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Published in Soil Sci. Soc. Am. J. 68:552-557 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Plant Competition Effects on the Nitrogen Economy of Field Pea and the Subsequent Crop

Y. K. Soon*,a, K. N. Harkerb and G. W. Claytonb

a Agriculture & Agri-Food Canada, P.O. Box 29, Beaverlodge, AB, T0H 0C0, Canada
b Agriculture & Agri-Food Canada, 6000 C & E Trail, Lacombe, AB, T4L 1W1, Canada

* Corresponding author (soony{at}agr.gc.ca).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We evaluated weed competition effects on the N economy of field pea (Pisum sativum L.) and the subsequent crop to address the paucity of such information. Plots were seeded to pea, canola (Brassica napus L.) and barley (Hordeum vulgare L.) in 1997 and 1998. Weeds, augmented by cross-seeding experimental plots with oat (Avena sativa L.), were removed with herbicides one and four weeks after crop emergence (WAE). The subsequent barley crop received 0 or 6 g N m–2. Mean percentage of N derived from the atmosphere (%Ndfa) for the 2 yr, estimated by 15N isotopic dilution, was 81% for the 4-WAE treatment and 51% for the 1-WAE treatment, indicating that a pea plant subjected to greater weed competition derived more of its N from symbiotic fixation. Total N fixed by pea was not affected by the time of weed removal, however, and total N uptake and seed yield were greater with early weed removal due to less competition for soil N. Early weed removal resulted in net N export in pea seeds (because of higher production) while later weed removal resulted in gains of 1.1 to 1.3 g N m–2. However, time of weed removal during pea cultivation had no effect on the yield or N uptake of the subsequent barley crop. Higher barley yield and N uptake following pea than following barley were mostly the result of greater N availability. Nitrogen fertilization benefited the subsequent barley regardless of preceding crop type.

Abbreviations: DM, dry matter • %Ndfa, percentage of N derived from the atmosphere • WAE, weeks after emergence


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
FIELD PEA IS AN important rotation crop in many parts of the world, including western Canada. The rotational and nitrogen benefits to the following crop are well-documented (Stevenson and van Kessel, 1996a, 1996b), and can be a strong incentive for growing peas in rotations. Peas are generally less competitive with weeds and sustain higher yield loss than barley or canola (Harker, 2001); however, early removal of weeds can increase pea yields considerably (Harker et al., 2001). This is partly because legume seedlings require N from the growth medium within 10 d of germination to achieve early vigor (Kriegel, 1967; McWilliam et al., 1970). There is a strong correlation between the quantity of N fixed and the soil N balance, that is, the difference between fixed N and N harvested in the legume grain (Evans et al., 2001). Since dry matter (DM) production and the amount of N fixed by legumes are well-correlated (Armstrong et al., 1994), weed competition, by affecting crop growth, may influence symbiotic N2 fixation in peas and the N balance. Keatinge et al. (1988) reported that weed removal by hand increased the total N uptake and the amount of N fixed by several legumes. However, we are not aware of reports on effects of timing weed removal with herbicides on N2 fixation by field pea and N availability to the sequent crop. The bulk of the data on interspecific plant competition comes from intercropping studies which indicate that symbiotic N2 fixation by legumes can be affected by plant competition for resources. Intercropped legumes tend to have a lower total plant N content and higher %Ndfa than pure legume stands, although the differences were not always significant (Eaglesham et al., 1981; Ofori et al., 1987; Colwell et al., 1989). The quantity of N fixed by pulse crops under intercropping can also be lower than that fixed under pure stands (Danso et al., 1987; Ofori et al., 1987; Jensen, 1996a).

The study of weed competition on N2 fixation by legumes is of practical significance because of its possible impact on legume grain production, soil N balance and subsequent crop yields. Therefore, as part of a larger experiment to evaluate the rotational benefits of field pea to subsequent nonlegume crops, we conducted a study to determine whether the timing of weed removal had a measurable effect on the N nutrition and symbiotic N2 fixation of field pea, and the effect this would have on the production and N nutrition of the subsequent crop.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was located 8 km north of the Agriculture and Agri-Food Canada research farm in Beaverlodge, Alberta (55°20' N, 119°29' W). The soil is a fine montmorillonitic, frigid, Typic Cryoboralf (an Orthic Gray Luvisol in the Canadian soil classification) with a pH (in 0.01 M CaCl2) of 4.7 to 5.2, an organic C content of 21 to 25 g kg–1, and a clay loam texture in the surface 15 cm. Moisture deficit is usually the growth-limiting factor for this region.

