Understanding intra-plant variations in
Nitrogen (N) is the most limiting nutrient in many terrestrial ecosystems,
especially those in temperate and boreal regions (Vitousek, 1994). As
atmospheric CO
Compared with the prolific studies on variations in
Several mechanisms have been proposed to explain intra-plant variations in
The impact of NH
Besides the mechanisms discussed above, other fractionating processes such
as transamination, redistribution of relatively enriched or depleted
metabolites, differential losses of N from plant organs, and resorption and
remobilization of N from senescing leaves have been suggested as potential
causes of intra-plant variations in
Thus, there is a strong need for systematical evaluation of
The joint analysis of N and P is important for understanding variations in
The present study builds upon the earlier efforts reviewed above and fills a
gap in systematic investigation of intra-plant variations in
Pen drawings of typical nebkha formed by
We previously described in detail the biological and environmental
characteristics of
Our field work was carried out in two desert locations. The first site was
in Dengkou County, Inner Mongolia Autonomous Region, China. Dengkou County
is at the junction between the Hetao Plain and Ulan Buh Desert of the
Mongolian Plateau in the middle reaches of the Yellow River. The mean annual
temperature is 8.84
The second study site was in Minqin County, Gansu Province, China. Minqin
County is located in the lower reaches of Shiyang River, surrounded by the
Badain Jaran Desert in the west and north and the Tengger Desert in the
east. The mean annual temperature is 8.87
In August 2012, three nebkhas were excavated at each study site. The
geometrical and biometrical characteristics of the six nebkhas were
summarized in Table 1 of Zhang et al. (2015). At the Dengkou site, the three
nebkhas were excavated in a sampling area of 40 m
We separated the whole plant biomass into groups of leaves, stems, in-sand
roots, and below-plain roots. The in-sand roots, which were roots found
inside the nebkha sands but above the plain formed by the underlying clay
layer, were further separated into in-sand fine roots (diameter
All categories of
The nitrogen isotope compositions were analyzed at the Stable Isotope Ratio
Mass Spectrometer Laboratory of the Chinese Academy of Forestry (SIRMSL,
CAF), Beijing, China. The instrument used was a Delta V Advantage Mass
Spectrometer (Thermo Fisher Scientific, Inc., USA) coupled with an elemental
analyzer (FlashEA 1112; HT Instruments, Inc., USA) in the continuous flow
mode. Isotope compositions were expressed using the delta notation (
Two-way ANOVA analyses (organ by site) were performed with SPSS (Ver.17.0).
C, N, and P contents,
Our regressional analyses were based on fixed effects models. Fixed effects
models are appropriate for the present study because we attempt to explain
variations in
A comparison of
Changes of
For comparing
Even though intra-plant and between-site variations in
Although intra-plant variations in
Changes of
Intra-plant nitrogen cycling and flux exchanges with external environments.
Since fine roots differ from other organs in that fine roots are the primary
organs for nitrogen acquisition, we re-calculated the organ
This study appears to be the first to report that the strongest predictor of
intra-plant variation in
To facilitate our discussion, we group potential mechanisms into three
categories: external factors only, internal factors only, and external and
internal factors together. Potential external factors include different
sources of nitrogen (e.g., NO
Plants can emit a vast number of nitrogenous compounds to the atmosphere,
which is known to affect atmospheric secondary aerosol formation and climate
(Sintermann and Neftel, 2015). These compounds are formed in metabolic
processes such as the decarboxylation and transamination of amino acids
(Bagni and Tassoni, 2001; Dudareva et al., 2013). Emissions of these compounds
from fruits and flowers are readily noticed without needing sensitive
measurements. In addition to fruits and flowers, leaves and stems can also
emit nitrogenous compounds. Like many physical and biochemical processes, it
is probably not unreasonable to assume that fractionation occurs in the
emission of nitrogenous compounds from plants. Unfortunately no isotope
fractionation measurements have ever been made on the emission of most of
these compounds. However, considerable isotopic knowledge exists in the
plant–atmosphere exchange of NH
As shown in Fig. 5, NH
We are not aware of any reports that stems and aerial roots may emit or
absorb NH
But how can we explain the positive
Our emphasis on photorespiration in the relationships of foliar
It is more challenging to include stems and roots in the equation. Clearly a
photorespiration-based mechanism alone is not sufficient to explain the
