Institute of Earth Sciences, Academia Sinica, China-Taipei
Stable isotopes studies could contribute much to our
understanding of elemental cycles in Asian ecosystems. As an example of an ongoing study in
the Asia region that uses stable isotope techniques, results from an inventory
of 15N natural abundance of soils and vegetation in a forested
watershed in Taiwan are discussed.
Soil d15N values in the
forest floor ranged from –1.8 to 1.8 ¢¶.
Mineral soils had higher d15N values (4.1 to
6.0 ¢¶). Foliage d15N values for
overstory trees ranged from –6.6 to –2.0 ¢¶, and understory foliage d15N values ranged
from –5.0 to – 1.2. Foliar d15N values of both
groups were generally lower at the upper hillslope site. Both among species and among site
differences are likely related to differences in plant-mycorrhizal
interactions. A smaller difference
between plant and soil d15N values at the
lower elevation site is possibly related to increased N availability at this
site, either through differences in mineralization/nitrification rates or
differences in plant and fungal N cycling. These results highlight the need for more information
regarding the relationships between N stable isotopes and processes affecting
rates of N cycling, especially as related to wider scale patterns in forest
ecosystems within the East Asian region.
Introduction
Biogeochemistry is by definition a multi-disciplinary field. This adds to the complexity of studying challenging problems such as climate change, environmental pollution, and protecting the multiple functions of the earth¡¯s biological diversity. When we consider the fact that ecosystem processes, pollution, and natural catastrophic events ignore political boundaries, these problems become even greater challenges. Asia, the world¡¯s largest and most populated continent, offers such challenges at the highest level. Despite the seemingly universal agreement regarding the notion that scientists and policy makers require more information about ecosystems in regions other than the temperate zone, relatively few examples of cross-site, comparative ecological studies can be found in the region. Many gaps remain in our understanding of Asian subtropical and tropical ecosystem characteristics and function (Schlesinger, 1997, Howarth et al. 1996). This report is intended to present a simple review of stable isotope techniques in ecosystem and biogeochemical research. This approach is fairly new in ecosystem research and might be used to improve our understanding of patterns and function in a diverse range of ecosystems. Some results from an ongoing project in Taiwan are given as examples.
Human activity has changed global processing and patterns of many materials: nutrients, sediments, metals, pollutants, and organic matter (e.g. Smil, 1985, Galloway, et al., 1995, Berner and Berner, 1996). Several trends indicate these changes will become more pronounced in Asia (and South America) in the future. Global population estimates show that Asia has about 60 per cent of the world¡¯s people (World Resources Institute, 1996). The 1995 World Population Trends report indicate that most of the future population growth will take place in developing countries (United Nations Population Division, 1995). This pattern has many implications for ecosystem nutrient cycling and global biogeochemistry. Galloway et al. (1989) estimated that sulfur and nitrogen emissions from Asia could increase by 650 and 530 per cent, respectively, by 2020. These trends, when taken together, reinforce the importance of understanding the present status and future projections for ecological systems in Asia. One approach that can be incorporated into ongoing, as well as future, environmental monitoring networks and research programs is stable isotope ecology.
Isotopes are atoms of the same element that have a
different number of neutrons.
Stable isotopes do not decay into radioactive daughter products, and
this is advantageous since there are no concerns about radioactive
contamination when preparing materials for stable isotope analysis. Stable isotope compositions are measured
using an isotope ratio mass spectrometer.
By convention, the stable isotopic composition is expressed as the per
mil (¢¶) deviation relative to a
standard:
d = (Rsample
/ Rstandard – 1) x 1000, where R refers to the ratio of heavy
isotope to the light isotope.
In ecological
research, we are usually concerned with stable isotopes of carbon (13C/12C),
nitrogen (14N/15N), and sulfur (32 S/ 34S). Several recent reviews on stable
isotopes in environmental and ecological studies have been published (Peterson
and Fry, 1987, Lajtha and Michener, 1994, Griffiths, 1998). One research area in biogeochemistry
that has exploited stable isotope approaches especially well is in ecosystem
nitrogen cycling studies.
