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REGULAR ARTICLE
Recovery of nitrogen fertilizer by traditional and improved
rice cultivars in the Bhutan HighlandsBhim Bahadur Ghaley &Henning Hgh-Jensen &
Jrgen Lindskrog Christiansen
Received: 13 July 2009 /Accepted: 11 January 2010 /Published online: 13 February 2010# Springer Science+Business Media B.V. 2010
Abstract The recovery of soil derived nitrogen (NDFS)
and fertilizer N (NDFF) was investigated in highland
rice (Oryza sativa L.) fields in Bhutan, characterized
by high inputs of farmyard manure (FYM). The effect
of 60 kg N ha1 (60 N) applied in two splits to a
traditional and an improved cultivar, popular among
the farmers, was investigated using the 15N isotope
dilution technique. No differences were found between
cultivars with respect to the uptake of NDFSand NDFF,
but the improved cultivar yielded 27% more (P0.05)
grain compared with the traditional cultivar. This was
largely due to its greater harvest index (HI). The mean
percentage recovery of fertilizer N (REN) applied at
45 days after transplanting (DAT) was 34% compared
to 22% at 7 DAT, resulting in 56% greater uptake of
NDFF at 45 DAT. The overall REN for both the
improved and the traditional cultivars were 25.7% and
30% respectively, with no difference between cultivars,
but REN decreased with increasing FYM inputs.Fertilizer N recommendations that allow for previous
FYM inputs combined with applications timed to
coincide with maximum crop demand (45 DAT), and
the use of improved cultivars, could enhance N
fertilizer recoveries (REN) and increase rice yields in
the Bhutan Highlands.
Keywords Added nitrogen interaction .
Oryza sativa L. . 15N isotope dilution . Farmyard
manure . Environmental index .Nitrogen fertilizer
Introduction
Asian irrigated rice (Oryza sativa L.) constitutes 70%
of global rice production and feeds almost half of the
worlds population (Bouman et al. 2007). Given the
growth in world population and rising food demand, rice
yields need to be increased (Jing et al. 2008). Across
Asia, rice yields vary from 215 Mg ha1 depending
Plant Soil (2010) 332:233246
DOI 10.1007/s11104-010-0288-3
Responsible Editor: Elizabeth (Liz) A. Stockdale.
B. B. Ghaley
Renewable Natural Resources Research centre, Yusipang,
Council for Renewable Natural Resources Research
of Bhutan, Ministry of Agriculture,
Thimphu, Bhutan
J. L. Christiansen
Department of Agriculture and Ecology,
Faculty of Life Sciences, University of Copenhagen,
Hjbakkegrd All 13,
2630 Taastrup, Denmark
H. Hgh-Jensen
Department of Policy Analysis, National EnvironmentalResearch Institute, Aarhus University,
Frederiksborgvej 399,
4000 Roskilde, Denmark
Present Address:
B. B. Ghaley (*)
Department of Agriculture and Ecology,
Faculty of Life Sciences, University of Copenhagen,
Hjbakkegrd All 30,
2630 Taastrup, Denmark
e-mail: [email protected]
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on soil fertility, genotype, environment and manage-
ment practice (Romyen et al.1998; Cooper et al.1999;
Dobermann et al.2003; Whitbread et al.2003; Tang et
al.2007). In Asia, recovery of applied nitrogen (N) is
only 3050% (Cassman et al. 1993; Cassman et al.
1996b; Dobermann and Cassman2002; Dobermann et
al.2003) due to losses from volatilization, leaching andimmobilization in combination with sub-optimal appli-
cation procedures in terms of timing and placement of
N. Hence, improving N recovery efficiency may
increase crop yield, reduce production costs and limit
negative downstream environmental effects. The com-
mon method of N application is topdressing over the
standing crop, which leads to significant losses (Peng
and Cassman 1998). Hence, there is a need to
understand the demand-supply dynamics of N to make
the best use of the available N. On-farm studies have
demonstrated that 60% of the variation in crops
Nuptake from the soil (NDFS) is due to spatial variation
within single fields and temporal variation over seasons
(Cassman et al. 1996a). However, such variations are
not taken into account when making blanket N
recommendations, which may lead to over-fertilization
in fertile soils and under-fertilization in poor soils.
