The green manure value of seven clover species grown as
annual crops on low and high fertility temperate soils
S. M. Ross1, J. R. King1, R. C. Izaurralde2, 4, and J. T. O’Donovan3
1
Department of Agricultural, Food and Nutritional Science, 4-10 Agriculture-Forestry Centre, University of Alberta,
Edmonton, Alberta T6G 2P5; 2Joint Global Change Research Institute, 8400 Baltimore Ave., Suite 201, College
Park, MD 20740-2496, USA; and 3Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail,
Lacombe, Alberta T4L 1W1. Received 30 September 2008, accepted 23 December 2008.
Ross, S. M., King, J. R., Izaurralde, R. C. and O’Donovan, J. T. 2009. The green manure value of seven clover species grown
as annual crops on low and high fertility temperate soils. Can. J. Plant Sci. 89: 465476. Annual and perennial clover species
may differ in green manure value. Seven clover (Trifolium) species were grown as annual crops on low fertility (Breton) and
high fertility (Edmonton) soils in Alberta. Four annual clovers [balansa (T. michelianum Savi), berseem (T. alexandrinum
L.), crimson (T. incarnatum L.), and Persian (T. resupinatum L.)], three perennial clovers [alsike (T. hybridum L.), red
(T. pratense L.), and white Dutch (T. repens L.)] and a non-legume reference crop [fall rye (Secale cereale L.)] were
ploughed-down as green manure in autumn, and followed by barley (Hordeum vulgare L.). Annual clovers had greater
biomass yields than perennial clovers, and berseem clover had the highest yield. At Breton, mean biomass N content was
greater for perennial clovers (2.9 g N kg1) than annual clovers (1.9 g N kg 1). Clover biomass at Breton yielded an
average of 77 kg N ha1, with N derived from the atmosphere averaging 88% by N difference method and 75% by 15N
natural abundance method. At Edmonton, the green manures had few effects on soil nitrate and subsequent barley yields.
At Breton, all clover green manures except balansa increased barley yields, and grain yields were greater following
perennial clovers than annual clovers in one year. Annual clovers will provide forage biomass and add N in areas where
rainfall is adequate, and they may be preferable under zero tillage. However there is no advantage of annual clovers,
relative to perennial clovers, in terms of N supply.
Key words: Green manure, clover, Trifolium species, nitrogen fixation
Ross, S. M., King, J. R., Izaurralde, R. C. et O’Donovan, J. T. 2009. Utilité comme engrais vert de sept espèces de trèfle
cultivées annuellement sur des sols tempérés peu ou très fertiles. Can. J. Plant Sci. 89: 465476. Les espèces annuelles et
vivaces de trèfle (Trifolium) pourraient ne pas avoir la même utilité comme engrais vert. Pour le savoir, les auteurs en ont
cultivé sept espèces comme une culture annuelle sur des sols peu fertiles (Breton) ou très fertiles (Edmonton) de l’Alberta.
Quatre espèces étaient annuelles [trèfle de Micheli (T. michelianum Savi), trèfle d’Alexandrie (T. alexandrinum L.), trèfle
incarnat (T. incarnatum L.) et trèfle de Perse (T. resupinatum L.)] et trois vivaces [trèfle hybride (T. hybridum L.), trèfle
rouge (T. pratense L.) et trèfle rampant (T. repens L.)]; s’y ajoutait une culture témoin ne faisant pas partie des
légumineuses [seigle d’automne (Secale cereale L.)]. Les cultures ont été enfouies à l’automne pour servir d’engrais vert,
puis on a semé de l’orge (Hordeum vulgare L.). Les espèces de trèfle annuelles ont donné plus de biomasse que les espèces
vivaces, le trèfle d’Alexandrie enregistrant le meilleur rendement. À Breton, la teneur en N moyenne de la biomasse était
plus élevée pour les espèces vivaces (2,9 g de N par kg) que pour les espèces annuelles (1,9 g de N par kg). Toujours à
Breton, la biomasse du trèfle a donné en moyenne 77 kg de N par hectare, l’azote issu de l’atmosphère en représentant en
moyenne 88 % quand on calcule sa proportion par soustraction et 75 % quand on la calcule en fonction de l’abondance de
l’isotope 15N dans la nature. À Edmonton, les engrais verts ont eu peu d’incidence sur les nitrates du sol et le rendement
subséquent de l’orge. À Breton, tous les engrais verts à base de trèfle sauf le trèfle de Micheli ont accru le rendement de
l’orge et le rendement céréalier était plus important après la culture du trèfle vivace qu’après celle du trèfle annuel. Les
espèces annuelles de trèfle produisent de la biomasse fourragère et ajoutent de l’azote au sol des endroits où il pleut
suffisamment; il pourrait être préférable de les cultiver quand on pratique le non-travail du sol. Néanmoins, cultiver des
espèces annuelles plutôt que des espèces vivaces de trèfle ne présente aucun avantage en ce qui concerne l’apport de N.
Mots clés: Engrais vert, trèfle, Trifolium sp., fixation de l’azote
Legumes are key components of sustainable cropping
systems. Legume green manures provide N to cropping
systems, and they can also reduce weeds and pests,
increase nutrient retention, improve N-use efficiency,
and reduce soil erosion (Cherr et al. 2006; Fageria 2007).
Annual legume crops and green manures can add
diversity to temperate cropping systems where cereals
4
Present address: Joint Global Change Institute, 5825
University Research Court, Suite 3500, College Park,
MD 20740, USA.
Abbreviations: NA, natural abundance of 15N; ND,
N difference procedure; WAP, weeks after planting
465
466 CANADIAN JOURNAL OF PLANT SCIENCE
and oilseeds predominate (Rice et al. 1993). Green
manures can improve the physical, chemical and biological properties of soil (Biederbeck et al. 1998; Abdallahi
and N’Dayegamiye 2000). The primary benefit of green
manures may be the replenishment of organic N reserves
in soil (Janzen et al. 1990).
The use of green manures may be more economically
justified when they provide multiple benefits such as N
supply, pest reduction, and improvement of soil quality
(Cherr et al. 2006). Long-term use of legume green
manure was more cost effective than fallow (in a 3-yr
rotation) due to increases in subsequent grain protein,
improvement in the N-supplying power of soil, and
weed suppression (Zentner et al. 2004). In areas with
adequate rainfall, such as the Parkland and Boreal
regions of Alberta, forage legumes offer potential to
provide both forage yield and green manure benefits.
Berseem clover provided similar benefit to a subsequent
crop when managed solely as a green manure or when
harvested once for forage and regrowth was ploughed
down (Westcott et al. 1995; Shrestha et al. 1999).
The effects of green manures on subsequent crops
vary among studies in Canada and northern USA.
Under dryland conditions, legume green manures may
deplete soil moisture and reduce subsequent grain yields
(Schlegel and Havlin 1997). In long-term experiments
under semiarid conditions, Zentner et al. (2004) observed gradual increases in grain N yields with early
seeding and July plough-down of green manures. In
northern Alberta, subsequent barley grain yields varied
with legume green manure species, year, and timing of
incorporation (Rice et al. 1993). Barley grain yields were
higher with green manures in some cases, and lower in
others. Legume green manures usually resulted in
greater barley dry matter yield and N uptake, than
with non-legumes in a study in Alaska (Sparrow et al.
