leaching potential of the released N (Izaurralde et al., 1995; Weed and Kanwar, 1996). Working on a 30-yr crop rotation study in western Canada, Campbell et al. (1994) determined that NO3–N leaching could occur with a cropping rotation that included alfalfa, especially if legume plowdown was followed by a fallow period. Mohr et al. (1999) found that termination of alfalfa with herbicides improved the synchrony between N release and N demand of two subsequent spring wheat crops, thus improving N use efficiency and reducing NO3–N available for leaching. Hoyt and Hennig (1971) determined that mineralizable N to a 15-cm depth existed in a much greater concentration in the 1st yr after alfalfa compared with fallow–wheat rotation, red fescue (Festuca rubra L.), or bromegrass; however, differences in N concentration diminished somewhat after four wheat crops.
The objectives of this study were: (i) to determine the pattern of NO3–N extraction by alfalfa vs. an annual crop rotation over a 6-yr period, (ii) determine how rotating from alfalfa to wheat after two or four alfalfa crops affected the soil NO3–N profile, (iii) determine how N fertilizer additions to wheat crops after alfalfa affected the soil NO3–N profile, and (iv) compare alfalfa-based systems with a native-grass system. The study was initiated after discovering very high concentrations of NO3–N to a 300-cm soil depth at the University of Manitoba Field Research Station. This site, therefore, presented a unique opportunity to examine phytoremediation potential of alfalfa-based cropping systems.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The experimental design was a randomized complete block with four replications. Treatments consisted of nine land use systems including annual cropping [wheat (‘Katepwa’), field pea (‘Victoria’), and barley (‘Heartland’)] with and without N fertilizer added, combinations of alfalfa (‘OAC Minto’) and annual crops (alfalfa for 2 or 4 yr followed by wheat) with and without N fertilizer added, continuous alfalfa hay, a perennial native-grass system (big bluestem and western wheatgrass), and continuous fallow (Table 1). Plot size was 5.5 by 8 m.
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Annual grain crops were seeded at rates of 135, 130, and 180 kg ha-1 for spring wheat, barley, and field pea at a 15-cm row spacing using a no-till offset disc drill (Swift Machinery Co., Swift Current, SK). Annual rotations received fertilizer N at soil test recommended levels (from 70–80 kg N ha-1) (+N) or no fertilizer N (-N). No N was added to annual crops in the year immediately following alfalfa. Weed control consisted of typically used herbicides registered for use in Manitoba. To determine grain yield at crop maturity, crops were harvested with a small-plot combine from a 6-m2 area within each plot.
The tillage regime consisted of intensive tillage to terminate alfalfa crops (two passes with a rotary tiller). In subsequent years, and in all annual crop rotations, a minimum tillage regime was used that involved one to two passes in autumn with a field cultivar. In spring, annual crops were direct-seeded into untilled soil. Continuous fallow plots were tilled with a field cultivator twice during the 6-yr period; weeds were controlled by hand weeding.
Soil samples (in 30-cm increments) to a depth of 240 cm (1990 and 1991) or 300 cm (all other years) were collected each year in early October using a soil auger (Giddings Machinery Co., Akron, CO). Two samples were taken from each plot and combined immediately after collection. Soil samples were frozen immediately after collection and subjected to chemical analysis within 2 mo. Soil samples were air-dried and then ground (<2 mm) using a rotating sieve. Soil inorganic N was extracted with 2 M KCl, and the concentration of NO3 was determined by automated colorimetric procedure (Keeney and Nelson, 1982). No soil data was collected in the 5th yr of the study (i.e., 1994).
Volumetric soil water content (in 20-cm increments) between 10 and 150 cm was determined periodically in each season using a field-calibrated neutron moisture gauge (Model 4330, Troxler, Research Triangle Park, NC). One neutron access tube was positioned in the center of each plot. Surface soil water content (0–10 cm) was measured using a neutron moisture gauge with a surface shield as illustrated by Chanasyk and Naeth (1988). Environmental data was collected 100 m from the plot area. Long-term average air temperatures at Winnipeg for May through September are 11.9, 16.6, 19.4, 18.1, and 12.3°C, respectively (Atmospheric Environment Branch, Environment Canada, Winnipeg, MB). Precipitation data is given in Table 2.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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A reduction in soil NO3–N concentration was observed in the annual rotation for the 90- to 150-cm soil zone in 1993 and 1995 (Fig. 1d and 1e, respectively) relative to 1992. It may be that NO3–N was leached in 1993 due to very high precipitation levels, and the leached NO3–N was not replaced in subsequent years because NO3–N that was higher in the soil profile was extracted by annual crops (Fig. 1). Such a conclusion is supported by Campbell et al. (1984), who pointed out that NO3–N leaching in dryland systems usually occurs only during very wet periods.
