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Published online 5 June 2006
Published in Agron J 98:1030-1040 (2006)
DOI: 10.2134/agronj2005.0277
© 2006 American Society of Agronomy
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Intercropping

Nitrogen Yield and Land Use Efficiency in Annual Sole Crops and Intercrops

Anthony R. Szumigalski* and Rene C. Van Acker

Department of Plant Science, Univ. of Manitoba, Winnipeg, MB, R3T 2N2, Canada

* Corresponding author (umszumig{at}cc.umanitoba.ca)

Received for publication September 30, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Nitrogen is the most limiting nutrient for crop production on the northern Great Plains of North America. This study was initiated to determine if N yield and land use efficiency for N could be improved by manipulating crop diversity using three annual crops (wheat, Triticum aestivum L.; canola, Brassica napus L.; and field pea, Pisum arvense L.) commonly grown on the Canadian Prairies. The study included all combinations of the crops (sole crops and intercrops) and compared their effects on soil N depletion, plant N concentration, plant N yield, and land equivalent ratios for dry matter and grain N yield (NLER) at two field sites in Manitoba, Canada. The pea sole crop treatment tended to result in higher fall soil nitrate (NO3–)–N concentrations compared to other treatments, indicating greater potential for post-season NO3– leaching after this treatment. There were often greater N concentrations in wheat, canola, and weeds when grown in association with field pea, suggesting that soil N could have been made available for nonlegume uptake through the NO3––N sparing effect. On average, most intercrop treatments resulted in more efficient land use for N compared to component sole crops, with overall mean intercrop NLER values ranging from 1.10 to 1.20. The wheat–canola–pea and canola–pea intercrop treatments tended to produce the highest and most consistent NLER values for crop dry matter and grain yield, respectively. The results of this study suggest that intercrops could be used for more efficient use of N on a per land area basis.

Abbreviations: ANOVA, analysis of variance • C, canola • CP, canola–pea • LER, land equivalent ratio • NLER, land equivalent ratio for nitrogen yield • P, pea • W, single density wheat • WC, wheat–canola • WCP, wheat–canola–pea • WP, wheat–pea • WW, double density wheat


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
NITROGEN is the most limiting nutrient for plant production in agroecosystems (Jarrell, 1990). Considering the relatively high cost of N fertilizer and environmental concerns [e.g., nitrate (NO3–) leaching] associated with excessive N application, increasing N use efficiency of cropping systems is of particular interest (Mohr et al., 1999; Przednowek, 2003). Intercropping (growing two or more different crops together on the same field) can result in greater than expected yields (i.e., overyielding) because of enhanced use of resources such as nutrients, and/or through suppression of crop pests (Willey, 1979; Vandermeer, 1989; Liebman and Dyck, 1993). Intercrops have been observed to compete more efficiently for soil nutrients than sole crops, thereby preempting weeds in the use of these resources (Abraham and Singh, 1984; Hauggaard-Nielsen et al., 2001). The apparent increase in resource use efficiency of intercrops suggests that these systems could be useful for adoption into low input or organic farming systems where options for chemical crop inputs are limited or nonexistent.

One of the most commonly used intercropping mixtures is the legume/nonlegume (usually cereal) combination. The legume can provide N benefits to the nonlegume directly through mycorrhizal links (e.g., Vankessel et al., 1985), root exudates, or decay of roots and nodules; or indirectly through a sparing effect, where the legume fixes atmospheric dinitrogen (N2), thereby reducing competition for soil NO3– with the nonlegume (Vandermeer, 1989; Anil et al., 1998). Another possible mechanism is that legumes can absorb large quantities of soil N which might otherwise have leached out of the system and supply it to the nonlegume companion crop later or to a subsequent crop (Vandermeer, 1989; Midmore, 1993); however, legumes tend to be poor competitors for soil nutrients compared to nonlegumes (Francis, 1989). Rooting patterns differ greatly between cereals and legumes (Anil et al., 1998), which could lead to more efficient exploration of the soil volume by cereal–legume mixtures. However, when fertilizer N is limited, biological nitrogen fixation is the major source of N in legume–cereal mixed cropping systems (Fujita et al., 1992), but the amount of N2 fixed by legumes generally declines with increasing soil N availability (Anil et al., 1998), and if legumes are continuously shaded their ability to fix N2 is further impaired (Willey, 1979).

Vandermeer (1989) summarized several intercropping experiments involving a legume and nonlegume component. In most cases, the crop mixture contained a greater quantity of N than did the monocultures (sole crops), indicating synergism in N use for the intercrops. Other studies confirm the N uptake and efficiency benefits of growing a legume with a nonlegume (Reynolds et al., 1994; Nissen et al., 1999; Lehmann et al., 1999). However, Chowdhury and Rosario (1993) found that N absorption by both maize (Zea mays L.) and mungbean [Vigna radiata (L.) Wilczek] was lower in intercrops compared with that in the sole crops, but they concluded that the N absorption efficiency of the intercrops was higher than that of the sole crops.

To determine the land use advantage in terms of N yield for intercrops compared to sole crops, a modification of the land equivalent ratio (LER), which shows the relative area under sole crops to achieve intercrop yields under the same conditions (Willey, 1979), can be employed. Thus, a LER based on N (Kwabiah, 2004) or protein yield (Chen et al., 2004) with a value exceeding unity indicates greater land utilization efficiency for intercropping compared to sole cropping with regard to N production. Conversely, a LER for N yield (NLER) that is less than one indicates that sole cropping would be more advantageous in terms of land utilization efficiency for N.

