Agronomy Journal Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (21)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Grant, C. A.
Right arrow Articles by Campbell, C. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Grant, C. A.
Right arrow Articles by Campbell, C. A.
Agricola
Right arrow Articles by Grant, C. A.
Right arrow Articles by Campbell, C. A.
Related Collections
Right arrow Crop Rotation Systems
Right arrow Dryland Cropping Systems
Right arrow Nutrient Cycling
Right arrow Nutrient Management
Right arrow Soil Fertility and Productivity
Agronomy Journal 94:186-198 (2002)
© 2002 American Society of Agronomy

SYMPOSIUM PAPERS

Nutrient Considerations for Diversified Cropping Systems in the Northern Great Plains

Cynthia A. Grant*,a, Gary A. Petersonb and Constantine A. Campbellc

a Agriculture and Agri-Food Canada, Brandon, MB, Canada R7A 5V3
b Colorado State Univ., Fort Collins, CO 80523-1170
c Agriculture and Agri-Food Canada, ECORC, Ottawa, ON, Canada K1A 0C6

* Corresponding author (cgrant{at}em.agr.ca)

Received for publication January 14, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 NITROGEN
 PHOSPHORUS
 SPECIAL NUTRIENT CONSIDERATIONS
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
The impacts of recent changes in cropping systems in the northern Great Plains on nutrient dynamics are described in this review paper. Cropping intensity and diversity are increasing in the Great Plains because reduced tillage systems have improved water conservation and, subsequently, crop yields. Higher annualized crop yields increase nutrient removal, which must be balanced by increased nutrient inputs to ensure optimal crop yield and quality while avoiding soil depletion. Furthermore, diversification of crops alters the pattern and degree of nutrient removal and influences microbiological activity and soil quality. Although cropping intensification and diversification will influence most plant nutrients, N and P are the nutrients most commonly deficient for crop production in the Great Plains and most likely to be affected by management practices. Cropping intensification increases crop residue return to the soil, which in turn increases the soil organic C pool. This can lead to higher soil organic matter levels and a greater potential for nutrient cycling, an effect that will increase with time. Cropping systems that include legumes have the potential for contributing N to following crops and may moderate NO3 levels in the soil to avoid potential for NO3 leaching. Phosphorus availability may be influenced by depletion or accumulation of P, based on past cropping and fertilizer management. In addition, preceding crop may influence P availability through residue effects and impacts on vesicular-arbuscular mycorrhizae activity. Synchrony of nutrient supply with crop demand is essential in order to ensure optimum crop yield and quality while avoiding negative environmental impacts.

Abbreviations: VAM, vesicular-arbuscular mycorrhizae • ZT, zero-tillage


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
NITROGEN
 PHOSPHORUS
 SPECIAL NUTRIENT CONSIDERATIONS
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
CROP PRODUCTION ON THE GREAT PLAINS has historically reflected the constraints imposed by available moisture and growing-season temperatures as well as technological constraints in seedbed preparation, weed control, and fertilizer management. These limitations have restricted diversification in rotation; thus, crop–fallow sequences dominated in the past. This is increasingly true as moisture deficit (the difference between precipitation and evapotranspiration) increases with decreasing latitude. In the semiarid Canadian prairies, the limited amount of moisture available for crop production has favored a spring wheat (Triticum aestivum L.)–fallow rotation, which was widely adopted by producers in the Brown and Dark Brown soil zones (Ardic and Typic Borolls). In the Black (Udic Borolls) and Grey Luvisols (Alfisols) soil zones, where moisture is less limiting, more intensive and diversified crop rotations, such as a fallow–spring wheat–spring wheat–coarse grain, were widely practiced, and forage legumes, green manures, and oilseeds such as canola (Brassica napus) were included more as fallow substitutes (Campbell et al., 1990; Dryden et al., 1983; Spratt et al., 1975).

In recent years, conservation tillage systems have emerged on the northern Great Plains as a means of increasing yield potential via increased moisture conservation. Moisture conservation under a minimum- or no-till system facilitates intensified cropping (Lafond et al., 1992; Izaurralde et al., 1994; Larney and Lindwall, 1995; Peterson et al., 1996; Halvorson et al., 1999a, 1999b). The added water conserved through use of reduced tillage compared with more intensive conventional tillage allows a grower to take full advantage of the often low and erratic growing-season precipitation of the Great Plains. For example, in studies conducted by Tanaka and Anderson (1997), precipitation storage efficiency during the 14-mo fallow period between winter wheat harvest and planting of the following winter wheat crop was increased 16% by no-till or minimum till compared with stubble-mulch plots. To take the greatest advantage of increases in stored water while reducing the risk of nutrient leaching and salinization, producers must shift away from rotations that include high proportions of monoculture cereals and summer fallow to more intensified and diversified cropping rotations. Intensified rotations allow the use of the extra water retained from reduced tillage and take advantage of the predominantly summer precipitation pattern prevalent in the northern Great Plains. In addition, increases in available water allow greater crop diversification in the rotation and movement from rotations dominated by cereals to rotations with greater proportions of pulse crops, oilseeds, and in the subhumid areas, forages, whose major water requirement occurs at a time different from cereal crops.

Intensification and diversification of cropping systems influence soil physical, chemical, and microbiological characteristics affecting soil quality. Increasing crop production increases the amounts of plant biomass produced and returned to the soil as surface residue or root material. This has potential to increase soil organic matter content (Wood et al., 1990; Halvorson et al., 1999c) and improve the soil's overall structure and stability (Campbell and Zentner, 1993; Lafond et al., 1992). Soil microbiological diversity, microbial biomass, and respiration are all influenced by intensity and diversity of cropping (Lupwayi et al., 1998, 1999). Changes in such soil quality attributes can influence crop yield potential and the amount and distribution of roots in the soil profile.

