Preceding crops and nitrogen fertilization influence soil nitrogen cycling in no-till canola and wheat cropping systems

Field Crops Research 191 (2016) 20–32

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Preceding crops and nitrogen fertilization influence soil nitrogen cycling in no-till canola and wheat cropping systems Mervin St. Luce a , Cynthia A. Grant b , Noura Ziadi a,∗ , Bernie J. Zebarth c , John T. O’Donovan d , Robert E. Blackshaw e , K. Neil Harker d , Eric N. Johnson f , Yantai Gan g , Guy P. Lafond h,1 , William E. May h , Sukhdev S. Malhi i , T. Kelly Turkington d , Newton Z. Lupwayi e , Debra L. McLaren b a

Quebec Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Boulevard, Quebec City, QC G1V 2J3, Canada Brandon Research and Development Centre, Agriculture and Agri-Food Canada, 2701 Grand Valley Road, Box 1000A, R.R. #3, Brandon, MB R7A 5Y3, Canada c Fredericton Research and Development Centre, Agriculture and Agri-Food Canada, 850 Lincoln Road, PO Box 20280, Fredericton, NB E3B 4Z7, Canada d Lacombe Research and Development Centre, Agriculture and Agri-Food Canada, 6000 C and E Trail, Lacombe, AB T4L 1W1, Canada e Lethbridge Research and Development Centre, Agriculture and Agri-Food Canada, 5401-1 Avenue South, PO Box 3000, Lethbridge, AB T1J 4B1, Canada f Scott Research Farm, Agriculture and Agri-Food Canada, PO Box 10, Scott, SK S0K 4A0, Canada g Swift Current Research and Development Centre, Agriculture and Agri-Food Canada, PO Box 1030, Swift Current, SK S9H 3X2, Canada h Indian Head Research Farm, Agriculture and Agri-Food Canada, PO Box, 760, R.R. #1 Gov Road, Indian Head, SK S0G 2K0, Canada i Melfort Research Farm, Agriculture and Agri-Food Canada, PO Box 1240, Melfort, SK S0E 1A0, Canada b

a r t i c l e

i n f o

Article history: Received 10 November 2015 Received in revised form 12 February 2016 Accepted 15 February 2016 Keywords: Cereals Crop rotation Fertilizer Legumes Soil fertility

a b s t r a c t Crop rotation and nitrogen (N) fertilization can influence soil N cycling, however, less is known about their interactive effects under varying soil and climatic conditions. We examined the interactive effects of preceding crops and N fertilizer rates on soil N cycling in canola (Brassica napus L.) and spring wheat (Triticum aestivum L.) cropping systems in no-till soils in the Canadian prairies. Field pea (Pisum sativum L.), lentil (Lens culinaris Medik), faba bean (Vicia faba L.; faba bean-seed), canola and wheat grown for grain, and faba bean green manure (faba bean-GRM) were the preceding crops and were direct-seeded at 7 and 6 locations in 2009 and 2010, respectively. Canola and wheat were seeded in 2010 and 2011 (2012 for wheat at one site), respectively with N fertilizer applied at 0, 30, 60, 90 and 120 kg N ha−1 . Above-ground residue N returned was greatest for faba bean-GRM and lowest for canola and wheat. On average across sites, apparent in-crop N mineralization (ANM) under canola was greater following faba bean-GRM than all other preceding crops, except field pea, while under wheat, ANM was greater following all the legumes than following canola and wheat. Crop N uptake increased with N fertilizer rate, but the response was generally lower following faba bean-GRM and lentil. The N budget showed that 40–65% of crop N uptake (35–100% at highest ANM site) was possibly derived from ANM, more so following legumes, as compared to 35–60% from N fertilizer. The surplus and unaccounted N were particularly pronounced when canola and wheat were preceded by legumes rather than by canola or wheat. Our findings indicate that legumes can enhance soil N supply in no-till soils, and also highlight the importance of adjusting N fertilizer rates based on preceding crops to minimize the potential for N losses. Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.

1. Introduction

Abbreviations: ANFR, apparent nitrogen fertilizer recovery; ANM, apparent incrop nitrogen mineralization; EONR, economic optimum nitrogen rate; faba beanGRM, faba bean grown as green manure; faba bean-seed, faba bean grown for seed; field pea-GRM, field pea grown as green manure. ∗ Corresponding author. E-mail address: [email protected] (N. Ziadi). 1 Deceased. http://dx.doi.org/10.1016/j.fcr.2016.02.014 0378-4290/Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.

The adoption of agricultural practices that increase soil fertility and enhance nutrient cycling is critical for the long-term sustainability and productivity of agroecosystems. In the Prairie Provinces of western Canada, canola (Brassica napus L.) and spring wheat (Triticum aestivum L.) are the two most dominant crops grown, with annual plantings on about 8 and 7 million hectares of land in 2011, respectively (Statistics Canada, 2014). Moreover, western Canada accounted for 98% of the total seeded area in Canada in 2011

M. St. Luce et al. / Field Crops Research 191 (2016) 20–32

for canola and wheat, respectively (Statistics Canada, 2014). Like in most semi-arid to sub-humid regions, nitrogen (N) and water availability are the major limiting factors for optimum crop production (Hooper and Johnson, 1999; Campbell et al., 2007; Sadras and Richards, 2014). Hence, N fertilizer application is required in large quantities to attain optimum crop yields, particularly when new high-yielding modern cultivars are grown (Harker et al., 2012). Nitrogen fertilizers therefore represent a significant cost to producers in this region. However, previous studies reported that N fertilizer-use efficiency, both globally and in this region, is usually less than 40% (Fageria and Baligar, 2005; Soon et al., 2006; Malhi et al., 2014). This suggests that a significant proportion of the N fertilizer applied in crop production systems may be immobilized within soil organic matter (SOM), lost from the soil-plant system during the growing season or fixed in clay lattices, thus resulting in potential environmental and economic consequences (Ladha et al., 2005). A large proportion of the N required by the crop may be derived from residual fertilizer N, if the residual N is maintained in the rooting zone in a plant-available form, or mineralized from SOM during the growing season. Therefore, there is a need for alternative and sustainable approaches to enhance the capacity of soils to provide a greater proportion of the N needed by crops, which in turn will reduce the dependence on mineral N fertilizers. In 2011, pea, lentil and beans seeded in western Canada represented 98% of the total pulses seeded in Canada (Statistics Canada, 2014). These legumes are frequently included in wheat and canola cropping systems in western Canada and have been shown to increase subsequent crop yields and reduce N fertilizer requirements (O’Donovan et al., 2014; St. Luce et al., 2015). Legume benefits are generally due to increased soil N supply through biological N2 fixation (Chalk, 1998; Walley et al., 2007; Peoples et al., 2009; Angus et al., 2015), and the return and mineralization of their N-rich above- and below-ground residues (Arcand et al., 2014; Angus et al., 2015; O’Dea et al., 2015). Legumes also provide non-N benefits such as increased water availability to subsequent crops (Miller et al., 2003; Cutforth et al., 2013; Angus et al., 2015), reduction in pests and diseases (Stevenson and van Kessel, 1996; Kirkegaard et al., 2008; Angus et al., 2015), enhanced soil tilth (Grant and Lafond, 1993) and the growth of beneficial soil microorganisms (Biederbeck et al., 2005; Tiemann et al., 2015), which in turn can enhance crop uptake of nutrients and water (Lupwayi and Kennedy, 2007). Due to differences in biological N2 fixation potential (Peoples et al., 2009; Williams et al., 2014), biomass and seed yield (Peoples et al., 2009; Williams et al., 2014), and straw and root C/N ratios (Gan et al., 2011b) among legumes, it is not entirely clear how different legume species, and legumes grown for seed or green manure, affect soil N cycling and N budgets in canola and wheat cropping systems. A better understanding of the influence of various legume preceding crops on soil N cycling and budget in semi-arid to sub-humid environments, and the role of inherent soil and environmental characteristics, is required for implementing sustainable strategies to enhance agroecosystem productivity. Soil N cycling is almost entirely driven by biological processes. Hence, changes in microbial community structure and activity due to variations in above- and below-ground residues, including root exudates, can have a significant impact on soil N cycling and dynamics (Biederbeck et al., 2005; Tiemann et al., 2015). The addition of crop residues is expected to stimulate microbial activity, but the mineralization-immobilization turnover will depend on the quality (e.g., C/N ratio) of the crop residue (Kirkegaard et al., 2008; St. Luce et al., 2014). Nitrogen mineralization and subsequent availability for crop uptake is expected to be lower for greater C/N ratio residues such as canola and wheat than for lower C/N ratio residues provided by legumes. However, the method of placement of the crop residues may affect the rate of decomposition and the extent to which different preceding crops affect soil N cycling.