observed overall relationships as they hold across leaves, stems, and roots
(Figs. 3 and 4). Assuming there are no N-and-P-mediated fractionating
processes that directly exchange nitrogenous compounds between stems (and
roots) and the ambient air, is it possible for the leaf–atmosphere exchanges
of nitrogen isotopes to affect
We believe it is possible. Mature leaves export nitrogen and phosphorous to
other organs of plants (e.g., Aerts, 1996; Killingbeck, 1996; Jeschke et al., 1997; Hörtensteiner and Feller, 2002; Masclaux-Daubresse et al., 2010;
Brant and Chen, 2015). In particular, plants resorb and remobilize essential
nutrients to storage tissues in stems and roots during leaf senescence. In
this process, proteins, particularly those involved in photosynthesis, are
degraded, providing an enormous source of mobile nutrients. Resorption and
remobilization of nutrients from senescing leaves are a vital strategy for
plant survival for multiple reasons. First, it requires energy to absorb and
assimilate new nutrients from soil solutions, and thus recycling extant
nutrients makes economic sense. Second, nutrient availability in the soil
may be low and the rate of absorption at the root–soil interface may not be
able to meet the instantaneous demand by new growth in the next spring. In a
survey of published values, Brant and Chen (2015) found that leaf nitrogen
and phosphorus resorption efficiencies are generally over 60 % for a wide
variety of plant species ranging from grasses and forbs to deciduous and
evergreen trees (see Table 1 in that paper). Franklin and Ågren (2002)
showed that a 70 % leaf nitrogen resorption efficiency is needed to
predict observed leaf area indices of several plant communities. Because of
methodological limitations, these estimates do not generally consider
volatilization losses to the atmosphere and thus are considered “apparent
remobilization” (Masclaux-Daubresse et al., 2010). Nevertheless, there is
little doubt that foliar nitrogen metabolism can affect stem and root
nitrogen status. The foliar nitrogen and phosphorus remobilized to storage
organs will support the growth of not only new leaves but also new tissues
in stems and roots. Given that large amounts of N and P participate in
reactions in leaves and are processed through leaves, it is reasonable to
assume that the relationships of
To summarize our fairly detailed reasoning above, the observed patterns in
intra-plant variations in leaf–atmosphere exchanges of nitrogenous compounds, particularly NH nitrogen and phosphorus resorption and remobilization from senescing leaves,
and mixture of resorbed and remobilized nitrogen and phosphorus with existing
pools in stems and roots.
Nevertheless, we emphasize that this is a hypothesis only and it remains a
research task to ascertain how N, P, and N
It is interesting to compare the intra-plant relationships found here with
the previously reported correlations of foliar
Positive foliar correlations of
Another possibility to consider concerns the situation when nitrate is the
source of N for plants. If soil supply of nitrate is low, all nitrate
absorbed by roots may be assimilated in the roots and no enriched nitrate
pool is left for transport to other parts of the plant. As soil supply of
nitrate increases, the proportion of the nitrate pool that is unassimilated
by roots and thus is available for transport to other parts of the plant may
not only increase in size but also become more enriched in
We are not aware of any previous studies that systematically evaluated
variations in root
A systematical evaluation of nitrogen isotope composition in the desert
plant species
Knowledge of how plants acquire, transport, and transform N is crucial for
understanding how plants use this crucial resource for production, growth,
and reproduction and how the terrestrial N cycle operates. Intra-plant
variations in
Data are available by contacting the author at lianhong-gu@ornl.gov.
Field work, data acquisition, and analyses were conducted at the Institute of Desertification Studies with support from the National Key Technology R&D Program of the Ministry of Science and Technology of China (2012BAD16B01), the National Natural Science Foundation of China Youth Fund Project (31400620), the State Forestry Administration of China Forestry Public Welfare Scientific Research Funding (201404304), the Science and Technology Foundation (CAF201202), and the Lecture and Study Program for Outstanding Scholars from Home and Abroad of the Chinese Academy of Forestry (CAFYBB2011007). Data analyses and manuscript writing were partly carried out at Oak Ridge National Laboratory (ORNL) with support from US Department of Energy, Office of Science, Biological and Environmental Research Program, Climate and Environmental Sciences Division. ORNL is managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725. Edited by: N. Ohte Reviewed by: three anonymous referees