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Nitrogen‐15 in NO 3 − characterises differently reactive soil organic N pools
Author(s) -
Oelmann Yvonne,
Wilcke Wolfgang,
Bol Roland
Publication year - 2005
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.2023
Subject(s) - chemistry , nitrogen , reactive nitrogen , environmental chemistry , organic chemistry
Intercropping with legumes is known to increase the plant‐available nitrogen (N) in soil, but can also increase leaching of NO 3 −to the groundwater. To minimise NO 3 −leaching risks, knowledge of the N‐release processes is essential, including an estimate of the contribution of legumes to total NO 3 −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 3 −could be calculated with the help of natural abundance 15 N in NO 3 − . We sampled soil monoliths of a Medicago x varia 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 3 − , and NH 4 +concentrations. Six of the 13 sampling dates were chosen for N isotope analysis in NO 3 − . Nitrate was separated as AgNO 3 . 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 3 ‐N (mg N initial ) −1 ) > disturbance‐roots (2.6 µg NO 3 ‐N (mg N initial ) −1 ) > disturbance (1.9 µg NO 3 ‐N (mg N initial ) −1 ), indicating that mineralisation of legume roots did not play a major role in N mineralisation. The δ 15 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.