Wheat straw enclosed in mesh bags was buried for periods up to 1 year over two seasons in Scottish, Danish and Portuguese soils treated with 15NH4NO3 or NH4 15NO3. Scottish soils were: Terryvale, a poorly drained sandy loam; and Tipperty, an imperfectly drained brown forest soil with a higher clay content. The Danish soil (Foulum) was a freely drained sandy loam and the Portuguese soils were a sandy soil (Evora) and a clay soil (Beja). During the first month, 15N was being incorporated into the straw in the Tipperty, Terryvale and Foulum soils simultaneously as the total N content was decreasing. Subsequently, the straws began to show net immobilization and the total N content of the original straw was exceeded in Tipperty and Foulum soils after 4 months and 8 months, respectively. Net immobilization in Terryvale was detected only in the second season and did not occur in the first because of high soil moisture content. The rates of 15N incorporation were similar in the two Portuguese soils, and a loss of N was only detected after 8 months. After 1 month, in the two clay soils, Beja and Tipperty, 15NO3 – was incorporated into straw to a greater extent than 15NH4 + and this was attributed to 15NH4 + fixation by clay minerals. In contrast, 15NH4 + was more efficiently incorporated than 15NO3 – under waterlogged conditions (Terryvale) and NO3 – loss could be attributed to denitrification. The proportion of added 15N in the straw residue after 1 month varied between 6% and 18% for 15NH4 + and 2% and 23% for 15NO3 – and immobilization of N in the longer term tended to be greater in soils from northern Europe than from Portugal.