Attention has been focused on the possibility of ¡°nitrogen saturation¡±
in forested ecosystems, especially in North America (Aber et al. 1989;
Stoddard, 1994) and Europe (Skeffington, 1990; Gundersen et al., 1999). Recent work has indicated that
differences in hydrology and patterns of N cycling from those in watersheds in
North America and Europe were responsible for the differences in temporal
patterns in stream chemistry found in several watersheds in Japan (Mitchell et
al. 1997). Much more information
about rates of N cycling from areas other than North America and Europe is
needed to increase our understanding of the variation in forest ecosystem
nitrogen cycling processes among different forest types and regions. In this example, I analyze patterns of d15N natural
abundance for soils and vegetation in a forested watershed in Taiwan. The objective is to investigate 15N
natural abundance of soils and vegetation collected along a hillslope gradient
in a forested watershed.
Since there have been few stable isotope studies
conducted in forests within the East-Asia Pacific region, data from tropical
and temperate forests in other regions are used to compare with results from
this subtropical forest. Overall,
there is much evidence that there are differences in rates and patterns in N
cycling between tropical and temperate forests. Because many N cycling processes result in isotopic
fractionation, some predictable patterns in d15N among
components of forested ecosystem occur (Nadelhoffer and Fry, 1994). Recently, Martinelli et al. (1999) used
this approach to test the hypothesis that tropical forests, in general, contain
N that is more enriched in 15N. This should occur because tropical forests are often
relatively rich in N and the mechanisms for N losses in forests leave the N
remaining in the forest (soil and vegetation N pools) enriched in 15N.
The Fu-shan Experimental Forest is located about 40
km southeast of Taipei, Taiwan (24¡Æ34¡¯ N, 121¡Æ34¡¯ E). Fu-shan is a moist, subtropical,
evergreen forest with a rich flora of over 500 species (Mabry et al.,
1998). Soil and vegetation samples
for this study were collected from experimental watershed 1 (37 ha). The watershed has a SE facing aspect
and elevation ranges from about 670 to 1100 m. Dominant tree species in the study area include Castanopsis carlesii (Helmsl.) Hayata, Litsea acuminata (Blume) Kurata, and Diospyros morrisiana Hance. Common understory plants are Lasianthus microstachys, Helicia formosana, Alsophila podophylla Hook, and Blatus
cochinchinensis Lour. Mean
annual precipitation is about 4200 mm and mean annual temperature is 20.4 ¡ÆC.
Three hillslope locations were selected and soil
pits were excavated by hand to a
depth approaching 100 cm. Mineral
horizons were sampled at 10 cm intervals, sieved, and oven-dried prior to
analysis. Foliage samples were
collected from the middle third of the tree canopy in dominant tree species in
the immediate area adjacent to each soil pit. Understory foliage was sampled in a similar manner from two
to four randomly clipped branches or bracts and composited. Foliage samples were oven-dried prior
to analysis. Foliage was
pulverized to a fine mesh using a dental amalgamator device. Elemental and stable isotopic
composition were determined using a Finnigan Delta-Plus isotope ratio mass
spectrometer interfaced to an elemental analyzer. Typical precision for duplicate isotopic analyses is 0.2 ¢¶.
Soil d15N profiles showed
a sharp increase below the litter horizon at each location (Figure 1). Soil d15N values in the
litter horizon ranged from –1.8 to 1.8 ¢¶.
For mineral soils, higher d15N values were
found (4.2 to 6.0 ¢¶). Below the
litter horizon, d15N values changed
very little with depth at all sampling locations. Soil N concentration in the forest floor was lowest at the
middle hillslope location and ranged from 0.80 to 1.86 % N (Figure 1). In mineral soil horizons, soil N
concentrations did not change notably with depth or among the three soil
profiles and ranged from 0.11 to 0.83 % N.
