Media type: E-Article Title: Nitrogen‐15 in NO characterises differently reactive soil organic N pools Contributor: Oelmann, Yvonne; Wilcke, Wolfgang; Bol, Roland Published: Wiley, 2005 Published in: Rapid Communications in Mass Spectrometry, 19 (2005) 22, Seite 3177-3181 Language: English DOI: 10.1002/rcm.2023 ISSN: 0951-4198; 1097-0231 Keywords: Organic Chemistry ; Spectroscopy ; Analytical Chemistry Origination: Footnote: Description: <jats:title>Abstract</jats:title><jats:p>Intercropping with legumes is known to increase the plant‐available nitrogen (N) in soil, but can also increase leaching of NO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-2.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-2" /> to the groundwater. To minimise NO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-3.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-3" /> leaching risks, knowledge of the N‐release processes is essential, including an estimate of the contribution of legumes to total NO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-4.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-4" /> concentrations in soil. Our objectives were to answer the questions: (1) whether the presence of legume roots increases N mineralisation, and (2) whether the proportion of legume‐derived N in NO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-5.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-5" /> could be calculated with the help of natural abundance <jats:sup>15</jats:sup>N in NO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-6.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-6" />. We sampled soil monoliths of a <jats:italic>Medicago x varia</jats:italic> Martyn monoculture in August 2004 and set up three treatments: ‘disturbance’ (sieved to <2 mm), ‘disturbance‐roots’ (sieved to <2 mm and visible roots removed), and ‘control’ (left untreated). During an incubation period of 70 days, an N‐free nutrient solution was leached through the samples weekly. In the leachates we measured total N, total organic carbon, NO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-7.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-7" />, and NH<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-8.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-8" /> concentrations. Six of the 13 sampling dates were chosen for N isotope analysis in NO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-9.gif" xlink:title="urn:x-wiley:09514198:media:RCM2023:tex2gif-stack-9" />. Nitrate was separated as AgNO<jats:sub>3</jats:sub>. During the incubation, 3 to 6% of the initial total mass of total N (192 to 274 mg N) in soil was mineralised. Nitrogen mineralisation followed zero‐order kinetics independent of treatments. Mineralisation rates decreased in the order control (day 70: 3.7 µg NO<jats:sub>3</jats:sub>‐N (mg N<jats:sub>initial</jats:sub>)<jats:sup>−1</jats:sup>) > disturbance‐roots (2.6 µg NO<jats:sub>3</jats:sub>‐N (mg N<jats:sub>initial</jats:sub>)<jats:sup>−1</jats:sup>) > disturbance (1.9 µg NO<jats:sub>3</jats:sub>‐N (mg N<jats:sub>initial</jats:sub>)<jats:sup>−1</jats:sup>), indicating that mineralisation of legume roots did not play a major role in N mineralisation. The <jats:italic>δ</jats:italic><jats:sup>15</jats:sup>N values jumped from ca. 3‰ to ca. 8‰ after 2 weeks of incubation, which we attributed to the contribution of two N pools. An exponential two‐pool model could not be fitted to the data. Legume‐derived soil organic matter, SOM (pool 1), was mineralised at the same rate as SOM accumulated before establishment of the legumes (pool 2). Fresh legume roots did not contribute significantly to N mineralisation. Copyright © 2005 John Wiley & Sons, Ltd.</jats:p>