Apparent Recovery and Physiological Efficiency of Nitrogen in Sesbania Incorporated before Rice

Abstract

AbstractWhen incorporated for lowland rice (Oryza sativa L.), the agronomic efficiency of green manure N exceeding 100 kg ha−1 is low. Apparent N recovery and physiological efficiency of green manure N, ranging to 200 kg N ha−1 were determined on a fine, mixed, isohyperthermic Aerie Tropaqualf in the Philippines. Sesbania sp. accumulated 100 kg N ha−1 in 48 d and 160 kg N ha−1 in 60 d. Nitrogen uptake (NU, kg N ha−1) from green manures flooded during the last 25 d before incorporation was a linear function of incorporated N (NU = 49 + 0.23N). Uptake from green manures that were not flooded diminished as incorporated N increased (NU = 35 + 0.51N – 0.0012N2). The flooding effect was attributed to inhibition of NO−3 accumulation on submerged control plots. Nitrogen uptake was linearly related to extractable NH+4‐N determined 7 d after green manure incorporation. As N uptake increased, however, physiological efficiency declined. Dry‐season rice recovered only 6% of the green manure N not recovered by wet‐season rice. Because both apparent recovery and physiological efficiency are low, and residual green manure N recovery is minor, the advantage to growing Sesbania sp. for more than 48 d to accumulate N in excess of 100 kg ha−1 is limited.