Sustainability of cropping systems requires that nutrients removed from the soil be balanced by nutrient replacement so that soils are not depleted of their fertility. Crops differ in their yield potential and in the amounts of nutrients that they remove from the soil (Table 1; Can. Fert. Inst., 1998). Therefore, the rate of nutrients applied must be adjusted to the nutrient demand of the crop. Intensification and diversification of cropping systems influence nutrient demand, cycling, and distribution within the soil profile, affecting nutrient requirements and dynamics throughout the crop rotation. Judicious nutrient management requires that nutrient availability be matched to crop demand. When this relationship is not maintained, there can be a loss of plant yield and quality and/or a loss of nutrients to the environment, leading to reduced nutrient use efficiency and a potential for degradation of air, water, and soil quality. This review paper will describe effects of intensification and diversification of cropping systems on nutrient demand and dynamics, concentrating on N and P, and will illustrate the impact that a cropping system may have on nutrient management decisions.


View this table:
[in this window]
[in a new window]
  		Table 1. Amounts of plant nutrients removed in the harvested portion of various crops. (Adapted from Canadian Fertilizer Institute, 1998.)

 

   NITROGEN
TOP
 ABSTRACT
 INTRODUCTION
 NITROGEN
PHOSPHORUS
 SPECIAL NUTRIENT CONSIDERATIONS
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Nitrogen is the nutrient that is most frequently limiting to crop production and the nutrient applied in the greatest amounts in the northern Great Plains (Campbell et al., 1986). Nitrogen is also required for assurance of optimum crop quality as protein content of crops is directly related to N supply (Grant and Flaten, 1998). It is also of major concern with regards to environmental sustainability because NO3 leaching can reduce ground water quality and N2O emissions can contribute to the greenhouse gas effect and global warming (Campbell et al., 1995). Therefore, N management plays a key role in improving crop yield and quality, environmental safety, and economics of crop production (Campbell et al., 1995). An efficient cropping system will attempt to balance crop demands for N with timing and rate of N supply so that crop yield is optimized while N is neither overdepleted from the soil nor accumulated in quantities that result in the potential for contamination of ground or surface waters. Source, rate, timing, and placement of fertilizer N can be chosen in an effective package to meet these goals. However, optimal management will be influenced by crop type and crop rotation.

Impact of Cropping Intensification
As cropping intensity increases, annualized grain yield increases (Campbell et al., 1990). For example, in studies conducted by Ridley and Hedlin (1968), total amount of spring wheat produced per hectare was increased by moving from fallow to continuous cropping every second year. Yield of spring wheat following sweetclover (Melilotus officinalis Lam.), flax (Linum usitatissimum L.), potato (Solanum tuberosum L.), corn (Zea mays L.), or oat (Avena sativa L.) hay, on two soil types in Manitoba, was from 65% to more than 100% of the wheat yield on fallow, with the lower yields related to problems with effective weed control (Spratt et al., 1975). In studies in Colorado, total annualized grain production and crop production per unit of water was increased when moving from a winter wheat–fallow rotation to a winter wheat–corn–fallow or wheat–corn–millet (Panicum miliaceum L.)–fallow rotation, with the greatest increases in productivity associated with the most water-favorable environments in a landscape catena (Peterson et al., 1993).

As crop production increases, so does N removal from the system (Peterson, 1996). Therefore, total nutrient removal with continuous cropping will be substantially higher than with a fallow cropping system. Kolberg et al. (1996) showed that inclusion of corn in a more intensive winter wheat–corn–fallow rotation led to greater depletion of soil N than did a winter wheat–fallow rotation, particularly at lower rates of applied N. With increased nutrient removal, responses to fertilizer applications become more likely (Campbell et al., 1991a, 1991d). For example, changing from a wheat–fallow to a wheat–corn–fallow rotation required a 44% increase in N fertilizer inputs over a 6-yr period (Kolberg et al., 1996). Therefore, in intensive cropping systems, N fertilization becomes increasingly more important.

Nitrogen returned to the system via crop residues from previous years of cropping must also be considered because this serves to replenish the organic nutrient pool in the soil. Historically, fallow systems have relied on N mineralized from the soil organic matter to provide N for the succeeding crop. Over time, insufficient crop residues were returned to the soil to compensate for the loss in N-supplying capacity of the soil due to fallowing. This, combined with the soil erosion associated with fallowing, led to soil organic matter depletion and an overall decline in the capacity of the soil to mineralize N (Campbell et al., 1993a; Campbell and Zentner, 1993). While crop removal of nutrients is increased by cropping intensification, the amount of organic residues returned to the system is enhanced by more frequent cropping, which can increase the potential for nutrient release from organic matter residues, particularly in fertilized systems. In studies conducted at Indian Head, SK, average yield, organic N, and initial potential rate of N mineralization after 34 yr was increased by continuous cropping compared with fallow systems (Table 2), particularly where N and P fertilizers were applied (Campbell et al., 1993a). Nitrogen mineralization rates at Mandan, ND, were increased after 10 yr by reducing tillage intensity and by annual cropping compared with a crop–fallow system (Weinhold and Halvorson, 1999). In eastern Colorado, increasing the cropping intensity from a wheat–fallow to a wheat–corn–millet–fallow rotation under no-till increased potential N mineralization capacity and N mineralization in the surface 5 cm of soil after 3.5 yr (Wood et al., 1990). The differences were related to greater surface organic matter concentrations caused by higher plant residue additions with more intensive cropping although there also may have been an impact of differential erosion under the varying cropping systems. The extended rotation also received greater amounts of fertilizer N applications. Similarly, in a 9-yr study of continuous no-till spring wheat conducted by Campbell et al. (1993b), N-supplying capacity of the soil was improved by a combination of fertilizing, reducing tillage, and cropping more frequently. Therefore, although continuous cropping will reduce the amount of residual mineral N in the soil, in the long-term, it may increase the potential ability of a soil to supply N to a crop via mineralization during the growing season. There is still some question as to how many years of good management it will take before the potential for greater N mineralization will be reflected in situ.