Two experiments, which were merely replications in time, were performed in 1997 to 1999, each running for 2 yr. Experimental treatments were factorial combinations of the three crop sequences, two fertilizer N rates (nominally 0 and 6 g N m–2 as urea), and two dates of weed removal which were imposed only in the first year. Barley, canola, and field pea were grown and subjected to the various treatments in the first year of experimentation, and all plots were seeded to barley in the second year. Pea was inoculated with a granular inoculant, received no N fertilizer other than 16.8 g m–2 of 4-17-35-11 blended fertilizer that was banded 2.5 cm below and beside the seed row into all plots at seeding, and had the same number of plots as barley or canola. The barley and canola grown on the zero-N plots served as the reference crop for estimating N2 fixation by pea. Each treatment combination was replicated four times in a randomized complete block design. Each plot was 4 by 25 m. The experiments were managed under a no-till system. Unconfined microplots were set up in each zero-N plot for estimating N2 fixation by field pea by the 15N isotopic dilution method. Each microplot was five crop rows wide (0.23 m between rows) and 1.2 m long. On seedling emergence, microplots received the equivalent of 0.71 g N m–2 as (NH4)2SO4 labeled with 10 atom% 15N. The 15N solution was applied with a pressurized sprayer on the soil surface at a rate of 1 L per microplot, which was subsequently irrigated with 2 L of water.

In the spring, the experimental plot area was given a preseeding treatment of glyphosate [N-(phosphonomethyl)glycine] at 90 mg a.i. m–2. Plots were seeded in early May each year: barley (‘Falcon’) at 12.3 g m–2, canola (‘Quest’) at 0.8 g m–2, and field pea (‘Swing’) at 22 g m–2. Oat (‘Grizzly’) was seeded acrossall plots at 7.6 g m–2 to supplement the natural weed infestation. The two levels of weed competition were attained by applying herbicides 1 or 4 WAE. Herbicides used were: 4.1 mg a.i. m–2 of a mixture of imazamox {2-[4,5-dihydro-4-methyl-4-(1-methyl-ethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridine-carboxylic acid} and imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridine-carboxylic acid} on pea plots; 45 mg a.i. m–2 of glyphosateon canola (glyphosate-resistant) plots; and 20 mg a.i. m–2of tralkoxydim {2-[1-(ethoxyimino)propyl]-3-hydroxy-5-(2,4,6-trimethylphenyl)cyclohex-2-enone} + 28 mg a.i. m–2 of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) + 28 mg a.i. m–2 of MCPA [4-chloro-2-methyl-(phenoxy) acetic acid] ester on barley plots.

Weed infestation at 4 WAE was assessed by the number and dry weight of plants per square meter. In the first year, all crops and weeds were sampled for determination of DM and N content at (i) flag-leaf of barley, (ii) podfill of pea, and (iii) maturity. Wet plot areas and inaccessibility delayed sampling in 1997. Plants were cut approximately 2 cm above ground level from four 1-m rows, with the exception that at maturity, samples from zero-N plots were taken from the three center rows of the microplots. Weeds were separated from crop samples. Subsamples were taken for determination of moisture and N content. The bulk of the 15N-labeled straw was returned as soon as possible to the microplots that they originated from, and were held in place against wind blow with wire netting until seeding the following spring.

In the second year of each experiment, barley was seeded to all plots and fertilized with 0 or 6 g m–2 of urea-N. Half of the plots previously growing field pea (and thus received no fertilizer N in Year 1) now received 6 g N m–2. Barley yield and total N uptake at mid-dough and maturity were determined.

Soil to 1-m depth was sampled before seeding and following harvest, and sectioned into depth increments of 0 to 20, 20 to 50, and 50 to 100 cm. Field-moist soil (equivalent to approximately 6 g of dry weight) was extracted with 30 mL of 0.33 M K2SO4, and the extract analyzed for NH4– and NO3–N by autoanalyzer techniques. Plant subsamples were dried at 65°C and ground with a Wiley mill (Model 3383-L10, Thomas Scientific, Swedesboro, NJ) for N analysis using a LECO N-analyzer (Model FP428, Leco Corp., St. Joseph, MI). Plant subsamples from microplots were subjected to further grinding by ball mill (Model D-42781, Retch GmbH & Co. KG, Haan, Germany) and analyzed for 15N/14N ratio as well as total N content. The 15N/14N analysis was performed by dual inlet, continuous flow (Dumas combustion) isotope ratio mass spectrometry at the Stable Isotope Facility of the University of California at Davis. Atom% 15N excess was calculated relative to atmospheric enrichment of 0.3663. Nitrogen in pea derived from the atmosphere (%Ndfa) was calculated using the atom% 15N excess of plant shoots as follows:

Shoot atom% 15N excess was calculated as the weighted average of grain and straw materials. All data were analyzed using general linear models in the SAS system (SAS Institute, 1990). Treatment effects and interactions were considered significant at P ≤ 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Crop and Weed Growth and Nitrogen Content in the First Year
Soil NO3–N before sowing in 1997 and 1998 was between 2.5 to 3.0 g m–2 in the top 50 cm and 0.5 to 0.9 g m–2 in the 50- to 100-cm depth (data not shown). There was no measurable difference in soil NO3 at harvest due to crop types or time of weed removal treatments (data not shown). Nitrogen fertilizer applied at the higher rate increased crop production and N uptake of the nonlegume crops. These effects have been well-established in the literature, and no further discussion will be necessary here since the results were similar. There was no N rate x weed removal interaction on crop growth and N uptake in 1997. In 1998, the interaction was absent in the mature crops; however, at the two earlier samplings, crop DM was significantly higher at 6 than at 0 g N m–2 with early weed removal, whereas N fertilizer rate had no effect with later weed removal (data not shown).

Aboveground DM of the three crops at podfill of pea are typical (Table 1) of the three sampling dates; therefore, the other sampling dates are not shown. In 1997, the 1-WAE treatment resulted in higher pea DM accumulation than the 4-WAE treatment, while the converse applied to canola growth. Early removal of weeds also benefited barley growth, although the difference was not significant. Crop yields followed a similar pattern as shoot DM in 1997 (Table 2). In 1998 shoot DM (Table 1) and yields of all crops (Table 2) were significantly higher with early weed removal. At 4-WAE, the day of the second weed removal treatment, weed (oat) DM was six times higher than that where weeds had been removed 3 wk earlier in 1997 and 10 times higher in 1998 (Table 1). Broad-leaved weeds showed similar trends and were present at lower densities (data not shown). Weed DM at crop maturity in 1997 indicated that continued cool and wet weather through July favored late emerging weeds under canola and may partly account for the different results for canola growth that year (Table 2). Wet soil and the associated cool temperatures, because of rainfall nearly twice the amount of normal precipitation in July, were probably the main reasons for the low yields of barley and canola in 1997 (Table 2). June and July of 1998 had below-normal amounts of rainfall; however, because of adequate soil moisture stored during the previous winter, the crops likely suffered only a short period of moisture stress, and yields were only slightly below average.


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  		Table 1. Dry matter of crops at podfill of pea, and weed (oat) at four weeks after crop emergence (WAE) in 1997 and 1998, as influenced by time of weed removal.{dagger}

 

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  		Table 2. Effect of time of weed removal on grain yield and weed dry weight at crop maturity in 1997 and 1998.{dagger}

 
The dynamics of N uptake in the aboveground crop biomass, as affected by time of weed removal, are shown in Fig. 1 . Early weed removal increased N uptake by pea. In 1997, pea N uptake was essentially flat between the flag-leaf and podfill samplings and sharply increased at maturity. The flag-leaf sampling in 1997 was late (at approximately 10 wk after seeding) because the plots were too wet then to be accessible. The barley was starting to head when sampled. The podfill sampling was only 2 wk later, and the maturity sampling another 4 wk later. Nitrogen uptake in 1997 was low throughout the entire growth period for barley and canola, and was no higher at maturity than at the flag-leaf sampling (i.e., at actual heading of barley). Loss of soil N by denitrification likely occurred because of wet soil, and may have contributed partly to the crop stress that year.



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  		Fig. 1. Effect of time of weed removal on aboveground N content of the three crops at flag-leaf of barley (FL), podfill of pea (PF), and maturity (MAT) in 1997 and 1998. WAE, weeks after crop emergence. Data are averages of two N rates (0 and 6 g m–2). SEs are 1.17 (FL), 0.91 (PF), and 1.22 (MAT) for 1997, and 0.31 (FL), 0.68 (PF), and 0.51 (MAT) for 1998.