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No-till systems are rapidly becoming common practice in western Canada as a mitigation strategy to reduce soil erosion, sequester more C and conserve soil moisture (Malhi et al., 2001; Halvorson et al., 2002). Hence, gaining a better understanding of soil N dynamics in no-till soils due to management practices is extremely relevant. Placement of crop residues on the soil surface in no-till soils may limit the contact between the residues and decomposer communities, thereby reducing crop residue decomposition and N mineralization (Drinkwater et al., 2000; Van Den Bossche et al., 2009). Nitrogen fertilizer application can also impact soil N cycling, by providing a readily available source of N for microbes and thus enhance N mineralization, particularly in situations where N was initially a limiting factor (Recous et al., 1995). As such, interactions between the crop residues, N fertilizer application rates, soil properties and environmental conditions may exert major influences on cycling and dynamics of N in no-till soils. The objective of this study was to examine the interactive effects of preceding crops and N fertilizer rates on soil N cycling in canola and wheat cropping systems in western Canada. We achieved this objective by examining the quantity of above-ground biomass and crop residue N returned, pre-plant soil NO3 -N content, aboveground crop N uptake, apparent in-crop N mineralization (ANM), and by calculating simple N budgets.

2. Materials and methods 2.1. Site description The canola experiment was conducted on seven no-till sites from 2009 to 2010, while the wheat experiment was conducted on six no-till sites from 2010 to 2011. Both experiments were conducted at Brandon, Manitoba; Indian Head, Saskatchewan; Lacombe, Alberta; Lethbridge, Alberta and Scott, Saskatchewan. The canola experiment was also conducted at Beaverlodge, Alberta and Swift Current, Saskatchewan, while the wheat experiment was also conducted at Melfort, Saskatchewan. Due to flooding in 2010 at Lethbridge, the wheat experiment at this site was delayed by 1 year and was therefore conducted from 2011 to 2012. Soil characteristics and climatic conditions at each site during the study period for the canola and wheat experiment are given in Tables 1 and 2, respectively. The soils varied in texture, with clay content greatest at Indian Head (572–622 g kg−1 ) and lowest at Melfort (160 g kg−1 ). The SOC content ranged from 14 to 54 g kg−1 across the sites, being greatest at Lacombe and lowest at Swift Current and Lethbridge. Soils were slightly acidic (pH 5.1–6.7) at five sites, with greater pH values (pH > 7) observed at the other three sites. Pre-plant soil NO3 -N content was lowest at Lethbridge (16 mg N kg−1 ) and greatest at Beaverlodge (113 mg N kg−1 ) in 2009, and lowest at Scott (10 mg N kg−1 ) and greatest at Lacombe (33 mg N kg−1 ) in 2010. Total precipitation in the 2009 growing season was near normal at Indian Head, Lethbridge, Melfort and Scott, but was slightly above normal at Beaverlodge (14%), Brandon (14%), Lacombe (18%), and Swift Current (18%). The 2010 growing season was very wet with above normal precipitation at all sites, except Beaverlodge, including more than 60% above normal precipitation at Lacombe, Melfort and Swift Current and over 100% above normal precipitation at Scott (Table 2). In 2011, total precipitation during the growing season was above normal at all sites (18–37%), but much lower than normal at Melfort (38%). Mean air temperature during the growing season was within 0.9 ◦ C of normal in most cases with few exceptions (Table 2). Mean air temperature was 1.9, 2.2, 1.9 and 1.2 ◦ C above normal in the 2009 growing season at Brandon, Indian Head, Melfort and Swift Current, respectively. In the 2010 growing season, mean air temperature was 1.5 ◦ C above normal at Swift Current.

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M. St. Luce et al. / Field Crops Research 191 (2016) 20–32

Table 1 Soil characteristics at each site before the establishment of the preceding crops for each experiment. Site

SOCa

Location

−1

g kg

Sandb

Clayb

(0–15 cm)

pHc

Soil classification Canadian

FAO

NO3 -Nd kg N ha−1

Canola experiment (spring 2009) 55◦ 28 N; 119◦ 48 W Beaverlodge 50◦ 02 N; 99◦ 89 W Brandon Indian Head 50◦ 32 N; 103◦ 40 W Lacombe 52◦ 28 N; 113◦ 44 W Lethbridge 49◦ 41 N; 112◦ 46 W 52◦ 21 N; 108◦ 51 W Scott 50◦ 38 N; 107◦ 78 W Swift Current

44 36 26 54 16 24 14

326 340 125 477 370 310 314

231 330 622 203 330 270 182

Gray Luvisol Black Chernozem Orthic Black Chernozem Black Chernozem Dark Brown Chernozem Dark Brown Chernozem Brown Chernozem

Albic Luvisol Chernozem Chernozem Chernozem Haplic Kastanozem Haplic Kastanozem Aridic Kastanozem

5.5 7.5 7.3 6.5 7.4 6.2 6.7

113 39 37 104 16 30 50

Wheat experiment (spring 2010) Brandon 50◦ 02 N; 99◦ 89 W Indian Head 50◦ 32 N; 103◦ 40 W 52◦ 28 N; 113◦ 44 W Lacombe 49◦ 41 N; 112◦ 46 W Lethbridge Melfort 52◦ 44 N; 104◦ 47 W Scott 52◦ 21 N; 108◦ 51 W

23 32 40 14 18 25

418 139 480 453 400 318

300 572 275 282 160 266

Black Chernozem Orthic Black Chernozem Black Chernozem Dark Brown Chernozem Black Chernozem Dark Brown Chernozem

Chernozem Chernozem Chernozem Haplic Kastanozem Chernozem Haplic Kastanozem

7.5 7.5 5.1 7.5 6.6 6.1

22 16 33 24 NDe 10

a b c d e

SOC, soil organic carbon was determined by dry combustion (Carlo-Erba Instruments, Milan, Italy). Soil texture was determined using the pipette method after organic matter and carbonate removal (Kroetsch and Wang, 2008). Soil pH (0–15 cm) was measured in a 1:1 soil: water slurry (Hendershot et al., 2008). Pre-plant soil NO3 -N (0–60 cm) was determined after 2 M KCl extraction (Maynard et al., 2008) and converted to content using measured bulk density. ND, Not determined.

Table 2 Growing season (May–August) total precipitation and mean air temperature at each site during the study period. Mean air temperature (◦ C)

Total precipitation (mm)

Beaverlodge Brandon Indian Head Lacombe Lethbridge Melfort Scott Swift Current a

a

2009

2010

2011

30-year average (1981–2010)

2009

2010

2011

30-year average (1981–2010)a

201 241 248 222 234 219 182 176

168 406 307 460 347 404 464 410

293 328 292 382 278 141 352 288

235 279 244 278 220 226 200 215

13.0 14.0 13.4 12.9 15.1 13.3 15.1 14.6

12.9 15.6 14.7 12.7 14.7 15.0 14.7 14.3

13.0 16.5 15.5 13.7 15.6 16.0 15.6 15.1

13.4 15.9 15.6 13.6 15.8 15.2 14.4 15.8

Environnent Canada (http://climate.weather.gc.ca/climate normals/index e.html).