Rice is the staple food grain in Bhutan and rice is
cultivated in terraces by smallholder farmers primarily
to meet the food needs of the family (Ghaley and
Christiansen 2009). In the highlands of Western
Bhutan, farmyard manure (FYM), consisting of com-posted bedding material from cowsheds and leaf litter
from forest, is the main source of N input to the
terraces, supplemented with 3580 kg N ha1 urea
topdressing applied, at or shortly after transplanting the
rice seedlings. However, there is a lack of knowledge
regarding the optimal timing and dose of topdressed
urea and the inherent soil N supply in such highland rice
production systems. Availability of such information
may help to match the inherent N supply of the soil with
the corresponding requirement for additional N input,
based on local yield levels. Improved cultivars arereleased by the Bhutanese National Agricultural Re-
search System, based mainly on grain yield performance
and constitute 35% of total rice area. However,
knowledge of cultivar differences in N uptake and
utilization efficiency are lacking in both on-farm and on-
station cultivar evaluation.
Based on the hypothesis that cultivars have different
N utilization potentials and the farmersfields differ in
N supply potentials due to N management and soil
fertility, the objective of the current study was to
quantify N recovery efficiency in a traditional and an
improved rice cultivar under field conditions using 15N
labelling techniques.
Materials and methods
Site characterization
The trials were conducted in 2007 in the fields of ten
rice (Oryza sativa L.) farmers in Paro and Thimphu,
two of the main rice-growing districts in Western
Bhutan and representative of the region. The trial fields
were located within an altitude range of 2,2002,300 m
asl in Thimphu and 2,3002,500 m asl in Paro,
distributed over four different village clusters within
two districts in order to capture the variation in the ricecropping environments. The trial sites represented the
target population of environments with regard to differ-
ences in input access, irrigation facilities, rainfall, slope,
altitude and management factors. More than 90% of the
farmers at the trial sites grow rice for subsistence and
both traditional and improved cultivars are grown.
Improved cultivars are grown due to their high yield
whereas traditional cultivars are grown for their aroma,
good taste and for ritual offerings according to tradi-
tional cultural norms.
Rice is grown in terraced bunds along sloping hillswith irrigation water drawn from the nearby rivers
and supplemented with seasonal monsoon rainfall.
Soil fertility is traditionally maintained by annual
applications of 520 Mg ha1 of FYM, consisting of
composted bedding material from cowsheds and
forest leaf litter. The dominant cropping system is a
single rice crop from May to October and vegetables
like chilli, peas, potatoes and tomatoes in the spring
(FebruaryMay) for household consumption as well
as for sale. From November to May, farmers cultivate
winter wheat and barley in the paddy terraces as themain winter food crops.
Trial layout
All the participating farmers were supplied with seeds
of Janam, a traditional cultivar, and Khangma Maap,
an improved cultivar and established their own
nurseries. The terraces for transplanting were pre-
pared by puddling, levelling and removal of weeds,
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after which standing water was maintained in the
terraces. Transplanting took place from mid-May to
the third week of June, varying from village to
village. The seedling nurseries were watered a few
days before transplanting to ease removal of the
seedlings. The seedlings were packed in small
bundles and transported to the fields and 1
3 seed-lings were transplanted in a single hill.
Field trials were laid out in two parallel split plot
designs at each site. The first split plot set (cultivar
N supply) consisted of two N fertilizer levels as main
plots and two genotypes as subplots. The two main
plots were two paddy terraces supplied with 0 kg N
ha1 (0 N) and 60 kg N ha1 (60 N) and the subplots
consisted of two genotypes, Janam, and Khangma
Maap. Urea was the N source, and the traditional
cultivar with 0 N was treated as the farmers practice.
There were four treatments in the first set of splitplots. The second split plot set (cultivar topdress-
ing) was superimposed on 60 N plots with two
cultivars as the main plots and two 15N microplots
in each cultivar. The two microplots in each cultivar
consisted of two topdressing treatments at 7 and
45 days after transplanting (DAT), respectively. There
were four treatments in the second set of split plots.
Hence, at each site, there were eight treatments with
one replication per site. The first split plot set was
necessary to obtain a higher degree of precision in
varietal differences in N uptake under both fertilizedand non-fertilized conditions and the second split plot
set was designed to determine a higher precision in
the efficiency of topdressing timing relative to the
varietal differences. The traditional cultivar (Janam) is
a tall cultivar that has been cultivated for generations
and is valued for its aroma, taste and high straw yield.
Khangma Maap is a semi-dwarf, early-maturing
cultivar released by the National Agricultural Re-
search System of Bhutan in 1999.
