| Aerobic rice yields of 5-8.7t/ha were reported in China and abroad. Compared with flooded rice, aerobic rice had on the average 20-30% lower yields, about 60% lower total water use, and 1.6-1.9 times higher water productivity. However, the continuous cropping obstacle limits the adoption of monocropping of aerobic rice. The term "soil sickness" was used to describe the reduction in growth and yield caused by continuous monocropping of aerobic rice. Soil sickness may be related to the buildup of nematodes or soil-borne pathogens, changes in nutrient availability in the soil, or growth inhibition by toxic substances from root residues. The causes of continuous cropping obstacle in aerobic rice system are still unclear.The objectives of this dissertation were: 1) to examine the growth response of aerobic rice to oven heating of soil with a monocropping history, 2) to identify if nutrient deficiency is responsible for the yield decline in continuous aerobic rice system, 3) to compare the effects of different N forms on the growth of aerobic rice grown under continuous aerobic rice soil, 4) to determine if management strategies can mitigate the yield decline of continuous aerobic rice, and 5) to examine the genotypic variations in tolerance to the "soil sickness" due to continuous cropping of aerobic rice. In this study, field, micro-plot and pot experiments were conducted at the International Rice Research Institute (IRRI) experimental farm at Los Banos, Laguna, Philippines. The following results were obtained:(1) Oven heating of "sick soil" with an aerobic history increased plant growth significantly over the unheated control. A growth increase in a continuous aerobic soil was observed with heating at 90℃for 12 hours or at 120℃for as short as 3 hours. Highest growth response of aerobically grown rice was observed with heating at 120°C for 12 hours. Leaf area was the most responsive to soil heating, followed by total biomass and stem number. Heating of soil increased plant growth greatly in soils with an aerobic history but a relatively small increase was observed in soils with a flooded history. Soil oven-heating provides a simple and quick test to determine whether a soil has any sign of sickness caused by continuous cropping of aerobic rice. The bigger the magnitude of growth enhancement by soil heating, the more severe is the "sickness" of the soil. (2) The micro-plot experiment arranged in the 9th-season aerobic rice plots of the long-term field experiment and a series of pot experiments indicated that additional N application significantly improved plant growth and leaf N nutrition and increased the grain yield of aerobic rice under continuous aerobic cropping. P, K, and micronutrients from Yoshida solution had no effect. The effects of different P sources on plant growth in the continuous aerobic rice soil showed that Ca-Mg phosphate, rock phosphate or monosodium phosphate dihydrate did not improve plant growth or leaf N nutrition compared with the control. Calcium superphosphate slightly increased plant height, stem number, leaf area, total biomass, SPAD value, and aboveground N uptake compared with the control. Solophos application had significant effects on plant growth and leaf SPAD value. Chemical analysis of P fertilizers revealed that solophos and calcium superphosphate contained 2.91% and 1.70% N, respectively. The N content in Ca-Mg phosphate and rock phosphate were negligible. Hence, the effect of solophos and calcium superphosphate on plant growth in this study was probably not associated with the improvement in P nutrition but N nutrition. In our long-term field experiment with continuous cropping of aerobic rice, 60 and 30 kg P ha-1 was applied during dry and wet season, respectively, since 2001. Soil Olsen P increased from 9.0 mg kg-1 at the beginning of the experiment to 22.5 mg kg-1 after the 7th-season aerobic rice was grown in the dry season of 2004. Therefore, it was unlikely that P nutrition was associated with the soil sickness in this continuous aerobic rice system.Oven heating of the aerobic soil increased the release of NH4+/ by 62% compared with untreated soil but did not change the total N content of the soil. In a pot experiment, increasing the rate of urea application from 0.23 to 0.90 g N pot-1 increased the vegetative growth parameters, chlorophyll meter readings, and aboveground N uptake consistently. Our results suggested that N deficiency due to poor soil N availability or reduced N uptake ability of the plant might cause the yield decline of continuous cropping of aerobic rice.(3) Different N forms had different effects on plant growth in continuous aerobic rice soil. Among N forms, ammonium sulfate was the most effective on plant growth improvement. In pot experiments, both 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. Another pot experiment showed that ammonium sulfate and urea were much more effective on the growth of aerobic rice in the "sick" continuous aerobic soil than in the "healthy" continuous flooded soil. The soil pH of the control, urea, and ammonium sulfate treatments in the aerobic soil was 6.93 (±0.01 standard deviation), 6.65 (±0.02), and 6.29 (±0.04), respectively. Therefore, the application of ammonium sulfate and urea in the aerobic soil reduced soil pH and the reduction was greater for ammonium sulfate than urea. Changes in nutrient availability and microbial community caused by the acidification of soil due to the application of ammonium sulfate could be associated with its greater effect on plant growth. Our experiments suggested that ammonium sulfate may be used to mitigate the yield decline caused by continuous cropping of aerobic rice and that it is possible to reverse the yield decline by developing improved N management strategies.(4) Fallow and crop rotation mitigated the continuous cropping obstacle in aerobic rice. The yields of aerobic rice after two seasons of fallow and after three seasons of flooded rice were significantly higher than that of continuous aerobic rice. The effects of fallow and aerobic-flooded rice rotation on yield improvement were attributed to higher total biomass production, bigger sink size (spikelet per m2), and greater 1000-grain weight. In addition, fallow and aerobic-flooded rice rotation increased N response, soil water holding capacity, and root activity and decreased 0-10 cm soil bulk density. When grain yields were compared among different rotation patterns, the aerobic rice yielded more after two seasons of upland crops than after two seasons of fallow. Among the three upland crops, relatively higher yield of aerobic rice was observed after two seasons of soybean, but the difference was not statistically significant.(5) Different varieties showed variations in plant growth in the continuous aerobic rice soil. Application of ammonium sulfate and soil oven-heating consistently improved plant growth and N nutrition compared with the untreated control across all three varieties. The two new aerobic rice lines (IR80508-B-7-3-B and IR78877-208-B-1-2) produced much more vigorous root systems and much more biomass than other varieties in all three treatments. The genotypic variations were greater in continuous aerobic soil than in flooded soil. The genotypic variations in plant growth in continuous aerobic soil were associated with the variations in root systems. The root biomass of IR80508-B-7-3-B and IR78877-208-B-1-2 was three and two times higher than that of Apo, respectively. The root morphological factors such as length, thickness, surface area and volume have profound effects on the plant's ability to acquire and absorb water and nutrients from soil. These parameters affect the ability of the roots to penetrate deep soil layers, to tolerate drought stress, and deficiencies and toxicities of elements. Therefore, selection of rice cultivars with a large and deep root system was considered to be an important strategy for sustaining the yield stability of rice under aerobic conditions. The results of this study showed that the continuous cropping obstacle in aerobic rice may be mitigated by ammonium sulfate application, fallow, crop rotation, or adoption of new aerobic rice varieties. Studying the mechanisms of the effects of these crop management practices will help find the causes of the continuous cropping obstacle in aerobic rice. The best way is to combine these crop management strategies properly depending on the soil properties, irrigation, fertilization, and local management technologies in the real aerobic rice production conditions. |
PDF Full Text Request