Alfalfa System
In October of the establishment year, soil NO3–N concentrations in the 30- to 90-cm profile under alfalfa were approximately one-quarter of the levels found in continuous fallow and similar to concentrations found in the annual rotation (Fig. 1a). Similar extraction patterns of establishment year alfalfa and annual crops suggest that, in the establishment year, alfalfa was no more capable of deep NO3–N recovery than annual crops. Working on the same soil type, Bonner (1997) observed that establishment year alfalfa was capable of extracting water only to a soil depth of 120 to 140 cm. Huang et al. (1996) and Mathers et al. (1975) observed NO3–N extraction by establishment year alfalfa to soil depths of 120 and 180 cm, respectively.
By the end of the second season (1991), the band of lower NO3–N concentration (<5 mg kg-1) in the alfalfa plots extended to the 150- to 180-cm soil zone (Fig. 1b). The change in NO3–N from Year 1 to Year 2 was significant at depths between 90 and 180 cm (Fig. 2). Decreases in soil NO3–N over time were attributed to alfalfa root growth below 90 cm in Year 2. By the end of the 2nd yr, the annual rotations extracted appreciable NO3–N only to 120 cm (Fig. 1b). Deeper root activity for alfalfa than annual crops after 2 yr is supported by soil water measurements. For example, soil water content at soil depth increments of 30 to 90 cm and 90 to 150 cm was significantly lower after 2 yr than for the annual crop and fallow treatments (Fig. 3). Mathers et al. (1975) reported a significant reduction in soil NO3–N to a soil depth of 360 cm after the second growing season. Izaurralde et al. (1995) also determined that 2nd yr alfalfa–bromegrass mixture in a wheat–oat–barley–hay–hay rotation prevented accumulation of NO3–N below 90 cm.
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No further increases in soil NO3–N extraction were observed between the 4th and 6th yr (Fig. 1). In fact, between the end of the fourth and sixth production years, soil NO3–N concentration increased for soil depth increments of 0 to 30 cm and 180 to 210 cm in the alfalfa system (Fig. 2). After 6 yr (i.e., in 1995), NO3–N concentrations in the 240- to 270-cm zone were not different for the annual and alfalfa systems (Fig. 1e) as had been the case after 4 yr (Fig. 1d). Also, the average NO3–N concentration between 0 and 120 cm increased by 249% from the 4th to 6th yr of the alfalfa stand. A possible explanation for these increases in surface and subsoil NO3–N concentrations may be greater mineralization and nitrification of organic N released by winterkilled alfalfa plants. Although alfalfa stand densities were not measured in this study, plant loss was visible each year; this was reflected in declining forage yields, especially in 1995.
Meek et al. (1989) found that the crown density of an alfalfa stand decreased from 190 to 44 crowns m-2 over a 3-yr period. Barley (1954) implied that as an alfalfa stand ages, the decomposition of roots from dead alfalfa plants results in an increased water infiltration rate. Unless other plants invade the field (which did not occur in the present study), NO3–N would be expected to increase when alfalfa plants die (Mohr et al., 1999). In 1994, Cavers (1996) established that the saturated soil hydraulic conductivity of the alfalfa plots in this study was 10 times greater than in either the annual grain or fallow systems. The combination of N release from alfalfa roots and this greater potential for soil water infiltration would dramatically increase the potential for NO3–N leaching (Bouma, 1991). The period from 1993 to July 1995 was also the wettest during the study period (Table 2), and soil water content in the 90- to 150-cm soil zone was usually not significantly different in the alfalfa and annual crop rotations (Fig. 3).