In the first split plot (cultivar N supply), the main
plots were 4560 m2
, varying with the size of thepaddy terrace available in the farmers field. The
subplots were 1520 m2 (54 m2) in size and two
genotypes were randomly placed within each main
plot. 60 N was split into two equal doses and applied at
7 DAT and 45 DAT to meet the N requirement in the
early establishment phase following transplanting and
at the active tillering stage. In the second split plot set
(cultivar topdressing), the main plots were 1520 m2
in size and two microplots were inserted in each of the
two cultivars. To each microplot, 60 N was applied in
two splits of 30 kg N ha1 (30 N) each at 7 DAT and
45 DAT. Of two splits in each microplot, one split was
labelled with 3 atom % 15N whereas the other split was
applied as unlabelled 14N urea. In the 7 DAT split, one
of the two microplots in each cultivar was labelled with15
N while the other microplot was supplied withunlabelled 14N urea, and vice versa in the 45 DAT
split. Each microplot was 0.5 m2 and consisted of
rectangular iron frames (18 mm thick) measuring 50
100 cm with a height of 50 cm. After transplanting of
the rice seedlings, the frames were driven 20 cm into
the soil to avoid exchange of water with the bordering
rice plants outside the microplots. The 15N microplots
were placed 1 m apart in the field to eliminate cross
contamination. The microplots were treated in precise-
ly the same way as the surrounding 60N plots.
Fertilizer application, weeding and irrigation in themicroplots followed the same timing and frequency as
in the 60N plots. This trial design enabled a direct
estimation of the partial fertilizer use efficiency of 30N
applied at each timing and the total fertilizer use
efficiency of two-split application of 60N.
Management practices
All other management practices were treated as non-
experimental variables and was left to the farmers in
order to enable evaluation of the treatments underrealistic farming conditions. There was no major pest
or disease pressure during the crop cycle, so no control
measures were taken. No potassium or phosphorus
fertilizers were applied in the terraces as the traditional
practice of 7 t ha1 FYM provided 76 kg N, 38 kg P
and 138 kg K ha1, which is sufficient to produce rice
yields in the range of 46 Mg ha1 (Chettri et al.2003).
Due to surface application of FYM 1 to 2 months
before land preparation, a considerable quantity of N is
liable to be lost (via volatilization, leaching and surface
run-off) and consequently N was expected to be alimiting factor for the crops.
Field data collection and sampling
During the entire crop cycle, data was recorded on the
different crop management practices of each farmer.
This included recording dates of different cultural
practices, such as nursery sowing, transplanting, top-
dressing, weeding and harvesting. In the first split plot, a
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harvest area of 6 m2 (32 m) was demarcated in each
subplot (15 m2) after excluding the border rows. Within
the harvest area, the crop was cut at 2 cm above ground
level and the fresh biomass was recorded. Threshed
grains were cleaned and grain weights were recorded
along with the moisture content of the grain samples, so
that all grain weights could be standardized to 14%moisture content. A subsample of 500 g grain and straw
was weighed and dried at 70C for 2 days to constant
weight to determine total dry matter (TDM).
In the second split plot, all plants within the micro-
plots were harvested and the fresh weights were
recorded. The samples were separated into straw and
grain, weighed, and dried at 70C for 2 days to constant
weight to determine TDM. The samples were powdered
(mess 0.2 mm) and 15N and N contents were analysed
using an ANCA-SL elemental analyser coupled to a
20
20 Tracermass mass spectrometer (SerCon Ltd.,Crewe, UK) using the Dumas combustion method.
One soil sample from each trial site (i.e. from each
farmer) was collected 3040 days before transplanting
of the rice seedlings. Each soil sample consisted of 56
subsamples taken from a depth of 020 cm topsoil and
bulked into one composite sample. The soil samples
were then air-dried, sieved and analysed for total soil N
(Kjeldhal), pH (H20), and cation exchange capacity
(CEC) (Table 1). The trial sites were characterized by
heavy soils, ranging from sandy loam to loamy with
mean sand, silt and clay proportions of 52.83.1, 32.52.8 and 14.61.7, respectively. Soil analysis showed
significant differences in soil fertility characteristics
among the sites (Table1). Total soil N content ranged
from 0.100.22%. Significant differences (P0.001) in
total soil N content were recorded among the sites. The
C/N ratio differed (P0.05) among the farms, ranging
from 13.1 to 19.1 with a mean value of 16.1. pH values
ranged from 5.06.0 with significant differences
(P0.05) between the farms, and lowest and highest
CEC range varied by more than two-fold between thefarms, ranging from 7.0 to 15.3 me 100 g1 soil.