1995). In Montana, legume plough-down had a greater
effect on a low fertility soil than a high fertility soil
(Westcott et al. 1995). In Quebec, N’Dayegamiye and
Tran (2001) reported that green manures accounted for
2531% of total N uptake in the subsequent wheat crop,
and suggested that plough-down in late summer or early
autumn allowed for better synchrony with the N needs
of the crop.
Annual clovers are not commonly grown in Canada.
Some annual clover species produced promising yields
in trials of annual forage legumes in southern Alberta
and northeast Saskatchewan (Fraser et al. 2004).
Further research was recommended to evaluate their
green manure potential. Research in Sweden identified
differences among five clover species grown as green
manures (Kirchmann 1988). Research on green manures
in western Canada has largely focused on medium to
large-seeded pulse crops (Janzen et al. 1990; Biederbeck
et al. 1993) and biennial sweetclover (Melilotus officinalis L. Lam.) (Blackshaw et al. 2001). A few green manure
studies in Canada and northern USA have included
annual clover species (Westcott et al. 1995; Shrestha
et al. 1999; Bullied et al. 2002), but further research is
needed.
The objective of this study was to test the green
manure value of seven clover species as measured by
their biomass productivity, nitrogen content, and their
effect on subsequent crops. The studies reported herein
compare the productivity of annual and perennial
clovers grown as annual crops as well as their effects
on soils in central Alberta that represent two extremes of
natural fertility.
MATERIALS AND METHODS
Field experiments were conducted at Breton (53807?N,
114828?W) and Edmonton (53825?N, 113833?W) from
1996 to 1998. Breton normally has higher rainfall and
lower mean temperatures than Edmonton, with longterm normals for 19511980 of annual precipitation
of 547 mm at Breton and 452 mm at Edmonton, and
frost-free days of 80 at Breton and 109 at Edmonton
(Izaurralde et al. 1993). Rainfall for May to September
was normal to above normal levels in 1996, 1997 and
1998 at Edmonton and Breton. The sites represented the
two main soil orders in north-central Alberta. The
experimental sites at Breton had less soil N and other
soil nutrients than the Edmonton sites (Table 1). The
soil at Breton is an Orthic Gray Luvisol (Albic Luvisol
in FAO soil taxonomy) that developed under boreal
forest vegetation. Gray Luvisols are acid, leached,
degraded soils with low organic matter content. These
soils form hard surface crusts upon drying, causing
Table 1. Soil test parameters for two depths at Breton and Edmonton sites
Breton
1996
Soil parameter
015 cm
1
NO3-N (mg kg )
Total N (%)
Organic matter (%)
pH
z
BDL, below detectible limit.
1
0.13
3.3
6.1
Edmonton
1997
1530 cm
1
0.07
2.3
6.1
015 cm
3
0.16
3.8
6
1996
1530 cm
z
BDL
0.08
2.5
6.1
1997
015 cm
1530 cm
015 cm
1530 cm
38
0.57
12.1
6.5
40
0.38
8.5
5.7
16
0.59
13.6
5.3
22
0.46
11.1
5.5
ROSS ET AL. * GREEN MANURE VALUE OF SEVEN CLOVERS
management problems such as water runoff and poor
seedling emergence. The soil at Edmonton is an Orthic
Black Chernozem (Haplic Chernozem in FAO taxonomy). The Black Chernozemic soils are the most fertile
of the prairie soils, with high organic matter, high
nutrient status, and good structure.
The experimental design was randomized complete
block, replicated four times, with nine crop treatments
(seven clover species, rye, and without crop). Plots were
2 m by 6 m in 1996 and 2 m by 5 m in 1997. Seeding
occurred on: Jun. 03 and 04 at Edmonton and Jun. 07
at Breton in 1996; and May 30 at Edmonton and Jun. 09
at Breton in 1997. The clovers and rye were seeded at
recommended rates for forage production (Table 2).
Clover seeds were inoculated with appropriate strains of
Rhizobium leguminosarum biovar trifolii, then broadcast
onto the soil surface by hand and incorporated by hand
raking. Kodiak fall rye was seeded with a four-row
Fabro drill at Edmonton and with a single-row cone
seeder at Breton at a rate of 70 kg ha1, at approximately 2 cm depth. Brown mustard (Brassica juncea L.)
was added to all plots, including mustard-only control
plots at 15 seeds m2. Mustard represented an annual
weed, and weed suppression by clovers was reported by
Ross et al. (2001). The 1996 and 1997 Breton experiments followed oats (Avena sativa L.) and barley
(Hordeum vulgare L.), respectively. Edmonton experiments followed tilled fallow. The 1996 Edmonton site
had been limed in 1995 to raise the pH. The 1996 and
1997 Breton sites were on adjacent areas that were
fertilized with 50 kg ha 1 P2O5 (0450) and 60 kg ha1
K2O (0060) in May 1996. No nitrogen fertilizer was
added.
To measure crop emergence and establishment, plant
numbers were counted in one 0.25-m 2 area in each plot
at 67 wk after planting (WAP) in 1996 and at 46 WAP
in 1997. In 1996, clover growth characteristics (number
of leaves and stems, average height, flowering) were
noted at 67 WAP and again at 10 WAP. Biomass dry
matter yields were measured by hand-harvesting a
1 m 2 area from each plot towards the end of the
growing season at 14 to 16 WAP. Clover and rye plants
Table 2. Common name, cultivar, seeding rate and seed weight for green
manure crops grown
Cultivar
Seeding rate
(kg ha1)
1000-seed
weight (g)
Annual clovers
Balansa
Berseem
Crimson
Persian
Paradana
Bigbee
Au Robin
Felix
8
15
15
12
0.83
2.88
5.10
1.23
Perennial clovers
Alsike
Red
White Dutch
Rye
Aurora
Altaswede
Common
Kodiak
8
12
8
70
0.81
1.76
0.67
Crops
467
were separated from weeds, put in bags, dried for 72 h at
528C and weighed. In the fall, the clover and rye growth
was ploughed down into the soil by rototilling to a depth
of about 9 cm. All plots, including control treatments,
were rototilled. Controls had no green manure crop, but
contained similar densities of weeds (mustard) as green
manure treatments. Prior to rototilling, the majority of
the weed (mustard) biomass was eliminated from all
plots by cutting and removal by hand. Plots were
ploughed down at 14 to 16 WAP in 1996 and at 18 to
20 WAP in 1997. Plots were rototilled with care, to
avoid mixing between plots. In the following spring, soil
samples were taken to assess the impact of the green
manure on soil nitrate. Soil was sampled at depths of
015 and 1530 cm from the eight green manure
treatments (seven clovers, rye) and control plots (no
green manure plough-down, previously mustard-only).