There have been very few studies on N dynamics in legume–oilseed mixtures (e.g., Waterer et al., 1994) or for more diverse three-crop mixtures, such as legume–cereal–oilseed intercrops (e.g., Andersen et al., 2004). For the northern Great Plains of North America, there has been very little work (e.g., Cowell et al., 1989) conducted on N dynamics for intercropping systems, in general. Furthermore, the land use efficiency benefits for N (protein) yield in intercrops are not known for this region. This is of economic importance to farmers, because N fertilizer represents one of the most expensive inputs, and grains and forage crops with greater protein concentrations are often of higher value. Environmental benefits (e.g., decreased NO3– leaching and denitrification rates) could also be realized with improved N land use efficiency.

The objective of this study was to investigate N yield and N land use efficiency in crop treatments composed of combinations of three different crops representing a cereal (spring wheat), oilseed (canola), and legume (field pea). These crops are commonly grown and adapted to production on the northern Great Plains of North America. The study includes all combinations of the three crops (sole crops and intercrops) and compares their use of N both in the presence and absence of in-crop herbicides (i.e., applied after crop emergence). This was accomplished by measuring soil inorganic N depletion, and crop and weed N yield. It is hypothesized that intercropping could lead to more efficient exploitation of the N resource, which is an important consideration in the development of low input or organic production systems.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Field Methods and Experimental Design
Experiments were conducted at two sites in Manitoba, Canada: JRI-Kelburn Research Farm (south of Winnipeg) and the University of Manitoba Carman Research Station. The trials were performed over 3 yr (2001–2003) at different locations within sites for each year. Site characteristics and climate data for the study are summarized in Tables 1 and 2, respectively. The soil at Carman was classified as a Typic Haplocryoll, while that at Kelburn was classified as a Boralfic Haploboroll. Experiments were designed as randomized-complete block/split block with six replicates at each site. The main plot treatment was crop treatment and included seven levels: spring wheat (‘BW755’ Clearfield, BASF, Toronto, Canada) sole crop (W), canola (‘46A76’ Clearfield) sole crop, field pea (‘DS-Stallworth’) sole crop, wheat–canola intercrop, wheat–pea intercrop, canola–pea intercrop, and wheat–canola–pea intercrop. The subplot (split block) treatments included the presence (sprayed) vs. the absence (unsprayed) of in-crop herbicide, with one-half of each block treated with the herbicide Odyssey (imazethapyr/imazamox, 1:1) at 30 g a.i. ha–1, to which all crops were tolerant. However, in 2002 and 2003 at Kelburn, the subplots that were sprayed with Odyssey were subsequently treated with the graminicide Horizon (clodinafop/propargyl) at 56 g a.i. ha–1 to further control large populations of grass weed seedlings. Plot size was 6 by 6 m; therefore, each subplot was 3 by 6 m. All treatments were initially planted at the same total plant density (144 seeds m–2) and all crops were planted in equal proportions within each treatment (e.g., the wheat–canola intercrop was sown at 72 wheat and 72 canola seeds m–2). In 2003, an extra sole crop treatment of wheat (WW) planted at a double density (288 seeds m–2) was added. Before crop emergence, weeds were controlled with a glyphosate application at 900 g a.e. ha–1 in all plots. Seeding of plots occurred in mid- to late May using a double-disc drill with 15-cm row spacing. Field pea inoculated with moistened peat-based Rhizobium were planted on a first pass to a depth of approximately 5 cm. Wheat and canola were planted together to a depth of approximately 2.5 cm on a second pass that was perpendicular to the direction of the first planting pass. In 2002 and 2003, background soil nutrient levels were judged to be adequate to achieve reasonable crop yields; however, in 2001, fertilizer (34–0–0) was broadcast onto all plots at a rate of 60 kg N ha–1 to compensate for low N levels (Table 1). See Szumigalski and Van Acker (2005) for more complete details on the field experimental design and treatments.


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  		Table 1. Comparison of physical and chemical properties in the top 15 cm (except for values in parentheses) of soil before crop emergence for the six site–years of the experiment at Manitoba, Canada.

 

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  		Table 2. Mean growing season air temperature and cumulative precipitation for six site–years compared to long-term values of field sites at Manitoba, Canada.

 
Sampling and Laboratory Procedures
Before crop emergence in the spring, soil from the main plots was sampled at two depths (0–15 and 15–60 cm) using a hand auger. These samples were pooled across all plots, mixed, subsampled, and then analyzed for major nutrients and characteristics (Table 1). Of the soil variables analyzed at spring sampling, only NO3––N and sulfate-S were determined for the 15 to 60 cm depth, because these tend to be more mobile than other nutrients (Vandermeer, 1989). In 2002 and 2003, but not in 2001, more detailed soil sampling was performed to determine inorganic N (NO3– and ammonium, NH4+) levels after crop harvest for the herbicide treated and untreated subplots separately. Soil samples consisted of a composite of three soil cores collected from each plot in the spring or from each subplot in the fall. After collection from the field, soil samples were immediately stored at 5°C (for <7 d) until they could be air-dried. Once air-dried, soil samples were placed in plastic bags and stored at room temperature until they were ground (to an aggregate size of <2 mm) using a rotating steel roller and sieve or soil pulverizer. Methods for soil chemical analyses followed those of Carter (1993). For determination of soil inorganic N, ground soil samples were extracted with 2 M KCl using a 5:1 extractant/dry soil equivalent ratio and extracts were analyzed by an autoanalyzer (Technicon Instruments Corp., Tarrytown, NY) for NH4+–N using a phenate colorimetric method and for NO3––N using a cadmium column reduction method. Phosphorus was determined by the stannous chloride method, K by the flame photometric method, sulfate-S by the turbidimetric method, pH by using a 1:2 soil/water ratio, and organic matter % by loss of ignition.