2.2. Experimental design and treatments The canola and wheat experiments used a split-plot arrangement of treatments in a randomized complete block design with four blocks. Main plots of preceding crops were established in 2009 and 2010, and five sub-plots of urea N fertilizer rates (0, 30, 60, 90 and 120 kg N ha−1 ) were established in 2010 and 2011, for the canola and wheat experiments, respectively. The exception was Lethbridge for the wheat experiment where the main plots were established in 2011 and the sub-plots in 2012. In both experiments, imidazolinone-resistant canola (45H73), faba bean (Snowbird; Vicia faba L.) grown for seed (faba bean-seed), faba bean grown as a green manure crop (faba bean-GRM), field pea (CDC Golden; Pisum sativum L.), lentil (CDC Imperial; Lens culinaris Medik) and spring wheat (CDC Imagine) were established as main plots (preceding crops) at all sites, except at Melfort. Faba beanseed and lentil were not established at Melfort, with field pea grown as a green manure crop (field pea-GM) being used instead. Sowing rates of 50, 100, 150, 150 and 300 seeds m−2 were used for faba bean, field pea, canola, lentils and wheat, respectively. The legumes were inoculated at each site with a suitable rhizobium inoculant to encourage nodulation and N fixation. All preceding crops were fertilized according to soil test recommendations at each site to produce average yields. Following grain harvest using a plot combine, the straw of canola, faba bean-seed, field pea-seed and wheat were chopped, returned and evenly spread on the soil surface of the plots. The faba bean-GRM and field pea-GRM crops were sprayed at the early flat-

pod stage with glyphosate (900 g a.e. ha−1 ) and clopyralid (300 g a.i. ha−1 ). They were then mowed in the fall with the entire plant biomass returned to the soil surface. The plot sizes varied according to location, based on the seeding equipment available (O’Donovan et al., 2014; St. Luce et al., 2015). In the canola experiment, glyphosate-resistant canola was sown across the entire experimental area at 150 seeds m−2 , while spring wheat was sown at 300 seeds m−2 in the wheat experiment. The control plots (0 kg N ha−1 ) received a small amount of N (3–15 kg N ha−1 ) as starter fertilizer depending on the site and the fertilizer source used. Details on plot sizes, fertilization practices and crop management for the preceding crops, and canola and wheat were previously reported by O’Donovan et al. (2014) and St. Luce et al. (2015). Briefly, N applied to canola and wheat was side-banded. Phosphorus was applied according to soil test recommendations as mono-ammonium phosphate with the canola and wheat seed at all sites, except Lacombe for wheat. A blend of mono-ammonium phosphate, ammonium sulphate and potassium chloride providing a ratio of 9-15-23-7 was used for wheat at Lacombe. Sulphur (0–15 kg S ha−1 ) was applied as ammonium sulphate where necessary for canola. For wheat, potassium and sulphate were applied as potassium sulphate at Melfort and Scott. The plots were monitored for pest and diseases, but little to no pests and diseases were observed, except for possible cutworm (Lepidoptera: Noctuidae) damage to the canola at Lacombe in 2010.

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2.3. Soil and plant sampling and analyses Before the establishment of the preceding crops, two random soil samples composited to form one sample per block were taken at each site for soil characterization (Table 1). Prior to planting and after harvesting of canola and wheat, a composite soil sample (two random cores) was taken at depths 0–15, 15–30 and 30–60 cm in each sub-plot. The soil samples were taken up to 60 cm since about 75–80% and 80–90% of canola and spring wheat roots, respectively, are within the 0–60 cm depth (Gan et al., 2011a; Cutforth et al., 2013). The samples were air-dried to constant weight and passed through a 2 mm sieve. Soil NO3 -N concentration at each soil depth was determined after 2 M KCl extraction (Maynard et al., 2008). Soil NO3 -N concentration was quantified on a Technicon autoanalyzer II (Technicon Industrial Systems, 1978). Soil NO3 -N concentration at each sampling depth was converted to kg N ha−1 using the average bulk density measured at each depth per site. Preliminary statistical analyses showed that the preceding crops did not significantly (P > 0.05) affect soil bulk density (data not shown). Canola and wheat were harvested at maturity with a small plot combine. The canola seed and wheat grain yields were previously reported (O’Donovan et al., 2014; St. Luce et al., 2015). Subsamples of the harvested straw, seed and grain were dried at 30 ◦ C and ground to pass a 1-mm screen. Straw yield was calculated on a dry weight basis. Total N concentrations in straw, seed and grain were determined by dry combustion with a vario MICRO cube (Elementar Americas) for the canola experiment and on a Carlo-Erba C and N analyzer (Carlo-Erba Instruments, Milan, Italy) for the wheat experiment. Above-ground N uptake (kg N ha−1 ) was calculated as: [Total N concentration in seed or grain × seed or grain yield (dry weight basis)] + [Total N concentration in straw × straw yield (dry weight basis)]. Above-ground N returned to the soil in the crop residue (kg N ha−1 ) was calculated as straw yield (kg ha−1 ) × total N concentration in straw. Apparent in-crop N mineralization (ANM; kg N ha−1 ) in the control treatment (treatments that received only starter N) was calculated as the difference between N recovered at harvest and N supplied at planting using the following equation:



ANM = (Nout + Nmin harvest ) − NF + Nmin planting



(1)

where Nout is the total above-ground plant N uptake (kg N ha−1 ), Nmin planting and Nmin harvest are the soil NO3 -N content (0 − 60 cm; kg N ha−1 ) at planting and harvest, respectively, and NF (kg N ha−1 ) is the starter fertilizer N applied. 2.4. Calculation of N budget The N budget was calculated separately for canola and wheat by considering N inputs [soil NO3 -N content at planting (0–60 cm), ANM, and fertilizer N applied] and N outputs [above-ground N uptake]. We assumed that N inputs through atmospheric deposition during the growing seasonwerenegligible. We did not estimate other specific N inputs such as biological N2 fixation and belowground inputs, nor specifically estimate N losses through leaching, denitrification and volatilization. We assumed that the pre-plant soil NO3 -N, plus the N from ANM were available to be taken up by the crop, and that the pre-plant soil NO3 -N and ANM were consistent across treatments. This allowed for the estimation of the proportion of plant-available N that was taken up, and does not distinguish whether the source is from applied fertilizer or from N mineralization. We also estimated the apparent N fertilizer recovery (ANFR, %) in the above-ground plant biomass using the following equation: ANFR = [

(NT − NC ) ] × 100% NF

(2)

23

where NT and NC are the above-ground N uptake in the fertilized and control treatment, respectively (kg N ha−1 ), and NF is the N fertilizer applied (kg N ha−1 ). The N budget was calculated using the average data across all sites, and was also calculated for the Brandon and Lacombe sites, which had the lowest and greatest values of ANM, respectively. In each case, the N budget was calculated using the average economic optimum N rate (EONR) for each site. The EONR for wheat was previously reported by St. Luce et al. (2015). The same procedure was used for estimating the EONR for canola, assuming that the cost of N fertilizer was 1.05CAN$ kg−1 N and the price of harvested canola seed was 0.42CAN$ kg−1 . 2.5. Statistical analyses Canola emergence in field pea and lentil plots at Lacombe for the canola experiment was non-existent or extremely poor and variable. As a result, canola data from the field pea and lentil plots at Lacombe in 2010 were excluded from statistical analyses. All statistical analysis was performed using SAS statistical software (SAS Institute, 2010). Data were checked for normality with the Kolmogorov–Smirnov test and were log or square root transformed when necessary to achieve normality. Analysis of variance (ANOVA) was done separately for the canola and wheat experiments across all sites for all parameters except above-ground residue N returned and pre-plant soil NO3 -N (0–60 cm), using PROC MIXED to determine the significance of preceding crops and N fertilizer rate effects and their interaction. For the quantity of above-ground residue N returned and pre-plant soil NO3 -N content, data for both experiments were combined with preceding crop and year treated as fixed effects and site, site × year, and replicate (site × year) treated as random effects. For ANM, preceding crop was the fixed effect and site, replicate (site) and preceding crop × site were random effects. For above-ground crop N uptake, preceding crop and N fertilizer rate were regarded as fixed effects with site, replicate (site), preceding crop × replicate (site) and site interaction with fixed effects as random effects. Field pea-GRM was excluded from this analysis for the wheat experiment since it was grown at only Melfort. By including site and site interactions with fixed effects as random, it became possible to make treatment inferences outside the study area. We also examined preceding crops and N fertilizer rate effects at individual sites. For the quantity of N returned in above-ground residues, pre-plant soil NO3 -N content and ANM, the ANOVA was conducted with preceding crops and replicates as fixed and random effects, respectively. Pre-plant soil NO3 -N results for the wheat experiment was previously reported by St. Luce et al. (2015). For above-ground crop N uptake, the preceding crops and N fertilizer rates were treated as fixed effects while replicates and replicate interaction with the fixed effects were treated as random. Differences were considered statistically significant at P < 0.05. Means were compared with a post-hoc least square means test at P < 0.05. The SLICE statement in SAS was used to partition the significant interactions. In addition, the significant interactions for N uptake were further examined by plotting all the data for N uptake against N fertilizer rates for each preceding crop using PROC MIXED whenever the response was linear for all preceding crops. Orthogonal comparisons were then used to compare the intercepts and slopes of the linear regression line. 3. Results 3.1. Above-ground biomass and residue N returned The quantity of above-ground biomass and residue N returned were significantly influenced by preceding crop and the preceding