To measure the impact of the green manure on a
subsequent crop, the plot areas were seeded to barley
in the spring of 1997 and 1998. A six-row barley variety
(Leduc) was used at Breton and a two-row variety
(Seebe) at Edmonton. Sampling of soil nitrate and
subsequent barley was conducted on only three of the
four replicate blocks, due to limitations of project
resources. At barley grain maturity, a 1-m 2 area of
barley was hand-harvested from each of the nine
treatments, at a cutting height of approximately 3 cm
above soil level.. For each 1-m2 area harvested, the
barley tillers were counted before placing them in a bag.
Barley plant numbers in each 1-m 2 area were determined by careful visual examination of the plant bases
after cutting. Barley samples were dried for 72 h at 528C
and weighed. Plant material was threshed to separate
seed grain, and weight of grain was determined.
Plant N yields and N derived from the atmosphere
(Ndfa) were determined in 1996 and 1997 for seven
clovers and a non-legume reference crop (rye) from three
replicate blocks at Breton, and for samples of berseem
clover and rye at Edmonton. Dried shoot samples were
ground and analyzed for total N and atom %15N
abundance using an ANA SIRA 10 mass spectrometer
(VG Isogas, Middlewich, UK).
The %Ndfa was calculated by the N difference
procedure (ND), assuming that the clovers and the
non-fixing reference crop (rye) absorbed equal amounts
of soil N:
% Ndfa [1 (total N (kg ha1 ) of rye=total
N (kg ha1 ) of clover)]100:
Symbiotic N fixation was also calculated using the
natural abundance of 15N (NA) method (Shearer and
Kohl 1986), assuming 15N abundance in the atmosphere
of 0.3663% and using B values of 0 for the clovers:
%Ndfa[(d15 Nrye d15 Nclover )=(d15 Nrye B)]100
468 CANADIAN JOURNAL OF PLANT SCIENCE
where d15Nrye and d15Nclover are the parts per 1000 15N
enrichment of N in rye and clover crops. The B value is a
measure of the isotopic discrimination that occurs
during N fixation, as determined in legumes that obtain
all of their N from biological N fixation. The NA
method has been accurate when compared with 15Nenriched techniques for a number of legume crops
(Rennie and Rennie 1983).
In 1997, barley grain samples were ground and
analyzed for total N using the Kjeldahl procedure.
Barley grain N was not measured in 1998 due to limited
resources and because substantial differences were not
found in 1997. Soil samples taken in the spring of 1997
and 1998 (prior to seeding barley) were dried, ground
and analyzed for NO3-N using the automated cadmium
reduction procedure (Technicon 1977).
Plant and soil data were subjected to analysis of
variance (ANOVA) using the Proc Mixed procedure of
SAS (SAS Institute, Inc. 2004). Crop species treatment
was considered as a fixed effect and block as a random
effect. Significant differences were determined at P B
0.05. Soil and barley data for Breton and Edmonton
are presented separately, to compare green manure
effects on low and high fertility sites. Data for each
year are presented separately, due to some significant
year treatment interactions. Simple linear regression
was used to test for relationships between yields and soil
nitrate.
RESULTS AND DISCUSSION
Dry Matter Yields of Green Manure Crops
The annual clovers (balansa, berseem, crimson, and
Persian) had greater above-ground biomass yields than
Table 3. Above-ground biomass dry matter yields of green manure crops
of clover species and rye at Breton and Edmonton
Breton
Crop treatment
1996
Edmonton
1997
———————(Mg ha
1996
1997
1
)————————
Annual clovers (A)
Balansa
Berseem
Crimson
Persian
3.6a
4.0a
3.8a
3.7a
2.3c
6.1a
5.2ab
4.6b
2.6bc
6.8a
2.1bcd
1.7cd
4.5b
10.3a
4.0bc
3.4bcd
Perennial clovers (P)
Alsike
Red
White
Rye
SED
ANOVA
Contrast: A vs. P
2.7b
2.3b
2.7b
0.6c
0.39
***
***
2.5c
2.1c
2.7c
0.5d
0.47
***
***
3.2bc
1.9bcd
0.8d
3.4b
0.73
***
**
4.7b
3.0cd
2.1d
2.8cd
0.66
***
***
ad Within columns, means followed by the same letter are not
significantly different at PB0.05.
**,*** Significant at PB0.01 and P B0.001 probability levels,
respectively.
the perennial clovers (alsike, red, and white clover)
(Table 3). Above-ground biomass DM yields (Mg ha1)
averaged 4.2 for annual clovers and 2.5 for perennial
clovers at Breton, and 4.4 for annuals and 2.6 for
perennials at Edmonton. Berseem clover had the highest
biomass yields, producing two to three times the
biomass of other clovers in many cases. Other studies
support the forage potential of berseem clover for
temperate regions (Shrestha et al. 1998; Fraser et al.
2004). Rye had lower yields than clovers at Breton, but
equalled many clover yields at Edmonton. Torbert et al.
(1996) reported lower biomass yields of rye than for
crimson clover without added fertilizer N, but fertilizer
additions of 134 kg N ha1 resulted in greater yields of
rye than crimson clover. Some clover yields at Edmonton were lower than at Breton due to much greater
competition from weeds (mustard) at Edmonton (Ross
et al. 2001). Mustard biomass DM yields (that were
removed before plough-down of clovers) averaged
0.9 Mg ha 1 at Breton and 7.5 Mg ha1 at Edmonton
(data not shown).
The clover species differed in rate of establishment
and flowering date. Higher growth rates for the annual
clovers than the perennial clovers were indicated by
greater number of leaves and longer stems in early stages
(data not shown). Balansa clover established ground
cover more quickly than the other clovers, began
flowering by 6 wk after planting, and growth then
slowed. Crimson clover also flowered relatively early,
blooming by 10 wk after planting. All of the clovers,
except red clover, had flowers when ploughed down in
autumn.
Nitrogen Yields and Fixation
At Breton, all the clovers derived the majority of their N
from the atmosphere (Table 4). Biomass N content
averaged 2.4 g N kg1 for clovers and 1.5 g N kg 1 for
rye. Perennial clovers had greater biomass N content
than annual clovers, averaging 2.9 g N kg1 for
perennial clovers and 1.9 g N kg1 for annual clovers.
Kirchmann (1988) observed less pronounced differences
among clover species, with biomass N content (g N
kg 1) values of 3.1 for white clover, 2.6 for red clover,
2.5 for berseem clover and 2.2 for Persian clover, at
101 d after planting. In 1996, there were no significant
differences among clovers in biomass N yield. In 1997,
annual clovers had greater biomass N yields than
perennials, largely due to the high N yields of berseem
clover. Symbiotic N fixation by clovers averaged 88%
Ndfa by ND method and 75% Ndfa by NA method.
The N yield of the above-ground biomass of clovers
averaged 77 kg N ha 1, with 5868 kg N ha1 derived
from the atmosphere. There were few significant differences in Ndfa among clovers.
