Chitrlada Thongsodsaeng
E-mail: Chitrladat@yahoo.com
………………………………
Experiments were conducted with the aims of measuring
the growth, yield and the amount of nitrogen fixation of pre-rice legumes.
Another objective was to study the residual N benefits of these pre-rice grown
crops on succeeding rice yield. These pre-rice crops included groundnut (Arachis
hypogaea), mungbean (Vigna radiata), sesbania (Sesbania rostrata),
grown in three densities, and a mixture of sesbania and multipurpose cowpea
(MCP; Vigna unguiculata).
The experiment was established on a characteristic
sandy loam of Roi Et soil series at Northeast Regional Agricultural Coordinating
Office (Tambon Tha Phra, Amphoe Muang, Khon Kaen Province) and laid out in a
Randomised Complete Block Design with four replications. There were eleven
treatments; i.e. high density sesbania (10x10 cm)-rice, medium density sesbania
(12.5x10 cm)-rice, low density sesbania (25x10 cm)-rice, sesbania intercropped
with multipurpose cowpea (MCP)-rice, mungbean-rice, groundnut-rice, non-nod
groundnut-rice, rice-rice, fallow-rice without nitrogen fertilizer, fallow-rice
receiving nitrogen fertilizer at 4.8 kg N/rai and fallow-rice receiving
nitrogen fertilizer at 9.6 kg N/rai. The per-rice crops were sown on 19 May
1996 and harvested on 2-27 August 1996. Succeeding rice, cultivar RD 6, was
grown on 9 September 1996 and harvested on 12-18 December 1996. The 15
N isotope dilution technique was used to quantify the amount of nitrogen
fixation by pre-rice legumes.
The results indicated that the intercropped sesbania+
MCP gave the highest total dry matter of 1,321 kg/rai (1,047 and 274 kg/rai for
sesbania and MCP respectively). Sesbania dry matter production increased with
increasing planting densities. Groundnut gave highest stover dry matter and
economic yields (1,042 and 274 kg/rai, respectively) comparing to other grain
legumes. It also fixed more nitrogen than other legumes (22.99 kg/rai, 82.31 %
Ndfa). Sesbania+MCP fixed almost the same amount of nitrogen (21.15 kg/rai,
78.76 % Ndfa). Sesbania at high density, medium density, low density and
mungbean fixed 16.66, 14.25, 13.84 and 7.64 kgN/rai (86.29, 89.33, 87.18, 69.96
% Ndfa) respectively.
All legumes, except non-nod groundnut, gave positive
net N benefits from fixation, when stover was returned to the soil, ranging
from 1.91 kgN/rai in the case of mungbean to 18.07 kgN/rai in the case of
sesbania+MCP. The pre-rice treatment with sesbania+MCP resulted in the same
yield benefit as the sole sesbania treatments. Groundnut contributed 8.90
kgN/rai to the land system, which was higher than the other grain legumes.
Rice growth and nitrogen content corresponded very
well with the net N benefit from fixation of each N2 fixing legume
i.e. being highest in rice grown after sesbania and lowest in that grown after
mungbean. Rice grown after sesbania at medium density gave the highest growth,
yield and nitrogen content. Nitrogen accumulation in rice was also
significantly affected by pre-rice crop. Again, the highest N yields were
obtained in the sesbania treatment. Pre-rice sesbania, sesbania+MCP and
groundnut increased rice yield production more than chemical N fertilizer
additions of 4.8 or 9.6 kgN/rai. About 13.57-44.09 % of the residue N applied
was recovered by rice.
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