摘要
国内外已有的研究结果表明旱稻产量可达5-8.7t/ha。总体上看,旱稻的产量比相同条件下的水稻低20%-30%,用水量比水稻低60%,水分利用效率比水稻高1.6-1.9倍。然而,旱稻的种植推广面积有限,究其原因,“连作障碍”是限制旱稻发展的主要因素。“连作障碍”是指旱稻在连作情况下的生长受抑和产量下降。这种现象可能与连作情况下线虫或土传病害的发生,由水分胁迫导致土壤结构和供肥能力的变化,根系分泌的有毒物质的累积等有关。到目前为止,造成旱稻“连作障碍”的具体原因尚不清楚。
本研究目的在于:(1)考查旱稻对土壤热处理的生长反应;(2)探明营养缺乏是否是造成旱稻“连作障碍”的主要原因;(3)比较旱稻对不同形态氮源的生长反应;(4)研究栽培管理措施对缓解“连作障碍”的作用;(5)研究在连作旱稻土壤中不同品种间是否存在基因型差异。在国际水稻研究所,利用大田、微区和盆栽试验进行了相关研究。取得的主要研究结果如下:
(1)与未经热处理的连作旱稻土壤相比,土壤热处理显著地促进了旱稻的生长。不同热处理温度和热处理时间的试验结果表明,在90℃下处理12h或在120℃下处理3h就已经观察到对旱稻生长显著的促进作用。在120℃下处理12h,旱稻的生长表现最佳。在所调查的参数中,旱稻的叶面积对土壤热处理的反应最敏感,总生物量和茎蘖数次之。比较不同土壤对热处理反应的结果发现,连作旱稻土壤经热处理后对旱稻生长的促进作用大于水稻土壤热处理。土壤热处理为判断旱稻土壤“连作障碍”的程度提供了一套简便、快速的检验方法。旱稻生长对土壤热处理的反应越大,表明土壤“连作障碍”的程度越严重。
(2)在第9季连作旱稻小区进行的大田微区试验和一系列的盆栽试验结果表明,增施氮肥显著地促进了连作旱稻的生长,改善了叶片的氮营养和提高了旱稻的产量;Yashida溶液中的微量元素、磷肥和钾肥对旱稻生长均没有起到显著的促进作用。不同磷肥对连作旱稻土壤中旱稻生长的促进作用不同,钙镁磷肥、磷矿粉和磷化学试剂(二水合磷酸二氢钠)对旱稻生长没有起任何的促进作用,过磷酸钙对早稻生长的促进作用较小,而solophos的效果较大。对这些磷肥化学成分的分析结果表明,过磷酸钙和sol~s分别含有1.70%和2.91%的氮。因此,过磷酸钙和solophos对旱稻生长的促进效果可能是由于改善了植株的氮营养,而不是磷营养。此外,大田连作土壤的有效磷含量随着连作季数的增加而增加。这就排除了磷素缺乏造成早稻“连作障碍”的可能性。热处理后连作旱稻土壤的总氮量保持不变,但使土壤中NH_4~+的释放量增加了62%。盆栽试验中,在0.23-0.90 g N pot~(-1)的施氮量下,旱稻的生长随着施氮量的增加而增加。增施氮肥对连作旱稻生长的促进作用表明氮素缺乏可能是旱稻“连作障碍”的主要原因。这种氮素缺乏可能与土壤供氮能力的降低或植株吸氮能力的下降有关。
(3)在旱稻连作土壤中,不同氮源对旱稻生长的促进作用不同,其中,硫酸铵对旱稻生长的促进作用最大。盆栽试验中,低施用量下的硫酸铵和尿素均显著地促进了旱稻的生长,然而,在较高施用量下,旱稻在硫酸铵处理中的生长表现明显优于尿素处理,并且这种差异随着施氮量的增加而增大。比较不同土壤对硫酸铵和尿素施入的反应发现,在连作旱稻土壤中,硫酸铵和尿素对旱稻生长的的促进作用大于水稻土壤。连作旱稻土壤的pH接近中性(6.93)。在1.2 g N pot~(-1)的施用量下施入硫酸铵和尿素后,土壤pH分别降低了0.64和0.28个单位。硫酸铵对土壤的酸化可能提高了连作土壤中养分的有效性或者改变了微生物的群落,从而促进了旱稻的生长。采取适当的氮肥管理措施,如施用硫酸铵可能缓解旱稻的连作障碍。
(4)休耕、水旱轮作、以及与旱地作物轮作均缓解了旱稻的连作障碍。在两季休耕、三季水稻后的田块中,旱稻的产量、生物量、库容(单位面积颖花数)和千粒重均显著地高于连作旱稻。经过两季休耕和三季水稻后,旱稻产量对施氮的反应增加,土壤的持水能力和根系活力增强,0-10cm土壤容重降低。旱稻-旱地作物轮作模式下的旱稻产量显著地高于旱稻-休耕模式。三种旱地作物中,与大豆轮作后的旱稻产量高于玉米和甘薯。
(5)在连作旱稻土壤中,不同品种在生长方面表现出显著的基因型差异。所有品种在硫酸铵处理下的生长表现明显优于土壤热处理和未处理对照。在热处理、施用硫酸铵处理和对照土壤中,两个新的旱稻品系IR80508-B-57-3-B和IR78877-208-B-1-2的生长表现均优于其它品种。其中,在未处理的对照土壤中,IR80508-B-57-3-B和IR78877-208-B-1-2的总生物量分别为Apo的4.2和3.4倍。与水稻土壤相比,在连作旱稻土壤中,不同品种所表现的基因型差异较大。品种在连作旱稻土壤中生长表现的差异与根系有关。在未处理的连作旱稻土壤中,IR80508-B-57-3-B和IR78877-208-B-1-2的根系生物量分别比Apo高约3和2倍。庞大的根系增加了与土壤的接触面积,从而能够更多地从土壤中吸收水分、矿质营养。同时,较大的根系还能分泌出较多的酸性物质来活化土壤养分,从而增加根系对养分的吸收。
本研究表明,硫酸铵的施用、休耕、轮作以及对连作障碍不敏感的高产旱稻新品种的选用等措施都可以用来缓解旱稻的“连作障碍”。对这些管理措施的生理机制的研究将有助于找到旱稻“连作障碍”的真正原因。在实际的旱稻生产中,要结合当地的土壤状况、灌溉条件、施肥和管理水平来综合考虑这些管理措施。
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 NH_4~+/ 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 m~2), 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.
引文
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