Improving nitrogen use efficiency by maize through the pigeon pea-groundnut intercrop-maize rotation cropping system in Malawi
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Date
2015
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Sokoine University of Agriculture
Abstract
A study was initiated in the 2011/12 cropping season with a parallel experiment mounted
along side in the second season to investigate the possibility of improving nitrogen use
efficiency (NUE) by the maize crop in a pigeon pea groundnut intercrop-maize rotation
cropping system at Chitedze Agricultural Research Station in Malawi. The parallel
experiment was conducted to compare the performance of legumes over two cropping
seasons. The experiments involved the planting of two pigeon pea varieties, namely; long
(ICEAP 04000) and medium duration (ICEAP 00557) and groundnut (CG 7) as
monocultures or as intercrops. The main experiment had eight treatments; 1) Sole maize
(control); 2) Medium duration pigeon pea; 3) Long duration pigeon pea; 4) Sole
groundnut; 5) Medium duration pigeon pea + groundnut; 6) Long duration pigeon pea +
groundnut; 7) Medium duration pigeon pea + groundnut (biomass not incorporated in
season two); and 8) Long duration pigeon pea + groundnut (biomass not incorporated in
season two). All the treatment plots except treatment plot number one were treated with
25 kg P ha -1 . The parallel experiment had ten treatments; 1) Long duration pigeon pea; 2)
Medium duration pigeon pea; 3) Sole groundnut; 4) Sole groundnut + 25 kg P ha -1 ; 5)
Medium duration pigeon pea 25 kg P ha -1 ; 6) Long duration pigeon pea + 25 kg P ha -1 ; 7)
Long duration pigeon pea + groundnut; 8) Long duration pigeon pea + groundnut + 25
kg P ha -1 ; 9) Medium duration pigeon pea + groundnut; and 10) Medium duration pigeon
pea + groundnut + 25 kg P ha -1 . Both experiments were laid in a randomized complete
block design replicated three times. Key parametres assessed during the experiment
included; legume biomass and grain yield, soil nitrate nitrogen ( NO 3- ‒N), maize stover,
and rachids yields; nitrogen and phosphorus partitioning both for the legumes and maize
NUE.ii
Soil characterization was conducted before treatment application in the first and second
year. Generally, the soil chemical characteristics for soil samples collected in all the
treatment plots both in the main and parallel experiment indicated that the soil was of low
fertility. The %OC and total N (%) was low, and was at 1.4 %, 0.12%, respectively, while
plant available phosphorus (Mehlich 3) was marginally adequate (19 mg P kg -1 to 25 mg
P kg -1 ). The soil texture which was predominantly sandy clay loam to sandy clay suggest
potential high leacheability of mobile nutrient ions more especially nitrogen as nitrate.
Inevitably, if the soil is not properly managed crop yield could be reduced drastically.
Total biomass yield assessment for the pigeon pea was conducted in the parallel
experiment in season two. Partial biomass yield assessment was done in season one in the
main experiment. In season two this involved assessment of litter, twigs, stems, fresh
leaves and roots for each treatment plot. The litter was collected from the ground on one
planting station (90 cm x 75 cm). This was done in September, 2013. Fresh leaves, twigs
and stems were also weighed from the 2 m x 2 m net plot. These were oven dried for 72
hours at 70 o C to constant weights. The assessment of the above ground groundnut
biomass indicate a low yield range of 479-656 kg ha -1 while the assessment of the total
above ground biomass yield of the pigeon pea varieties indicate a high yield range of
3,124-3,840 kg ha -1 . Nitrogen yield assessment indicate that the monoculture for
groundnut treated with P yielded more N (52.0 kg N ha -1 ) compared to the non treated
groundnut monoculture (40.0 kg N ha -1 ) while the P treated monoculture for the long
duration pigeon pea yielded higher soil returnable N (87.2 kg N ha -1 ) compared to the non