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M. St. Luce et al. / Field Crops Research 191 (2016) 20–32

Fig. 1. Apparent in-crop N mineralization under (a) canola and (b) wheat as affected by preceding crops at no-till sites in western Canada. Bars within each site with different lowercase letters are significantly different (P < 0.05). Error bars represent standard error of mean. Data for field pea and lentil were excluded at Lacombe under canola in 2010 due to extremely poor and variable canola emergence in these plots. Table 3 Interactive effect of preceding crops and year on the quantity of above-ground biomass and N returned, and on pre-plant soil NO3 -N content on average across 13 site-years in no-till sites in western Canada. Values are mean ± standard error. Preceding cropa

Biomass returned 2009

2010 −1

5688 ± 356ab 3197 ± 324c 4014 ± 285b 4091 ± 263b 3538 ± 331bc 4375 ± 519b

ANOVA (P value) Year (Y) Preceding crop (PC) Y × PC

nsc <0.0001 <0.0001

a b c

Pre-plant soil NO3 -N (0–60 cm)

2009

2010

2010 + 2011

19.3 ± 1.9c 39.5 ± 6.3b 127.8 ± 12.1a 34.9 ± 3.1b 39.7 ± 4.1b 20.5 ± 2.0c

17.8 ± 1.0c 18.4 ± 1.3c 26.8 ± 1.6a 23.3 ± 1.5b 25.2 ± 1.6ab 18.2 ± 1.3c

−1

kg ha Canola Faba bean-seed Faba bean-GRM Field pea Lentil Wheat

N returned in crop residues

kg N ha 3831 ± 298bc 3382 ± 350 cd 4595 ± 332a 2938 ± 240d 2594 ± 311d 4573 ± 472ab

29.9 ± 1.3 cd 28.1 ± 2.8d 113.0 ± 8.9a 42.2 ± 4.4b 40.8 ± 5.3bc 23.2 ± 1.8e ns <0.0001 0.005

Faba bean-seed, faba bean grown for seed; faba bean-GRM, faba bean grown as a green manure crop. Values followed by different lowercase letters within a column are significantly different at P < 0.05. ns, not significant (P > 0.05).

ns <0.0001 ns

M. St. Luce et al. / Field Crops Research 191 (2016) 20–32

crop by year interaction (Table 3). In 2009, above-ground biomass returned was greater for preceding canola than all other preceding crops, similar among faba bean-GRM, field pea, lentil and wheat, and lowest for preceding faba bean-seed. In 2010, above-ground biomass returned was greater for preceding faba bean-GRM than all other preceding crops, except wheat, while field pea and lentil returned the least quantity of above-ground biomass. Faba beanGRM returned 113 and 128 kg N ha−1 of above-ground residue N in 2009 and 2010, respectively, which were 3–6 times greater than all other preceding crops in 2009 and 2010. In 2009, a preceding wheat crop returned the least amount of N, while faba bean-seed returned less N than the other preceding legume crops. In 2010, canola and wheat returned similar amounts of N, which were lower than the amounts returned by faba bean-seed, field pea and lentil. Interestingly, the quantity of above-ground residue N returned by faba bean-seed was 40% greater in 2010 than 2009. 3.2. Pre-plant soil NO3 -N content Pre-plant soil NO3 -N content differed significantly among the preceding crops, but was not affected by year or the year × preceding crop interaction (Table 3). Across the 13 siteyears, pre-plant soil NO3 -N content was greater following faba bean-GRM, field pea and lentil than canola, faba bean-seed and wheat. Pre-plant soil NO3 -N content was also greater following faba bean-GRM than field pea. 3.3. Apparent in-crop N mineralization under canola On average across sites, ANM under canola was significantly influenced by the preceding crops (Fig. 1a). The ANM under canola was greater following faba bean-GRM than all other preceding crops except field pea. It was also greater following field pea and lentil than canola. For individual sites, there was a significant effect of preceding crops on ANM under canola at Beaverlodge, Indian Head, Lacombe and Scott, but not at Brandon, Lethbridge and Swift Current (Fig. 1a). At Beaverlodge and Indian Head, ANM under canola was greater following faba bean-GRM than all other preceding crops except lentil. The ANM under canola was also greater following field pea and lentil than canola, faba bean-seed and wheat at Beaverlodge and Indian Head. Preceding faba bean-GRM and faba bean-seed crops at Lacombe resulted in 80–110 kg N ha−1 more ANM than preceding canola or wheat crops. At Scott, ANM under canola was greater following faba bean-GRM than all other preceding crops. 3.4. Apparent in-crop N mineralization under wheat On average across sites, ANM under wheat was significantly influenced by the preceding crops (Fig. 1b). The ANM was greater following faba bean-seed, faba bean-GRM, field pea and lentil than following canola and wheat. In addition, ANM under wheat was 15% greater following faba bean-GRM than faba bean-seed. For individual sites, there was a significant effect of preceding crops on ANM under wheat at Lacombe, Lethbridge, Melfort and Scott, but not at Brandon (Fig. 1b). At Lacombe, ANM under wheat was similar among all the preceding legume crops, and lower for preceding canola and wheat crops than all the preceding legume crops except field pea. At Lethbridge, ANM under wheat was greater for preceding faba bean-seed and faba bean-GRM crops than canola, lentil and wheat crops. At Scott, ANM under wheat was similar among all preceding legume crops, greater for preceding faba beanGRM, field pea and lentil than canola and wheat crops, but similar between preceding faba bean-seed and canola crops.

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Table 4 Above-ground N uptake linear response parameters to N fertilizer rate for each preceding crop in no-till sites in western Canada, where preceding crop × N fertilizer rate effect was significant and the response was linear for each preceding crop. Preceding cropa

ab

bb

R2b

Above-ground canola N uptake Beaverlodge Canola 57.7d Faba bean-seed 67.3cd Faba bean-GRM 107.2a Field pea 79.1bc Lentil 87.5b Wheat 78.7bc

0.51a 0.49ab 0.39abc 0.39abc 0.27c 0.31bc

0.86 0.86 0.54 0.61 0.49 0.28

Above-ground wheat N uptake All sites 88.2d Canola 106.3c Faba bean-seed Faba bean-GRM 128.4a 116.4b Field pea 123.1ab Lentil Wheat 81.9d

0.55a 0.44abc 0.35c 0.40bc 0.43abc 0.48ab

0.26 0.13 0.12 0.16 0.12 0.21

a Faba bean-seed, faba bean grown for seed; faba bean-GRM, faba bean grown as a green manure crop. b a, b and R2 represent the intercept, slope and coefficient of determination, respectively. All a and b values were significant (P < 0.05).