There was a significant relationship between soil N
concentration and soil d15N value at each
of the hillslope positions. The
best fit regression lines for the three soil profiles were d15Nsoil
= -4.275 [%Nsoil] + 6.668 (p<0.001); d15Nsoil
= -5.858 [%Nsoil] + 6.445 (p<0.001); and d15Nsoil
= -4.345 [%Nsoil] + 6.559 (p<0.001) for the upper, middle, and
lower hillslope positions, respectively.
Forest soil profiles often show a general trend of increasing d15N with depth;
this pattern has been observed in many forest soils throughout the world
(Hogberg et al., 1996). This
pattern occurs because of isotopic fractionation during nitrogen cycling
leading to plant uptake of N that is depleted in 15N, and because of
internal N cycling through mycorrhizal fungi that also results in the transfer
of N that is depleted in 15N to host plants (Hobbie et al,
2000). Both of these mechanisms
result in the transfer N that is depleted in 15N to the soil surface via
litterfall. Thus, for nitrogen,
plants are nearly always isotopically lighter than the soil supporting their
growth (Nadelhoffer and Fry, 1994).
Fractionation during decomposition and foodweb processes may also
contribute to this pattern.
Foliage d15N values for
overstory trees ranged from –6.6 to –2.0 ¢¶, and understory foliage d15N values ranged
from –5.0 to – 1.2 (Figure 2).
Foliar d15N values of both
groups were generally lower at the upper hillslope site. The smaller difference between foliage
and soil re was a trend for a smaller difference between foliage and soil d15N values at the
lower hillslope position may be related either to differences in the d15N of available N
or differences in mycorrhizal-plant N cycling.
Rates of microbial N transformations should affect
the isotopic composition of soils (Garten, 1993; Hogberg et al., 1996). Topographic position may affect the
rates of many microbial processes in soils and thereby affect soil and foliar d15N values. For example, nitrification rates in
soils are influenced by variables such as temperature, moisture, and nitrate
concentration. During
nitrification, the remaining ammonium should become enriched in 15N
and the nitrate produced depleted in 15N (Hogberg et al., 1996). At
our study site, preliminary evidence suggests that net nitrification rates are
higher at the lower elevation site (Owen, unpublished data). Differences in net nitrification and
mineralization rates are likely related to the pattern we found in foliar d15N values. At the lower site, if net nitrification
rates are higher, then the isotopic composition of available NH4+
would be expected to be enriched in 15N. This mechanism may help explain the trend of higher foliage d15N values found at
the lower hillslope site. In
general, rates of net nitrification and mineralization are often higher in
tropical forests compared to forest soils in temperate regions (Martinelli et
al., 1999). Information about
stable isotopic composition of specific forms of N are needed to better
understand patterns in N cycling in forested ecosystems. The few data that are available (e.g.
Koba et al., 1998) suggest that measurements of both soil extractable and
solution NO3- and NH4+ could
contribute to understanding regional differences in rates and patterns in N
cycling. Investigating the
relationships between rates of microbial N transformations and soil isotopic
composition is one focus of our ongoing research at this site.
Differences in foliar d15N values among
the three hillslope sites can be plausibly explained by isotopic fractionation
during mycorrhizal transfer and differences in mineralization and nitrification
rates, with either greater effective mycorrhizal fractionation at the upper
hillslope site with lower N availability, or smaller losses of inorganic N that
is depleted in 15N
relative to that in the lower hillslope site.
In summary, these results for soil and vegetation
stable N isotopic composition from a subtropical forest in Taiwan are
consistent with similar data from other forest ecosystems. Isotopic patterns in this sub-tropical
forest seem to conform more closely to patterns in temperate forest than to
patterns in tropical systems. d15N values of bulk
N for soil profiles among the three hillslope positions did not vary
greatly. d15N values of
understory and overstory vegetation generally decreased from the lower site to
the upper hillslope site. The
difference between foliage and soil d15N values was
smaller at the lower hillslope site and this difference is probably related to
a complex suite of variables with differences in inorganic N availability,
mycorrhizal-plant N cycling, and rates of microbial N transformations among the
more important factors. Intensive studies on N cycling in subtropical forests
in the East Asia region, including stable isotope approaches can help improve
our understanding of patterns in global N cycling.
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