Calculations and statistics
Agronomic parameters (Bandyopadhyay and Sarkar2005;
Ghaley et al.2005) were calculated as given below:
% N derived from fertilizer% N DFF
15N excess in rice
15N excess in applied fertilizer 100% 1
Nitrogen derived from fertilizer NDFF inkgNha1
TDMN %NDFF
2
Nitrogen derived from soil NDFS inkgNha1
TDMN NDFF 3
Agronomic efficiency in N use AEN
in kg grain kg1N applied GYFGY0NF
4
District Site Village
clusters
Total soil
N (%)
C/N ratio pH CEC
(me 100 g1 soil)
Paro 1 Doteng 0.22 13.4 5.0 10.8
2 Doteng 0.11 19.1 5.1 7.0
3 Doteng 0.16 16.4 5.1 11.5
4 Shari 0.13 16.6 6.0 9.4
5 Chento 0.14 14.1 5.1 9.3
6 Chento 0.10 13.1 5.2 7.8
Thimphu 7 Khasa 0.13 15.2 5.5 14.5
8 Khasa 0.13 17.6 5.5 11.8
9 Khasa 0.14 18.4 5.7 10.6
10 Khasa 0.17 17.0 6.0 15.3
Mean 0.14 16.1 5.4 10.8
LSD 0.05 0.02 3.0 0.5 3.8
Table 1 Soil total nitrogen,
C/N ratio, pH and cation
exchange capacity (CEC) of
020 cm topsoil of ten
farmers fields (sites) in
Western Bhutan
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Recovery efficiency of applied N REN in %
NDFF
NF 100% 5
Physiological efficiency of N PEN
in kg grain kg1N uptake GYF GY0
NDFF
6
Partial factor productivity PFPN in kg grain kg1 soil
applied N GYF
NF7
Nitrogen harvest index NHI
GN
TDMN 8
where TDMN is total dry matter N, GY0 is grain yield
without fertilizer N application, GYFis grain yield with
fertilizer N application, NF is fertilizer N rate applied
(kg N ha1) and GN is grain N. Based on NDFSaccumulation, equivalent FYM quantity was calculated
based on 1.6% N content at 68% dry matter of FYM,
as reported in a similar field study in rice in Bhutan
(Chettri et al.2003). FYM content of P and K nutrients
are taken as 0.8% P and 2.9% K, respectively.
As there was only one replicate per site (per farmer),an analysis of variance over sites could not be carried
out to separate the effects of site on treatments. In such
situations, adaptability analysisprovides a good option
to determine site effects on the treatments (Hildebrand
and Russell 1996; IITA 1991; Mutsaers et al. 1997;
Mutsaers and Walker1990). Crop response in terms of
grain yield of a particular treatment in a given farmers
field is compared with the environmental index (EI),
which is the average of grain yields under all treatments
at a given site. The EI is thus an integrated index for
overall growing conditions in the field due to agronomic,biophysical and socioeconomic factors affecting crop
productivity.
Analysis of variance (ANOVA) was carried out for
combined data from ten sites with a nested design for
the main plots and the subplots. Separate ANOVA
for cultivar N supply (first split plot set) and
cultivar topdressing (second split plot set) was
carried out to determine the significance of treatment
effects on the respective dependent variables. EI-by-
treatment and EI-by-soil property interactions were
tested with multiple regressions to assess the signif-
icance of the relationship. Cultivar, N supply, top-
dressing timing as well as two-way interactions
between these factors were treated as fixed effects,
whereas sites and their interactions with treatment
factors were treated as random effects. All three-wayinteractions were treated as residuals and were
considered non-significant. Differences were consid-
ered significant if P0.05. Levels of significance
are denoted as follows: *** significant at P0.001,
** significant atP0.01, * significant atP0.05, ns =
not significant. Data were analysed with the Genstat
software package (Genstat 8.1 2005).
Results
Accumulation of dry matter and N in grain and straw
Grain and straw yield, harvest index (HI), grain and
straw N accumulation, TDMN, NDFS, and quantities
of estimated FYM are provided in Table2. Rice grain
yield (14% moisture content) was significantly affect-
ed by site (P0.001), cultivar (P0.001) and N
supply (P0.001), and there were interactions be-
tween N supply EI (P0.02) and cultivar EI (P
0.04). Among the farmers, Thimphu farmers 9 and 10
harvested significantly higher grain yields (6.07 and6.35 Mg ha1 respectively) compared with the other
farmers (3.365.85 Mg ha1). The difference between
the lowest and the highest average grain yield was as
high as 81% of the lowest average grain yield. Such
wide differences in average grain yields demonstrated
wide fertility differences between the farmers, a
measure of EI variability at the trial sites. Averaged
across sites and cultivars, the supply of 60 N
increased grain yields by 15% (P0.05) compared
to 0 N (Table 2). Averaged across site and fertilizer
treatments, farmers using the improved cultivarobtained 27% higher (P0.05) grain yields compared
with those using the traditional cultivar. This yield
improvement was mainly due to 27% higher HI of the
improved cultivar compared to the traditional cultivar.
Straw dry matter accumulation was significantly
affected by site (P0.001), cultivar (P0.001), N
supply (P0.01) and cultivar N supply (P0.04).