Aboveground weed and crop biomass samples were collected together before crop harvest in late July to early August in two 0.25 m2 quadrats randomly placed in each subplot. Biomass samples were separated by crop species and weeds, dried at 70°C for 48 h and massed. The weed and crop biomass data from the quadrats were pooled for each subplot before chemical and statistical analyses. For grain yield, crop plants were hand swathed at the time of crop maturity in late August within one 1.0 m2 quadrat per subplot. The swathed plants were collected into cloth bags and allowed to air-dry for several weeks before being threshed using stationary threshers. In 2001, most of the wheat grain samples from Carman were destroyed by mice (Mus musculus); therefore, wheat yield data from this site–year were discarded.

To determine N concentration (%N) of the aboveground crop (from both herbicide treatments) and weed (only from herbicide-free treatments) biomass samples, the oven-dried samples were ground to the extent that they could pass through a 2-mm sieve using a Wiley mill and the ground plant samples were analyzed for total N by combustion using a LECO Model FP-428 (LECO Lab Equipment Corp., St. Joseph, MI). The wheat and pea grain samples were analyzed for N by the same method, except that the grain samples were ground using blenders. The canola grain was analyzed for protein and oil concentration using a NIR-Systems 6500 Full Scanning Monochromator (FOSS North America, Eden Prairie, MN) and the protein content values were converted to %N values by dividing by 6.25. Total dry matter (biomass) N yield and grain N yield were calculated by multiplying dry matter and grain yield, respectively, by corresponding %N values. Total N yield values for each replicate were calculated by summing the N yield values for each component crop within a given crop treatment. Biomass N yield was not determined for 2001, while grain N yield was determined for all six site–years (except for wheat at Carman in 2001).

The N land equivalent ratio, which shows relative area under sole crops to achieve intercrop N yields under the same conditions, was calculated as follows for both crop dry matter N and grain yield N, based on the land equivalent ratio (Willey, 1979; Oyejola and Mead, 1982):

Formula 1[1]
where NLER is the N land equivalent ratio, I is intercrop N yield, S is sole crop N yield, and a, b and c refer to the component crops. A NLER value of more than one indicates greater land use efficiency for grain or biomass N production in the intercrop. For example, a NLER of 1.25 for a given intercrop indicates that it would require 25% more land to achieve the same N yield if sole crops were grown instead.

Statistical Analyses
Initially most data were subjected to analysis of variance (ANOVA) using a randomized split-block design (Gomez and Gomez, 1984) for each site–year separately, with crop treatment as the main plot treatment and herbicide use as the subplot treatment. Analyses were conducted using the PROC GLM procedure in SAS (SAS Institute, Cary, NC). Block (replicate) was considered to be a random effect in the models. If these analyses indicated significant (P < 0.05) herbicide treatment x crop treatment interactions, then in subsequent models data from the herbicide-treated and untreated plots were analyzed and presented separately within site–years. Log transformations were performed, where necessary, to improve the normality and homogeneity of variance of data (Gomez and Gomez, 1984). All significant (P < 0.05) ANOVA models were followed by a protected least squared difference (LSD) test for mean comparisons between treatments. In addition, to test the relationship between N and oil concentration for canola grain, linear regression models were employed for each site–year separately using PROC GLM in SAS.

The NLER values of the intercrop treatments were tested for significance following methods that Oyejola and Mead (1982) and Mohler and Liebman (1987) recommended for LER values. The NLERs were calculated separately for each intercrop replicate using the replicate biomass N or grain yield N values for the numerators and the mean sole crop values across all replicates for the denominators in Eq. [1]. The mean NLER values of the intercrop replicates were then compared to a NLER value of one using a one-tailed t test. Although this method tends to underestimate the true value of NLERs, because it eliminates the variation in the ratio which is due to variability in sole crop yields, it is preferable to using the individual sole crop biomass or yield values for each replicate (Oyejola and Mead, 1982; Vandermeer, 1989).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
Effects of Crop Treatment on Fall Soil Nitrogen
Crop treatment had no significant effect on soil NH4+–N concentration at either depth for any site–year (data not shown). For fall soil NO3––N concentration (mg kg–1), initial analyses conducted on data for each site–year indicated no significant crop treatment x herbicide treatment interaction for any site–year. Therefore, soil NO3––N data for the herbicide-treated and untreated subplots were pooled for subsequent analyses. Fall soil NO3––N levels were significantly lower in the absence of in-crop herbicides at three of four site–years, indicating additional uptake of soil NO3––N by weeds (data not shown). The exception was Kelburn–2002, where there was no difference between herbicide treatments for either depth.