 
In 1998, maximum N accumulation in the crops occurred at the podfill sampling (11 wk after seeding). The lower N accumulation at maturity was probably due to yield limitation because of dry weather, and loss of plant N between podfill and maturity as the crops were unable to utilize all the accumulated N. The crops reached maturity in 13 wk in 1998 compared with 16 wk in 1997. Early weed removal in 1998 resulted in significantly higher N uptake for all crops, mimicking the pattern observed for DM accumulation. Early weed removal also resulted in higher N concentrations in crop tissues. For example, N concentration in pea shoot at flag-leaf (of barley) was 21.9 mg g–1 with the 1-WAE treatment, as compared with 18.8 mg g–1 with the 4-WAE application. Weed N uptake also followed the pattern of its DM accumulation. Nitrogen uptake in weed shoots remaining at crop maturity ranged from 0.53 (under pea) to 0.72 g N m–2 (under barley) in 1997, and from 0.92 (under canola) to 1.24 g N m–2 (under pea) in 1998.

Dinitrogen Fixation and Nitrogen Balance due to Field Pea
As expected, the atom% 15N of pea shoot was significantly lower than those of the nonlegumes, owing to dilution by symbiotic fixation of atmospheric N2 (Table 3). The pea atom% 15N tended to be lower with the 4-WAE than the 1-WAE treatment; however, the differences were not significant in both years. Eaglesham et al. (1981) and Papastylianou (1988) reported that the atom% 15N of intercropped legumes tended to be lower as compared with sole-cropped legumes, although the differences were not always significant. Those data suggest that legumes tend to fix more atmospheric N2 when the competition for soil N is more intense. There was no significant difference in the atom% 15N of canola and barley, indicating that those crops were drawing on similar soil N pools.


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  		Table 3. Effect of time of weed removal on atom% 15N of mature crop shoot.

 
There was no significant difference in the proportion or amount of biologically fixed N in pea whether barley or canola was used as the reference crop, therefore data for N2 fixation were averaged across both reference crops (Table 4). This contrasts with the report of Giller and Witty (1987) that use of rape (Brassica rapa L.) as reference crop rather than barley resulted in lower estimates of N2 fixation. In both years, pea derived a higher proportion of its N from the atmosphere with the later weed removal (i.e., greater weed competition). Results from some intercropping studies showed that the %Ndfa in pulse crops was mostly similar between intercropped and sole-cropped systems (Ofori et al., 1987; Colwell et al., 1989), while others showed that intercropped legumes had higher %Ndfa (and a lower amount of fixed N) than sole-cropped legumes (Jensen, 1996a). The amount of N fixed in the pea crop was not significantly affected by the time of weed removal; however, total and grain N uptake by pea was higher with early weed removal (because of higher seed yield) (Table 4).


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  		Table 4. Effect of time of weed removal on percentage N derived from the atmosphere (%Ndfa), total amount of N fixed, and grain N uptake by field pea in 1997 and 1998.

 
Assuming that 50% of seed N was translocated to the shoot (Jensen et al., 1985), about 56% of pea N at maturity in 1997 was derived from the soil for the 1-WAE treatment compared with 21% for the 4-WAE treatment. In 1998, the percentages of N in mature pea that were derived from the soil were 33% and 11%, respectively, for 1 and 4 WAE. With early weed removal, more N was removed in the pea seed than was fixed by symbiosis (2.5 to 6.4 g N m–2 net) due to greater seed production, while later weed removal resulted in N gain of 1.1 to 1.3 g N m–2 due to reduced seed production (Table 4). Armstrong et al. (1994) had reported N balance of –2.9 to +4.0 g N m–2 among six pea genotypes grown in three locations in southwestern Australia.