At Edmonton, analysis of samples of berseem clover
and rye indicated that above-ground biomass of berseem clover yielded about 180190 kg N ha1, with
about 4050% Ndfa (data not shown). Biomass N yields
ROSS ET AL. * GREEN MANURE VALUE OF SEVEN CLOVERS
469
Table 4. Above-ground biomass nitrogen (N) content, N yield, and percentage of N derived from the atmosphere (Ndfa) by N difference (ND) and 15N
natural abundance (NA) method for seven clover species grown at Breton
Biomass N content (g N kg 1 )
Biomass N yield (kg N ha 1)
Ndfa by ND (%)
Ndfa by NA (%)
Crop treatment
1996
1997
1996
1997
1996
1997
1996
1997
Annual clovers (A)
Balansa
Berseem
Crimson
Persian
2.0c
2.1c
1.9c
2.4b
1.9c
1.8cd
1.6d
1.8cd
72a
84a
71a
90a
44e
110a
82bc
84b
88
90
86
91
81d
92a
90ab
90ab
84
59
76
80
89
86
66
82
Perennial clovers (P)
Alsike
Red
White
Rye
SED
ANOVA
Contrast: A vs. P
3.0a
3.2a
3.2a
1.5d
0.15
***
***
2.5b
2.5b
3.0a
1.6d
0.13
***
***
83a
72a
86a
8b
10.8
***
NS
64cd
51de
80bc
8f
9.2
***
**
90
88
90
87bc
84cd
90ab
68
86
78
64
68
70
2
NS
NS
2
***
NS
10.3
NS
NS
9.7
NS
NS
af Within columns, means followed by the same letter are not significantly different at PB0.05.
**, *** Significant at PB0.01 and P B0.001 probability levels, respectively; NS, not significantly different at P B0.05.
of berseem clover exceeded those of rye (9095 kg N
ha1), but the N content of rye biomass (2.83.3 g N
kg1) was equal to or greater than that of berseem
clover biomass (1.92.7 g N kg1).
Clover Ndfa results at Breton were higher by ND
method than by NA method (Table 4). A limitation of
the ND method is finding a non-fixing reference species
that has a pattern of soil N use similar to the legume.
Zhu el al. (1998) and Mueller and Thorup-Kristensen
(2001) concluded that the ND method could substitute
for the more expensive 15N isotope dilution method,
when annual or Italian ryegrass (Lolium multiflorum L.)
was used as reference species for herbage Ndfa in
annual Medicago and clover species. The ND method
may underestimate N fixation, compared with isotope
methods (Carlsson and Huss-Danell 2003). Grasses,
used as reference plants, are often more efficient than
legumes in taking up soil N. Sainju et al. (1998)
concluded that rye had greater root density and
scavenged more soil nitrate in early stages than crimson
clover, when grown as winter cover crops. Cereals often
have greater uptake of soil N than legumes during early
growth, but legumes may accumulate more soil N over
the growing season (Unkovich and Pate 2000).
Results for clover Ndfa at Breton were more variable
by NA method (standard error of difference of 10%
Ndfa) than by ND (SED 2% Ndfa) (Table 4). The
accuracy of the NA method is limited by the capability
of instruments to detect very small amounts of 15N and
the variability of soils. Significant changes in the natural
abundance levels of 15N occurred at the point of
maximal precision of the mass spectrometer.
The NA method is subject to variation from B values,
and these values vary with species, plant age, growing
conditions, and microsymbiont (Unkovich and Pate
2000). We used arbitrary B values of 0 for all seven
clover species. Some studies have measured B values
close to 0 for clover species: 0.3 for balansa clover in
Australia (Unkovich et al. 1994); 0.15 for red clover in
Quebec (Allahdadi et al. 2004). Carlsson et al. (2006)
found that B values for shoots of red, white and alsike
clover varied among strains of Rhizobium. They recommended using B values of 1.2 for alsike clover, 1.3
for red clover and 1.7 for white clover, for grasslands
typical of northern Scandinavia. Using these B values
for NA calculations for alsike, red and white clover at
Breton would give Ndfa values of 2851%, instead of
6486%. It is unlikely that Ndfa at Breton was less than
60%. In a long-term experiment at Breton, N uptake by
barley biomass was 2125 kg N ha1 yr 1 on unfertilized soil (Nyborg et al. 1995). If it is assumed that up to
25 kg N ha1 of the clover biomass N yield at Breton
could be obtained from the soil, the remaining N derived
from the atmosphere would average 5055 kg N ha1,
representing 6368% Ndfa. Hogh-Jensen and Schjoerring (1994) used B values of 0 for red and white clovers
in mixtures. They measured B values averaging 1.34
for whole plants of white clover, and they found that
using a B value of 1.20 for clovers gave Ndfa results
similar to those by isotope dilution method. Using B
values of 0.25 for alsike, red and white clovers at
Breton would give Ndfa results close to those by ND
method (averages of 86% by NA and 88% by ND).
We did not measure the biomass and N yields of
roots. Perennial clovers have greater root biomass than
annual clovers (Kirchmann 1988). To account for N
from stubble and below-ground plant materials, legume
biomass N fixation estimates should be increased by at
least 1025% (Mueller and Thorup-Kristensen 2001).
470 CANADIAN JOURNAL OF PLANT SCIENCE
Impact on Soil Nitrate
Our measurement of soil nitrate in spring provided a
single ‘‘snapshot’’ of the fate of N from green manures.
Although more extensive research would be required to
follow the fate of green manure N, a single sampling of
soil nitrate may indicate treatment differences. Janzen
et al. (1990) measured impacts of green manure and
fertilizer N inputs to soil depth 120 cm, and treatment
differences were more apparent in the 015 cm depth
than in deeper soil layers.
Soil nitrate (NO3-N) levels were increased by green
manures, compared with control treatments of no green
manure, in several cases at Breton and in a few cases at
Edmonton (Table 5). White clover treatments had the
most consistent increases in soil nitrate. The non-legume
green manure (rye) increased soil nitrate, compared with
the control, on the high fertility soil at Edmonton, but
not on a low fertility soil at Breton.
At Breton in 1997 at 015 cm soil depth, all clover
treatments, except balansa and crimson clover, had
higher soil nitrate than the control treatment (Table
5). In 1998, at 015 cm, only white and crimson clover
increased soil nitrate compared with the control. Green
manures may have had greater impact on soil nitrate in
1997 than in 1998 due to earlier plough-down in the
previous autumn (mid-September in 1996 versus
mid-October in 1997).
At Edmonton in 1998 at 015 cm, soil nitrate was
higher for white, berseem and Persian clover treatments
than the control (Table 5). Soil nitrate was higher for the
rye treatment than balansa and crimson clovers and the
control treatment. Soil nitrate levels in the spring of
1997 were likely affected by leaching and denitrification
losses at Edmonton. Prior to seeding in the spring of
1996, soil nitrate measured 3840 mg N kg 1 (Table 1).
Heavy rainfall in June 1996 resulted in pools of standing
water on some of the plots during early growth. Soil
nitrate levels in spring 1997 averaged 812 mg N kg1
and were lowest in areas that had been covered with
pools of water in 1996 (Table 5).
