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Nitrate Dynamics during the Aerobic Soil Phase in Lowland Rice-Based Cropping Systems

IRRI/NifTAL Collaborative Program

J. K. Ladha

IRRI, P.O. Box 933, Manila, Philippines

D. P. Garrity

ICRAF Southeast Asia Regional Programme, Jalan Gunung Batu 5, P.O. Box 161, Bogor 16001, Indonesia

R. J. Buresh

ICRAF, P.O. Box 30677, Nairobi, Kenya

*Corresponding author.

ABSTRACT

In tropical rice (Oryza sativa L.) lowlands, soil NO₃ is lost during the transition from the dry to the wet season. To understand how soil and crop management influences NO₃ loss, we examined NO₃ dynamics during a 2-yr period in an Alfisol in the Philippines: weedy, weed-free, and frequently tilled main plots during the February to May dry season (DS), and Sesbania rostrata (Brem. & Oberm), mungbean [Vigna radiata (L.) R. Wilczek var. radiata], weedy, and weed-free subplots during the May to July dry-to-wet transition (DWT). Weed-free plots were maintained by removing weeds as they emerged. Soil NH₄ (0–60 cm), which was not affected by management, averaged only 9 kg N ha^–1. While soil NO₃ increased under frequent tillage and weed-free fallowing, it decreased rapidly under weedy fallowing. On most sampling dates, NO₃ was the highest in DS tilled main plots. The widest range of NO₃ during the DS or DWT was 14 to 110 kg N ha^–1 in the first year, and 12 to 155 kg N ha^–1 in the second. During the second half of the DWT, NO₃ declined in all plots, but more markedly when plants were present than when not, indicating plant N uptake. Above-ground plant N prior to permanent flooding ranged widely from 31 kg N ha^–1 in weeds to 222 kg N ha^–1 in N₂-fixing S. rostrata plants in the first year, and 37 to 193 kg N ha^–1 in the second. The data also indicate NO₃ leaching following heavy rains. Further, the high water-filled pore space, exceeding 0.7 L L^–1 in the second half of the DWT and approaching 1 L L^–1 with permanent flooding, is presumed to have favored denitrification. Regardless of DS management or DWT plant N accumulation, the soil was virtually depleted of NO₃ soon after permanent flooding; NO₃ rarely exceeded 10 kg N ha^–1 when measured after 9 d (first year) and 11 d (second year) of permanent flooding. Our data indicate the immense capacity of this lowland soil to accumulate NO₃ and the marked effect of DS and DWT management on the amount of NO₃ that actually accumulates. In tropical rice lowlands, soil and crop management during the DS should be designed to limit NO₃ buildup so as to reduce NO₃ that is prone to loss during the DWT.

Joint contribution from the International Rice Research Institute (IRRI), the Nitrogen Fixation by Tropical Agricultural Legumes (NifTAL) Center, Paia, HI and the International Fertilizer Development Center, Muscle Shoals, AL.

Received for publication December 3, 1992.

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