View this table:
[in this window]
[in a new window]
  		Table 2. Average annual rate of change in yield trends over 34 yr, and organic N, available P, and potential N-supplying capacity of soil in a thin Black Chernozem at Indian Head, SK (Campbell et al., 1993a).

 
Effect of Diversification
Diversified crop rotations can increase yield potential by influencing plant diseases, weeds, root distribution, moisture utilization, and nutrient availability (Campbell et al., 1990). Ideally, crop diversification involves the inclusion of crop species with different resource requirements {e.g., a high N–using crop such as corn combined with low N–using or N-fixing crops such as soybean [Glycine max (L.) Merr.] or lentil (Lens culinaris Medikus)}. Badaruddin and Meyer (1994) determined that wheat yields following legumes were higher than yields following wheat and suggested that the benefit was due to a combination of N and non-N benefits. Rotational benefits also were seen in spring wheat grown after fababean (Vicia faba L. var. faba), lentil, and pea (Pisum sativum L.) over a range of N rates in studies conducted in Manitoba (Table 3) (Soper and Grenier, 1987).


View this table:
[in this window]
[in a new window]
  		Table 3. Impact of preceding crop and N management on grain yield of spring wheat in Manitoba (Soper and Grenier, 1987).

 
When water is limiting, rotating crops with different water use patterns and requirements can increase total crop yield and total nutrient removal. In studies conducted at Lethbridge, AB, Larney and Lindwall (1995) reported that available water in the fall for winter wheat establishment was least after canola (45 mm), followed by continuous winter wheat (59 mm), lentil–flax (74 mm), and fallow (137 mm). Thus, there is potential for higher crop yields following lentil or flax than following canola or continuous winter wheat if the major limiting factor is available water. To optimize crop yield, the nutrient supply rates, whether from chemical fertilizers or organic amendments, must be matched to the yield potential supported by the available water.

Diversified crop rotations will not only influence the demand for nutrients, but may also influence the supply of nutrients to the growing crop. Crops differ substantially in the amount of N returned in the crop residue for use by subsequent crops because N supplied will depend on the amount of crop residue, primarily, and on the concentration of N in the residue. Nitrogen concentration in the residue will determine the net balance between immobilization and mineralization. If the N concentration in the residue is below approximately 20 to 24 g N kg-1, immobilization will exceed mineralization, and the decomposing residues will tie up N rather than release it (Goos, 1995).

Over the long term, as decomposition proceeds, all residues will eventually release the minerals they hold. The time required for this to occur will increase as the initial N concentration in the residue decreases and the C/N ratio widens (Janzen and Kucey, 1988). Straw from a well-fertilized wheat crop could decompose at a similar rate and produce similar amounts of N as a legume residue. Janzen and Kucey (1988) reported that N concentration of lentil, rape (Brassica napus L. var. napus), and wheat residue had a dominating influence on rate of residue decomposition and nutrient release (Table 4). Increasing the N concentration of the residue by fertilization increased rate of residue decomposition. Some residues also may have more of the nutrient present in a readily soluble inorganic or readily mineralizable organic form and so would release nutrients more readily than those that hold most of the nutrients in a more recalcitrant form (Schoenau and Campbell, 1996). Therefore, species and nutrient management of the preceding crop will influence its nutrient content and the amount of nutrients it will release to the subsequent crop. Placement of residues and method of termination of the crop will also influence N release. Soil incorporation of residues reduces N loss by volatilization, enhances mineralization, and increases the short-term supply of plant-available N (Mohr et al., 1998a, 1998b, 1998c, 1999).


View this table:
[in this window]
[in a new window]
  		Table 4. Effects of crop species and crop N on net mineralization of residue N content after 2 and 4 wk of incubation as determined from 15N analysis (Janzen and Kucey, 1988).

 
The amount and type of crop residue and factors influencing its rate of decomposition (e.g., moisture) may also influence the availability of fertilizer N. If fertilizer N is applied with high C/N ratio residues, N immobilization may be substantial because the microbial population will utilize the fertilizer N to build biomass as the residue decomposes. Further, residues can enhance the loss of N by volatilization from broadcast urea [(NH2)2CO] because the urease enzyme present in the residues can increase the rate of NH3 release (McInnes et al., 1986). Therefore, where high amounts of crop residues are present, particularly if they contain a wide C/N ratio, separating the fertilizer from the residues by placing N fertilizers below the soil surface can increase fertilizer use efficiency (Malhi and Nyborg, 1991).

Role of Annual Legumes
Annual legumes, such as soybean, field pea, or lentil, are frequently incorporated into cropping sequences and can increase the amount of available N for the subsequent nonlegume crop. Legume crops can symbiotically fix N in association with Rhizobium spp. If the legume crop is used as a green manure, considerable amounts of N can be supplied to the succeeding crop as the legume residue decomposes (Badaruddin and Meyer, 1990; Welty et al., 1988). In the case of legume pulse crops, such as soybean, field pea, or lentil, where the seed is harvested and removed from the field, N fixation will reduce the fertilizer N requirement for optimum yield of the legume crop (Izaurralde et al., 1992). The amount of N removed from the system via the seed of pulse crops is generally similar to the amount of symbiotic N fixation. Despite this, N requirements are generally reduced and total N accumulation increased in crops following pulse crops, indicating that pulse crops increase the available N for subsequent nonlegume crops (Table 5). For example, in studies in Alberta, unfertilized barley (Hordeum vulgare L.) following fababean under a no-till system produced crop yields equivalent to fertilized continuous barley under a no-till system (Izaurralde et al., 1995a). Welty et al. (1988) reported that various annual legumes provided N contributions to the following crop ranging from 37 to 69 kg N ha -1, depending on the environment. Similarly, Zentner et al. (2000) determined in rotation studies at Swift Current, SK, that fertilizer N savings of 11 kg ha-1 were obtained for wheat following lentil compared with monoculture stubble wheat (Fig. 1) . Legume residues contain considerable amounts of N and have a relatively low C/N residue, leading to more rapid release of N than lower N–containing cereal residues (Janzen and Kucey, 1988). In rotation studies at Swift Current, N content of the top 60 cm of soil was increased by including lentil in rotation compared with continuous wheat production (Campbell et al., 1992). Work by Sawatsky and Soper (1991) indicated that up to 44% of N fixed by legumes remained in the soil after roots were physically removed from the soil, presumably present in irrecoverable root material or lost from the plant root by sloughing and exudation. Some of this fixed N remaining in the soil would become available for subsequent crops. Increased N availability to crops following legumes may also be due to reduced immobilization because legume crops generally produce lower amounts of crop residue than do cereal crops (Green and Blackmer, 1995).