Greater increases in soil nitrate with white clover than
with other clovers cannot be attributed to greater aboveground biomass or biomass N yield (Tables 3 and 4).
White clover green manures may have had smaller C:N
ratios, greater root N and more rapid N mineralization
than other clovers (Kirchmann 1988; Kirchmann and
Marstorp 1991; Brandsaeter et al. 2008). Net N mineralization has been shown to correlate with C:N ratios of
clover green manures (Marstorp and Kirchmann 1991).
In a study comparing plough-down of berseem clover
and alfalfa (Medicago sativa L.), soil N availability was
consistently greater for alfalfa, although above-ground
biomass N was higher for berseem clover than alfalfa
(Westcott et al. 1995). It was suggested that the higher
soil N availability with alfalfa may have been due to
greater N contributions from roots and crowns.
Compared with other clover green manures, white
clover may have greater potential to supply mineral N to
soil, but also greater potential for N losses by leaching.
Kirchmann (1988) initially recommended that red and
white clover may be preferable to Persian and berseem
clover as green manures, because the perennial clovers
Table 5. Soil nitrate concentration for two soil depths in spring following autumn plough-down of green manures of clover species, rye, and no green
manure (control) at Breton and Edmonton
Breton
Edmonton
1997
Green manure treatment
015 cm
1998
1530 cm
015 cm
1997
1530 cm
015 cm
1998
1530 cm
015 cm
1530 cm
————————————————— (mg NO3-N kg1) ————————————————————
Annual clovers (A)
Balansa
Berseem
Crimson
Persian
3.8cd
7.1ab
4.9bcd
6.7ab
1.2bcd
2.8ab
2.1abc
3.4a
3.4abc
2.9bc
5.0a
3.4abc
1.9ab
1.7abc
2.4a
2.0ab
8.2
7.3
7.1
7.8
12.9
10.6
13.0
10.8
16.9cd
22.5abc
13.4d
21.7abc
13.5
13.1
12.2
14.6
Perennial clovers (P)
Alsike
Red
White
Rye
Control
SED
ANOVA
Contrast: A vs P
5.5bc
5.3bc
8.8a
2.5d
2.4d
1.22
***
NS
2.0abcd
1.1bcd
3.1a
0.2cd
0.2d
0.88
*
NS
2.7c
2.1c
4.6ab
1.8c
2.0c
0.8
*
NS
1.2c
1.2c
2.3a
1.0c
1.3bc
0.33
**
*
8.4
8.1
8.5
9.5
7.1
0.97
NS
NS
11.1
10.6
16.6
17.5
9.0
2.8
NS
NS
19.1abcd
18.2bcd
25.5a
24.3ab
13.0d
3.13
**
NS
12.4
12.1
14.2
17.6
12.9
2.17
NS
NS
Within columns, means followed by the same letter are not significantly different at P B0.05.
*, **, *** Significant at PB0.05, P B0.01 and P B0.001 probability levels, respectively; NS, not significantly different at PB0.05.
ROSS ET AL. * GREEN MANURE VALUE OF SEVEN CLOVERS
accumulated considerable amounts of N in their root
systems. Slower release of N from root residues than
shoot tissues might reduce the risk of N losses. In a
subsequent study, Kirchmann and Marstorp (1991)
concluded that N leaching losses might be greater
from white clover due to greater N mineralization
than from red, Persian and berseem clover.
Low soil nitrate results for balansa and crimson
clover treatments at Breton in 1997 and Edmonton in
1998 may have related to early flowering and less
decomposable material. Ranells and Wagger (1992)
suggested that increased proportions of structural carbohydrates and lignin in full bloom crimson clover
caused slower N release than from vegetative crimson
clover. Differences in N supply from alfalfa and red
clover residues related to differences in C:N ratio, lignin
content and decomposability (Bruulsema and Christie
1987).
At Edmonton, rye green manure treatments had
relatively high soil nitrate levels, indicating greater N
mineralization than might be expected. In a study of
winter cover crops, Ranells and Wagger (1996) reported
that rye had less N release, lower N content and a higher
C:N ratio than crimson clover. Torbert et al. (1996)
attributed negative effects of rye cover crops on subsequent corn yields to immobilization of N by rye. At
Edmonton, it appeared that the N content of rye was
high enough to preclude significant immobilization of N.
Impact on Subsequent Barley
At Breton, all clover green manures except balansa
clover increased subsequent barley yields, compared
with rye and control treatments (Tables 6 and 7).
Compared with controls, clover treatments increased
barley biomass yields by an average of 66% in 1997 and
55% in 1998, and barley grain yields by an average of
62% in 1997 and 48% in 1998. In 1997, the trend of
higher barley grain N content in rye and control
treatments was an indicator of very low soil-N availability. Barley grain N concentrations increase both at
extremes of N deficiency and with high soil-N status,
showing a U-shaped response curve to soil N (Askegaard and Eriksen 2007). Treatment differences with
balansa clover were evident with more than one yield
component of barley. In 1997, the balansa clover
treatment had lower barley tiller biomass yields than
other clovers, and was the only clover that failed to
increase barley grain N uptake (compared with the
control). In 1998, the balansa clover treatment had less
barley tillering than other clover treatments.
Perennial clover treatments at Breton had greater
barley tillering (tillers plant 1) than annual clover
treatments in both years (Tables 6 and 7). In 1998,
barley grain yields were also greater for perennial clover
treatments than for annual clovers. Differences in barley
tillering and yield indicate differences in the timing and
amounts of N-release from annual and perennial
clovers. Greater effects of perennial clovers on subse-
471
quent barley may have been due to greater N content in
biomass, greater N mineralization and/or greater contributions to soil N from roots and rhizodeposition.
Greater or more rapid N mineralization likely occurred
from perennial clovers than annual clovers. Marstorp
and Kirchmann (1991) measured 3035% mineralization of total N from white clover green manure over
115 d, compared with only 20% from red and Persian
clovers, and 17% from berseem clover. Uptake of green
manure N by a test crop (ryegrass) was 27% of added N
from white clover, but only about 10% of added N from
red, Persian and berseem clovers. Askegaard and
Eriksen (2007) observed higher barley grain DM and
grain N yields following catch crops of white and red
clover than following Persian clover. The N fertilizer
replacement value of white clover exceeded the N
measured in shoots and roots, suggesting that substantial N was provided by rhizodeposition and fine roots.
Hogh-Jensen and Schjoerring (2001) found that 8087%
of total below-ground N of white and red clover was
from rhizodeposition (exudates and root tissues).
At Edmonton, green manure treatments had little
effect on subsequent barley yields (Table 8). In 1997,
barley grain yields for rye and alsike clover treatments
were greater than for balansa, crimson and red clover
treatments. Barley biomass yields were much greater at
Edmonton (average 10.5 Mg ha 1) than at Breton
(average 3.4 Mg ha1) (Tables 68).
