不同施氮量对套作大豆根系形态与生理特性的影响

在小麦/玉米/大豆套作模式中，利用挖掘法研究了施氮量在不同生育时期对套作大豆（贡选1号）一些根系形态与生理特性的影响。结果表明，在V3~R5时期，根干重、根瘤数、一级侧根长、伤流量均以低氮（纯氮45、90 kg hm^-2）处理最优；在R7时期，高氮（纯氮180、225 kg hm^-2）处理能够延缓大豆根系衰老，根干重、伤流量与施氮量呈正相关。根冠比与施氮量在V3~R5时期呈二次曲线关系，在生育末期呈直线递增关系。伤流量冠比(y)与三节期后天数(x)呈极显著负指数函数关系y=ae^-bx。伤流液氮素内含物中NO₃^-高于NH₄⁺，并随着施氮量增加而增加。在套作大豆整个生育时期中根系活力呈单峰曲线，低氮处理的套作大豆根系活力明显增强，高氮处理促使根系活力的峰值推迟至R3出现，并在生育末期（R7）保持高水平（＞116.00 μg g^-1 FW h^-1）。

Effects of Different Nitrogen Levels on Morphological and Physiological Characteristics of Relay-Planting Soybean Root

Relay-planting soybean pattern is dominant in southern China. However, studies on relay-planting soybean are so defective that it may influence the development of soybean. Soybean accumulates vast protein by itself; and grows well with adequate nitrogen fertilizer. Most of studies have been concentrated on the effects of nitrogen on the aerial parts, but less on the roots. The present experiment was carried out to study the effects of different nitrogen levels (0, 45, 90, 135, 180, 225 N kg ha^-1) on morphological and physiological characteristics in the relay-planting soybean roots by root-excavating method. The results showed that the optimal treatments on the dry root weight, root nodule number, length of first lateral root and bleeding sap were the low nitrogen levels (45 and 90 N kg ha^-1) at the growth stages of V3–R5. Length of first lateral root, dry root weight and bleeding sap all were positive correlated with the application amount of nitrogen at the growth stage of R7. The relation between the root-top ratio and nitrogen levels was a quadratic curve at growth stages of V3–R5, but root-top ratio had a linear relationship with the nitrogen levels at the very late growth stage. The relation between bleeding sap-top ratio (y=Bleeding sap/dry matters) and days (x) after V3 (11th, Aug, 2005) was a negative exponential correlation (y=1.0357e-0.03823x). The content of NO₃^--N in bleeding sap was more than that of NH₄⁺-N; it was also raised with the increasing of nitrogen amount. Root activity had a peak during the whole growth stage of relay-planting soybean. The activity of soybean roots was enhanced by low nitrogen levels (45 and 90 N kg ha^-1). High nitrogen levels(180 and 225 N kg ha^-1) postponed the appearance of the peak value of root activity at the growth stage R3 and maintained a high level(＞116.00 μg g^-1 FW h^-1) at the very late growth stage(R7). However, whether high nitrogen levels are propitious to increasing the yield of relay-planting soybean is not clear. The relationship between characteristics of roots and the aerial parts should be studied in the future.