3.5. Above-ground canola N uptake On average across sites, preceding crop and N fertilizer rate but not their interaction significantly influenced above-ground canola N uptake (Fig. 2a). Above-ground canola N uptake was greatest for a preceding faba bean-GRM and lowest for a preceding canola crop, while it was similar for preceding faba bean-seed, field pea, lentil and wheat crops (Fig. 2a). Above-ground canola N uptake across the seven sites increased with increasing N fertilizer rate up to the maximum N rate applied (Fig. 2a). There was a significant preceding crop by N fertilizer rate interaction on canola N uptake only at Beaverlodge (Fig. 2b). An increase in N fertilizer above 60 kg N ha−1 in preceding faba bean-GRM and lentil plots did not result in a significant increase in canola N uptake. In contrast, canola N uptake increased up to 90 kg N ha−1 following faba bean-seed and wheat, and up to 120 kg N ha−1 following canola and field pea. The intercept of the linear regression line, which represents the estimated N uptake in the absence of N fertilization, was greater for preceding faba bean-GRM than all other preceding crops, greater for preceding field pea, lentil and wheat than canola and lower for preceding faba bean-seed than lentil (Table 4). With a greater soil N supply, as indicated by a greater intercept, the response to applied N as indicated by the slope of the regression is expected to be lower. The slope was greater for preceding canola and faba bean-seed than lentil crops, and greater for preceding canola than preceding wheat crops (Table 4). Canola N uptake was significantly influenced by preceding crop and N fertilizer rate at Indian Head (Fig. 2d), Lethbridge (Fig. 2f) and Scott (Fig. 2g). About 14–49 and 21–35 kg N ha−1 more N was taken up by canola following faba bean-GRM than all the other preceding crops at Indian Head and Scott, respectively. Also at Indian Head, preceding field pea and lentil crops resulted in similar canola N uptake, which was 25–60% greater (17–34 kg N ha−1 ) than for preceding canola, faba bean-seed and wheat crops (Fig. 2d). At Lethbridge, preceding faba bean-GRM, field pea, lentil and wheat crops resulted in similar canola N uptake, which was also greater than for a preceding canola crop (Fig. 2f). In addition, about 42 kg N ha−1 more N was taken up by canola at Lethbridge following faba beanGRM compared to faba bean-seed. In contrast, canola N uptake was significantly influenced only by preceding crop at Lacombe (Fig. 2e), and only by N fertilizer rate at Brandon (Fig. 2c) and Swift Current (Fig. 2h). At Lacombe,

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M. St. Luce et al. / Field Crops Research 191 (2016) 20–32

Fig. 2. Total above-ground canola N uptake as affected by preceding crops and N fertilizer rates in no-till sites in western Canada. Error bars indicate standard error of mean at each N fertilizer rate. The range of values for above-ground canola N uptake varies among graphs.

canola N uptake was highly variable and followed the order of faba bean-GRM > faba bean-seed > wheat = canola. Above-ground canola N uptake increased with increasing N fertilizer rate up to the maximum N rate applied at Brandon, Indian Head, Lethbridge and Scott, but up to 60 kg N ha−1 at Swift Current.

3.6. Above-ground wheat N uptake On average across sites, preceding crop, N fertilizer rate and their interaction significantly influenced above-ground wheat N uptake (Fig. 3a). While wheat N uptake increased linearly with N fertilizer rate following all the preceding crops, the linear responses varied

M. St. Luce et al. / Field Crops Research 191 (2016) 20–32

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Fig. 3. Total above-ground wheat N uptake as affected by preceding crops and N fertilizer rates in no-till sites in western Canada. Error bars indicate standard error of mean at each N fertilizer rate. The range of values for above-ground wheat N uptake varies among graphs.

among the preceding crops (Table 4). The intercept was greater for preceding faba bean-GRM than all other preceding crops except lentil (Table 4). In addition, the intercept was lower for preceding faba bean-seed, canola and wheat crops than all other preceding crops, greater for preceding faba bean-seed than preceding canola and wheat, and similar for preceding field pea and lentil crops. Consistent with this, the response to applied N (slope) was greater for preceding canola and wheat than preceding faba bean-GRM, and greater for preceding canola than field pea crops (Table 4). Preceding crop, N fertilizer rate and their interaction influenced above-ground wheat N uptake at Lacombe, but it was not linear for all the preceding crops (Fig. 3c). At Lacombe, there was no response to applied N fertilizer for preceding faba bean-GRM. However, an increase in N fertilizer above 30 kg N ha−1 in preceding lentil plots, above 60 kg N ha−1 in preceding faba bean-seed plots and above 90 kg N ha−1 in preceding canola, field pea and wheat plots did not result in a significant increase wheat N uptake. Preceding crop had a significant effect on wheat N uptake at Brandon (Fig. 3b), Melfort (Fig. 3e) and Scott (Fig. 3f) but not at Lethbridge

(Fig. 3d). At Brandon, wheat N uptake was 17–31 kg N ha−1 more following faba bean-GRM, field pea and lentil crops than canola, faba bean-seed and wheat crops. At Melfort, wheat N uptake was 129 and 122 kg N ha−1 following field pea-GRM and faba beanGRM, respectively compared to 85, 107 and 112 kg N ha−1 following wheat, canola and field pea, respectively. At Scott, wheat N uptake was 5–38% greater for preceding faba bean-GRM, field pea and lentil crops than preceding canola, faba bean-seed and wheat crops. Wheat N uptake was also greater for a preceding lentil than a preceding field pea crop at Scott. Wheat N uptake increased with increasing N fertilizer rate at Brandon and Scott, but only up to 90 kg N ha−1 at Lethbridge and Melfort. 3.7. Nitrogen budget Mineral N fertilizer accounted for the largest proportion of the total N inputs for canola and wheat production across sites, but this varied according to the preceding crop (Table 5). On average across all sites, N fertilizer accounted for 52–60% of the total

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Table 5 Nitrogen budget on average across no-till sites, and in low (Brandon) and high (Lacombe) apparent N mineralization no-till sites for each preceding crop when the economic optimum N was applied in western Canada. Preceding cropa

N inputb EONR

N balancec

N output Pre-plant NO3 -N

NO3 -N at harvestd

Unaccounted Ne

ANM

Above-ground N uptake

131.3 (79.0)f 143.1 (97.9) 174.5 (125.9) 149.9 (108.0) 152.7 (105.4) 140.9 (96.0)

67.6 74.2 75.3 77.6 72.8 67.0

13.6 (9.2)f 14.0 (9.1) 22.0 (12.8) 15.7 (10.3) 18.4 (11.3) 13.3 (9.4)

54.0 60.2 53.3 61.9 54.4 53.7

124.6 (67.2) 129.6 (77.6) 147.1 (83.3) 145.7 (80.8) 146.0 (79.6) 137.2 (74.5)

60.0 65.0 56.4 53.5 56.1 55.9

16.9 (12.4) 16.9 (12.0) 23.7 (15.2) 18.4 (13.4) 23.8 (15.4) 19.5 (13.7)

43.1 48.0 32.8 35.1 32.3 36.4

−1

kg N ha Canola All sites Canola Faba bean-seed Faba bean-GRM Field pea Lentil Wheat

120 120 120 120 120 120

18.9 20.4 29.0 25.7 25.1 20.7

60.0 76.9 100.8 81.8 80.4 67.2

Brandon Canola Faba bean-seed Faba bean-GRM Field pea Lentil Wheat

120 120 120 120 120 120

22.5 28.1 38.6 35.1 36.9 28.4

42.1 46.4 44.9 44.1 45.2 44.8

Lacombeg Canola Faba bean-seed Faba bean-GRM Wheat

60 60 60 60

21.3 25.4 33.3 25.3

105.3 190.4 215.3 110.9

168.4 (129.4) 213.3 (204.1) 234.1 (234.2) 188.5 (165.4)

18.2 62.5 74.5 7.7

10.8 (12.4) 17.2 (14.0) 19.5 (21.3) 16.4 (11.4)

7.4 45.3 55.0 −8.7

139.3 (85.2) 149.5 (101.8) 161.3 (124.3) 158.0 (110.0) 164.1 (117.2) 124.9 (75.1)

34.8 46.0 55.5 45.2 41.1 42.5

9.7 (7.1) 15.9 (8.3) 19.4 (10.3) 13.6 (8.9) 17.1 (8.1) 13.2 (8.8)

25.1 30.1 36.1 31.6 24.0 29.3

95.7 (44.8) 96.8 (43.8) 127.9 (69.1 117.1 (63.4) 113.5 (63.0) 87.0 (41.1)

75.6 74.0 68.3 81.1 75.3 79.5

18.7 (12.5) 17.8 (13.0) 16.4 (13.0) 21.3 (16.0) 16.7 (11.9) 13.4 (10.8)

56.8 56.2 51.8 59.7 58.6 66.0

175.4 (140.7) 191.4 (165.9) 196.4 (183.1) 190.0 (167.2) 213.8 (185.9) 156.5 (134.2)

24.2 38.9 58.4 42.1 38.6 46.5

16.6 (7.8) 24.3 (13.3) 28.7 (20.8) 20.5 (13.9) 28.1 (15.5) 23.4 (17.8)