Averaged across sites and cultivars, 60 N increased
straw yields (12%) significantly compared to 0 N
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(Table 2). Averaged across site and fertilizer treat-
ments, the traditional cultivar produced 17% morestraw yields compared to the improved cultivar. HI
was significantly affected by site (P0.001), cultivar
(P0.001), and cultivar N supply interaction (P
0.02). HI differed widely among the farmers with a
difference of 34% between the highest and the lowest
HI (0.32 and 0.43). However, the N supply level had
no effect on HI. Accumulation of N in the grain was
significantly affected by site (P0.01), N supply (P
0.001) and cultivar (P0.001). Application of 60 N
increased grain N accumulation by 41% over 0 N, and
the improved cultivar accumulated 21% higher grainN compared with the traditional cultivar. Accumula-
tion of N in the straw fraction was affected by site
(P0.003) and N supply (P0.001). Farmer-to-
farmer differences in straw N accumulation were
recorded, with Thimphu farmer 9 harvesting the
highest quantity of straw N compared with other
farmers. 60 N increased straw N accumulation by
39% and there was no difference in straw N
accumulation between the two cultivars.
N accumulation in crop, soil N supply and FYM
application
Total accumulation of N in the whole crop (TDMN)
was affected by site (P0.01) and N supply (P
0.001). TDMN ranged from 82.7143.4 kg ha1
among the farms (Table 2) and 60 N supply resulted
in 40% increase (P
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by 100% and the improved cultivar had 7% higher
FYM application effect than the traditional cultivar.
Urea N uptake
Accumulation of N from the applied urea fertilizer
(NDFF) (kg ha1
) in grain + straw differed by site (P0.05), urea topdressing timing (P0.01), cultivar EI
(P0.05) and topdressing EI (P0.05) (Table 3).
Averaged over cultivars, NDFF (kg ha1) quantifica-
tion by 15N dilution in grain + straw was significantly
higher (56%) with top dressing at 45 DAT than at 7
DAT. Of 30 kg N ha1 top-dressed each at 7 and 45
DAT, the mean NDFF in grain + straw, averaged over
cultivars, were 10.2 kg N ha1 with 45 DAT
topdressing and 6.5 kg N ha1 with 7 DAT.
Correspondingly, the mean REN at 45 DAT was
34% compared to 22% at 7 DAT. Over two top-dressings, the mean REN in grain + straw in the
traditional cultivar and the improved cultivars were
30.0% and 25.7% respectively with no significant
difference between the cultivars. Compared to the 15N
dilution method, the mean REN based on N differ-
ence method (Table 3), were significantly higher in
both the traditional (56 .2%) and the improved
cultivars (46.2%) with no significant difference
between the cultivars.
N utilization efficiency
N utilization efficiency varied according to farm and
cultivar (Table 4). Agronomic efficiency in N use
(AEN) (kg grain kg1 N applied) varied among the
farmers (P0.01) from as low as 1.2 up to 22.9 kg
grain kg1 N applied (Table4). Across the farms, the
improved cultivar exhibited 66% higher AEN (P
0.001) than the traditional cultivar. Similarly, physi-
ological efficiency in N (PEN) (kg grain kg1 N
uptake) deviation was significant (P0.01) across the
farms and the PEN of the improved cultivar was
almost 100% more (P0.001) than the traditionalcultivar. Although, there was no cultivar differences
in fertilizer N uptake, higher PEN in the improved
cultivar resulted in higher AEN.
Partial factor productivity in N (PFPN) (kg grain
kg1 soil + applied N) differed significantly between
Table 3 N derived from fertilizer (NDFF in kg N ha1) and recovery of the fertilizer N (REN %) in grain + straw, from two
30 kg N ha1 splits applied at 7 and 45 days after transplanting (15N dilution) and REN% with N difference method in the traditional
and the improved cultivar in ten farmers fields (sites) in Western Bhutan
NDFF(kg N ha1)
by 15N dilution
REN by 15N
dilution (%)
REN by 15N dilution (%) REN by N
difference (%)
Traditional Improved Traditional Improved Mean
Traditional
Mean
Improved
Traditional Improved
Site (7) (45) (7) (45) (7) (45) (7) (45) (7+45) (7+45)
1 8.3 12.2 6.2 10.8 27.6 40.7 20.8 36.1 34.2 28.5 72.1 58.8
2 6.6 12.5 5.0 9.9 21.8 41.6 16.6 33.1 31.7 24.9 39.3 30.7
3 6.6 9.2 5.9 8.6 21.9 30.7 19.8 28.5 26.3 24.2 68.0 65.1
4 10.0 11.9 6.2 9.0 33.3 39.6 20.6 29.9 36.5 25.3 53.3 56.0
5 9.4 13.0 8.1 9.0 31.2 43.3 27.1 30.1 37.3 28.6 44.6 34.8
6 6.8 9.6 7.7 8.5 22.6 31.9 25.5 28.3 27.3 26.9 67.9 37.6
7 5.9 10.2 5.6 9.9 19.5 34.0 18.6 33.0 26.8 25.8 52.4 52.2
8 8.1 11.6 6.3 10.6 27.0 38.5 20.9 35.5 32.8 28.2 59.3 30.9
9 4.4 9.3 4.3 8.9 14.5 30.9 14.2 29.8 22.7 22.0 63.2 56.3
10 5.1 9.9 4.4 9.4 17.0 32.9 14.7 31.5 25.0 23.1 42.1 39.5
Mean (topdress) 7.1 10.9 6.0 9.5 23.6 36.4 19.9 31.6