It did not appear that rye had detrimental effects on
subsequent barley yields due to N immobilization or
allelopathy. With rye green manures at Breton and
Edmonton, soil nitrate levels were equal to or higher
than control treatments (Table 5) and barley yields were
equal to controls (Tables 68). At Edmonton, rye
treatments produced barley yields that were in the upper
range among treatments. With fertilizer uptake of 70 kg
N ha1, a winter cover crop of rye produced higher
subsequent corn biomass yields than with a crimson
clover cover crop (Torbert et al. 1996). In production
systems where N is less limiting, nonlegumes may
be more advantageous than legume green manures
(Cherr et al. 2006).
At Breton there were positive relationships between
soil nitrate at 015 cm and barley DM yields (1997 R2 0.35, P B0.01; 1998 R20.21, P B0.05). Similarly,
Westcott et al. (1995) concluded that green manure
effects on subsequent spring soil nitrate levels were a
determining factor in barley N uptake and yield
responses to treatments.
At Edmonton, there was no correlation between soil
nitrate and barley yields. Rice et al. (1993) observed a
lack of relationship between soil nitrate and subsequent
barley yields. They suggested that the amount of green
manure N available to subsequent barley may not have
been large enough to affect barley yields. Treatment
effects from legume plough-down can be difficult
to detect against high background levels of soil N
(Westcott et al. 1995), as at Edmonton. Westcott et al.
Barley yields (Mg ha1)
Barley tillering
Green manure
Annual clovers (A)
Balansa
Berseem
Crimson
Persian
Perennial clovers (P)
Alsike
Red
White
Rye
Control
SED
ANOVA
Contrast: A vs P
Biomass
Grain
Straw
2.7cd
3.8ab
3.2bc
4.2a
1.7cd
2.3ab
2.0bc
2.4a
1.1def
1.5abc
1.3cde
1.8a
4.1a
3.6ab
3.6ab
2.5d
2.2d
0.32
***
NS
2.4ab
2.2ab
2.2ab
1.6cd
1.3d
0.21
***
NS
1.7ab
1.4cd
1.4bc
1.0ef
0.8f
0.16
***
NS
Barley tiller yields
(g tiller 1)
Grain N content
Grain N uptake
(tillers plant 1)
Biomass
Grain
(g N kg1)
(kg N ha 1)
61
60
60
58
1.4bcd
1.5bc
1.5bc
1.6ab
1.17b
1.50a
1.47a
1.62a
0.72bc
0.90a
0.88ab
0.93a
14.7
14.9
14.8
14.3
25bc
34a
29ab
35a
58
62
61
62
61
2.8
NS
NS
1.7a
1.5abc
1.6ab
1.3cd
1.2d
0.11
*
*
1.55a
1.47a
1.43a
1.07a
1.04a
0.107
***
NS
0.90a
0.91a
0.87ab
0.67c
0.63c
0.082
**
NS
14.3
14.5
14.7
15.3
15.4
0.38
NS
NS
34a
32a
32a
23bc
21c
3.1
**
NS
Harvest index
af Within columns, means followed by the same letter are not significantly different at P B0.05.
*, **, *** Significant at P B0.05, PB0.01 and PB0.001 probability levels, respectively; NS, not significantly different at P B0.05.
472 CANADIAN JOURNAL OF PLANT SCIENCE
Table 6. Barley dry matter and grain yield, harvest index, tillering, barley tiller yields, and grain N content and uptake at Breton in 1997, following green manures of clover species, rye, and control
(no green manure)
ROSS ET AL. * GREEN MANURE VALUE OF SEVEN CLOVERS
473
Table 7. Barley dry matter and grain yield, harvest index, tillering, and barley tiller yields at Breton in 1998, following green manures of clover species,
rye, and control (no green manure)
Barley yields (Mg ha 1)
Barley tiller yields (g tiller 1)
Biomass
Grain
Straw
Harvest index
Barley tillering
(tillers plant 1)
Annual clovers (A)
Balansa
Berseem
Crimson
Persian
2.7cd
3.4bc
4.0ab
3.9ab
1.7cd
2.2bc
2.5ab
2.4ab
0.9bc
1.2ab
1.5a
1.5a
65abc
64bcd
63d
62d
1.4d
1.8ab
1.8ab
1.8abc
1.27bc
1.40ab
1.39ab
1.43ab
0.82bc
0.90ab
0.87ab
0.88ab
Perennial clovers (P)
Alsike
Red
White
Rye
Control
SED
ANOVA
Contrast: A vs P
3.8ab
3.7ab
4.3a
2.4d
2.4d
0.41
**
$
2.4ab
2.3ab
2.8a
1.6d
1.6d
0.26
**
*
1.4a
1.4a
1.6a
0.8c
0.8c
0.16
***
$
63d
63cd
64bcd
67a
66ab
1.1
**
NS
2.0a
1.9ab
2.1a
1.4cd
1.5bcd
0.18
*
*
1.36ab
1.35ab
1.48a
1.12d
1.12cd
0.077
**
NS
0.85b
0.85ab
0.94a
0.74c
0.74c
0.043
**
NS
Green manure
Biomass
Grain
ad Within columns, means followed by the same letter are not significantly different at P B0.05.
$,*, **, *** Significant atPB0.08,PB0.05, PB0.01 and P B0.001 probability levels, respectively; NS: not significantly different at PB0.05.
(1995) observed a linear relationship between barley N
uptake and soil mineral N levels for part of their dataset,
but N uptake was insensitive to increases in soil mineral
N beyond 100 kg N ha1. Janzen and Schaalje (1992)
reported that barley N uptake generally increased
linearly with N application rate, but green manures
caused additional non-nutritive increases in barley yields
that were independent of N uptake. It was suggested
that N benefits of green manures may dominate under
low N fertility, but non-nutritive benefits could conceivably exceed N benefits on soils with high N fertility.
It appeared that balansa clover was not a suitable
green manure for central Alberta, due to lack of benefit
to subsequent crop, perhaps related to early flowering of
balansa clover. As only one variety of each clover
species was tested in our study, some caution is needed
in interpreting results for individual clover species.
Green manure value differed among varieties or strains
of red clover (Bruulsema and Christie 1987; Christie
et al. 1992).
There was some regrowth of the perennial clovers in
subsequent barley crops, but reappearance of the annual
clovers (by re-seeding) was negligible. Annual clover
green manures may provide advantages over perennial
clovers in organic and reduced tillage systems where
continuation of a legume beyond 1 year is undesirable.
Residues of annual clovers could provide surface cover
and N benefit to a subsequent crop, without the need for
tillage or herbicide to terminate the green manure crop.
The Effects of Soil Type
The benefits of legume green manures to subsequent
crops were more evident on the low fertility soil at
Breton than on the high fertility soil at Edmonton. The
incorporation of green manures represented a large
addition to soil N pools at Breton, but a relatively small
addition at Edmonton. Higher rates of N mineralization
would be expected at Breton than at Edmonton.
Jans-Hammermeister et al. (1994) observed greater net
N mineralization rates from legume green manures in a
Luvisolic soil than a Chernozemic soil in Alberta,
indicating more rapid internal N cycling system and
greater microbial activity, consistent with a hypothesis
of higher mineralization rates on soils with lower clay
content. Mineralization of N from green manures may
contribute relatively little or substantial portions
(430%) of N uptake by a subsequent crop, depending
upon C:N ratio of residue, soil type and management
(Fageria 2007). Decomposition of green manures depends mainly on temperature and soil moisture, and is
influenced by soil texture, structure, acidity, microbial
activity and fertility.