7.5 14.6 29.7 21.6 10.5 23.0

Wheat All sites Canola Faba bean-seed Faba bean-GRM Field pea Lentil Wheat

90 90 90 90 90 90

16.7 16.6 24.5 20.9 25.2 15.8

67.4 88.9 102.3 92.3 89.7 61.6

Brandon Canola Faba bean-seed Faba bean-GRM Field pea Lentil Wheat

120 120 120 120 120 120

12.0 13.1 19.5 24.4 19.1 10.7

39.2 37.7 56.6 53.8 49.7 35.6

Lacombe Canola Faba bean-seed Faba bean-GRM Field pea Lentil Wheat

60 60 60 60 60 60

24.1 19.7 38.7 28.4 40.5 23.9

115.4 150.5 156.1 143.7 151.9 119.1

a b c d e f g

Faba bean-seed, faba bean grown for seed; faba bean-GRM, faba bean grown as green manure. EONR, economic optimum N rate; pre-plant NO3 -N, pre-plant soil NO3 -N content in the 0–60 cm soil depth; ANM, apparent in-crop N mineralization. N balance, N inputs − N output. NO3 -N at harvest, soil NO3 -N content in the 0–60 cm soil depth at harvest. Total N applied − N recovered (above-ground crop N uptake + NO3 -N at harvest). Corresponding data for the control treatment where starter N fertilizer was applied. Data for field pea and lentil were excluded at Lacombe under canola due to extremely poor and variable canola emergence in these plots.

N input for canola and wheat production when canola and wheat were the preceding crops. When canola and wheat were preceded by legumes, N fertilizer accounted for 42–55% of the total N input. Pre-plant soil NO3 -N content accounted for only 8–12% of the total N input across sites. For canola production across sites, ANM following the legumes was 76 (faba bean-seed) to 101 kg N ha−1 (faba bean-GRM), accounting for 35–40% of the total N input, but was 60–67 kg N ha−1 following canola and wheat, which accounted for 30–32% of the total N input. For wheat production across sites, ANM following the legumes was 88 (faba bean-seed) to 102 (faba bean-GRM) kg N ha−1 , accounting for 44–47% of the total N input, but was 61–67 kg N ha−1 following canola and wheat, accounting for 37–39% of the total N input. On average, about 66% (preceding canola) to 70% (preceding faba bean-GRM), and 74% (preceding faba

bean-GRM) to 80% (preceding canola) of the total available N was recovered in the above-ground canola and wheat biomass, respectively. In addition, ANFR was about 35–44% in the canola study on average across sites; lowest for preceding field pea and greatest for preceding canola. On average across sites, ANFR in the wheat study was 55% and 60% following wheat and canola, respectively. Conversely, 41–53% was recovered following the legumes; lowest for faba bean-GRM and greatest for faba bean-seed and field pea. On average across sites, there was a positive N balance (surplus) following all preceding crops in both the canola and wheat studies (Table 5). The N balance ranged from 67 to 78 kg N ha−1 in the canola study. In the wheat study, the N balance was 35 kg N ha−1 for preceding canola, 41– 46 kg N ha−1 for preceding wheat, lentil, field pea and faba bean-seed, and 56 kg N ha−1 for preceding faba bean-GRM.

M. St. Luce et al. / Field Crops Research 191 (2016) 20–32

About 19 to 29% of the N balance in the canola study remained as soil NO3 -N at harvest; greatest for preceding faba bean-GRM and lowest for faba bean-seed (Table 5). In the wheat study, we estimated that 28–31% of the N balance remained as soil NO3 -N at harvest in the preceding canola, field pea and wheat plots, while 35%, 35% and 42% remained as soil NO3 -N in the preceding faba bean-seed, faba bean-GRM and lentil plots, respectively. On average across sites, the unaccounted N represented 21–28% and 12–18% of the total N supplied in the canola and wheat study, respectively. The N fertilizer required for optimum canola and wheat production were lower at Lacombe than at Brandon (Table 5). The N fertilizer at Brandon accounted for 59–62% and 61–72% of the total N input for canola and wheat production, respectively. In contrast, N fertilizer at Lacombe accounted for 19–32% and 24–30% of the total N input for canola and wheat production, respectively. The ANM accounted for 22–29% of the total N input for canola and wheat production at Brandon but 56–70% at Lacombe. At Brandon, 67–73% of the total available N was recovered in the above-ground canola biomass. Also at Brandon, 52% and 56% of the total available N was recovered in the above-ground wheat biomass following wheat and canola, respectively, while 60% and 65% was recovered following lentil and faba bean-GRM, respectively. The ANFR was about 43% (preceding faba bean-seed) to 55% (preceding lentil) in the canola study, and 38% (preceding wheat) to 49% (preceding faba beanGRM) in the wheat study at Brandon. These results differed to what was observed at Lacombe where 50% less N fertilizer (60 kg N ha−1 ) was applied to canola and wheat. At Lacombe, 90–96% of the total available N was recovered in the above-ground canola biomass following canola and wheat, while about 76% was recovered following faba bean-seed and faba bean-GRM. About 77% (preceding faba bean-GRM and wheat) to 88% (preceding canola) of the total available N was recovered in the above-ground wheat biomass at Lacombe. In addition at Lacombe, ANFR was about 38% and 65% in the canola study following wheat and canola, respectively but about 15% following faba bean-seed; it was negative following faba bean-GRM. For wheat production at Lacombe, we estimated that the ANFR was 37% (preceding wheat) to 58% (preceding canola) following canola, lentil, faba bean-seed, field pea and wheat but 22% following faba bean-GRM. There was a positive N balance following each preceding crop at Brandon and Lacombe, which was generally greater at Brandon than at Lacombe. In the canola study at Brandon, the N balance ranged from 53 to 65 kg N ha−1 . The N balance in the wheat study at Brandon was 68 kg N ha−1 following faba bean-GRM but 81 kg N ha−1 following field pea. In the wheat study at Lacombe, the N balance was 7.7 and 18.2 kg N ha−1 for preceding wheat and canola, respectively but was up to 63 and 75 kg N ha−1 for preceding faba bean-seed and faba bean-GRM, respectively. In the wheat study at Lacombe, the N balance was only 24 kg N ha−1 for preceding canola but was 46 and 58 kg N ha−1 for preceding wheat and faba bean-GRM, respectively. For canola production at Brandon, about 42% of the N surplus remained as soil NO3 -N following faba bean-GRM and lentil compared to 26–28% following faba bean-seed and canola. For canola production at Lacombe, only 26–28% of the N balance remained as soil NO3 -N following faba bean-seed and faba bean-GRM, while about 59% remained as soil NO3 -N following canola; the N balance was smaller than the soil NO3 -N content for preceding wheat. The unaccounted N in the canola study at Brandon ranged from 32 kg N ha−1 (faba bean-GRM and lentil) to 48 kg N ha−1 (faba beanseed), representing 16–25% of the total N input. In the canola study at Lacombe, the unaccounted N for preceding faba bean-GRM and faba bean-seed was 6–7 times greater than for preceding canola; unaccounted N was negative for preceding wheat. For wheat production at Brandon, 17–26% of the N surplus remained as residual soil NO3 -N. About 73% and 49% of the N surplus in the lentil and faba bean-GRM plots, respectively, remained as residual soil NO3 -N at

29

Lacombe for wheat production. The unaccounted N for wheat production at Brandon ranged from 52 kg N ha−1 (faba bean-GRM) to 66 kg N ha−1 (preceding wheat), representing 26–40% of the total N input. At Lacombe, the unaccounted N for wheat production ranged from 7 to 30 kg N ha−1 ; lowest for preceding canola and greatest for preceding faba bean-GRM.