Crop treatment produced a significant affect on fall soil NO3––N concentration for both depths at all site–years, except at Carman–2003 for the 15 to 60 cm (lower) depth (Fig. 1 ). Although the field pea sole crop treatment resulted in the highest overall mean concentration of fall NO3––N for both the surface (13.4 mg kg–1) and lower (5.9 mg kg–1) depths, only for the lower depth at Carman–2002 was the concentration significantly greater after the pea sole crop compared to all other crop treatments. For the 0- to 15-cm (surface) depth, the field pea sole crop was not significantly different in fall soil NO3––N concentration from the wheat–W sole crop or wheat–pea intercrop treatments at Carman–2002, the wheat–W sole crop treatment at Kelburn–2002, and the canola sole crop, wheat–pea intercrop, canola–pea intercrop and wheat–WW sole crop treatments at Carman–2003. For the lower depth, fall soil NO3––N concentration was not significantly different between the pea sole crop treatment and the canola sole crop or wheat–pea intercrop treatment at Kelburn–2002. For both depths at Kelburn–2003, only the canola sole crop treatment resulted in significantly lower fall soil NO3––N concentrations than did the pea sole crop treatment. Fall NO3––N concentrations were lowest at both depths after the canola sole crop treatment in two (Carman–2002 and Kelburn–2003) of the four site–years; however, only at Kelburn–2003 for the 15 to 60 cm depth were concentrations significantly lower in the canola sole crop treatment than all other treatments. For the surface depth at Kelburn–2003, the canola sole crop treatment did not differ significantly from the canola–pea intercrop treatment. For Carman–2002, fall NO3––N concentrations for the canola sole crop treatment were significantly lower than wheat sole crop, pea sole crop and wheat–pea intercrop treatments at the surface depth, and only lower than the latter two treatments at the 15 to 60 cm depth. With the exception of Kelburn–2003, fall NO3––N levels for the canola sole crop treatment did not differ significantly from those in any of the other crop treatments that included a canola component (i.e., wheat–canola, canola–pea and wheat–canola–pea). Similarly, fall NO3––N concentrations in the wheat-W sole crop treatment were not significantly different from those in any of the other crop treatments that included a wheat component (i.e., wheat–WW, wheat–canola, wheat–pea, and wheat–canola–pea), except at Kelburn–2003.


Figure 1
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  		Fig. 1. Fall soil nitrate N (NO3––N) concentration after harvest for different crop treatments (W = wheat, C = canola, P = pea, WW = double density wheat) at two depths over four site–years at Manitoba, Canada. Means from same series with same letter were not significantly different according to protected LSD(0.05). Note different scale for Kelburn in 2003. There were no WW treatments in 2002.

 
When averaged across site–years, the pea sole crop resulted in 25% more fall residual soil NO3––N than did the wheat–W sole crop treatment, and from 35% (lower depth) to 54% (surface depth) more than did the canola sole crop. The relatively high concentration of fall soil NO3––N following the pea sole crop compared to the canola sole crop is probably related to the N2–fixing abilities of the field pea crop, but could also be related to high N requirements for canola (Heenan, 1995; Hocking et al., 2002). In crop rotations, field pea can provide N benefits to subsequent crops from the mineralization of N from pea crop residues or from the N sparing effect where a legume crop can fix atmospheric N2, thereby, reducing competition for soil NO3– with a nonlegume crop (Vandermeer, 1989; Anil et al., 1998; Przednowek, 2003). In our study, the higher levels of soil NO3––N after the pea sole crop were more likely attributed to a sparing effect than to mineralization since it is unlikely that significant decomposition of crop residues had occurred before the soil-sampling period in early September. However, there is a possibility of root exudates or the decay of roots and nodules causing the release of N from legumes into the rhizosphere during the cropping season (Vandermeer, 1989). Legumes could also provide N benefits to the nonlegumes directly through mycorrhizal links (Vankessel et al., 1985). Therefore, it is possible for mycorrhizal transfers to occur between pea and wheat; however, this would be unlikely to occur with canola, since members of the Brassicaceae are not known to be mycorrhizal. For example, Waterer et al. (1994) found no evidence of N transfer between pea and mustard (Sinapsis alba L.) in pea–mustard intercrops. The relatively high levels of NO3––N after the pea sole crop treatment at most site–years (excepting Kelburn–2003), suggests that the potential for NO3– leaching during the noncropped season could be greater after field pea than after nonlegume crops. The relatively lower ranking of the pea sole crop for fall NO3––N levels at Kelburn–2003 may be attributed to higher levels of NO3––N observed at this site–year (Table 1, Fig. 1), which might have inhibited N2–fixation (Cowell et al., 1989; Waterer et al., 1994). Although initial NO3––N levels were also high at Kelburn–2002, final levels were about half of those at Kelburn–2003. In general, however, the inclusion of companion crops such as canola and/or wheat could reduce the leaching risk associated with pea because the nonlegume component may take up excess soil N that is being spared by the field pea crop. For a study conducted in Denmark, Hauggaard-Nielsen et al. (2003) found that NO3– leaching was greater for the first year following a pea sole crop than a barley (Hordeum vulgare L.) sole crop or a pea–barley intercrop. For our study, caution should be employed when interpreting the results for NO3––N remaining after the growing season, because amounts of soil NO3––N lost from leaching, immobilization, and denitrification, and gained from nitrification were not determined.

Effects of Crop Treatment on Crop Nitrogen
Crop Nitrogen Concentration
Initial analyses conducted for each site–year showed that there were no significant crop treatment x herbicide treatment interactions for crop dry matter N concentration (%N), and for grain N concentration this interaction was significant (P = 0.0439) only for field pea at Carman–2003. Herbicide treatment also had no significant effect on crop dry matter %N (data not shown). Accordingly, data were pooled over herbicide treatments.