P treated counterpart (79.7 kg N ha -1 ). For the medium duration pigeon pea monoculture
higher soil returnable N was harvested in the P treated monoculture (95.6 kg N ha -1 ) thaniii
the non P treated monoculture (87.0 kg N ha -1 ). Similar soil returnable yield of N was
observed in the P (128.3 kg N ha -1 ) and non P treated (128.8 kg N ha -1 ) intercrop of
medium duration pigeon pea and groundnut. Higher soil returnable yield of N was
observed in the P (128.4 kg N ha -1 ) and non P treated (103.9 kg N ha -1 ) intercrop of long
duration pigeon pea and groundnut. Generally, the monocultures and intercrops treated
with P gave higher N yield when compared to the non P treated counterparts. This was
attributed to enhanced biological N fixation in the P treated treatments due to the
increased level of available P. Poor grain filling for the pigeon pea varieties was observed
both in the main and parallel experiment. For the groundnut shells’ yield ranged from 846
kg ha -1 to 1,985 kg ha -1 while grain yield ranged from 1,513 kg ha -1 to 3,025 kg ha -1 and
haulms’ yield ranged from 1,396 kg ha -1 to 2,463 kg ha -1 . N concentration in the shells
ranged from 0.9% to 1.5% while in the grain ranged from 2.9% to 3.2% while for haulms
ranged from 1.9% to 2.3%. N yield in the groundnut shells ranged from 10.2 kg N ha -1 to
25.2 kg N ha -1 while for grain ranged from 46.9 kg N ha -1 to 98.8 kg N ha -1 and for
haulms ranged from 29 kg N ha -1 to 52 kg N ha -1 . The concentration of N in the maize
grain ranged from 1.1% to 2.1% while maize grain yield ranged from 1,775 kg ha -1 to 5,
806 kg ha -1 and the N yield ranged from 23 kg N ha -1 to 115 kg N ha -1 . The concentration
of N in the maize stover ranged from 0.1% to 1.0% while stover yield ranged from 2,029
kg ha -1 to 4,413 kg ha -1 and the N yield ranged from 2.3 kg N ha -1 to 33.2 kg N ha -1 . The
concentration of N in the maize rachids ranged from 0.1% to 0.5% while the rachids yield
ranged from 405 kg ha -1 to 1,235 kg ha -1 and the N yield ranged from 0.7 kg ha -1 to 5.1
kg N ha -1 . The data indicated that more N in the groundnut and maize plant is
translocated to the grain as such there is net export of N from the field which might lead
to depletion of N in the soils.iv
Assessment of soil NO 3- ‒N was conducted in the main experiment in the 2012/2013
cropping season, after the emergence of the succeeding maize crop. This was done in
order to establish the effect of incorporating legume residues on soil NO 3- ‒N and the
implication this might have on nitrogen management and crop yield. Data was collected
over a period of three weeks. This was done before top dressing with urea. Over the study
period high levels (100 > mg L -1 ) of soil NO 3- ‒N were observed that were in most cases
statistically the same (p>0.05) across the treatment plots. In general, mean soil NO 3- ‒N
was higher between 20 cm to 40 cm than 0 to 20 cm, attributable to the soil texture which
is predominantly sandy clay loam both between 0 to 20 cm and 20 cm to 40 cm hence
high leaching of NO 3- . Most likely, the level of soil NO 3- ‒N, into the season, in treatment
plots in which no biomass was incorporated declined faster than in treatment plots where
no incorporation was done, as a result of uptake by the maize crop and leaching losses.
The high levels of soil NO 3- ‒N probably, lasted longer into the season for the latter
treatment plots, but might not have endured until the end of the cropping cycle due to
limited supply of N from the incorporated biomass. Therefore, supplementation of N
from mineral sources is requisite for the attainment of optimal maize grain yield. In
general the KCl method gave higher readings of NO 3- ‒N (0-20 cm=90.3 mg L -1 and 20
cm to 40 cm =108.5 mg L -1 ) compared to the nitrate meter (0 to 20 cm=68.1 mg L -1 and
20 cm to 40 cm=65.9 mg L -1 ). This could be attributed to the differences in the extraction
procedure for the two methods, a cause of the different results generated by each
procedure.
