Nitrogen‐15 in NO characterises differently reactive soil organic N pools

Abstract

Intercropping with legumes is known to increase the plant-available nitrogen (N) in soil, but can also increase leaching of NO3- to the groundwater. To minimise NO3- leaching risks, knowledge of the N-release processes is essential, including an estimate of the contribution of legumes to total NO3- 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 NO3- could be calculated with the help of natural abundance 15N in NO3-. 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, NO3-, and NH4+ concentrations. Six of the 13 sampling dates were chosen for N isotope analysis in NO3-. Nitrate was separated as AgNO3. 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 microg NO3-N (mg Ninitial)-1)>disturbance-roots (2.6 microg NO3-N (mg Ninitial)-1)>disturbance (1.9 microg NO3-N (mg Ninitial)-1), indicating that mineralisation of legume roots did not play a major role in N mineralisation. The delta15N values jumped from ca. 3 per thousand to ca. 8 per thousand 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.

15N natural abundance during early and late succession in a middle-European dry acidic grassland

“…Sand ecosystems in general are characterised by relatively low amounts of organic soil compounds, high water permeability and, therefore, a comparatively low storage capacity for water and nutrients (Scheffer & Schachtschabel 2002). Thus, the fraction of mineralised N that is washed into the groundwater (Oelmann et al. 2005) is particularly high in these systems.…”

“…Sand ecosystems in general are characterised by relatively low amounts of organic soil compounds, high water permeability and, therefore, a comparatively low storage capacity for water and nutrients (Scheffer & Schachtschabel 2002). Thus, the fraction of mineralised N that is washed into the groundwater (Oelmann et al 2005) is particularly high in these systems. Further, it is well known that drought and highsoil temperatures, both common events in open grassland ecosystems on sand, lower N 2 fixation efficiency (Carranca et al 1999;Serraj et al 1999;Zahran 1999;Valles de la Mora et al 2003) and thus reduce the speed of succession (Vitousek et al 2002).…”

Can differences in 15N natural abundance be used to quantify the transfer of nitrogen from legumes to neighbouring non-legume plant species?

“…The model was based on two labile organic N pools derived from litter of the tree and grass, and a recalcitrant organic N pool in the soil. According to these authors 15 N fractionation during mineralization could be estimated from the 15 N content of the total organic N and the mineral N obtained from laboratory or field incubation experiments (Natelhoffer and Fry, 1988;Oelmann et al, 2005), or from the 15 N content of the total organic N of different soil layers (Vervaet et al, 2002). Model simulations duplicated well the observed relationships between organic N, mineralized N, their 15 N content and apparent 15 N fractionation, the latter varying from þ1.2 to þ2.4‰.…”

Using δ15N and δ18O to evaluate the sources and pathways of NO in rainfall event discharge from drained agricultural grassland lysimeters at high temporal resolutions

“…Stable isotope abundances of N have been used extensively to provide information on the origins and transformations of N in soils, surface waters and ground waters 3–9. It has also been demonstrated by a number of studies that a wide variety of chemical and biological processes (i.e.…”

“…Taken alone, however, δ 15 N values fail to distinguish among the various N sources due to overlapping ranges and though isotopic fractionations which occur during N transformations. Further studies have suggested that the oxygen (O) isotope content of NO may provide additional information on microbial processes, such as denitrification,12 as well as better distinguishing the sources of NO in waters, particularly discrimination between atmospheric, microbial and synthetic fertiliser NO 8, 13–19. Hence, one can expect that a dual isotope approach with respect to the identification of NO sources may provide more reliable information with respect to the NO sources and biogeochemical interactions during NO production.…”