View this table:
[in this window]
[in a new window]
  		Table 5. Effects of pulse crops on the N status of soils (Soper and Grenier, 1987).

 


View larger version (44K):
[in this window]
[in a new window]
  		Fig. 1. Fertilizer N requirement for wheat in continuous spring wheat (W) and wheat–lentil (W–L) cropping systems at Swift Current, SK, Canada (Zentner et al., 2000).

 
In long-term studies conducted at Swift Current, SK, Campbell et al. (1992) showed that including lentil as a pulse crop in a cropping system increased the potential mineralizable N in the soil and reduced the fertilizer N requirement for the subsequent wheat crop. The effect was small in the initial years of the study but increased over time, indicating a cumulative enhancement of the soil's N-supplying capacity, which was probably due to incorporation of a portion of the N fixed by lentil. Prolonged inclusion of pulse crops such as lentil in rotation with cereals can increase the soil's N-supplying capacity and reduce fertilizer N requirements for the following nonlegume crop (Fig. 1). Thus, crop rotations with legumes, as compared with continuous monoculture cropping systems with nonlegumes, can reduce inorganic N requirements. If the fertility management is not adjusted for this effect, or if soil tests are not conducted to determine fertilizer requirements, it could lead to overfertilization, thereby reducing economic efficiency and producing excessive losses of N to the air and water. This is especially true if legume-containing rotations are compared with continuous cropping of high N–use crops such as corn and grain sorghum [Sorghum bicolor (L.) Moench]. In studies conducted in Nebraska, Yamoah et al. (1998) reported that continuous sorghum responded consistently to N application, but sorghum grown in rotation with soybean did not generally respond to fertilizer N. This suggests that the relatively high fertilizer N rates required for continuous cereal production may not be needed in some rotational systems incorporating legume crops. Thus, producers should use soil tests to guide their N applications. The authors also observed that the N contribution from soybean to the following sorghum crop was low when the preseason was dry and moderately high when the preseason was wet (Table 6). They suggest that this was due to improved mineralization of residues throughout the cropping season when moisture was adequate and that N fertilization of the nonlegume crop may be required when the preseason is dry to compensate for the low N supply from the soybean crop.


View this table:
[in this window]
[in a new window]
  		Table 6. Nitrogen contribution of soybean (Ns ± SE) and combined N response equations{dagger} for sorghum yield in different rainfall–temperature environments (Yamoah et al., 1998).

 
Diversification of the crop rotation may influence the amount of N fixed by legume crops. On a thin Black Chernozem (Udic Boroll) at Indian Head, SK, Matus et al. (1997) found that grain yield and N fixation in lentil were greater when grown using more diverse crop rotations compared with less diverse rotations. The highly diversified rotations included three different crop species before seeding lentil while in the less diversified rotations, wheat occupied 50% of the rotation.

Protein Content of Crop
Nitrogen is an integral component of protein, and so is required for protein production in a crop. Grain protein content generally increases with increasing available N and decreases with increasing crop yield potential (Campbell et al., 1997; Grant and Flaten, 1998). Late uptake of N by crops can increase protein content compared with N absorbed by the crop earlier in the growth period (Grant and Flaten, 1998). Grain protein content is often high after fallow due to the accumulation of N in the soil profile (Campbell et al., 1990; Spratt et al., 1975). Residual soil N unused by the preceding crop may also increase protein content (Campbell et al., 1992). For example, wheat grown on flax stubble in a continuous cropping rotation had the highest grain protein concentration among all rotations measured in a Brown Chernozem at Swift Current (Campbell et al., 1983). The elevated protein content was presumably due to access of wheat to N at depth, which had not been utilized effectively by the previous flax crop (Campbell and Zentner, 1996). This N would presumably not be used by the wheat until later in the season (near anthesis), and would therefore primarily contribute to protein increase in the grain. Nitrogen released during the growing season from decomposing legume residues can increase the protein content of crops (Campbell et al., 1992; Zentner et al., 2000). Hedlin et al. (1957) showed that protein concentration of first-crop wheat after alfalfa (Medicago sativa L.) or sweetclover green manure was higher than protein concentration after cereals or fallow; however, protein concentration after grass was lower than that after other crops. They attributed the protein concentration effects to N released gradually by mineralization throughout the growing season. Crop residues that mineralize rapidly and release greater amounts of available N during the growing season (e.g., legume residues) result in higher protein concentration than those that mineralize slowly and release smaller amounts of available N. Grain protein content of wheat grown after lentil was consistently higher than wheat grown on wheat stubble in studies conducted at Swift Current in the Brown soil zone (Campbell et al., 1992; Zentner et al., 2000). The crops started the season with the same amount of available N in the soil, and yields did not differ; thus, the higher protein content of wheat in the lentil rotation suggests an improved synchrony between N availability and N uptake by wheat in the wheat–lentil rotation. Other studies with annual legume crop residues corroborate this principle (Beckie et al., 1997; Stevenson and van Kessel, 1996; Wright, 1990).