LSD0.05 (topdress effect) 1.7 2.3 5.8 7.7
Mean (cultivar) 9.0 7.7 30.0 25.7 56.2 46.2
LSD0.05 (cultivar effect) 1.5 5.1 11.0
Table 3 N derived from fertilizer (NDFF in kg N ha1) and
recovery of the fertilizer N (REN %) in grain + straw, from two
30 kg N ha1 splits applied at 7 and 45 days after transplanting
(15N dilution) and REN% with N difference method in the
traditional and the improved cultivar in ten farmersfields (sites)
in Western Bhutan
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the farms (P0.05), with the improved cultivar
producing over 20 kg more grain kg1 N (P0.001)
applied than the traditional cultivar. Nitrogen harvestindex (NHI) (proportion of grain N to total TDMN)
varied across the farms (P0.05) according to farm
environments, and the improved cultivar showed 15%
(P0.001) higher allocation of N to the grain than the
traditional cultivar.
Adaptability analysis
Grain yields of the traditional and the improved
cultivar differed with EI with significant cultivar
EI interactions (P0.04) (Fig. 1a), demonstratinggenotypic differences in grain yield performance
across the environments. The improved cultivar
produced higher treatment grain yields across the
environments compared to the traditional cultivar.
Across the farms, grain yield of 60 N plots were
consistently higher than in 0 N plots (N supply EI)
across the EI (Fig. 1b). Grain yield differences
between the 0 and 60 N plots were higher in low-
yielding environments than in high-yielding environ-
ments, and grain yields differed significantly (P
0.02) between the N treatments.
Discussion
The current study tested the hypothesis that cultivarshave different N utilization potentials and that the
cultivars manifest such inherent potentials across a
wide range of environmental conditions. The study, to
our knowledge, is the first to combine the rigour of
the 15N dilution technique with the flexibility of field-
oriented adaptability analysis, based on large envi-
ronmental variations and under conditions prevailing
for highland rice farmers in the Himalayan region.
On-farm data with single replication per site is suited
to adaptability analysis, and EI-by-treatment interaction
analysis can be consistent over a number of years ifcertain requirements are met in a single years trial
(Hildebrand and Russell 1996; Meertens et al. 2003;
Fukai et al. 1999; Kajiru et al.1998; Sall et al. 1998).
Firstly, for linear and quadratic response estimation of
EI-by-treatment interaction, ten environments are rec-
ommended with four treatments in each environment
(Hildebrand and Russell 1996), and the trial set-up in
the current study contained ten environments with
eight treatments in each environment (site). Secondly,
EI values should represent distribution of the target
population of environments at the trial site forimproved data quality and reliability. In the present
study, EI values reflected a normal distribution with
most environments clustered around the average
environments, representative of actual farming envi-
ronments in the trial sites (Fig. 1). Thirdly, crop yield
levels should represent average yields in the area,
typical of a normal cropping season. According to the
national agriculture production census in Bhutan,
average yields reported for rice at the trial sites were
in the 3.84.2 Mg ha1 range (MOA2001,2004), and
yield data from the current study conforms to theselevels. Our dataset therefore meets the requirements for
carrying out adaptability analysis.
Cultivars differ in response to environmental
conditions
Cultivar EI (P0.05) values indicate that NDFFvaried
across the range of EI depending on the field
conditions. Differences in field characteristics among
Table 4 Agronomic efficiency in N (AEN) (kg grain kg1 N
applied), physiological efficiency in N use (PEN) (kg grain
kg1 uptake), partial factor productivity in N use (PFPN) (kg
grain kg1
soil + applied N) and nitrogen harvest index (NHI)
in the traditional and the improved cultivar in ten farmers
fields (sites) in Western Bhutan
Site AEN PEN PFPN NHI
1 22.9 73.3 59.6 0.48
2 6.6 24.2 66.9 0.48
3 19.0 75.9 71.0 0.44
4 11.5 38.6 86.4 0.48
5 1.2 2.6 68.5 0.59
6 7.7 28.4 68.9 0.54
7 15.7 60.2 86.0 0.55
8 7.6 27.1 87.6 0.55
9 9.7 43.9 95.8 0.45
10 13.9 58.1 93.9 0.56
Mean 11.6 43.2 78.5 0.51
Cultivar
Traditional 8.7 29.1 68.3 0.48
Improved 14.5 57.4 88.6 0.55
LSD0.05 (site main effect) 7.8 32.7 18.2 0.08
LSD0.05 (cultivar main effect) 2.8 10.3 3.1 0.03
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phase of the seedlings (Zhang et al. 2007; Peng and
Cassman1998). A similar study on transplanted rice in
China found that the indigenous soil N supply fulfilled
the N requirement at the early rice seedling stage (Xue
and Yang2008).