4. Discussion 4.1. Effect of preceding crop on above-ground biomass and N returned, pre-plant soil NO3 -N and ANM The generally greater above-ground biomass returned by preceding canola and wheat than faba bean, field pea and lentil was expected. Excessive rainfall at most sites in 2010 substantially reduced the biomass yields, particularly for canola, field pea and lentil. The consistently greater quantity of above-ground residue N returned by faba bean-GRM was due to the fact that the entire above-ground biomass was returned as residues in comparison with the other preceding crops, particularly the legumes harvested for seed. On average across the 13 site-years, 72, 119, 122, 75 and 69 kg N ha−1 were exported as seeds from the soil-plant system for preceding canola, faba bean-seed, field pea, lentil and wheat crops, respectively (data not shown). The greater pre-plant soil NO3 -N content and ANM following legumes than non-legumes could be related to inputs of biologically N2 fixed N (Chalk, 1998; Walley et al., 2007; Peoples et al., 2009), greater mineralization of above- and below-ground residues of the legumes due to their quality, e.g., C/N ratio (Kirkegaard et al., 2008; Gan et al., 2011b; Arcand et al., 2014; St. Luce et al., 2014; O’Dea et al., 2015), the residual carry-over of unutilized (“spared”) soil mineral N, and/or reduced rate of immobilization of soil NO3 N by the legume residues (Angus et al., 2015). Also, there were possible changes in microbial community structure and activity (Biederbeck et al., 2005; Tiemann et al., 2015), which in turn can influence N mineralization rates. However, N mineralization during the growing season may not necessarily be greater following legumes than following non-legumes (Gan et al., 2010; BedardHaughn et al., 2013). For example, Bedard-Haughn et al. (2013) found similar gross N mineralization rates after lentil, field pea and wheat at two sites in Saskatchewan, Canada. They suggested that the higher quality (C/N ratio) of the legume residues appears to be offset by their smaller quantities. We did not report the C/N ratios of the preceding crops in our study, but the N concentrations in the biomass returned were greater for the legumes than nonlegumes. However, the quantity of biomass returned was greater for the non-legumes than legumes. Our ANM values were within the range reported in this region (Beckie et al., 1997; Soon et al., 2006; Gan et al., 2010). We expected to see greater ANM following faba bean-GRM than the other preceding legumes. However, this was not always the case, suggesting that the greater amount of above-ground residue N returned by a green manure crop may not necessarily translate into greater N mineralization in the subsequent growing season compared to a legume where the seed is exported. It is likely that the impact of faba bean-GRM on ANM was reduced due to slower than expected decomposition of the substantially greater amount of N-rich residues since they were left on the soil surface. Nonetheless, it is important to note that preceding faba bean-GRM always resulted in the highest subsequent crop N uptake in the absence of N fertilizer than the other preceding legumes, possibly due to additional non-N benefits (Stevenson and van Kessel, 1996; Angus et al., 2015). While we did not determine below-ground residue N addition, which varies according to plant species and may account for up to 70% of total plant N returned to soils (Gan et al., 2010;

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Arcand et al., 2013, 2014), below-ground residue N can supply a substantial amount of N to succeeding crops (Arcand et al., 2014). The lack of consistent preceding crop effect on ANM could be due to the wetter than normal conditions at some sites, which probably limited N mineralization. The wetter than normal conditions probably also increased the risk of N losses. Background N mineralization was high at some sites, and therefore variation in N mineralization due to the preceding crops was difficult to detect. The similarity in ANM among the preceding crops at Brandon under both canola and wheat, and at Lethbridge and Swift Current under canola could be due to the low SOM and initial NO3 -N content at these sites. At Indian Head, the high clay content probably fixed a portion of the mineralized N to a greater extent for the preceding non-legumes than the legumes. Nonetheless, our method used to determine N mineralization was probably not entirely precise to account for actual N mineralization. A more precise measurement of the degree of decomposition and N supply using 15 N labelling would have allowed us to better determine the impact of the preceding crops on N dynamics and the magnitude of soil N supply. 4.2. Effect of preceding crops and N fertilizer rate on above-ground N uptake Canola grown after canola and wheat grown after wheat consistently resulted in the lowest above-ground crop N uptake. There was possibly increased N immobilization in preceding canola and wheat plots due to the higher C/N ratio of these residues, coupled with the fact that the crop residues were left on the soil surface in these no-till soils (Malhi et al., 1996, 2011). Although pest and disease levels in our study were generally low, it is likely that pest and disease incidence were slightly higher in the canola-canola and wheat-wheat plots. Another explanation could be the possible lower water availability (Miller et al., 2003; Cutforth et al., 2013; Angus et al., 2015), and other non-nutritional benefits (Janzen and Schaalje, 1992) provided by the non-legumes. Water availability in these semi-arid to sub-humid regions is usually a major limiting factor for crop production (Campbell et al., 2007; Sadras and Richards, 2014). Hence, greater average above-ground crop N uptake following faba bean-GRM could also be due to increased water conservation resulting from the greater soil cover relative to that provided by the other preceding legume residues (Wang et al., 2015). By relating wheat grain yield to above-ground wheat N uptake, we previously showed that wheat grain yield at Brandon was predominantly due to increased N availability following the legumes as compared to the non-legumes (St. Luce et al., 2015). However, at the other sites, factors other than N played a role in influencing wheat grain yield following the legumes (St. Luce et al., 2015). Other studies also reported that greater crop response to N fertilizer rate following legumes than non-legumes may be due to either increased N availability, factors other than N, or a combination of both (Chalk, 1998). If the legume N benefit was sufficiently large relative to the contribution from native SOM, there would be a significant preceding crop by N fertilizer rate interaction in all cases, with the benefit of the legume decreasing as N application rate increases. The interactive effect of preceding crops and N fertilizer rate on above-ground N uptake only occurred at Beaverlodge for canola, and at Lacombe and on average across sites for wheat. If we consider the lower N rates (0–30 kg N ha−1 ), average canola N uptake at these rates was greater following faba bean-GRM than all other preceding crops, while average wheat N uptake was greater following faba bean-GRM than all other preceding crops, except field pea at 30 kg N ha−1 and lentil at both N rates. In addition to the influence of preceding crops, crop N uptake response to N fertilizer application is also dependent on a combination of several factors, including background SOM and soil available N content, soil tex-

ture, pH, the quantity and distribution of precipitation, temperature during the growing season and pest and disease incidence (Fageria and Baligar, 2005; Schillinger et al., 2008; Harker et al., 2012; St. Luce et al., 2013). The greater pre-plant soil NO3 -N, ANM and SOM levels at Beaverlodge and Lacombe may also account for the generally lower crop N uptake response at these sites. Conversely, crop N uptake response to N fertilizer rate was greater in sites with lower pre-plant soil NO3 -N content and ANM, such as Brandon and Indian Head. At Indian Head, the high clay content at Indian Head probably also fixed a portion of the available N, thereby reducing the N available for crop uptake. As previously reported for canola and wheat yield (O’Donovan et al., 2014; St. Luce et al., 2015), the ability of faba bean-seed to enhance soil N supply was usually lower than the other preceding legumes. Although faba bean is reported to have a greater N2 fixation potential than field pea and lentil (Peoples et al., 2009; Williams et al., 2014), our results was due to lower ANM following faba bean-seed than the other preceding legumes. This could be related to poor crop performance of the faba bean-seed that significantly impacted its biomass production and thus, the quantity of N-rich residues returned, particularly in 2009. The increase in above-ground N uptake with N fertilizer rate was expected since canola and wheat have high N demands, especially when high-yield modern cultivars are grown (Harker et al., 2012). However, there was a limited response to N fertilizer rate at Lacombe, which was more pronounced in 2010 than 2011. This was due in part to high ANM; on average, ANM was about 10% greater in 2010 than 2011. The Lacombe site also had the highest particulate organic matter N (POM) (St. Luce et al., 2013), a labile form of SOM that could contribute to soil N supply through mineralization (St. Luce et al., 2014). The high SOM and sand contents at Lacombe are ideal for rapid N mineralization and easier access to available N. In addition, excessive moisture due to high precipitation in 2010 and possible cutworm damage to canola may have also reduced the impact of N fertilizer rate on canola N uptake at Lacombe (O’Donovan et al., 2014). Similarly, O’Donovan et al. (2014) found no effect of N fertilizer rate on canola seed yield at Lacombe.

4.3. Nitrogen budget The N budget is a useful tool for understanding N cycling in the soil-plant system in various cropping systems, and for assessing the potential for environmental pollution caused by inefficient or excessive N application (Ross et al., 2008; Pathak et al., 2010). Our results, averaged across various sites in western Canada, suggest that optimal canola and wheat production can be achieved with lower quantities of N fertilizer input when preceded by legumes compared to canola and wheat, due to greater pre-plant soil NO3 -N content and ANM following the legumes. This was in agreement with previous findings for canola and wheat (Stevenson and van Kessel, 1996; O’Donovan et al., 2014; O’Dea et al., 2015; St. Luce et al., 2015), as well as for other crops (Cela et al., 2011; N’Dayegamiye et al., 2015). Moreover, the N surplus was particularly pronounced when canola and wheat were preceded by legumes rather than by canola or wheat, with an even greater magnitude in sites with higher SOM content and ANM. The N surplus is an indicator of increased N availability. Other studies also reported greater N balances in cropping systems with legumes than nonlegumes (Ross et al., 2008; Malhi et al., 2009). On average across sites, ANFR was about 35–60%, which was in agreement with studies showing that N fertilizer-use efficiency is usually less than 40% (Fageria and Baligar, 2005; Soon et al., 2006; Malhi et al., 2014). This therefore suggests that about 40–65% of crop N uptake was possibly derived from ANM.

