Bremer, E.

Rennie, D.A.

Rennie, R.J.

Voroney, R.P.

English

University of Saskatchewan Research Archive

Nitrogen Pb:atioa of Several Grain Legumes in Saskatchewan E. Bremer, D.A. Rennie, R.j. Rennie, and R.P. Voroney Saskatchewan Institute of Pedology University of Saskatchewan Grain legumes have become increasingly important as a cash crop in many areas of Saskatchewan over the past decade. Por example, the area seeded to lentils has increased from less than 8,000 ha in the early seven-ties to 60,000 ha last year. Similarly, the area planted to field peas has increased from less than 4,000 ha to 28,000 ha over the same time period (Saskatchewan Agriculture). Further increases are likely. There is also increasing interest in using grain legumes as green manures. An important consideration in growing grain legumes is their ability to obtain atmospheric nitrogen through a close association with llbizobium spp. The plant supplies nodules containing rhizobia with carbohydrates in exchange for simple amino acids containing nitrogen fixed from the atmosphere. The amount of nitrogen that is fixed this way may be quite large, depending on environmental conditions, crop variety and rhizobia! strains. Pew estimates of the amount of nitrogen fixed by grain legumes in Saskatchewan exist as accurate field estimates of nitrogen fixation are difficult to obtain. Methods A study was initiated under the Innovative Acres program to measure the growth and nitrogen fixation of several grain legumes under a range of conditions in Saskatchewan. The crops tested were Eston and Laird lentils, 320 Trapper and Tara peas, and A!adin and Outlook fababeans. johnston barley and Columbus wheat were grown as non-fb:.ing controls. They were tested at four locations in 1984 and at sa in 1985, with all the major soil zones being represented by at least one site in each year. Inoculated and uninoculated strips (2m I 50m) of each cu1tivar were planted at each site. just after emergence urea (51 15N abundance) was injected in one meter sections of a row in each treatment at an equivalent rate .of 7.5 kg ha-t. At harvest two of these meters were oombined for a."flalysis, with five replications in each treatment. Additional forage samples of lentils and barley were taken from the inoculated treatments at earlier dates. All samples were weighed and analysed for 'IN and i5N abundance. Soil samples were analysed for nitrate at an sites. Soil moisture depletion was measured at the 198 ~ locations. As blocks were not randomized the validity of these estimates requires that environmental variations be randomly distributed across blocks. In particular, no gradients, in available soH nitrogen or moisture should eJist across blocks. This assumption appears reasonable for most of the sites tested. Nitrogen fixation could be estimated from this data by either the classical difference method or the 15N isotope dilution method. Estimates of nitrogen fb:ation by the dassicai difference method were calculated as follows: 321 where: N ... yield of N (kg ha-l) fs ... fixing crop nfs ... non-fixing crop This method assumes that the fs and the :nfs assimilate the same amount of soil nitrogen. This rarely achieved as the control crop often explores a different soil volume and/or assimilates soil nitrogen with a different efficiency than the fixing crop (Rennie and Renrue.1983). The fs may simply substitute fixed nitrogen for soil nitrogen, with no change in total nitrogen. The 15N isotope dilution is based on the difference in the abundance of 15N between the different :nitrQ<ge:n sources (Fig.l ). The fixing crop obtains it's nitrogen requirements from air (nitogen fixation), soil and fertilizer nitrogene while the nfs only obtains it's nitrogen requirements from soil and fertilizer nitrogen. As atmospheric nitrogen has a lower concentration of 15N than soil or fertilizer nitrogen. the abundance of 15N in plant material decreases with increasing dependence on fixed nitrogen. Nitogen fixation can therefore be calculated as follows: ~ Ndfa'"' ( 1 _ "15Nex. infs ) * 100 ~~5N ex. in nfs N filed ... N yield 1: INdfa/1 00 where %Ndfa = % of crop nitrogen derived from atmospheric nitrogen fs "' fixing crop nfs .. non-fixing control 322 ATMOSPHERIC N 1~ X N Ab.:::0.3663 fERTiliZER N 15 eg.: NAb. = 5.000 SOil SOlUTION son. N 15 Z NAb. :::0. 3700 Fig.J. " 15N Ab. of nitrogen sources for a fixing and a non-fixing crop. 323 This calculation assumes that the the control crop obtains the same ratio of 15N :14N from soil solution as the filing crop. This assumption is met if the 15N abundance or available soil nitrogen is constant for both crops, or if both crops ut.mze soil nitrogen in a similar pattern (Rennie and Rennie, 1983). As i5N enriched urea was injected at a localized point the enrichment of 15N in the soil solution varied both spatially and temporally, altb.ough in a similar pattern for both the fs and the nts. Samples taken in june, approx-imately siJ weeks after planting and three to four weeks after the addition of 15N enriched urea, showed that barley had a much lower i5N abundance than lentils (Table 1 ). This is due to the barley obtaining tess of it's nitrogen requirements from fertilizer than lentils as barley explores a larger soil volume to meet it's larger nitrogen requirements, and perhaps as barley accumulates more nitrogen prior to the addition of l5N enriched urea. Thus, estimates of nitrogen fixation using barley as a control were invalid at these earlier sampling dates. At harvest lentils contained a lower l5N abundance than barley. This is due to both increased nitrogen fllatlon and greater assimilation of soU nitrogen by lentils during the period between the june and August samplings. Estimates based on t5N dilution showed significant quantities of nitrogen were filed. These estimates assume that barley obtained the same ratio of tSN:•~N from soil solution over the whole growing season as lentils. but this assumption could not be checked from the information collected. As the uninoculated treatments were also effectively nodulated, they could not be compared with barley as non-filing controls. 