For aboveground crop dry matter or grain %N, crop treatment had a significant effect on %N in wheat for three of four site–years, while for canola and field pea, crop treatment had a significant effect in two of four site–years for each (Table 3). Crop treatment produced no significant effect on crop dry matter %N at Kelburn–2002 for any crop. For wheat dry matter %N, in site–years where crop treatment did produce a significant effect, the wheat–canola–pea intercrop was always greater than the wheat sole crop treatments, the wheat–pea intercrop treatment was greater than the wheat–W sole crop at Carman–2002 and the wheat–WW sole crop treatment at Kelburn–2003, and the wheat–canola intercrop treatment was never greater than the wheat–W and only greater than the wheat–WW sole crop treatment at Kelburn–2003. At Kelburn–2003 there was significantly greater N concentration for wheat in the W sole crop treatment than in the WW sole crop treatment; however, at Carman in 2003 there was no difference between the two wheat sole crop treatments. For canola, only at Carman did crop treatment produce a significant effect on dry matter %N, where both the canola–pea and wheat–canola–pea intercrop treatments resulted in greater N levels than did the canola sole crop treatment, whereas the wheat–canola intercrop did not differ from the sole crop. Therefore, it is possible that more soil N was made available to wheat and canola growing with a field pea crop through the sparing effect for some site–years (mainly at Carman). For field pea, however, intercropping treatments tended to produce lower N concentrations than did the sole crop treatment at Carman, but not at Kelburn. At Carman–2002, %N was significantly higher in field pea for the sole crop treatment than for the wheat–pea intercrop treatment, while at Carman–2003, %N in pea was higher in the sole crop treatment than in any of the intercrop treatments. Increased competition for soil N may be responsible for the reduced N level in field pea within intercrops at Carman, since precropping levels of soil NO3––N (and all other macronutrients) were higher at Kelburn than at Carman (Table 1). These results suggest that nonlegume components in intercrops could gain N at the expense of a legume companion under conditions of lower soil N availability; however, this hypothesis needs to be tested in the same soil type with varying rates of N-fertilizer application before some conclusion can be made.


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  		Table 3. The effect of crop treatment (W = wheat, C = canola, P = pea, WW = double density wheat) on mean aboveground crop dry matter N concentration (%) before grain harvest in late July to early August over four site–years at Manitoba, Canada.

 
For grain N concentration, the trends did not always follow those for dry matter N concentration. Crop treatment had a significant effect on wheat grain %N in four of five site–years, on canola grain %N in four of six site–years, and on pea grain %N in three of six site–years (Table 4). The trends for wheat grain %N were similar to those for wheat dry matter %N (Table 3) in that there was a significant crop treatment effect on wheat grain %N at all site–years except at Kelburn–2002. For three of the site–years (Kelburn–2001, Carman–2002, and Carman–2003), the wheat–pea and wheat–canola–pea intercrop treatments produced significantly greater wheat grain N concentrations than did the wheat–W sole crop treatment; however, at Carman–2003, neither intercrop treatment differed significantly from the wheat–WW sole crop treatment. At Kelburn–2003, the only difference was that wheat grain %N was significantly lower in the wheat–WW sole crop treatment compared to all other treatments. For canola grain %N, crop treatment had a significant effect in 2001 and 2002, but not in 2003. At both sites in 2001 and at Carman–2002, the canola–pea and wheat–canola–pea intercrop treatments resulted in significantly higher canola grain %N than did the canola sole crop treatment; however, at Kelburn–2002, the sole crop treatment produced significantly greater %N than did the wheat–canola–pea intercrop treatment. The wheat–canola intercrop never differed significantly in canola grain %N from the canola sole crop treatment, except for Carman–2001 where the wheat–canola treatment was greater than the sole crop treatment. For pea grain %N, there was a significant crop treatment effect for all years at Kelburn, but not at Carman, which was opposite to the trend for pea dry matter %N, where crop treatment had a significant effect only at Carman (Table 3). The reason for these differences is not known, but could be related to altered resource allocation patterns in field pea due to interspecific competition (Harper, 1977).


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  		Table 4. The effect of crop treatment (W = wheat, C = canola, P = pea, WW = double density wheat) on crop grain N concentration (%) and canola grain oil concentration over six site–years at Manitoba, Canada.

 
Intercropping legumes with a cereal has been shown to increase the N content of cereal grains in other studies (Reynolds et al., 1994; Bulson et al., 1997). Besides the sparing effect or any direct N transfers, the increase in cereal grain N could be a result of lower cereal grain yields because of competition with the legume, resulting in increased cereal protein content through lower yields (i.e., the inverse of the dilution effect of yield on protein content; Fowler et al., 1990). Similarly, the lower wheat grain %N for the high density WW vs. the low density W sole crop treatment at Kelburn–2003 (Table 4) could be related to the dilution effect of grain protein in the higher density treatment. However, competition with weeds in the absence of in-crop herbicides produced no effects on grain N concentration for any of the crops compared to treatments in the presence of in-crop herbicides (data not shown).

Crop treatment had a significant effect on canola seed oil concentration in four of six site–years (Table 4). Although the canola sole crop treatment produced significantly greater oil concentration than canola within all intercrop treatments at Carman–2001, it did not differ from any intercrop treatment at three of six site–years. The canola sole crop exhibited significantly greater oil content than the wheat–canola–pea intercrop treatment for three site–years, than the canola–pea intercrop treatment for two site–years, and than the wheat–canola treatment for only one site–year. Overall, there was a significant inverse linear relationship between canola seed oil concentration and %N at all six site–years. Although this relationship was fairly weak at Carman in 2002 (r2 = 0.14) it was strong for the other site–years (generally r2 ≥ 0.9). Taylor et al. (1991), Rathke et al. (2005) and others have also reported an inverse relationship between B. napus seed oil concentration and N concentration. The physiological explanation for this negative correlation, commonly found in oilseeds, can be related to the competition for C skeletons during carbohydrate metabolism (Bhatia and Rabson, 1976). Because the carbohydrate content of proteins is lower than that of oils, increased N availability amplifies protein synthesis at the expense of fatty acid synthesis, thereby reducing the oil content of the seed (Bhatia and Rabson, 1976; Rathke et al., 2005). Our results suggest that there may be tradeoffs between protein and oil concentrations when intercropping oilseeds with legumes, but not under all conditions.