Environmental Considerations
Soil Organic Matter
Soil organic matter content has a large impact on both soil quality and nutrient cycling (Schoenau and Campbell, 1996). Organic matter losses from soil as a result of cultivation have been recognized as a serious long-term concern for many years (Campbell et al., 1986, 1990; Peterson and Vetter, 1971). Changes in soil organic C are of increasing importance because organic matter can serve both as a source and sink for atmospheric C. Therefore, the potential for reducing greenhouse gases by sequestering atmospheric C in soil organic matter storage is under investigation. If soil erosion and the addition of organic amendments are ignored, the soil organic C balance is the result of the difference between C inputs from organic residues and C loss by soil respiration (Janzen et al., 1997). Crop residue return to the soil is the major method of replenishing soil organic matter, as demonstrated by Campbell et al. (2000a)(2000b). Thus, systems with frequent fallow will deplete organic matter content more rapidly than will continuous cropping (Campbell et al., 1990, 2000a, 2000b; Larney et al., 1997).

In studies across the entire Great Plains region of the United States (Peterson et al., 1998), crop intensification, particularly when combined with reduced tillage, increased crop residue production and organic C storage in the soil. Eliminating fallow (Nyborg et al., 1995) and increasing the cropping frequency increases inputs of both above- and belowground residues to the soil (Peterson et al., 1998; Campbell et al., 2000a, 2000b), resulting in higher soil organic matter content (Table 7). After 11 yr of cropping at Mandan, ND, total soil organic C content was greater under annual cropping compared with a crop–fallow system (Wienhold and Halvorson, 1998). Ridley and Hedlin (1968) showed that organic matter content was lower when a row crop, such as corn, was grown continuously (5.0%) or rotated with wheat (5.1%) than when cereal crops seeded with narrow row spacing, such as wheat (7.2%), oat (6.3%), or barley, (6.8%) were grown. Intertilling of the corn rows likely resulted in more rapid breakdown of the organic matter and possibly more erosion.


View this table:
[in this window]
[in a new window]
  		Table 7. Effect of cropping frequency on crop residue inputs and soil organic C content in the 0- to 15-cm depth of a Brown Chernozem at Swift Current, SK.

 
Changes in organic matter content may depend on the level of organic matter initially in the soil. In a thick Black Chernozem soil at Melfort, SK, with high organic matter (61–67 t C ha-1), Campbell et al. (1991b)(1991d) could not detect differences in organic matter due to crop rotation or fertilization and suggested that it is difficult to increase soil organic matter in a soil that already has a very high organic matter content. However, in the drier, Brown Chernozem area at Swift Current, SK (Campbell and Zentner, 1993), and with a thin Black Chernozem at Indian Head, SK (Campbell et al., 1991c), soil organic matter content increased with extended rotations and adequate fertilization. Rates of increase in organic C will vary with available water, inherent fertility of the soil, rates of fertilizer applied, and the length of time that management is imposed (Peterson et al., 1998). Increasing residue production through N fertilization increased crop residue production and soil organic C after 11 yr of cropping at Akron, CO (Halvorson et al., 1999c). Thus, intensification of cropping combined with reduced tillage systems and fertilizer management targeted to the production level of the system can increase organic matter content and improve the quality of soils on the Great Plains (Campbell et al., 1996). If soil organic matter is to be maintained or increased, sufficient N must be added to the system, either from fixation by legumes or from chemical or organic inputs, to compensate for the amounts that are removed in the grain and by erosion.

Nitrate Leaching
Accumulation of NO3 in ground water is an environmental and health concern. Nitrate leaching may occur if NO3 is present in the soil profile and water moves through the profile to locations below the rooting zone. Under native-grass ecosystems, NO3 rarely accumulates in the soil; thus, the risk of leaching is low (Izaurralde et al., 1995b; Reynolds et al., 1995). However, under cultivated systems, NO3 leaching below the rooting zone can occur even under semiarid conditions (Campbell et al., 1975, 1984). The problem of NO3 leaching in fallow systems can be particularly serious if fallowing after a dry year results in downward movement of residual NO3 left by a drought-restricted crop. Numerous studies have shown accumulation of NO3 at depth in soils under fallow (Lamb et al., 1985; Spratt et al., 1975; Campbell et al., 1984; Grant and Lafond, 1994). Shallow-rooted crops with a low N demand may increase the risk of NO3 leaching. Campbell and Zentner (1996) and Campbell et al. (1996) reported that the amount of NO3–N located in the 60- to 120-cm depth following a flax crop was consistently higher than following spring wheat. Flax takes up much less N than wheat, leading to a greater amount of residual NO3 in the soil. In a fallow–crop–crop system at Swift Current, SK, the residual N after flax had no effect on yield or protein content of the following spring wheat crop, indicating that the excess N may have been leached (Campbell and Zentner, 1996); however, in a flax–spring wheat–spring wheat system, this excess NO3 contributed to increased grain protein of wheat (Campbell et al., 1983). Including legume green manure crops or breaking forage stands that included alfalfa also may lead to accumulation of NO3 in the subsoil due to mineralization of the accumulated organic N in the root and crown residue (Spratt et al., 1975; Campbell et al., 1995).