In the present study, a 2- to 6-yr alfalfa stand was capable of maintaining the soil NO3–N concentrations below 10 mg kg-1 to depths of up to 240 cm (Fig. 1). Maximum NO3–N extraction benefits obtained from alfalfa were observed in the 4th yr (1993) of this study at a maximum soil profile depth of 270 cm (Fig. 1d), indicating that under the conditions of this study, the alfalfa stand length of 4 yr provided the greatest NO3–N extraction benefits. Increasing the stand length from 4 to 6 yr did not result in further decreases in soil NO3–N concentrations (Fig. 1c) and may have increased NO3–N leaching potential. Unfortunately, no information from the 5th yr was available, and so we cannot determine whether the 5-yr alfalfa stand may have been better or worse than the 4-yr stand.
Soil Nitrate-Nitrogen Profile after Six Years as Affected by Crop Rotation
Soil samples were collected at the end of Year 6 to assess NO3–N profiles for several additional treatments, including wheat following either 2 (AA-WWWW) or 4 (AAAA-WW) yr of alfalfa and a native-grass system.
0- to 30-cm Soil Depth
As expected (Campbell et al., 1975), NO3–N in the top 30-cm soil depth was lowest for the grass treatment (Table 3). Interestingly, two arable systems, the unfertilized annual crop and unfertilized AA-WWWW rotations had surface NO3–N concentrations similar to the grass system. All other systems, including continuous alfalfa, had higher surface NO3–N concentrations (Table 3). Fertilizer N additions consistently increased NO3–N relative to the grass system; however, effects tended to be localized in the 0- to 30-cm soil depth.
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These results indicate that adding N in a cropping system, either through additions of inorganic fertilizer or legumes, increased accumulation of NO3–N in the 30- to 150-cm zone, supporting a similar observation by Campbell et al. (1994). The highest NO3–N concentrations in the 30- to 150-cm zone were found in systems that contained both alfalfa and N-fertilized wheat crops.
150- to 300-cm Soil Depth
Treatment effects for NO3–N concentration were much different in the subsoil (150- to 300-cm soil depth increment) than in the upper soil zones. For example, while NO3–N concentrations were higher for alfalfa than for grasses in the 0- to 120-cm zone, NO3–N concentrations for the two systems were similar between 150 and 300 cm (Table 3). Muir et al. (1976) also found no differences in soil NO3–N between alfalfa and native grassland below 100 cm. Roots of big bluestem and western wheatgrass may grow to depths of 210 and 150 cm, respectively (Weaver, 1968). Others (Catchpoole, 1986; Standley et al., 1990) also have identified grasses as being important in deep NO3–N recovery; however, Weaver (1968) observed that in the natural prairie, tap-rooted forbs had deeper roots than grasses. This is supported by observations of greater water use between 90 and 150 cm by tap-rooted alfalfa compared with fibrous-rooted grasses (Fig. 3). It is important to note, however, that soil water use is positively correlated with plant dry matter production, and dry matter production in the present study was much lower for the grass system compared with alfalfa-based systems (Table 4).
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Interestingly, the AAAA-WW systems had significantly less NO3–N in the 270- to 300-cm zone than either the continuous alfalfa or grass systems (Table 3). Superior performance of the AAAA-WW system over continuous alfalfa may be attributed to the combination of an optimum alfalfa stand length (for deep NO3–N recovery) and two subsequent non-N-fixing crops (resulting in minimum NO3–N leaching). The combination of alfalfa and wheat in the cropping system may also have enhanced deep NO3–N recovery. For example, alfalfa may have increased soil porosity ("biological tillage"; Dexter, 1991), thereby increasing root activity in the postalfalfa wheat crops. Groenvelt et al. (1984) determined that the roots of corn following alfalfa grew deeper than in annual systems. Deeper wheat root growth would be expected to reduce NO3–N leaching potential. Cook and Veseth (1991) demonstrated that a diversified rotation resulted in less subsoil NO3–N accumulation than a monoculture because of better root health. Superior performance of AAAA-WW over the grass system in deep NO3–N extraction is more difficult to explain. Perhaps lower NO3–N extraction in the grass system is due to less N removal, which is due to lower dry matter production (Table 4) and less water use (Fig. 3) than the alfalfa-containing systems. It is also possible that 4 yr of growth may not have been enough for these grasses to fully express their deep-root activity.