Crop Nitrogen Yield
Preliminary analyses for crop dry matter N yield indicated that the crop treatment x herbicide treatment interaction was not significant for both sites in 2002; however, there were significant interactions for both sites in 2003. Crop dry matter N yield (i.e., N uptake) was significantly greater in the presence of in-crop herbicide applications at all site–years, except Carman–2002, reflecting greater crop biomass when weed control was applied (data not shown). For grain N yield, there was a significant crop x herbicide treatment interaction only for the Kelburn–2003 site–year. Thus, the data were analyzed and presented accordingly with data for herbicide treatments pooled for the site–years where there was not a significant interaction between crop and herbicide treatments.

Crop treatment had a significant effect on crop dry matter (biomass) N yield at all four site–years; however, at Carman–2003 crop treatment was only significant in the presence of in-crop herbicides, and at Kelburn–2003, only in the absence of in-crop herbicides (Fig. 2 ). Crop treatment effect on biomass N yield varied considerably between years, and in 2003, between herbicide treatments. At Carman–2002, the pea sole crop treatment produced the greatest, while the canola sole crop treatment produced the lowest, crop dry matter N yield. At Kelburn–2002, an opposite trend was observed, whereby the greatest biomass N yield occurred in the canola sole crop treatment, and lowest occurred in the wheat–W and pea sole crops and the wheat–pea intercrop; however, the canola sole crop did not differ significantly from any other crop treatment with a canola component. In the absence of herbicides at Kelburn–2003, the greatest biomass N yield occurred in the wheat sole crop (W and WW) and wheat–canola intercrop treatments (Fig. 2). At Carman–2003, in the presence of herbicides, the greatest dry matter N yield occurred in the crop treatments that included pea. For a similar tri-component intercropping study conducted in Denmark using field pea, barley, and rape (B. napus), Andersen et al. (2004) reported that the pea sole crop consistently accumulated greater levels of dry matter N compared to all other sole or intercrop treatments. The great variability among site–years in dry matter N yield for crop treatments in our study can be related to environmental differences among site–years (i.e., differences in soil fertility, precipitation, soil temperature, soil texture, weed competition, and their interactions). Despite this variability, the most diverse crop treatment (wheat–canola–pea) tended to maintain a relatively moderate to high level of biomass N yield at all site–years compared to less diverse treatments (especially the sole crop treatments).


Figure 2
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  		Fig. 2. Effect of crop treatment (W = wheat, C = canola, P = pea, WW = double density wheat) on crop dry matter N yield over four site–years at Manitoba, Canada. Means from same series with same letter were not significantly different according to protected LSD(0.05). In 2002, in-crop herbicide treated and untreated subplots were pooled due to nonsignificant crop treatment x herbicide treatment interactions. There were no WW treatments in 2002.

 
Crop treatment had a significant effect on grain N yield at all site–years, except at Kelburn–2002 (Table 5). Grain N yield was higher in the pea sole crop treatment than in any other crop treatment at four of six site–years; however, at Kelburn–2003, only in the presence of herbicide applications did the pea sole crop produce the highest grain N yield. At Kelburn–2003, in the absence of herbicides, the wheat sole crops resulted in highest, while the pea sole crop produced among the lowest grain N yields. The large difference in response of the pea sole crop between herbicide treatments at Kelburn–2003 was related to poor survival and growth of pea because of the large weed infestation and intense competition in the herbicide-free treatments (Szumigalski and Van Acker, 2005). At Kelburn–2001, the pea sole crop was only significantly greater than the wheat–W sole crop and grain N yield was highest for all crop treatments with a field pea component and for the canola sole crop. Grain N yield was significantly lower in the canola and wheat sole crop treatments compared to the pea sole crop treatment at most of the site–years. Similarly, Andersen et al. (2004) reported that field pea sole crops produced higher grain N yields than all other sole or intercrop treatments for an experiment using pea, barley, and rape as component crops. In our experiment, the difference in grain N yield observed between sole crop treatments could be attributed to differences in seed size and grain yield between crops (Szumigalski and Van Acker, 2005), in addition to differences in N concentration (Table 4).


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  		Table 5. The effect of crop treatment (W = wheat, C = canola, P = pea, WW = double density wheat) on total crop grain N yield over six site–years at Manitoba, Canada.

 
Land Equivalent Ratios for Nitrogen Yield
The land equivalent ratio for crop dry matter N yield (biomass NLER) was significantly greater than one (i.e., significant N overyielding) for about one-third (12/32) of the site–year–treatment combinations (Table 6). In the presence of in-crop herbicides, the wheat–canola–pea and canola–pea intercrop treatments resulted in the greatest mean biomass NLER values (1.23 and 1.13, respectively) and significant N overyielding occurred at two out of four site–years for each. However, significant biomass N underyielding (NLER < 1.0) also occurred for the canola–pea treatment in the presence of herbicides at one site–year (Kelburn–2003). For the wheat–pea intercrop treatment, significant biomass N overyielding only occurred at one site–year (Carman–2003), while for the wheat–canola treatment there were no instances of significant overyielding with herbicide applications.


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  		Table 6. Nitrogen land equivalent ratios (NLER) based on crop dry matter or grain yield for different intercrop treatments (W = wheat, C = canola, P = pea) in the presence and absence of in-crop herbicide over six site–years at Manitoba, Canada.