324 Table 1 Nitrogen yield and 1 15N abundance at Tisdale for two sampling dates of barley and lentils. Sampling Cultivar N Yield 115N Ab. I NdfFert Calculated date (kg ha-l) INdfA june 27 Barley 73 0.53 3.5 Eston 26 0.86 10.6 -201 Laird 25 0.86 10.6 -201 August 22 Barley 112 0.49 2.6 Eston 153 0.42 1.1 55 Laird 167 0.43 1.3 40 325 The 15N isotope dilution method gave better estimates of nitrogen faation than the classical difference method. Fewer anomalous values were obtained by the dilution method than by the difference method as only about 61 of the dilution estimates were significantly below zero while about 191 of the difference estimates were significantly below zero. Wheat and barley had significant differences in nitrogen yield at 6 sites, but only at 2 sites for 1 15N abundance. At sites where the uninoculated treatments were ineffectively nodulated the uninoculated treatments were more similar in I 15N abundance than in nitrogen yield to the cereal controls. These observations indicate that the differences between filing and non-filing crops were greater for nitrogen uptake than for the ratio of 15N:l4N assimilated from soil solution, suggesting that the assumptions of the dilution method were more closely met than those of the difference method. On average, estimates of INdfa derived from the t5.N isotope dilution method were about 201 higher than the estimates based on the classical difference method, but this did vary quite widely. Factors affecting nitrogen fixation Nitrogen faation requires the presence of adequate numbers of effective rhizobia. At three of the ten sites inoculation did not increase the amount of nitrogen filed significanUy as native rhizobia were able to nodulate the uninoculated treatments, but at the remainder of the sites inoculation was effective (Fig. 2). High soil nitrate levels inhibit nitrogen fixation as the plant preferentially assimilates soil nitrogen. A negative correlation was observed between spring soil N03 1evels and INdtA for most crops tested (Table 2). 326 80 70 60 N2 F1xed 50 (kg ha-1) 40 30 20 10 0 II Uni noculsted II Inoculated Gun Rf!gin.a 'VIakot5 Tisd.o~l~ 'w'hit4\' G1~nb.ain RE>g'ina Abt>r- Foam lakf' Fox 84 dl!'~n lak~ SITE F1g:-2: Effect of inoculation on the amount of nHrogen f1xed by Laird lentns at 91ocauons in 1984 and 1985. 327 Table 2: Coefficients of correlation of water use and spring soil N03 with pea and lentil crop parameters in 1985 (5 sites). Cultivar Eston Laird Trapper Tara Independent Coefficients of Correlation of Crop Parameters factor Total Yield NYield WUl 0.74 0.66 N~2 0.64 0.69 wu 0.70 0.68 N03 0.66 0.70 wu 0.76 0.83 N03 0.63 0.52 wu 0.70 0.77 N03 0.62 0.57 I WU • Total water use INdfA NFixed 0.81 0.69 0.39 0.64 0.19 0.80 -0.71 0.20 0.58 0.89 -0.66 0.01 0.25 0.85 -0.88 0.12 If r> 0.88 then P < .05 If r> 0.73 then P < .1 2 N03 - Spring soil nitrate (ppm) at 0-15 em Correlation coefficient of WU to N03 is 0.146 328 Water use was positively correlated with not only yield but also with the dependence of most crops on filed nitrogen (Table 2). This may be due to an increase in both the amounts of nitrogen required and the availability of energy (photosynthate) under less water stressed conditions. Although the correlations of water use and soil nitrate with INdf A were generally not significant, together they accounted for about 801 of the observed variation in INdfA of peas and lentils in 1985. There were significant differences in growth and nitrogen fixation between cultivars. At higher levels of available moisture fababeans had the highest yields of dry matter and filed nitrogen. followed by peas and then lentils (Table 3). Under more drought stressed conditions peas had the highest yields of dry matter ·and filed nitrogen, followed by lentils, and then fababeans, which failed to survive at most of these sites. Nitrogen fixation ranged from 5 to 25 kg ha-l for peas and lentils in the Brown soil zone to 140 to 160 kg. ha-t for fababeans and 70 to 100 kg ha-t for peas and lentils in the Gray and Gray-Black transition soil zones. ConclUSionS The t5N isotope dilution method was a better estimater of nitrogen fixation than the classical difference method. Dilution estimates averaged about 201 higher than difference estimates. However, further work is still required to establish appropriate controls. Significant quantities of nitrogen were filed by grain legumes in Saskatchewan, ranging from 1 0 to 40 kg ha-t in the Brown soil zone to 80 to 160 kg ha~• in the Gray and the. Gray-Black Transition soil zones. The 329 Table 3: Yields of above ground dry matter and filed nitrogen or lentils. peas and fababeans in SaskatcJlewan. Area of province South (Brown soil zone) North (Gray and Transition soil zones) Q-op Lentils Peas Fababeans Lentils Peas Fababeans 330 Yield (I ha-t) 0.5-1.5 0-3 0-0.8 5-8 5-9 7-10 INdfA 30-50 40-60 50-70 60-80 75-85 N filed (kg ha-l) 5-30 0-40 60-80 80-120 120-160 amount of nitrogen !lied Increased with inoculation. reduced water stress. and lower sou nitrate levels. Temperature may also have been a factor, but was not documented in this study. Fababeans fixed the most nitrogen under wetter conditions, but peas and lentils fixed more under water stressed conditions. Rennie, R.j. and D.A. Rennie. 1983. Can. j. Microbic!. 29:1022-35. Saskatchewan Agriculture. 1985 Specialty Crop Report. 331 

Nitrogen fixation of several grain legumes in Saskatchewan

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Nitrogen fixation of several grain legumes in Saskatchewan

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