One unexplained observation was lower NO3–N concentration between 270 and 300 cm for AAAA-WW in 1995 (Table 3) than 4-yr alfalfa stand in 1993 (Fig. 2). Because these observations were taken in different years, a statistical comparison was not possible. However, one possible explanation for this observation may be upward movement of NO3–N between 1993 and 1995 due to high water demand by wheat. The conditions for upward movement of water and NO3–N described by Boswell and Anderson (1964) (i.e., evaporation > water available) were observed in the dry season of 1995 (Table 2), and the greater soil hydraulic conductivity in alfalfa-based rotations (Cavers, 1996) may have enhanced any vertical soil water movement.
The AA-WWWW system did not perform as well as AAAA-WW system. The unfertilized AA-WWWW system did have lower subsoil NO3–N concentrations than the annual grain system; however, this benefit only occurred to a maximum soil depth of 210 cm (Table 3). It was interesting to note, however, that after 6 yr, subsoil NO3–N concentrations were no different for AA-WWWW and continuous alfalfa treatments (Table 3). Therefore, a 2-yr alfalfa stand in a 6-yr rotation provided similar long-term NO3–N recovery benefits as a 6-yr alfalfa crop. Campbell et al. (1994) found that a 3-yr alfalfa–bromegrass stand in a 6-yr rotation resulted in minimum buildup of subsoil NO3–N. Similar observations were reported by Izaurralde et al. (1995).
Campbell et al. (1993) indicated that inadequate nutrient supply could contribute to NO3–N leaching due to insufficient plant growth, leaving additional moisture to leach NO3–N downward in the soil profile. In the present study, no evidence of subsoil NO3–N accumulation was observed in the unfertilized systems (Table 3) despite the fact that grain yield of the annual (-N) treatment was 36% lower than grain yield of the AA-WWWW (-N) treatment and 53% lower than that of the AAAA-WW (-N) treatment (Table 4). One important difference between our two studies was that only N fertilizer was eliminated from the present study while no fertilizers of any kind were used in the Campbell et al. (1993) study.
Others have suggested that low subsoil NO3–N concentrations in alfalfa-based cropping systems are due to either recycling of NO3–N to the upper soil profile by alfalfa (Izaurralde et al., 1995) or reduced leaching potential of N released from alfalfa residue compared with fertilizer N (Papendick et al., 1987). The present study was not established to specifically compare these two mechanisms; a more detailed accounting of N (e.g., using 15N) would be required to establish the relative importance of these two NO3–N transport mechanisms. However, it did appear that both NO3–N extraction and NO3–N leaching played a role in the present study. For example, deep NO3–N extraction was important to reduce soil NO3–N concentrations, and a 4-yr alfalfa stand effectively reduced NO3–N to a soil depth of 270 cm. However, rotating to wheat after 4 yr of alfalfa appeared to further reduce subsoil NO3–N concentrations relative to continuous alfalfa. This observation was attributed to reduced NO3–N leaching losses in the alfalfa–wheat system compared with the continuous alfalfa system. It is important to note that results may have been different had the alfalfa stand not been in decline.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The AAAA-WW system resulted in the lowest soil NO3–N concentrations of any treatment tested in this study, even lower than the continuous alfalfa and native-grass systems. Subsoil NO3–N extraction was less with only 2 yr of alfalfa; however, after 6 yr, subsoil NO3–N patterns were similar for the AA-WWWW and continuous alfalfa systems. Based on these observations, especially the shortcomings of continuous alfalfa relative to the AAAA-WW system, it was concluded that cropping-system diversity is important for reducing subsoil NO3–N accumulation and that monoculture alfalfa is inferior to some alfalfa–wheat systems.
From the data available in this study, it was concluded that NO3–N extraction from the subsoil contributed more to the environmental quality benefit of alfalfa than reduced NO3–N leaching from alfalfa. However, both processes (i.e., extraction and reduced leaching) appear to have occurred in this study. The superiority of the AAAA-WW system over the AA-WWWW system was attributed to greater initial subsoil NO3–N extraction while the superiority of AAAA-WW over continuous alfalfa was attributed to less leaching.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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