 
In the absence of in-crop herbicides, the highest mean biomass NLER values were observed for the wheat–canola–pea (1.36) and wheat–canola (1.33) intercrop treatments. However, with the exception of the wheat–canola treatment, the biomass NLER values of the intercrop treatments did not differ greatly between the herbicide treated and untreated subplots (Table 6). There was an increase of 0.27 for the mean wheat–canola treatment biomass NLER in the absence vs. the presence of in-crop herbicides, and at two of four site–years significant biomass N overyielding occurred for this intercrop treatment when herbicides were not applied compared to no instances of overyielding when herbicides were applied. The improved NLER values for the wheat–canola intercrop in the absence of herbicides may be attributed to relatively greater weed suppression observed in this treatment compared to that of its component sole crops (Szumigalski and Van Acker, 2005). Without herbicide applications, significant biomass N overyielding occurred in three of four site–years for the wheat–canola–pea treatment and in only one of four site–years for the canola–pea treatment. Significant biomass N overyielding also occurred at only one site–year for the wheat–pea intercrop treatment (Carman–2003), whereas significant underyielding occurred at Kelburn–2003 for this treatment, which produced the lowest mean biomass NLER (0.98) of all intercrop treatments when herbicides were not applied. The wheat–pea intercrop treatment was generally found to be less competitive against weeds compared to the other intercrop treatments (Szumigalski and Van Acker, 2005).

Significant grain N overyielding occurred in about one-quarter (11/42) of the site–year–treatment combinations (Table 6). A greater proportion of significant grain N overyielding occurred in the presence (8/21) vs. the absence (3/21) of in-crop herbicides. In the presence of in-crop herbicides, the canola–pea intercrop treatment produced the greatest mean grain NLER (1.21) and significant overyielding occurred at four of six site–years. In the presence of herbicides, significant grain N overyielding occurred only at one site–year each for the wheat–canola (Kelburn–2003) and wheat–canola–pea (Kelburn–2001) intercrop treatments. For the wheat–pea treatment, when in-crop herbicides were applied, significant grain N overyielding occurred at two site–years, while significant N underyielding occurred at two other site–years.

The mean grain yield NLER values of the intercrop treatments did not differ greatly between the herbicide treated and untreated subplots, with the exception of the wheat–pea treatment, where the mean NLER value was reduced by 0.19 in the absence of in-crop herbicides (Table 6). The wheat–pea treatment resulted in the lowest mean grain yield NLER (0.91) and significantly underyielded at Kelburn–2003. In the absence of herbicides, the wheat–canola–pea intercrop treatment produced a mean grain yield NLER value of 1.20; however, overyielding was significant only at one site–year (Carman–2002). The canola–pea intercrop treatment resulted in a mean grain yield NLER value of 1.16 and overyielded significantly at two site–years in the herbicide-free treatments. The wheat–pea and wheat–canola intercrop treatments did not produce a grain yield NLER significantly greater than one at any site–year in the absence of herbicides.

At Kelburn–2003, very high dry matter and grain yield NLER values (1.60–1.77) observed for the wheat–canola and wheat–canola–pea intercrop treatments in the absence of herbicides did not translate into statistically significant N overyielding. This lack of significance was likely a result of the high variability in yield across plots due to the extreme patchiness of weeds at this site–year. However, at this same site–year, significant biomass and grain underyielding for the wheat–pea intecrop treatment was achieved with NLER values of 0.67 and 0.60, respectively. This attainment of significance could be related to the higher yield stability observed in the wheat–pea compared to the wheat–canola and wheat–canola–pea intercrop treatments in the absence of herbicides at this site–year (Szumigalski, unpublished data, 2005).

The NLER values largely reflected corresponding LER values from Szumigalski and Van Acker (2005); however, the biomass NLER values tended to be slightly greater than corresponding biomass LER values, especially in the presence of in-crop herbicides. For example, when in-crop herbicides were applied, the wheat–canola–pea intercrop treatment had greater biomass NLER values (mean of 1.23) than biomass LER values (mean of 1.08) at all four site–years and produced significant biomass N overyielding at Carman–2002, whereas there was no significant biomass overyielding for the wheat–canola–pea treatment at this site–year (Szumigalski and Van Acker, 2005). A higher NLER value compared to that for LER is indication that N may have been better exploited by intercropping (Willey, 1979). The grain yield NLER values, however, tended to be quite similar to corresponding grain yield LER values for each site–year. As for the LERs (Szumigalski and Van Acker, 2005), the biomass NLERs did not consistently reflect the grain yield NLERs for a particular site–year. This lack of congruence could be related to differential resource allocation patterns (e.g., leaf vs. seed accumulation) in the intercrop treatments. For example, plants might devote a greater proportion of energy to support structures in a competitive environment and their reproductive output could be influenced by inter- or intraspecific density stress (Harper, 1977).

