Effect of climate and soil type on the immobilization of nitrogen by decomposing straw in northern and southern Europe

 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. Received: 19 January 1998