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Sustainable Impact of In Situ Leguminous Green Manuring on Grain Yield and N Utilization Patterns of Rainfed Lowland Rice (Oryza sativa) Grown Under Different Cropping Geometries

ABSTRACT Rice needs adequate nitrogen, particularly under rainfed lowland situation, to ensure its growth for optimum grain yield. Traditional practice of applying entire dose of N fertilizer during sowing causes its low recovery. Nonetheless, it does not sustain over subsequent growth of rice, mainly due to uncontrolled waterlogged situation. Green manuring along with chemical N application was reported as advantageous ensuring sustained availability of adequate N over the period. A field study on green manuring rice under rainfed lowland situation was conducted during wet seasons in 2001 to 2004 in the Central Rice Research Institute, Cuttack, India. The photosensitive, tall and long-duration rice variety, Durga was grown along with green gram (Phaseolus radiatus) for green manuring under three mixed cropping stands, that is, additive system, replacement system, and broadcast-sown stands. Green gram, also known as mung bean, is a widely grown pulse crop. As a leguminous crop, it is capable of contributing considerable amount of green matter and N into the soil. Thus it can be used for green manuring to facilitate N nutrition to the rice grown in its association. Rice was sown at 15 × 20 cm spacing (plant × row), except in broadcast-sown stands where rice and green gram seeds were mixed thoroughly before sowing and broadcasted evenly without maintaining any plant/row arrangement. In the additive system, one row of green gram was intercropped after every two consecutive rice rows, when usual row spacing was changed therein. Thus the adjacent rice rows were 10 cm apart from green gram row. In the replacement system, one rice row after every two consecutive rice rows was replaced with one row of green gram without changing usual row spacing, that is, the adjacent rice rows were 20 cm apart from green gram row. These green-manured mixed stands were compared with two pure stands of rice, that is, rice grown with usual dose of 40 kg N/ha and rice grown without N. Similar management practices were followed for both the crops altogether and no additional fertilizers were applied in green gram. Growth dynamics of green gram as evidenced from its plant height and dry matter recorded at regular interval showed no variation over the years. It was incorporated into the soil at its flowering stage accumulating 7.00 to 25.60 kg N/ha over the years under different stands. Rice growth due to green manuring in the replacement system remained consistently better than other stands in all the years. As a consequence, rice under the replacement system significantly out-yielded other stands producing 2.41 to 2.52 t/ha over the years, which was at par with that treated with fertilizer N alone (2.48 and 2.60 t/ha), while grain yield (2.12 to 2.20 t/ha) under the additive system was significantly higher than broadcast-sown crop (1.77 and 1.84 t/ha). In addition, N utilization pattern revealed that crops under the replacement system derived higher Agronomic efficiency (43.50 to 53.50 kg grain/kg N applied), N uptake (27.75 to 29.30 kg/ha), and N recovery (56.25 to 66.00%, respectively) over the years as compared with that in other cropping geometry.