The Effects Of Ammonia Volatilization In Soil On Growth Of Aerobic Rice And The Regulation Strategies

The reduction in growth and yield of aerobic rice was found in monocropping system. The cause may be the nutrient deficiency. Previous researches reported that ammonium sulfate and urea significantly increased the plant growth, however, the plant growth was better with ammonium sulfate than that with urea and the difference increased as the N rates increased. While, the soil pH increased after long term monocropping of aerobic rice. The causes that the effect with ammonium sulfate is better than with urea are still unknown. The objectives of this research were:To evaluate the effects of urea on seeds germination and plant growth of aerobic rice;to examine the effects on soil nutrient availability, plant nutrient and growth of aerobic rice in response to acidification of soil with a monocropping history. Experiments were conducted at the International Rice Research Institute (IRRI).The following are the main results:(1)Across all soils, the application of ammonium sulfate had little or no effect on seedling growth at 10 d, while the use of urea had a clear inhibitory effect. This observation indicated strongly that toxicity induced by urea during the early growth stage is the main cause of growth differences found between the two N sources. Both N fertilizer and the soil to which it was added had significant effects on the amount of ammonia released over a 4 d period. No significant differences between soils or N rates were observed when ammonium sulfate was the N source. However, when urea was the N source ammonia volatilization increased by an additional order of magnitude (all soils released>12mg NH3kg-1) and significant differences between the soils. The application of ammonium sulfate at progressively higher N rates had no affect on the germination percentage and root:shoot ratio of seeds germinating in close proximity to the four soils. In contrast, the application of urea at increasing rates had a severe negative effect on germination percentage, root:shoot ratio, and number of mesocotyl roots on all soils.(2) Both N rate and soil had significant effects on ammonia volatilization. Ammonia was low when no N was added, ranging from 0.037 to 0.209mg N kg-1 depending on the soil.For each soil, measured ammonia increased significantly at progressively higher N rates. While ammonia levels always increased with N rate, the total amount released differed considerably between soils.When no urea was added, germination was high(> 88%) and no significant differences were observed between soils. As N rate increased, seed germination declined for all soils. Across all treatments there was an inverse relationship between volatilized ammonia and germination percentage (r=0.96). Germination declined gradually as volatilized ammonia increased from 0 to 50mg N kg-1 and germination was completely inhibited above this threshold. We found that ammonia in excess of the critical limit where much more likely when soil clay was<15%, CEC was<10cmol kg-1 and BC was<2.5cmol kg-1 pH-1.(3)In addition to the positive effects on the overall growth of aerobic rice, both soil and plant N status improved significantly with acid treatments when no N was applied. A sharp increase in soil-available ammonium and nitrate(a 5.5-fold and 1.5-fold increase in ammonium and nitrate over the zero-acid control, respectively) was observed following adding 50mL of 0.05 mol L-1 sulfuric acid per pot induced. In the soil, total C, total N, available P and K, exchangeable Al and Na, free Fe and Mn, and the cation exchange capacity were not affected by a reduction in soil pH. Soil acidification was found to increase soil-exchangeable Mg and Ca, but this was not reflected in the leaf tissue. A positive effect of soil acidification on plant growth andl N uptake was also showed when N was applied as either urea or ammonium sulfate. However, the response to soil acidification generally decreased with increases in N rates from 0.6 to 1.2g per pot. This further suggests that acidification of aerobic soil improved soil N availability and/or plant N uptake ability and that these benefits were less important at the higher N rates. The relative increase in plant growth and N uptake following acidification was greater when urea was the N source than with ammonium sulfate. Soil acidification and the application of N improved both plant growth and N uptake of rice grown in aerobic soil.In general, N application was more effective in increasing plant growth and N uptake than soil acidification, particularly when ammonium sulfate was the N source.(4) Urea incorporated with soil and deep placement of urea resulted in a significantly larger growth as compared with the control and surface application. Almost growth parameters were best with USG application among all treatments. When ammonium sulfate was as nitrogen source, plant growth was no influenced by application methods. Plant growth and N uptake were much poorer with urea than with ammonium sulfate.The results of this study showed that a reduction in soil N availability following an increase in soil pH can contribute to observed declines in the yield of monocropped aerobic rice. The possibility of ammonia toxicity that aerobic rice suffer increases with soil pH. It can improve growth of aerobic rice by nitrogen management.