Assessment of NUE for the maize crop was conducted in order to determine how
efficient the crop utilized applied N from urea. NUE was determined using the recoveryv
efficiency (RE), agronomic efficiency (AE) and partial factor productivity (PFP) indices.
Under the conditions of this study RE ranged between 20% and 88%, AE ranged between
7 and 32 kg yield increase per kg of nitrogen applied and PFP ranged from 27 to 104 kg
grain yield per kg nutrient applied. RE values of 50% to 80% , AE values of 10–30 kg kg -
1
and PFP values of 40–80 kg kg -1 are often encountered with values >25 kg kg -1 for AE
and >60 kg kg -1 for PFP being common in well-managed systems or at low levels of N
use, or at low soil N supply. The linear increase in grain yield with application of N and
the presence of a diminishing-return relationship between maize grain yields (grain yield
was near the yield potential of the maize variety at high N input) and increasing nitrogen
supply, suggest that the RE, AE and PFP values emerging from this study might apply
both to low and high levels of N use, or at low and high soil N supply.
From the study, the following conclusions were made; the soils on which the experiments
were conducted were of low fertility status evidenced by the low nitrogen and
phosphorus. A situation that calls for soil N and P management for increased crop
productivity. Furthermore, the study confirmed the viability of the pigeon pea-groundnut
intercropping system. The nitrogen yields for the cropping system were deemed to be
reasonably high. Employing this system in rotation with maize can reduce to an extent
the amounts and hence the costs of mineral fertilizers required for maize production. On
the effect of incorporating legume biomass into the soil on soil NO 3- ‒N, it was noted that
apparently the soil had high NO 3- ‒N in the soil solution attributable to residual N from N-
fertilization and legume cropping over years. Soil NO 3- ‒N was higher between 20 cm to
40 cm than between 0 to 20 cm in the soil. This was attributable to the soil texture which
is predominantly sandy clay loam with low to medium level of SOM. Leaching of NO 3- isvi
high under such soil conditions. It is likely that soil NO 3- ‒N levels in all the treatment
plots would decline in all the treatment plots along the season principally due to crop
uptake of N and leaching losses. This for the Malawian smallholder farmers implies that
in this cropping system N supplementation from mineral fertilizer is not optional if
reasonably high maize yield is to be realized. Additionaly, comparative analysis of two
soil NO 3- ‒N analysis procedures indicated that the KCl method gave higher readings of
NO 3- ‒N compared to the nitrate meter. This accrued from the differences in the extraction
procedure for the two methods.
The study served to confirm that more N yield in groundnut is exported from the field in
form of shells and grain and less is returned to the soil upon incorporation of the haulms.
Over and above, it was observed that in the pigeon pea much of the N contribution to the
soil N pool comes from the above ground biomass as compared to the below ground
biomass. Additionally, supply of P to legumes increases N accumulation and yield
through enhanced biological N fixation. The legumes, however, do not yield enough P for
the correction of soil P deffiencies that are prevalent across Malawi. The PFP ( 27 to 104
kg grain yield kg -1 N applied ) values obtained under the conditions of this study, which
are higher than that ( 20 kg grain yield kg -1 N applied ) reported under smallholder farms
in Malawi, seem to suggest that legume biomass incorporation into the soil does improve
NUE of the suceeding maize crop. The NUE values generated fall within the range of
values that are often encountered in well-managed systems or at low levels of N use, or at
low soil N supply.
Ratooning of the pigeon pea in this environment appears to be the solution to the
observed poor grain filling for the pigeon pea. Furthermore, the low P yields from thevii
legumes indicate the need to supply P using mineral fertilizer sources in addition to N for
optimal maize grain yield. Further studies in this cropping system should focus on
understanding the decomposition and mineralization pattern of the incorporated legume
biomass for the assertion of the time and amount of N release. This is critical inorder to
establish if this is in syncrony with nutrient demand by the maize crop.
Description
Keywords
Nitrogen fixation, Pigeon pea-groundnut, Intercrop-maize rotation, Cropping system, Pigeon pea, Malawi