Yield and Nitrogen Nutrition of the Subsequent Crop
Although the time of weed removal during pea cultivation affected the cropping system N balance, it had no measurable effect on N availability to the subsequent barley crop. The predominant treatment effects on the growth and N uptake of the subsequent crop were the preceding crop type and N fertilizer rate. Analysis of variance showed that the preceding crop accounted for 13 to 29% of yield variation, and N rate 7 to 26%. Fertilizer N rate accounted for 39 to 48% of the variation in total N uptake by barley compared with 25 to 39% for the preceding crop. There was no preceding crop x N rate interaction. Grain yield and total N uptake were higher when barley followed pea than when barley followed barley, regardless of N application rate (Table 5). Barley yield in 1999 was low, due mostly to moisture stress caused by a severe drought (96 mm of rainfall May–August, compared with a 30-yr mean of 228 mm). Grain yields of barley following canola were mostly similar to or slightly higher than those of barley following barley; that is, the rotational effect of a canola crop was slight. Barley yield following a nonlegume was significantly higher by an average of 82 g m–2 in 1998 and by 26 g m–2 in 1999 when N was applied at 6 g m–2 compared with no N. Barley following pea yielded 43 g m–2 more in 1998 and 24 g m–2 more in 1999 when N was applied at 6 g m–2 compared with no N applied. The N effect of a previous legume seemed to render the following cereal crop less responsive to N fertilizer. However, apparent fertilizer N recovery [defined as N uptake (fertilized) minus N uptake (check)/fertilizer N applied x 100] at maturity was similar among crop sequences: averaging 61% in 1998 and 34% in 1999. Barley following pea has shown a yield response to up to 10 g m–2 of fertilizer N (Wright, 1990). Thus, in most years, adding a modest amount of N fertilizer to crops sequent to field pea would be a suitable management option.


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  		Table 5. Effect of preceding crop and N fertilizer application on grain dry weight and total N uptake of barley at maturity in 1998 and 1999.

 
The preceding crop effects on barley yield and N uptake at mid-dough (data not shown) were similar to those at maturity. The difference in barley N uptake between the canola–barley sequence and the barley–barley sequence may be attributed to a rotational benefit, while the difference in N uptake between the pea–barley sequence and the barley–barley sequence may be attributed to the rotational + N benefit. The N uptake accruing from the rotational benefit was slight, ranging from 0.2 to 0.4 g N m–2; the N+ rotational benefit was 1.1 to 2.9 g N m–2. Since there was no N x previous crop interaction, it can be assumed that the N and rotational benefits are additive. Thus, the N benefit of pea to barley was 0.9 to 2.5 g N m–2, and was likely derived from several sources such as crop residues and rhizodepositions.

The higher soil NO3 level following field pea than barley or canola in spring 1998 (Table 6) was likely responsible for the greater barley growth and N uptake on those plots. In 1999, however, soil NO3 was relatively high throughout the experimental plot area, especially where canola was grown previously. This and the drought that limited the growth of barley may have resulted in the diminished N effect observed in 1999. Soil NO3 in the 50- to 100-cm layer was essentially similar between previous crops. Nitrogen returned in crop residues the previous fall (Table 6) appears to influence the level of soil NO3 the following spring, presumably through mineralization. Soon and Arshad (2002) found that N mineralization during residue decomposition in the fall and spring was virtually nil for wheat straw, approximately 0.1 g m–2 for canola straw, and 0.5 to 0.6 g m –2 for pea straw. It is also likely that greater mineralization of N from rhizodeposits and roots after pea as compared with barley also contributed to more available soil N following pea (Jensen, 1996b).


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  		Table 6. Nitrogen returned in straw residues of preceding crop and its influence on NO3 content in two soil layers in the following spring.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Early removal of weeds resulted in the pea deriving a significantly lower proportion of its total N from symbiotic N2 fixation than later weed removal. However, the amount of N fixed was not significantly different between the two weed removal treatments. Early weed removal resulted in higher total pea N accumulation when compared with later weed removal due to greater DM production and utilization of soil N. Seed production was also higher with early than with later weed removal. The N balance following a pea crop was 1.1 to 1.3 g N m–2 with late weed removal, whereas more N was removed in pea seed than was fixed symbiotically (i.e., a negative N balance) with early weed removal. However, weed-removal treatments during pea cultivation did not affect N availability to the subsequent barley crop. The previous crop, as well as fertilizer N rate, were the main determinants of barley production. There was no N rate x time of weed removal interaction on barley growth or N uptake. Grain yield and N uptake were higher when barley followed pea than when barley followed barley or canola. At this site, the N benefit to barley derived from pea was considerably greater than the rotational benefit. Barley fertilized with 6 g N m–2 outyielded unfertilized barley, regardless of preceding crop. Therefore, early weed removal during pea cultivation is recommended, as it results in higher pea yields.


   ACKNOWLEDGMENTS
 
The research was partly supported by a grant from the Alberta Pea Growers Commission. The technical assistance of J. Drabble, G. McLean, A. Haq, and S. Neighbor is gratefully acknowledged. We also thank D. Harris of the University of California, Davis, for timely 15N analysis.

Received for publication May 27, 2003.


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




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