【摘要】 <正> 氮肥对作物的增产效果,决定于作物对氮肥的吸收率(即氮肥的利用率)和作物体内累积的氮素转化成经济产量的效率。水稻对氮肥的利用率一般显著低于旱作,未被吸收利用的部分从土壤中的损失一般也较旱地多。例如,就太湖地区来说,硫铵做返青肥表施更多还原

【Abstract】 The present article deals with the patterns of soil N supply, the uptake of soil and fertilizer N by rice plant and the fate of fertilizer N in relation to the forms and methods of placement of N fertilizer in the paddy soils of Suchow District, Jiangsu Province. Field experiment was carried out with ¹⁵N-tracer technique. The soils used for the experiment are developed from the alluvial-lacustrine deposits with a texture of clay loam. The results obtained are summarized as follows: 1. The characteristic of N supply of a paddy soil depends on both the mineralization of soil N and the uptake of the mineralized soil N by rice plant. The mineralization rate in well-structured soil was much higher than that in poor-structured soil during the growing period of early rice, though the difference between their total N contents was insignificant. In addition, it was shown by auxiliary experiment of ³²P that the rice roots extended much faster in the well-structured soil than in the poorstructured soil. Therefore, the total uptake of soil N by rice plant as well as the growth rate of rice plant grown in the well-structured soil during tillering stage were much greater than that in the poor-structured soil. However, in the reproductive stage, the uptake of soil N by rice plant grown on both soils was so effective that the mineralization rate of soil N might become a factor limiting the uptake of soil N by the rice plant.2. The fate of fertilizer N was affected significantly by the forms and methods of placement of the N-fertilizers. The results obtained in full-heading stage revealed that when ammonium sulfate, urea and ammonium bicarbonate were broadcasted on the surface of a neutral soil before transplantion, the N recovery by rice plant amounted to 50.1, 27.5 and 24.0% and the N loss amounted to 28.5, 53.9 and 57.4% respectively. Both the placement of urea at a depth of 6 cm and the application mixed with the top soil of 6 cm in depth increased the N recovery to 37—38%, whereas the N loss was decreased to 40—43.5%. A much higher percentage of plant-recovery （64.7%） was obtained when urea was top-dressed in the reproductive stage, but the amount remained in soil was much less, and the amount of loss was no less than 30%. The amount of plant recovery obtained by the application of prilled urea at a depth of 6 cm was as high as 74.5%, while that of loss was only 13.1%. No obvious difference in N recoveries by plants was found between the well-structured and poor-structured soils, whether urea was broadcasted on soil surface or prilled and deeply applied.3. Loss and immobilization of fertilizer N occured quickly after its surface application. Eleven days after dressing, both processes attained or approximated to the maximum. At that time, about 37—47% of applied N was lost and 29—45% was immobilized. 14—22% of the immobilized N was remineralized during the growing period of early rice.4. At the tillering stage of rice plant, the absorption rate of fertilizer N either applied on soil surface or mixed with top soil as basal dressing was much higher than that of soil N. This shows that fertilizer N contributed more to N supplying intensity and the promotion of plant growth than soil N in the early growing stage of rice plant. In addition, the absorption rates varied considerably with methods of application. For example, most part of fertilizer N applied on the soil surface or mixed with the top soil before transplanting was absorbed by rice plant within the first 20 days after dressing, whereas the peak of N uptake of prilled urea by rice plant was delayed to the period of 11—21 days （well-structured soil） or 11—41 days （poor-structured soil） after dressing. The diffences metioned above may perhaps be due to the differences in distribution patterns between soil-derived NH₄-N and fertilizer NH₄-N, and that among various methods of application. Moreover, there was no difference between the rates of fertilizer N absorbed by rice plant grown in the well-structured and the poorstructured soils when urea was surface ap更多还原