全 文 ：Received 28 May 2003 Accepted 6 Oct. 2003
Supported by the State Key Basic Research and Development Plan of China (G1998040800).
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Long-term Partitioning of Ammonium and Nitrate Among Different
Components in an Alpine Meadow Ecosystem
XU Xing-Liang, OUYANG Hua*, PEI Zhi-Yong, ZHOU Cai-Ping
?(Institute of Geographical Sciences and Natural Resources Research, The Chinese Academy of Sciences, Beijing 100101, China)
Abstract: The fate of 15N-labeled ammonium and nitrate was determined in a Kobresia pygaea C. B.
Clarke meadow after 11-13 months following 15N additions in order to understand the role of alpine
meadows in retention of deposited nitrogen (N). It was shown that the fate of NO3--15N or NH4+-15N was
distinctly different. In 11-13 months after 15N additions, total 15N recovery of NO3--15N was 92.83%, 92.64%
and 79.96% while 15N recovery of NH4+-15N was 49.6%, 63.33% and 66.22%. Their different fates were
obviously shown on partitioning of 15N among different components in alpine meadows. After 11-13 months
following 15N additions, more NO3--15N was recovered in plants than in soil organic matter or in soil
microorganisms. 15N retained in soil organic matter was also high and increased with time. 15N immobilized
in soil microorganisms was close to that retained in soil organic matter within 11-12 months while it
decreased significantly after 13 months. When NH4+-15N was added, more 15N was retained in soil organic
matter than in plants or in soil microorganisms. 15N taken up by plants was constant while that immobilized
in soil microorganisms altered greatly. Eleven to thirteen months after 15N additions, plants and soil
microorganisms took up more NO3--15N than NH4+-15N while soil organic matter recovered more NH4+-15N
than NO3--15N. It indicates that alpine meadows play different roles in retention of deposited NO3- and NH4+
after a long time.
Key words: NO3--15N; NH4+-15N; Kobresia pygaea meadow; soil organic matter; soil microorganisms;
alpine plants
Since atmospheric N2 can be transferred into biologi-
cally reactive nitrogen (N), human activities have changed
the N cycling. With application of N-fertilizers more and
more anthropogenic N has deposited into all kinds of ter-
restrial ecosystems. Because plant growth is limited by N
availability in many ecosystems（Louseck and Howarth,
1991), this input of N will result in the changes of many
ecological processes in terrestrial ecosystems, especially,
in those ecosystems lacking of available N. Therefore, the
fate of deposited N in these ecosystems has been concerned.
Recently many short-term (within two months) and long-
term (over two months) 15N experiments have been con-
ducted on the grasslands (Hart et al., 1993) and forests
(Mead and Pritchett, 1975; Melin et al., 1983; Schimel and
Firestone, 1989; Hart and Firestone, 1991; Hart et al., 1993;
Zogg et al., 2000) as well as tundra ecosystems (Marion et
al., 1982), in order to explore the ability of different ecosys-
tems on retention of added N for better understanding of N
cycling in N-limited ecosystems. We reported the fate of
short-term (within two months) 15N-labelled nitrate and
ammonium in an alpine meadow ecosystem distributed in
the Qinghai-Xizang Plateau, China (Xu et al., 2003).
However, little is known about partitioning of added
ammonium and nitrate among different components during
a long period of time in alpine meadow ecosystems. In this
research we attempt to understand the role of alpine mead-
ows in retention of added N within about one year and ex-
plore whether there is great difference between the fate of
added 15NO3- and 15NH4+.
1 Materials and Methods
The research was carried out at Haibei Alpine Meadow
Ecosystem Station of The Chinese Academy of Sciences,
Qinghai Province (37° 3660 N, 101° 1914 E). Site descrip-
tions are available elsewhere (Zhou, 1982; 2001). The soil is
an alpine meadow soil of Mat Cryogenic Cambisols (Chinese
Soil Taxonomy Research Group, 1995), some features of the
soil were determined and given in Table 1.
Two parallel plots were established in an area chosen for
Acta Botanica Sinica
植 物 学 报 2004, 46 (3): 279-283
Table 1 Total N, inorganic N, moisture, C/N ratio and pH of
the soil before the tracers added
Date
Total N Inorganic N Soil moisture
C/N
pH
(%) (g/m2) (%) (H2O)
28 July 0.48±0.01 1.3±0.2 23.4±1.1 18.2±0.7 7.5
All data are for the top 15 cm of soil in the field. Means ±SE of
three replicates are shown.
Acta Botanica Sinica 植物学报 Vol.46 No.3 2004280
uniformity in species diversity and covered in a Kobresia
pygaea C. B. Clarke meadow in July 2000. Each plot was a 9-
m2 with a 5-m distance between them and was enclosed by
a wire. We applied 0.44 g N/m2 (Na15NO3, 99.26 atom %)
and 0.56 g N/m2 (15NH4)2SO4, 99.40 atom %) dissolved in
H2O and sprayed them to each plot. Another amount of
H2O was sprayed on each plot in order to prevent more 15N
from being absorbed on the leaves. The total amount of
H2O was equivalent to 2 mm of rain. Samples of soils, and
aboveground biomass were collected on each plot on the
11th, 12th and the 13th week after 15N additions.