Proper management of the cropping sequence can reduce the potential for NO3 leaching. Increasing cropping intensity to increase N uptake and reduce the risk of water percolation below the root zone will reduce the risk of downward movement of NO3. Peterson and Westfall (1994) showed that subsoil NO3 was 27% lower under wheat–corn–fallow and 42% lower under wheat–corn–millet–fallow cropping systems than under wheat–fallow. Improvements in nutrient use efficiency may be obtained through selection of sequential crops that have varying rooting patterns and timeliness of growth and N demand. Mobility of NO3 in soil is similar to that of water, and roots may attract NO3 from as far away as 35 cm in the soil solution (Barber, 1962). Increased nutrient use efficiency may result if the sequential crops explore different portions of the soil profile. Rapid and deep rooting are important for accessing mobile nutrients such as NO3 and SO4 before their movement below the maximum rooting depth while intensity of rooting in the surface soil and interactions with mycorrhizae may be more important than depth of rooting for less mobile nutrients, which tend to accumulate near the soil surface (e.g., P, K, and Zn). In long-term studies conducted in Alberta (Izaurralde et al., 1995b), 5-yr rotations that included alfalfa–brome (Bromus inermis Leyss.) for 2 yr provided less opportunity for NO3 leaching than did a spring wheat–fallow rotation. Muir et al. (1976) reported that alfalfa was an effective scavenger of NO3 that may have accumulated under prior annual crops. A crop such as alfalfa, which combines deep rooting with the presence of feeder roots near the soil surface, is well-adapted to remove NO3 from the deeper portions of the soil profile as well as from surface soil horizons. Mathers and Stewart (1975) reported that alfalfa removed NO3 as deep as 3.6 m in the soil profile by the second year of crop production. However, there is a concern for NO3 leaching when the alfalfa stand is terminated, particularly if the land is subsequently fallowed, because of the rapid mineralization of the organic N in the alfalfa root system (Campbell et al., 1995). Other deep-rooted nonlegume crops, such as sunflower (Helianthus annuus L.) or safflower (Carthamus tinctorius L.), may be effective at recovering deep-leached NO3 from below the rooting depth of cereal crops such as spring wheat (Halvorson and Black, 1985a; Halvorson et al., 1999b).

Synchronizing the availability of N to the uptake requirements of the crop will reduce the risk of NO3 leaching (Fig. 2) . In annual rotations, significant NO3 leaching may occur between growing seasons, particularly if there is poor synchronization between N inputs (from soil N mineralization and fertilizer N additions) and plant N uptake (Izaurralde et al., 1995b). Deep-rooted winter annual crops in rotation can be used to reduce accumulation of NO3 at depth (Campbell et al., 1975, 1984; Grant and Lafond, 1994). Fall-seeded cereal crops use soil N and water efficiently because the plants have a well-established root system by spring, using water and mineral N before the spring rains arrive. Roots of fall-seeded crops also reach the subsoil more quickly than those of spring-seeded crops, resulting in lower potential for movement of NO3 into the deeper soil depths. For example, following a crop such as flax, which tends to have a major concentration of roots in the soil surface, and thus may leave excess NO3 and water in the subsoil (Campbell et al., 1996), with a crop such as fall rye (Secale cereale L.) or winter wheat, which roots deep in the soil profile and removes moisture and NO3 in late fall and early spring, would improve NO3 utilization and reduce the risk of NO3 leaching into the profile (Campbell et al., 1984). However, in crop rotation studies (Campbell et al., 1983), a large portion of the NO3 located in the 90- to 120-cm depth remained unused by the plant, even under rye rotations. With the short growing season of the Canadian prairies, soil temperature at depth is slow to increase. Root density of most crops at this depth is low, and roots are only active at this depth for a short period of the growing season, leading to minimal uptake of N from this depth. Thus, such NO3 may be leached in wet years (Campbell et al., 1984).



View larger version (32K):
[in this window]
[in a new window]
  		Fig. 2. A contrast of the quantity of NO3 present in soils under a continuous spring wheat system and a wheat–lentil system (Reynolds et al., 1995).

 
Inclusion of annual legumes, such as soybean, in rotation with cereal crops may be important in terms of an environmental buffer for NO3 leaching (Reynolds et al., 1995; Varvel and Peterson, 1992). High-yielding, high-protein pulse crops, such as soybean or field pea, remove large amounts of N in the harvested seed. The pulse crop will utilize soil or fertilizer N, if it is present in the system, but in the absence of available N, the crop can symbiotically fix N2 to satisfy its needs. In studies by Varvel and Peterson (1992), residual soil NO3 levels under soybean grown with no fertilizer N were similar to levels when high rates of N fertilizer were applied. Nonfertilized legumes, in rotation with highly fertilized cereal crops, could utilize residual N left in the profile after the cereal crop without risking yield and quality losses through N deficiency. Studies by Campbell et al. (1992) showed that soil test N in the top 60 cm of soil following lentil was higher than under monoculture stubble wheat. However, leached NO3 was lower under the two continuously cropped rotations than under fallow–wheat and lower under wheat–lentil than under fertilized continuous wheat (Zentner et al., 2000). Improved synchrony between the release of N from mineralization of lentil residues and N uptake by the following wheat crop, compared with N uptake by continuous wheat from fertilizer, can reduce the risk of N accumulation in subsoil and NO3 leaching (Campbell et al., 1992). The beneficial effect of legumes on reducing NO3 leaching was illustrated by direct measurement of lower NO3 levels in drainage water under corn–soybean or soybean–corn rotations compared with continuous corn (Kanwar et al., 1991; Varvel and Peterson, 1992).

Nitrous Oxide
Accumulation of NO3 in the soil can also increase the potential for N2O emissions. In Alberta, Lemke et al. (1999) showed that the highest overall N2O emissions occurred on fallow plots, followed by continuous spring wheat fertilized with broadcast urea. Similarly, in Saskatchewan, Aulakh et al. (1982b) reported 300% higher N2O losses from a summer-fallowed field compared with a spring wheat field; they suggested the higher losses on fallow were partly due to the higher available moisture. In another study, annual gaseous losses (N2O + N2) from a zero-tillage (ZT) wheat–fallow rotation were 24 kg N ha-1 compared with 13 kg N ha-1 from continuous spring wheat. Higher losses occurred where water storage was improved by ZT compared with conventional tillage, and the authors suggested that water appeared to be the primary factor affecting gaseous N losses and that short-term increases in soil moisture led to large increases in the gaseous N flux. Hilton et al. (1994) also measured higher N2O losses under no-till compared with moldboard plow tillage systems and suggested that the lower air-filled porosity, higher bacterial populations, and higher amounts of C substrate near the soil surface can enhance denitrification under no-till.

