Abstract
Abstract Recent studies have documented adverse affects of urea on the establishment and growth of aerobic rice when applied at seeding. The following experiments were conducted to examine the relative importance of ammonia and nitrite (NO $ _2^- $ ) toxicities as mechanisms contributing to poor germination and early growth of aerobic rice. Soil was collected from an experiment in the Philippines where aerobic rice was grown continuously for 7 years. Subsamples of the soil were: (1) pretreated with sulfuric acid (0.5 M H 2 SO 4 added at 75 mL kg –1 ), (2) oven‐heated at 120°C for 12 h, or (3) left untreated. In a greenhouse study N was applied to the untreated, acidified, and oven‐heated soils as either urea or ammonium sulfate (0.0 or 0.3 g N kg –1 ). Plant height, root length, total biomass, and number of seminal roots were evaluated after 10 d. Microdiffusion incubations were used to assess the effects of soil pretreatment, N source, and N rate (0, 0.5, 1.0, 1.5 g N kg –1 ) on ammonia (NH 3 ) volatilization and germination. Nitrite incubations were conducted to establish a critical level for NO $ _2^- $ toxicity and measure the extractable NO $ _2^- $ and germination trends as affected by soil pretreatment, N source, and N rate. On untreated soil, urea reduced early growth and germination while ammonium sulfate caused no adverse effects. Progressively higher rates of urea increased NH 3 volatilization and inhibited germination, while oven‐heating and acidification minimized the adverse effects. All treatment combinations (soil pretreatment, N source, N rate) had extractable NO $ _2^- $ levels below the critical level of 0.2 g N kg –1 , suggesting that ammonia and not NO $ _2^- $ toxicity was the principal cause of inhibition. Since the risk of NH 3 toxicity is highest just following urea hydrolysis, strategies to optimize the timing and placement of urea should be considered.
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Claims 9
Claim · model-written summary
Urea reduced early growth and germination on untreated soil.
Verbatim
e0“On untreated soil, urea reduced early growth and germination while ammonium sulfate caused no adverse effects.”
t0“aerobic rice”
Claim · model-written summary
Ammonium sulfate caused no adverse effect on early growth and germination on untreated soil.
Verbatim
e0“On untreated soil, urea reduced early growth and germination while ammonium sulfate caused no adverse effects.”
t0“aerobic rice”
Claim · model-written summary
Higher urea rates increased ammonia volatilization.
- Taxon
- aerobic ricet0
- Treatment
- Progressively higher rates of ureae0
- Outcome
- NH 3 volatilizatione0
- Direction
- increasede0
Not reportedComparator
Verbatim
e0“Progressively higher rates of urea increased NH 3 volatilization and inhibited germination, while oven‐heating and acidification minimized the adverse effects.”
t0“aerobic rice”
Claim · model-written summary
Higher urea rates inhibited germination.
- Taxon
- aerobic ricet0
- Treatment
- Progressively higher rates of ureae0
- Outcome
- germinatione0
- Direction
- inhibitede0
Not reportedComparator
Verbatim
e0“Progressively higher rates of urea increased NH 3 volatilization and inhibited germination, while oven‐heating and acidification minimized the adverse effects.”
t0“aerobic rice”
Claim · model-written summary
Oven heating minimized the adverse effects of urea.
Not reportedComparator
Verbatim
e0“Progressively higher rates of urea increased NH 3 volatilization and inhibited germination, while oven‐heating and acidification minimized the adverse effects.”
t0“aerobic rice”
Claim · model-written summary
Acidification minimized the adverse effects of urea.
Not reportedComparator
Verbatim
e0“Progressively higher rates of urea increased NH 3 volatilization and inhibited germination, while oven‐heating and acidification minimized the adverse effects.”
t0“aerobic rice”
Claim · model-written summary
Extractable nitrite remained below the critical level in all treatment combinations.
Verbatim
e0“All treatment combinations (soil pretreatment, N source, N rate) had extractable NO $ _2^- $ levels below the critical level of 0.2 g N kg –1 , suggesting that ammonia and not NO $ _2^- $ toxicity was the principal cause of inhibition.”
t0“aerobic rice”
Claim · model-written summary
The authors infer ammonia toxicity was the principal cause of inhibition.
Verbatim
e0“All treatment combinations (soil pretreatment, N source, N rate) had extractable NO $ _2^- $ levels below the critical level of 0.2 g N kg –1 , suggesting that ammonia and not NO $ _2^- $ toxicity was the principal cause of inhibition.”
t0“aerobic rice”
Claim · model-written summary
The authors recommend optimizing timing and placement of urea.
Verbatim
e0“Since the risk of NH 3 toxicity is highest just following urea hydrolysis, strategies to optimize the timing and placement of urea should be considered.”
t0“aerobic rice”
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