Environmental variation and soil fertility
The measured soil properties were found to have
varying degrees of effects on EI (Fig. 2a, b, c, d).
Total soil N % and EI were not correlated (P0.24)
(Fig. 2a), which conforms to the results in other on-
farm studies of rice-wheat and ricerice systems
(Adhikari et al. 1999; Cassman et al. 1996b). A
possible reason for the non-correlation is that the
warm monsoonal climate during paddy growth is
conducive for sufficient mineralization of N, masking
the soil N status measured before transplanting(Adhikari et al. 1999). Soil C/N ratio, however, was
positively correlated with EI (Fig. 2b), indicative of a
better proxy for potential soil N mineralization than
total soil N content. Similarly, soil pH was positively
correlated with EI (Fig. 2c), indicating that the soils
with neutral pH (5.56) had higher yield levels
compared to the slightly acidic soils (55.5). CEC
was not found to be correlated (P0.31) to EI(Fig. 2d), in contrast to other studies (Ruth and
Lennartz 2008; Yanai et al. 2000, 2001). However,
the high fertility of the soils and the tradition of
consistently applying FYM to the fields may elimi-
nate or reduce the importance of CEC effects.
Environmental variation and management
FYM application and EI were not correlated (R2=
0.25), but the direct role of FYM in supplying N to
the crops (Fig. 3a) was significant in line with thefindings from other studies, including a study in
y = 1.44x + 8.68
R2= 0.41*
0
10
20
0 2 4 6 8
Environmental Index (Mg ha-1)
SoilC/Nratio
(b)y = -0.01x + 0.22
R2
= 0.17
0.00
0.10
0.20
0.30
0 2 4 6 8
Environmental Index (Mg ha-1
)
0 2 4 6 8
Environmental Index (Mg ha-1
)
0 2 4 6 8
Environmental Index (Mg ha-1
)
(a)
y = 0.33x + 3.74
R2 = 0.67**
0
2
4
6
8
SoilpH
(c) y = 1.02x + 5.59
R2= 0.13
0
5
10
15
CEC(me100g-1s
oil)
(d)
TotalsoilN%
Fig. 2 Relationship
between environmental
index and (a) total soil N %
(b) soil C/N ratio (c) soil pH
and (d) cation exchange
capacity (me 100 g1 soil )
in ten farmers fields in
Western Bhutan
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Bhutan (Ghosh and Sharma1999; Chettri et al.2003;
Satyanarayana et al. 2002). FYM may thus have a
priming effect or added nitrogen interaction (ANI)
(Azam 1990; Ghosh and Sharma 1999; Jenkinson et
al.1985). Consequently, NDFS and EI were correlated
(Fig. 3b). However, there was a negative correlation
bet ween FYM quant ity and REN (Fi g. 3c) atrelatively high FYM input levels, found in some sites
(Table 2, site 3, 10) compared with others (Table 2).
Correspondingly, REN values were low (Table3, site
3,10) in high fertility sites, and FYM input therefore
needs to taken into account when formulating
fertilizer N recommendation for farmers. The tenden-
cy of decreasing REN from fertilizer with increasing
availability of N from indigenous sources has been
reported in similar rice field studies in the Philippines
and Bangladesh (Hossain et al. 2005; Cassman et al.
1996c).
Soil N supplying capacity
Nitrogen derived from soil constituted 7791% of
TDMN (Table2), corresponding with the results from
a very large study covering 179 sites over 2 years in
eight key rice domains in Asia (Dobermann et al.
2002). In flooded rice paddies, NDFS constitutes Ncontribution from the soil organic matter fraction
(FYM), atmospheric deposits, irrigation water, stubble
residues from preceding crops and N2 fixation. N
contribution from atmospheric deposits and irrigation
water are in the range of 13 kg ha1 per rice crop and
considered negligible (Cassman et al. 1996b). Nitro-
gen contributions from stubble residue and N2fixation are interrelated as availability of crop residue
enhances N2 fixation due to increased carbon sub-
strate for microbial activity (Roper and Ladha1995).