It is likely that a substantial proportion of the green
manure N would have been retained in soil. Janzen et al.
(1990) observed that 24 to 59% of N applied in green
manures remained in stable organic N reserves in the
surface layers of Chernozemic soils. At Breton, study of
the fate of organic N inputs in a long-term agroecological cropping system concluded that 44% was
retained in soil (Ross et al. 2008). Substantial inputs of
organic N and C are needed to maintain and improve
the agronomic performance of low fertility Luvisolic
soils (Izaurralde et al. 2001).
CONCLUSION
Compared with perennial clovers, annual clovers are not
advantageous in supplying N to a subsequent crop.
However the biomass yield advantages and lifecycle of
annual clovers may make them preferable to perennial
clovers for short-term forage production and in reduced
Barley yields (Mg ha1)
Green manure
Harvest index
Barley tillering
(tillers plant 1)
Barley tiller yields
(g tiller 1)
Biomass
Grain
Grain N content
(g N kg 1)
Grain N uptake
(kg N ha1)
Biomass
Grain
Straw
Annual clovers (A)
Balansa
Berseem
Crimson
Persian
10.7
11.3
10.1
11.1
5.3b
5.8ab
5.1b
5.8ab
5.4
5.5
5.1
5.3
49
52
50
52
3.7
3.3
3.3
3.2
2.76
2.83
2.65
2.57
1.36
1.46
1.33
1.33
22.5
22.6
22.5
20.5
119
132
115
120
Perennial clovers (P)
Alsike
Red
White
Rye
Control
SED
ANOVA
Contrast: A vs. P
12.9
11.2
11.5
12.7
11.8
0.99
NS
NS
6.6a
5.3b
5.7ab
6.5a
5.8ab
0.45
*
NS
6.2
5.8
5.8
6.2
6.0
0.6
NS
NS
52
48
50
51
49
1.6
NS
NS
3.9
3.1
2.8
3.4
3.7
0.37
NS
NS
2.90
2.62
2.53
2.93
2.83
0.216
NS
NS
1.50
1.25
1.26
1.51
1.39
0.127
NS
NS
22.6
20.3
21.3
23.3
21.8
1.15
NS
NS
150
109
121
152
127
14.5
NS
NS
1998
Clover mean
Rye
Control
SED
9.5
10.1
8.8
1.12
4.4
4.7
4.1
0.57
5.1
5.5
4.7
0.6
46
46
47
1.9
5.1
4.7
4.4
1.0
1.45
1.50
1.39
0.121
0.67
0.69
0.66
0.069
ANOVA
NS
NS
NS
NS
NS
NS
1997
NS
a, b Within columns, means followed by the same letter are not significantly different at PB0.05.
* Significant at P B0.05; NS: not significantly different at P B0.05.
474 CANADIAN JOURNAL OF PLANT SCIENCE
Table 8. Barley dry matter and grain yield, harvest index, tillering, barley tiller yields, and grain N content and uptake at Edmonton in 1997 and 1998, following green manures of clover species,
rye, and control (no green manure)
ROSS ET AL. * GREEN MANURE VALUE OF SEVEN CLOVERS
tillage systems. Annual clovers can add N and C to the
soil without the need to terminate the clover. Where
rainfall is adequate, annual clovers will provide forage
biomass and add N and organic matter to the soil.
ACKNOWLEDGEMENTS
We are grateful for the assistance of many technical staff
at the University of Alberta. We acknowledge financial
support from the Alberta Agriculture Research Institute, and the Natural Sciences and Engineering Research Council of Canada (NSERC) scholarship
program.
Abdallahi, M. M. and N’Dayegamiye, A. 2000. Effects of green
manures on soil physical and biological properties and on
wheat yields and N uptake. Can. J. Soil Sci. 80: 8189.
Allahdadi, I., Beauchamp, C. J. and Chalifour, F. P. 2004.
Symbiotic dinitrogen fixation in forage legumes amended with
high rates of de-inking paper sludge. Agron. J. 96: 956965.
Askegaard, M. and Eriksen, J. 2007. Growth of legume and
nonlegume catch crops and residual-N effects in spring barley
on coarse sand. J. Plant Nutr. Soil Sci. 170: 773780.
Biederbeck, V. O., Bouman, O. T., Looman, J., Slinkard, A. E.,
Bailey, L. D., Rice, W. A. and Janzen, H. H. 1993. Productivity
of four annual legumes as green manure in dryland cropping
systems. Agron. J. 85: 10351043.
Biederbeck, V. O., Campbell, C. A., Rasiah, V., Zentner, R. P.
and Wen, G. 1998. Soil quality attributes as influenced by
annual legumes used as green manure. Soil Biol. Biochem. 30:
11771185.
Blackshaw, R. E., Moyer, J. R., Doram, R. C., Boswall, A. L.
and Smith, E. G. 2001. Suitability of undersown sweetclover as
a fallow replacement in semiarid cropping systems. Agron. J.
93: 863868.
Brandsaeter, L. O., Heggen, H., Riley, H., Stubhaug E. and
Henriksen, T. M. 2008. Winter survival, biomass accumulation
and N mineralization of winter annual and biennial legumes
sown at various times of year in northern temperate regions.
Eur. J. Agron. 28: 437448.
Bruulsema, T. W. and Christie, B. R. 1987. Nitrogen contribution to succeeding corn from alfalfa and red clover. Agron. J.
79: 96100.
Bullied, W. J., Entz, M. H., Smith, S. R. and Bamford, K. C.
2002. Grain yield and N benefits to sequential wheat and
barley crops from single-year alfalfa, berseem and red clover,
chickling vetch and lentil. Can. J. Plant Sci. 82: 5365.
Carlsson, G. and Huss-Danell, K. 2003. Nitrogen fixation in
perennial forage legumes in the field. Plant Soil 253: 353372.
Carlsson, G., Palmborg, C. and Huss-Danell, K. 2006. Discrimination against 15N in three N2-fixing Trifolium species as
influenced by Rhizobium strain and plant age. Acta Agric
Scand., Sect. B Soil Plant Sci. 56: 3138.
Cherr, C. M., Scholberg, J. M. S. and McSorley, R. 2006.
Green manure approaches to crop production: A synthesis.
Agron. J. 98: 302319.
Christie, B. R., Clark, E. A. and Fulkerson, R. S. 1992.
Comparative plowdown value of red clover strains. Can. J.
Plant Sci. 72: 12071213.
Fageria, N. K. 2007. Green manuring in crop production. J.
Plant Nutr. 30: 691719.
Fraser, J., McCartney, D., Najda, H. and Mir, Z. 2004. Yield
potential and forage quality of annual forage legumes in
475
southern Alberta and northeast Saskatchewan. Can. J. Plant
Sci. 84: 143155.