These results indicate that, on average, most intercrop treatments resulted in more efficient use of land area for N (protein) production compared to component sole crops. Overall mean NLER values indicate that intercrop treatments overyielded by 10 to 20%, depending on weed control and whether dry matter (i.e., forage) or grain N yield was of interest. The most diverse intercrop treatment (wheat–canola–pea) generally produced the greatest dry matter N overyielding, resulting in mean overyielding of 23 and 36% in the presence and absence of herbicides, respectively. For grain N yield, however, the canola–pea intercrop was generally the most efficient and consistent intercrop treatment, with mean N overyielding of 21 and 16% in the presence and absence of herbicides, respectively. Andersen et al. (2004) also reported relatively strong overyielding for barley–rape–pea and rape–pea intercrops under different levels of soil N, but found that, under conditions of low N fertilization, the relative amount of N2 fixed from the atmosphere by pea grown with rape was higher compared to that fixed by pea in other crop treatments. A lower relative amount of N2 fixed by pea in association with barley was attributed to the greater competitiveness of barley compared to rape; thereby reducing the effectiveness of pea in fixing N2 and resulting in decreased overyielding for barley–pea intercrops (Andersen et al., 2004). In another study, Chen et al. (2004) reported LER values for dry matter protein yield from 1.05 to 1.26 for barley–pea intercrops and stated that high N rates could reduce the overyielding potential of this intercrop. In our study, the NLER for grain and dry matter yield of the wheat–pea intercrop treatment tended to be inconsistent between different environments, but treatment results were more consistent when in-crop herbicides were applied. The overall poor response for the wheat–pea intercrop treatment in the absence of herbicides could be related to the effects of weed competition, since generally greater weed biomass values were observed in this treatment compared to in other intercrop treatments (Szumigalski and Van Acker, 2005). The NLER values greater than one observed for the wheat–canola intercrop treatment in some environments demonstrated that improved land use efficiency for N cannot always be attributed to the presence of a legume component in intercrops. Andersen et al. (2004) showed that barley–rape intercrops could also produce overyielding, and suggested temporal complementarity between the crops with respect to N uptake as a mechanism.

Effects of Crop Treatment on Weed Nitrogen
In the absence of in-crop herbicides, crop treatment had a significant effect on weed N concentration (%N) in two of four site–years (Carman–2002 and Kelburn–2003) (Table 7). At Carman–2002, the pea, canola–pea and wheat–W crop treatments resulted in the highest %N levels in weeds compared to the other treatments. At Kelburn–2003, all crop treatments with a pea component (except wheat–canola–pea) and the wheat–canola intercrop treatment produced the highest, whereas the wheat and canola sole crops produced the lowest %N levels in weeds. Higher weed N concentration in the presence of field pea observed for some site–years was not surprising given the tendency for greater soil NO3––N to occur in the pea sole crop treatment, thereby providing an N source available for uptake by weeds (Fig. 1). Similarly, Weik et al. (2002) suggested that weeds could benefit from the N2–fixing properties of lupin (Lupinus polyphyllus L.), a perennial legume.


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  		Table 7. The effect of crop treatment (W = wheat, C = canola, P = pea, WW = double density wheat) on mean aboveground weed dry matter N concentration (%N) and accumulation (kg N ha–1) before grain harvest in late July to early August in the absence of in-crop herbicide over four site–years at Manitoba, Canada.

 
Crop treatment had a significant effect on weed biomass N uptake in the absence of herbicides at both sites in 2003, but not in 2002 (Table 7). At Carman–2003, the greatest weed N uptake was observed for the pea sole crop and wheat–pea and canola–pea intercrop treatments, and at Kelburn–2003, for the pea sole crop and canola–pea intercrop treatments. The crop treatments lacking a pea component resulted in the lowest weed N uptake at both sites in 2003; except that the canola sole crop resulted in higher weed N uptake than the wheat sole crops and wheat–canola intercrop at Kelburn for that year. At Kelburn–2003, there seemed to be a correlation between weed %N and total N uptake, whereas at Carman–2003, there was no evidence of such a correlation. The high concentration and uptake of N by weeds in the pea sole crop observed at Kelburn–2003 were probably related more to effective competition for soil N by weeds than by a NO3––N sparing effect produced by the legume. Very low levels of pea biomass in the absence of herbicides (Fig. 2) and a lack of evidence for excess soil NO3––N after the pea sole (Fig. 1) at this site–year indicated that N2–fixation rates would have been minimal for this treatment. In general, weed N uptake seemed to be more strongly related to weed biomass (Szumigalski and Van Acker, 2005) than weed %N. Weed community species composition could have influenced weed N uptake levels (Blackshaw et al., 2003), but this effect was not tested in our study.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
The results of this field study indicated that N use differs between annual crop treatments and may be related to crop diversity. In intercrops, the utilization of different chemical forms of N (N2 vs. NO3–) by companion plants could provide a means of N use complementarity. The presence of field pea in crop treatments reduced unit area demands for soil NO3–, thereby freeing up this resource for nonlegume crop uptake, and possibly for weed uptake as well. The potential for post crop NO3– leaching in field pea crops can, thus, be reduced when a nonlegume companion crop is included with pea. In some, but not all environments, there were greater N concentrations in wheat, canola, and weeds in the presence of pea, suggesting that background soil N levels and other environmental factors were important in determining whether legumes provide N benefits to companion plants. Nitrogen resource partitioning could contribute to the generally greater land use efficiency for N of intercrops compared to component sole crops. The most diverse crop treatment (wheat–canola–pea) tended to have the greatest land use efficiency for N on a whole plant basis, while the canola–pea intercrop treatment tended to be the most efficient in terms of grain N yield. Nitrogen overyielding observed for the wheat–canola intercrop treatment in some instances demonstrated that improved land use efficiency for N cannot always be attributed to the presence of a legume component in intercrops. The results of this study suggest that intercrops could be used for more efficient land use of N, thereby providing both economic and environmental benefits. However, additional research is required on mechanized harvesting, rotational effects, and the effects of varying rates of N fertilization for intercropping systems before the scope of these benefits can be fully determined.


   ACKNOWLEDGMENTS
 
Funding for this project was provided by Agriculture and Agri-Food Canada through the Manitoba Rural Adaptation Council, and graduate fellowships for A. Szumgalski from The University of Manitoba, Winnipeg, Canada and The Land Institute, Salina, KS. The authors would like to thank Rufus Oree for technical assistance, Lyle Friesen for statistical assistance, and Dr. Don Flaten for advice and providing lab facilities for the soil N analyses.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 




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