Legumes in cropping systems can also enhance N2O production (Aulakh et al., 1982a). Substantial losses occurred from field pea residue, particularly in a ZT system where the residue was not soil incorporated (Lemke et al., 1999). In that case, losses were higher from field pea residue than from fertilizer urea. In theory, winter cereals should reduce risk of N2O emissions by reducing the accumulation of NO3 and water in soils over the fall and during the early spring period when N2O losses are often high.


   PHOSPHORUS
TOP
 ABSTRACT
 INTRODUCTION
 NITROGEN
 PHOSPHORUS
SPECIAL NUTRIENT CONSIDERATIONS
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Phosphorus, N and other nutrients need to be available to the crop in balance to optimize crop yield and quality and efficiency of crop production (Halvorson and Black, 1985b; Grant et al., 1996). Phosphorus supply is commonly limiting for crop yield on the northern Great Plains (Halvorson and Black, 1985b). Cropping intensification and diversification will influence both P supply and demand in cropping systems.

Impact of Cropping Intensification
Phosphorus dynamics can be affected by cropping intensity and diversification. Intensified cropping in the absence of P inputs from fertilizer or organic amendments will result in a depletion of soil P. McKenzie et al. (1992a)(1992b) evaluated the effect of cropping system and fertilizer management on P in two long-term rotation studies in Alberta. They found that without fertilizer addition, continuous cropping resulted in the greatest reduction of almost all soil organic and inorganic P pools. By increasing the frequency of fallowing, the drain on the soil P pools was generally reduced due to less crop uptake of P. Similarly, in a 24-yr study in the Brown soil zone at Swift Current, SK, annual P removal in the grain in fertilized systems decreased with an increase in fallow frequency, increasing soil P from 4.8 to 5.3 kg P ha-1 for continuous wheat, 3.5 to 3.8 kg P ha-1 for fallow–wheat–wheat, and 3.1 kg P ha-1 for fallow–wheat (Selles et al., 1995). However, if continuous cropping was coupled with the addition of N and P fertilizers, there was a positive effect of cropping on P availability, as a balance of P calculation indicated that all P-fertilized systems received more P than was exported in the grain (Table 8). The residual P fertilizer enriched the inorganic labile pools (inorganic resin P and inorganic P HCO3), the P held in the microbial biomass, and the moderately labile inorganic-P NaOH (Table 9). Bowman and Halvorson (1997) also reported increases in P availability under a continuous cropping system compared with wheat–fallow systems even through P inputs were generally greater in the latter system. The authors attributed the increased P availability to redistribution of soil P from lower depths through biocycling in residue and litter production.


View this table:
[in this window]
[in a new window]
  		Table 8. Phosphorus balance for rotations during 24-yr period (1967–1990) at Swift Current, SK (Selles et al., 1995).

 

View this table:
[in this window]
[in a new window]
  		Table 9. Phosphorus fractions (0–15 cm depth) in three selected cropping systems at Swift Current, SK, based on the Hedley fractionation procedures (Selles et al., 1995).

 
The type of crop grown will also influence P depletion because crops differ in their yield potential and in the amount of P removed in the harvested portion (Table 1). Selles et al. (1995) reported that P exported from the system was higher in cereals (4.9–7.4 kg ha-1 yr-1) than in the lower yielding flax and lentil (3.3–3.7 kg ha-1 yr-1). Increasing crop yield will increase P removal, but there may not be as great an impact on the P fertilizer requirements as there is with N because the amount of P removed by crops is small relative to the total available P in most soils. For example, in the Brown soil zone, the soil-available P has been constant over 30 yr of cropping (Roberts et al., 1999).

The preceding crop may have an important influence on P nutrition of crops due to its effect on mycorrhizal activity. Vesicular-arbuscular mycorrhizae (VAM) are fungi which form symbiotic relationships with many plant species. The extended hyphae of the fungi can penetrate into the soil considerably further than the root hairs of the plant, thereby increasing the zone of absorption of immobile nutrients such as P. Mycorrhizal interactions are important for uptake of P and Zn, particularly under low-fertility conditions (Kucey and Paul, 1983). Severe early growth problems can occur due to P deficiency when corn is planted on fields that were fallowed the previous year (O'Halloran et al., 1986; Kucey and Paul, 1983). The presence of living plants is required for multiplication of the VAM; thus, fallowing reduces the incidence of mycorrhizae (Kucey and Paul, 1983) and may lead to lower root colonization in the next crop. Vivekanandan and Fixen (1991) reported that early dry matter production and P uptake were higher in a ridge planted corn–soybean rotation than in a moldboard plowed corn–fallow system, when no fertilizer P was added, and that early growth responses to P were inversely related to mycorrhizal colonization. Phosphorus fertilization tends to decrease mycorrhizal colonization (Clapperton et al., 1997). The type of preceding crop also has an effect on VAM dynamics and diversity. Johnson et al. (1991) reported that diversity of mycorrhizal population was higher on land that had grown corn than on land that had grown soybean. Where differences existed, total VAM spore counts and root colonization in soybean were higher when it was gown on land with corn history than with soybean history. Soil P also tended to be higher in plots with a soybean history compared with a corn history.