Hogh-Jensen, H. and Schjoerring, J. K. 1994. Measurement of
biological dinitrogen fixation in grassland: comparison of the
enriched 15N dilution and the natural 15N abundance methods
at different nitrogen application rates and defoliation frequencies. Plant Soil 166: 153163.
Hogh-Jensen, H. and Schjoerring, J. K. 2001. Rhizodeposition
of nitrogen by red clover, white clover and ryegrass leys. Soil
Biol. Biochem. 33: 439448.
Izaurralde, R. C., Juma, N. G., McGill, W. B., Chanasyk, D.
S., Pawluk, S. and Dudas, M. J. 1993. Performance of
conventional and alternative cropping systems in cryoboreal
subhumid central Alberta. J. Agric. Sci. 120: 3341.
Izaurralde, R. C., McGill, W. B., Robertson, J. A., Juma, N. G.
and Thurston J. J. 2001. Carbon balance of the Breton
Classical Plots over half a century. Soil Sci. Soc. Am. J. 65:
431441.
Jans-Hammermeister, D. C., McGill, W. B. and Jensen, T. L.
1994. Nitrogen accumulations and relative rates of mineralization in two soils following legume green manuring. Can. J. Soil
Sci. 74: 2328.
Janzen, H. H., Bole, J. B., Biederbeck, V. O. and Slinkard, A.E.
1990. Fate of N applied as green manure or ammonium
fertilizer to soil subsequently cropped with spring wheat at 3
sites in western Canada. Can. J. Soil Sci. 70: 313323.
Janzen, H. H. and Schaalje, G.B. 1992. Barley response to
nitrogen and non-nutritional benefits of legume green manure.
Plant Soil 142: 1930.
Kirchmann, H. 1988. Shoot and root growth and nitrogen
uptake by six green manure legumes. Acta Agric. Scand. 38:
2531.
Kirchmann, H. and Marstorp, H. 1991. Calculation of N
mineralization from six green manure legumes under field
conditions from autumn to spring. Acta Agric. Scand. 41:
253258.
Marstorp, H. and Kirchmann, H. 1991. Carbon and nitrogen
mineralization and crop uptake of nitrogen from six green
manure legumes decomposing in soil. Acta Agric. Scand. 41:
243252.
Mueller, T. and Thorup-Kristensen, K. 2001. N-fixation of
selected green manure plants in an organic crop rotation. Biol.
Agric. Hortic. 18: 345363.
N’Dayegamiye, A. and Tran, T. S. 2001. Effects of green
manures on soil organic matter and wheat yields and N
nutrition. Can. J. Soil Sci. 81: 371382.
Nyborg, M., Solberg, E. D., Izaurralde, R. C., Malhi, S. S. and
Molina-Ayala, M. 1995. Influence of long-term tillage, straw
and N fertilizer on barley yield, plant N uptake and on the soilN balance. Soil Tillage Res. 36: 301310.
Ranells, N. N. and Wagger, M. G. 1992. Nitrogen release from
crimson clover in relation to plant growth stage and composition. Agron. J. 84: 424430.
Ranells, N. N. and Wagger, M. G. 1996. Nitrogen release from
grass and legume cover crop monocultures and bicultures.
Agron. J. 88: 777782.
Rennie, R. J. and Rennie D. A. 1983. Techniques for quantifying N2 fixation in association with nonlegumes under field and
greenhouse conditions. Can. J. Microbiol. 29: 10221035.
Rice, W. A., Olsen, P. E., Bailey, L. D., Biederbeck, V. O. and
Slinkard, A. E. 1993. The use of annual legume green-manure
crops as a substitute for summerfallow in the Peace River
region. Can. J. Soil Sci. 73: 243252.
476 CANADIAN JOURNAL OF PLANT SCIENCE
Ross, S. M., King, J. R., Izaurralde, R. C. and O’Donovan, J.
T. 2001. Weed suppression by seven clover species. Agron. J.
93: 820827.
Ross, S. M., Izaurralde, R. C., Janzen, H. H., Robertson, J. A.
and McGill, W. B. 2008. The nitrogen balance of three longterm agroecosystems on a boreal soil in western Canada.
Agric. Ecosyst. Environ. 127: 241250.
Sainju, U. M., Singh, B. P. and Whitehead, W. F. 1998. Cover
crop root distribution and its effects on soil nitrogen cycling.
Agron. J. 90: 511518.
SAS Institute, Inc. 2004. Version 9.1 ed. SAS Institute, Inc.,
Cary, NC.
Schlegel, A. J. and Havlin, J. L. 1997. Green fallow for the
central Great Plains. Agron. J. 89: 762767.
Shearer, G. and Kohl, D. H. 1986. N2-fixation in field settings:
estimations based on natural 15N abundance. Aust. J. Plant
Physiol. 13: 699756.
Shrestha, A., Hesterman, O. B., Squire, J. M., Fisk, J. W. and
Sheaffer, C. C. 1998. Annual medics and berseem clover as
emergency forages. Agron. J. 90: 197201.
Shrestha, A., Hesterman, O. B., Copeland, L. O., Squire, J. M.,
Fisk, J. W. and Sheaffer, C. C. 1999. Annual legumes as green
manure and forage crops in winter canola (Brassica napus L.)
rotations. Can. J. Plant Sci. 79: 1925.
Sparrow, S. D., Cochran, V. L. and Sparrow, E. B. 1995.
Residual effects of harvested and green-manured legumes on a
subsequent barley crop in a sub-arctic environment. Can. J.
Plant Sci. 75: 453456.
Technicon. 1977. Nitrate and nitrite in soil extracts. Industrial
method no. 49777A. Technicon Industrial Systems, Tarrytown, New York, NY.
Torbert, H. A., Reeves, D. W. and Mulvaney, R. L. 1996.
Winter legume cover crop benefits to corn: rotation vs. fixednitrogen effects. Agron. J. 88: 527535.
Unkovich, M. J., Pate, J. S., Sanford, P. and Armstrong, E. L.
1994. Potential precision of the d15N natural abundance
method in field estimates of nitrogen fixation by crop and
pasture legumes in south-west Australia. Aust. J. Agric. Res.
45: 119132.
Unkovich, M. J. and Pate, J. S. 2000. An appraisal of recent
field measurements of symbiotic N2 fixation by annual
legumes. Field Crops Res. 65: 211228.
Westcott, M. P., Welty, L. E., Knox, M. L. and Prestbye, L. S.
1995. Managing alfalfa and berseem clover for forage and
plowdown nitrogen in barley rotations. Agron. J. 87:
11761181.
Zentner, R. P., Campbell, C. A., Biederbeck, V. O., Selles, F.,
Lemke, R., Jefferson, P. G. and Gan, Y. 2004. Long-term
assessment of management of an annual legume green manure
crop for fallow replacement in the Brown soil zone. Can. J.
Plant Sci. 84: 1122.
Zhu, Y., Sheaffer, C. C., Russelle, M. P. and Vance, C. P. 1998.
Dry matter accumulation and dinitrogen fixation of Medicago
species. Agron. J. 90: 103108.