Fifteen soil cores (2.7 cm in diameter, 15 cm in depth)
were collected randomly from each plot in every sampling
time. Every five-soil cores were combined as one sample.
Then these samples were put in an ice-box and brought
into the laboratory. Immediately the samples were mixed
well by hand and sieved (< 2 mm), respectively. Bigger
roots were picked carefully out from these soil samples
and the remainders after sieving rinsed carefully by water
and used to estimate below ground biomass. The sieved
soils were used for the necessary measurement. Five
subsamples on field-moisture from the sieved soils were
placed in beakers. One subsample was used for gravimetric
moisture determination (over 48 h at 60 °C). The second
one was used to measure pH values with a glass electrode
using a 1:2 soil-to-water ratio. The third one was dried in an
oven at 60 °C over 48 h, and was used to estimate organic
carbon (Kalembasa and Jenkinson, 1973) and to measure
total N by Kjeldahl digestion of a salicylic acid modifica-
tion (Pruden et al., 1985). The other two subsamples were
used to estimate microbial biomass N. Microbial biomass N
was determined using a chloroform fumigation-direct ex-
traction technique (Brookes et al., 1985; Davidson et al.,
1989). NH4+-N and NO3--N with 0.5 mol/L K2SO4 extracts
were measured by stream distillation with MgO, and using
Devarda’s alloy to reduce NO3- to NH4+ (Bremner, 1965).
Aboveground biomass was measured by harvesting
three 25 cm × 25 cm plots. These plant materials including
green and roots were dried at 60 °C over 48 h and milled,
with more than 90% passing a 60 mesh sieve. Part of them
was used to analyze total organic carbon, total-N and their
15N/14N ratios.
For 15N analysis, all the samples were digested by
Kieldahl digestion of a salicylic acid modification except
NH4+-N and NO3--N samples. To avoid errors from
“memory” effect, H2O-free alcohol was distilled after each
digest was distilled. The 15N/14N ratios were measured on a
Finnigan MAT-251 mass spectrometer.
The applied 15N recovered in plants and inorganic N
(NH4+-N + NO3--N) was measured by multiplying the N
concentration of the pool by the mass of the component
per square meter and its atom% excess 15N. The 15N recov-
ered in soil microbial biomass was calculated as the differ-
ence in the 15N recovered in non-fumigated and fumigated
soil samples. The 15N recovered in soil organic matter was
calculated as the difference between the 15N recovered in
bulk soil and the 15N in soil microbial biomass and extract-
able inorganic-N pools (Zogg et al., 2000).
2 Results and Discussion
Concentrations of inorganic N were very low in
Kobreasia pygaea meadows (Fig.1). Compared with inor-
ganic N, soil microbial biomass N was higher (Fig.2). NO3-
and NH4+ had different impacts on soil microbial biomass
N. Soil microbial N was higher under NO3- addition than
NH4+ addition. And microbial N in June or in July was higher
than in August. Plant N was also higher and remained al-
most constant in this period (Fig.3) and it was higher when
NH4+ was added. The reason is that NO3- can stimulate
alpine plant growth more than NH4+ can do. So plant growth
results in a dilution of plant N when NO3- is added. It sounds
be that alpine plants determine the fate of deposited NO3-
while soil organic matter determines the fate of deposited
NH4+ after about one year of N additions.
After 11-13 months following 15N additions, there was
still a small quantity of 15N in organic N pool. Recovery of
15N from NO3--15N was 0.73% ±0.17% (11 months), 1.04%
±0.12% (12 months) and 0.26% ±0.04% (13 months) while
it was 0.4% ±0.06%, 0.83% ±0.08% and 0.52% ±0.05%
from NH4+-15N. The recovery of 15N from NO3--15N was
very different from that from NH4+-15N. The total recovery
from NH4+-15N increased with time (49.6% for 11 months,
Fig.1. Concentrations of inorganic N in soils 11 months, 12
months and 13 months following 15N additions. Values are means
(±SE) from three replicates per treatment.
281
XU Xing-Liang et al.: Long-term Partitioning of Ammonium and Nitrate Among Different Components in an Alpine
Meadow Ecosystem
63.33% for 12 months and 66.22% for 13 months,
respectively) while the total recovery from NO3--15N de-
creased with time (92.83%, 92.64% and 79.96%, respectively).