Availability of P may also be affected by residue type and management. Controlled-environment incubation studies demonstrated that sodium acetate–extractable P in the soil increased with increasing rates of residue applied and with increasing temperature and soil water potential (Li et al., 1990). Alfalfa residue provided the greatest amount of extractable P, with pea providing intermediate amounts and wheat providing the least. The concentration of P in the plant tissue was 0.30, 0.38, and 0.25% in the alfalfa, pea, and wheat residue, respectively, while the N concentrations were 2.50, 2.99, and 1.16%, respectively. The differences among the plant sources were likely due to differences in the initial P concentration and differences in the rates of decomposition of the materials as the high N concentration of the legume residues would hasten decomposition. Residues may also influence P availability by decreasing the precipitation of P as insoluble compounds in soil, desorbing the fixed soil P, or possibly blocking adsorption sites on the surface of soil colloids. While the type of residue is important, the overriding factor is the amount of residue applied. This implies that crops that return large amounts of residue to the system will lead to greater availability of P for subsequent crops. In studies conducted by Riedell et al. (1998), soils under a corn–soybean rotation had lower P levels than those under continuous corn. Removal of P would have been higher under the continuous corn system; therefore, the higher extractable P is not a reflection of differences in nutrient removal between the two systems. In contrast, in a soybean–corn rotation, Copeland et al. (1993) reported that soil test P decreased with more years of corn in the rotation, likely because of greater nutrient removal. But, they also reported that shoot P concentration was higher in first year of corn compared with monoculture corn, suggesting that there was a general improvement in corn nutrition by growing corn after soybean.

Another consideration is the amount of erosion that has occurred. Studies by O'Halloran et al. (1987) showed that changes in P in long-term studies at Sidney, NE, were mainly related to changes in sand content brought about by erosion and by mixing due to plowing. Inorganic and organic P are both present in the soil. Bringing sandier material to the surface does not change the total P content of the soil, but it does change the amount of P that is present in apatite-like materials. Therefore, changes in management practices that influence erosion can affect the amount and form of P present in a soil.

Biocycling of immobile nutrients through uptake from deep soil horizons by deep-rooted crops and the redeposition near the soil surface through decomposing plant residues could be important, particularly in cropping systems with minimal inputs of nutrients from external sources, minimal disturbance, or both. The amount of biocyling would increase with frequency of cropping and biomass production (Bowman and Halvorson, 1997).


   SPECIAL NUTRIENT CONSIDERATIONS
TOP
 ABSTRACT
 INTRODUCTION
 NITROGEN
 PHOSPHORUS
 SPECIAL NUTRIENT CONSIDERATIONS
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Moving to a diversified rotation may involve growing crops that differ in nutrient requirements from those traditionally grown by a producer. For example, the S requirement of canola is substantially greater than that of cereal crops (Grant and Bailey, 1993). Producers accustomed to growing cereal crops need to recognize the potential for yield loss due to S deficiency on soils with S levels that may be sufficient for optimum wheat production. Sulfur depletion by canola may hasten the occurrence of S deficiency in subsequent wheat crops on soils containing marginal levels of available S. Conversely, if canola is grown with adequate S to optimize crop yield, cereal crops following the canola may be adequately supplied with S on marginal soils by mineralization of S from the high S–containing canola residue. This could translate into higher grain protein content and/or quality for cereal crops.

Requirements for other nutrients may also differ among crops. For example, Cl responses may occur in spring and winter wheat, and frequency of response may vary with crop cultivars (Engel and Mathre, 1988; Engel et al., 1994; Lamond et al., 2000). Boron requirements of both canola and seed alfalfa are greater than those of cereal crops (Grant and Bailey, 1993; Grant and McCaughey, 1991). Mahler et al. (1985) indicated that cereal crops such as wheat have greater Cu requirements than pea, alfalfa, and red clover (Trifolium pratense L.). Further cereal roots deplete soil Cu to lower levels and to greater depths than do noncereal crops. Corn, beans, and flax tend to have higher Zn requirements than small-grain crops such as wheat or barley. Oat crops are particularly sensitive to Mn deficiency.

Even optimal placement of nutrients may vary with crop type. For example, canola tends to be more effective than flax at accessing P from fertilizer granules due to its ability to proliferate roots when it contacts a high P reaction zone (Strong and Soper, 1974). Small-seeded crops, such as canola and flax, tend to be more sensitive to damage from seed-placed fertilizers than are cereals (Nyborg and Hennig, 1969). Pulse crops, such as field pea, may also be sensitive to seed-placed fertilizer (Henry et al., 1995). Thus, rates of seed-placed fertilizer must be reduced for sensitive crops to avoid the risk of seedling damage and consequent loss in crop competitiveness, resulting in late maturity, lower yield, and reduced grade. To optimize crop yield and quality, the specific nutritional needs of each crop in the rotation must be carefully considered.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 NITROGEN
 PHOSPHORUS
 SPECIAL NUTRIENT CONSIDERATIONS
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Diversification and intensification of cropping systems on the northern Great Plains will have major effects on nutrient dynamics. Removal of nutrients will increase because of higher annualized crop yields. This increased removal must be balanced by increased nutrient inputs to ensure optimal crop yield and quality while avoiding soil depletion. Increased return of crop residue from greater annualized crop production can lead to higher soil organic matter levels and a greater potential for nutrient cycling, an effect that will increase with time. Cropping systems that include legumes have a greater potential for contributing N to following crops and may be used to moderate soil NO3 levels to avoid potential for NO3 leaching. Phosphorus availability may be influenced by depletion or accumulation of P, based on past cropping and fertilizer management. In addition, preceding crop may influence P availability through residue effects and impacts on VAM activity. Synchrony of nutrient supply with crop demand is essential to ensure optimum crop yield and quality while avoiding negative environmental impacts.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 NITROGEN
 PHOSPHORUS
 SPECIAL NUTRIENT CONSIDERATIONS
 SUMMARY AND CONCLUSIONS
 REFERENCES