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The unaccounted N represents N that was probably lost during the growing season through various pathways or immobilized within SOM, and may reflect an excess of N fertilizer applied. On average, the unaccounted N tended to be elevated following the legumes than following canola or wheat due to greater supply of available N following the legumes. The higher precipitation at most sites when canola was grown compared to when wheat was grown, as well as higher N application rates in the canola than the wheat study could explain the greater quantity of unaccounted N in the canola than the wheat study, on average across sites. The lower N fertilizer requirements at Lacombe than at Brandon resulted in lower quantities of unaccounted N at Lacombe. Given that aboveground canola N uptake was influenced only by N fertilizer rate at Brandon, it was not surprising that the N balance was similar among the preceding crops, except for preceding faba bean-seed. The greater N surplus following faba bean-seed in the canola study at Brandon was likely due to low ANFR and low recovery of total available N. In contrast, the lower N surplus at Brandon in the wheat study following faba bean-GRM than the other preceding crops could be related to the higher N use-efficiency following faba bean-GRM. Although root diseases were not severe, the root diseases were possibly lower in the faba bean-GRM-wheat plots, thus enabling the wheat to recover more N fertilizer from the soil (Angus et al., 2015). Apparent recovery of total available N and N fertilizer by wheat at Brandon were lowest for preceding wheat. Cereals such as wheat are also known to absorb more water from the soil and at greater depths than legumes, thus leaving less water available for subsequent crops (Miller et al., 2003; Cutforth et al., 2013). Low water availability can adversely affect crop growth and nutrient uptake efficiency. Increased N availability coupled with lower ANFR following faba bean-GRM and faba bean-seed in the canola study, and following faba bean-GRM in the wheat study could explain the much greater N balance for these legume preceding crops at Lacombe. In the wheat study at Lacombe, the N balance was 2 times greater, with the unaccounted N being 4 times greater, following faba bean-GRM than canola. Another interesting finding at Lacombe was that the N balance was twice as high when canola was grown after canola than wheat, and when wheat was grown after wheat than canola. This was probably due to lower diseases and better N use-efficiency when canola was preceded by wheat and when wheat was preceded by canola. One of the most interesting findings in our study was that ANM was the primary contributor to total N inputs (up to 70%) and crop N uptake (up to 100%) at Lacombe, due mostly to a high concentration of labile SOM, particularly POM (St. Luce et al., 2013). Moreover, this high background soil N supply at Lacombe was enhanced further through the inclusion of legumes in the rotations, whereby aboveground N uptake for optimum yield could be achieved by reducing the N fertilizer input by > 50%, and in some cases in the absence of N fertilizer. However, in sites with low inherent soil fertility or low N mineralization potential, such as Brandon, the use of legumes to reduce the dependence on N fertilizer input may not be attainable after 1 year. Given that the N fertilizer was banded beneath the soil surface, N losses through volatilization were most likely very low to negligible in our study (Fenn and Miyamoto, 1981; Malhi et al., 1996). A portion of the unaccounted N was probably immobilized within SOM (Malhi et al., 2009), especially in sites with lower available N such as Brandon. In addition, a portion of the N input may have been lost through denitrification and leaching below 60 cm (Malhi et al., 2009; Bedard-Haughn et al., 2013), particularly in 2010 when precipitation was higher at most sites. Nitrous oxide emissions through autotrophic nitrification, particularly in soils with high N contents (Wrage et al., 2001), is an important pathway of N loss in this semiarid to sub-humid environment (Ma et al., 2008; Bedard-Haughn et al., 2013). Hence, a portion of the N was probably lost through

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this pathway at all the sites. At Lacombe, particularly in 2010, N was probably lost to a greater extent through leaching than through denitrification due to the high sand content at Lacombe. Interestingly, there was a lower potential for N loss at Lacombe than at Brandon since external N inputs were 50% lower at Lacombe due to greater N input through N mineralization. The potential for N losses at Lacombe were further elevated by legumes. 5. Conclusion In this study conducted in no-till soils in the Canadian prairies, we were able to demonstrate how preceding crops influence preplant soil NO3 -N content and ANM, how they affect N budgets and the response of subsequent canola and wheat to N fertilizer application. Our results indicate that preceding faba bean-GRM, field pea and lentil crops can increase pre-plant soil NO3 -N content and ANM, and hence reduce the reliance of canola and wheat on external N inputs as compared to preceding canola and wheat crops. Our N budget provided further evidence to highlight the importance of N mineralized from SOM and crop residues in meeting crop N requirements. In a site with high labile SOM and ANM, ANFR was 0–65%, suggesting that about 35–100% of crop N uptake was possibly derived from ANM. Given that the surplus and unaccounted N were generally greater for preceding faba bean-GRM, field pea and lentil, reduction in fertilizer rate to compensate for increased soil N availability is therefore critical for reducing the N surplus in cropping systems, which may be at risk to be lost from the soil-plant system. Overall, our findings indicate that the inclusion of legumes can increase soil fertility and enhance soil N cycling in canola and wheat cropping systems, but the magnitude of the benefit and the time required to achieve such benefits is dependent to a large extent on the legume species, the purpose for which it was grown (seed vs. green manure) and site-specific conditions. Acknowledgements This project was funded by the Sustainable Agricultural Environment Systems (SAGES) program of Agriculture and Agri-Food Canada (Grant No. 1475). We thank Mike Svistokski, Ray Smith, Randy Shiplack, Jennifer Zuidhof, Ginette Decker, Karen Terry, Lorne Nielson, Arlen Kapiniak, Kevin Willoughby, Orla Willoughby, Larry Michielson, Greg Semach, Liz Sroka, Patty Reid, Irene Murray, Randall Brandt and Lee Poppy of Agriculture and Agri-Food Canada for their excellent technical support. References Angus, J.F., Kirkegaard, J.A., Hunt, J.R., Ryan, M.H., Ohlander, L., Peoples, M.B., 2015. Break crops and rotations for wheat. Crop Pasture Sci. 66, 523–552. Arcand, M.M., Knight, J.D., Farrell, R.E., 2013. Estimating belowground nitrogen inputs of pea and canola and their contribution to soil inorganic N pools using 15 N labeling. Plant Soil 371, 67–80. Arcand, M.M., Lemke, R., Farrell, R.E., Knight, J.D., 2014. Nitrogen supply from belowground residues of lentil and wheat to a subsequent wheat crop. Biol. Fertil. Soils 50, 507–515. Beckie, H.J., Brandt, S.A., Schoenau, J.J., Campbell, C.A., Henry, J.L., Janzen, H.H., 1997. Nitrogen contribution of field pea in annual cropping systems. 2. Total nitrogen benefit. Can. J. Plant Sci. 77, 323–331. Bedard-Haughn, A., Comeau, L.P., Sangster, A., 2013. Gross nitrogen mineralization in pulse-crop rotations on the Northern Great Plains. Nutr. Cycl. Agroecosyst. 95, 159–174. Biederbeck, V.O., Zentner, R.P., Campbell, C.A., 2005. Soil microbial populations and activities as influenced by legume green fallow in a semiarid climate. Soil Biol. Biochem. 37, 1775–1784. Campbell, C.A., Zentner, R.P., Basnyat, P., Wang, H., Selles, F., McConkey, B.G., Gan, Y.T., Cutforth, H.W., 2007. Water use efficiency and water and nitrate distribution in soil in the semiarid prairie: effect of crop type over 21 years. Can. J. Plant Sci. 87, 815–827. Cela, S., Santiveri, F., Lloveras, J., 2011. Optimum nitrogen fertilization rates for second-year corn succeeding alfalfa under irrigation. Field Crops Res. 123, 109–116.

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