Nitrogen fixation is reported in the range of 28
y = 11.04x + 11.83
R2= 0.74
y = 15.30x + 10.29
R2= 0.88***
0
40
80
120
0 5 10
TraditionalImprovedLinear (Traditional)Linear (Improved)
(d)
y = 12.80x + 0.66
R2
= 0.95***
0
40
80
120
0 3 6 9 12
Farmyard manure (Mg ha-1)
Farmyard manure (Mg ha-1)
Nitrogenderivedfromsoil(kgNha)
(a)y = 13.36x + 21.51
R2
= 0.44*
0
40
80
120
0 2 4 6 8
Nitrogenderivedfromsoil(kgha-
1) (b)
Environmental Index (Mg ha-1)
Environmental Index (Mg ha-1)
y = -0.02x + 0.40
R2
= 0.49*
0
20
40
0 4 8 12
(c)
Recoveryefficiencyof
Nin%(REN)
PFPN(kggrainkg-1so
il+appliedN)
-1Fig. 3 Relationship
between nitrogen derived
from soil (NDFS) and (a)
farmyard manure (FYM)
and (b) environmental index
(EI); (c) farmyard manure
(FYM) and recovery
efficiency of N (REN %)
and (d) environmental index
(EI) and partial factor
productivity in N use
(PFPN) in ten farmersfields in Western Bhutan
Plant Soil (2010) 332:233246 243
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51 kg Nha1 per crop cycle with sufficient crop
residue and phosphorus availability, and N2 fixation
might have contributed significantly due to the
abundance of FYM as substrate for microbial activity.
After N2 fixation is accounted for, the remaining
organic soil N must have come from mineralization of
FYM, constituting the major bulk of soil N, whichcorresponds with the strong correlation (R2=0.95***)
between the NDFS and quantity of FYM applied
(Fig. 3a).
Quantification of REN
Quantification of REN can vary according to the
method used. The N difference method may in some
cases give a higher estimate of REN than the 15N
tracer method in transplanted rice (Bronson et al.
2000). In concurrence with other studies (Rao et al.1991; Bronson et al. 2000), REN estimated by the
difference method in our study was 2026% greater
than that obtained by the 15N dilution method. This
positive Added Nitrogen Interaction (ANI) may be
real or apparent. A real ANI may arise due to greater
accumulation of soil N in plots receiving fertilizer N
compared with unfertilized plots, while an apparent
ANI may arise as a result of pool substitution. A real
positive ANI may arise due to enhanced soil organic
matter mineralization following the application of N
fertilizer. It may also occur as a result of greater soilexploration by plant roots and enhanced uptake of
unlabelled N in fertilized plots. Both of these
mechanisms may have operated in this study, but
uptake of N mineralized from previous inputs of FYM
may also have enhanced the potential for pool
substitution. Hence, REN estimation by 15N dilution
method may have been affected by both real and
apparent ANI.
Conclusions
The wide variability in rice grain yields across the
environments are related to differences in soil
properties, FYM application and cultivar response to
fertilizer N. Agronomic efficiency of N use was
relatively low across the environments, with wide
differences between farmers, highlighting opportuni-
ties for the transfer of knowledge on better N
management to the farmers. The improved cultivar
yielded significantly more grain compared with the
traditional rice variety given the same amount of
fertilizer N. Consequently, the wider use of improved
cultivars may enhance rice yields without the need for
greater N inputs. The decreasing REN with increasing
FYM application is a matter of concern as N lost from
fields may contribute to eutrophication of rivers andstreams, as well as increase the cost of rice production
due to the decreasing marginal rate of return at higher
FYM input. Nitrogen application at 45 DAT resulted
in 56% greater uptake of NDFF than at 7 DAT,
indicating that the first topdressing could be delayed
until 45 DAT (maximum tillering) to coincide with
the maximum crop N demand and enhance the
opportunity for N uptake. To make the best use of
both fertilizer and soil-derived N, the fertilizer N
recommendations for rice farmers in the Bhutan
highlands should to take account of previous inputsof FYM and the timing of maximum crop N demand.
Acknowledgements We gratefully acknowledge the financial
assistance by the Consultative Research Committee for Devel-
opment Research/Danish International Development Agency,
under the Ministry of Foreign Affairs in Denmark. The study
would not have been possible without the help of Dr. Lungten
Norbu, Mr. Padam Lal Giri, Hema Devi Nirola and Kalpana
Rai, who provided full assistance in terms of planning and
implementing the field activities with the farmers in Thimphu
and Paro districts in Western Bhutan. Thanks are due to Mrs.
Marie Bcker Pedersen for language editing and feedback on
the paper and to the anonymous reviewer for the significantcontributions in improving the scientific content of the
manuscript.
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