The reason is that we did not consider 15N recovered in the
litter and dead roots when we estimated the total recovery
of 15N. In this study NO3- stimulated the alpine plant growth
more than NH4+ did. Also, more NO3--15N was recovered in
the alpine plants while more NH4+-15N was retained in the
soil organic matter. Therefore a great decrease in the total
recovery of 15N appeared under NO3--15N addition, whereas
a little increase happened under NH4+-15N addition. Clark’s
(1977) showed that the recovery of 15N was 80% after 46
months following addition of K15NO3 in a short grass prairie.
Our results agreed with Clark (1977)’s study but disagreed
with the results Zogg et al. (2000) obtained from a northern
hardwood forest ecosystem. They showed that the recov-
ery of added 15N was 93% after 2 h following application of
the NO3--15N but rapidly dropped to 29% within one month.
The total recovery of 15N from NH4+-15N after 11-13
months of added 15N was similar to that Hart et al. (1993)
obtained in an annual grassland and a young mixed conifer
forest. Mead and Pritchett (1975) reported that the recov-
ery of 15N from NH4+-15N still reached 55% after 18 months
of 15N addition. Our results show that the K. pygaea
meadow plays a more important role in retention of NO3--N
than in retain of NH4+-N during a longer period of time.
Moreover, the total recovery of added NH4+-15N and
NO3--15N give us a clear insight into the utilization effi-
ciency of different forms of N in alpine meadows. In this
study, there was little amount of added NO3--15N to be
lost, whereas more added NH4+-15N was lost in the alpine
meadow. Higher total recovery of NO3--15 N means that
the lost N is very lower through eluviation and surface
flows. The average emission rate of N2O under added
NO3--15N or NH4+-15N was 1.6 mg.m-2.mol-1 or 3.7
mg.m-2.mol-1 during the two months following N addi-
tions (Xu et al., 2003). According to these emission rates,
the lost N by denitrification in this study period was esti-
mated to be 4.5% of added NO3--15N and 8.0% of added
NH4+-15N. This seems that the lost N by denitrification was
also very lower. More NH4+-15N might be lost due to am-
monia volatilization in alpine meadows (Cao and Zhang,
2001) because the soil of the alpine meadow had high pH
values (Table 1).
Different fate of NO3- and NH4+ was shown obviously
on the partitioning of 15N in different components in the
alpine meadow ecosystem. When NO3--15N was added,
the soil organic matter retained 24.3% (after 11 months),
26.3% (after 12 months) and 32.4% (after 13 months) of the
added 15N. Correspondingly, the soil microorganisms im-
mobilized 25.2%, 26.6% and 13.9% of the added 15N while
the plants were taken up 42.6%, 39.2% and 33.8% of the
added 15N (Fig.4). 15N taken up by plants declined with
time but 15N retained in the soil organic matter increased.
15N immobilized by the soil microorganisms remained al-
most constant after 11 and 12 months but decreased obvi-
ously after 13 months following 15N addition. When NH4+-
15N was added, the soil organic matter retained 30.6% (after
11 month), 32.3% (after 12 months) and 42.6% (13 months)
of the added 15N. The soil microorganisms immobilized
7.4%, 15.9% and 9.4% of the added 15N and the plants took
up 11.2%, 14.5% and 13.7% of the added 15N (Fig. 4). 15N
retained in the soil organic matter increased with time while
15N uptaked by the plants altered a little, but 15N immobi-
lized in the soil microorganisms changed significantly. These
results indicate that the components in the alpine meadow
Fig.2. Soil microbial biomass N in soils after 11-13 months
following 15N additions. Values are means (±SE) from three repli-
cates per treatment.
Fig.3. Plant N after 11-13 months following 15N additions.
Values are means (±SE) from three replicates per treatment.
Acta Botanica Sinica 植物学报 Vol.46 No.3 2004282
preferred to retain different forms of added N in the long
period. Concretely, plants and soil organic matter prefer to
retain NO3- while soil microorganisms prefer to immobilize
NH4+ after about one year of N additions.
Why does the fate of NO3- and NH4+ appear differently
in many terrestrial ecosystems? The reason rests with the
different attributes of NO3- and NH4+ ions. In soil solu-
tions NO3- ion was more mobile than NH4+ ion (Owen and
Jones, 2001), so it was easier for NO3- ion to be transferred
to the surface of roots through water flow (Nye and Tinker,
1977) while NH4+ was easier to be absorbed to negative
ions of soil organic matter or soil granules. This is probably
the reason why more NH4+-15N was retained in the soil
organic matter in Kobresia meadow. Another reason may
be that alpine plants preferentially use NO3--N, which re-
sults in more 15NO3--N recovered in alpine meadows. This
can rise to a question, i.e., why alpine plants prefer to take
up NO3--N? A further research should be conducted on
this question, which will be great of help to understand the
mechanism of plant acquisition of N in Kobresia meadows.
Acknowledgements: We thank CAO Guang-Min for help-
ing to select the experimental site and for assistance during
experiments.
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(Managing editor: HAN Ya-Qin)