中国南西部のカルスト地域における天然林を移動する水の水質及び養分収支
誌名誌名 森林立地
ISSNISSN 03888673
巻/号巻/号 532
掲載ページ掲載ページ p. 61-71
発行年月発行年月 2011年12月
農林水産省 農林水産技術会議事務局筑波産学連携支援センターTsukuba Business-Academia Cooperation Support Center, Agriculture, Forestry and Fisheries Research CouncilSecretariat
~'.H:tl0t!J53(2), 61 ~71 2011 Jpn J For Environ 53 (2), 61-71, 2011
Hydrochemistry and nutrient budgets in a natural forest
in a karst area of southwestern China
Xiaoqiang Lu 1.*, Hiroto Toda2, Fangjun Ding3, Dongsu Choi2 and Shengzuo Fang4
lUnited Graduate School of Agriculture Science, Tokyo University of Agriculture and Technology
2Faculty of Agriculture, Tokyo University of Agriculture and Technology
3Guizhou Academy of Forestry Sciences, China
4College of Forest Resources and Environment, Nanjing Forestry University, China
Bulk precipitation, throughfall, stemflow, and stream water were collected and the <;oncentrations of major ions were determined
from samples collected from a subtropical natural evergreen and deciduous broadleaf mixed forest in Maolan, a karst area in south
west China, in order to quantify the input-output budgets of major dissolved nutrients and to examine changes in the chemical com
position of precipitation after passing through the canopy. Calcium ion and Mg2+ derived predominantly from carbonate weathering
were the major contributors to the hydrologic system's compartments. Low ion concentrations and a high pH value characterized bulk
precipitation quality that was influenced by natural rather than anthropogenic sources in Maolan. Calcium ion and Mg2+ had negative
annual input-output budgets in contrast to K+, Na+, Cl-, S042--S, and dissolved inorganic nitrogen (DIN) that had annual positive
input-output budgets. We used a canopy budget model (Na+ tracer) to estimate amounts of dry deposition and canopy exchange of
ions in throughfall. Calcium ion had the largest annual dry deposition, followed by SOl- and NH4+. Annual atmospheric deposition
of DIN amounted to 12.3 kg ha-' yr-'. Eighty-six percent of DIN input in the forest was retained in the plant-soil system, indicating
effective immobilization by vegetation and/or soil microflora.
Key words: bulk precipitation, dry deposition, input-output budgets, karst, natural forest
JM'l 1!l\\~~· plIlil'iA· T ll1jJft. -'ill :J.lUru;·/j ~·fli: ICj:tOOr+JlLIiIffIO):iJ]vA r:lt!l1i.%~::;f:Ht;.,~r&1't:a-~ill}r9;"*0)7M;g:1iHl"~ 5tJNx. M1[:lt!l53: 61-71, 2011.
riJOOO)i¥IjJgj'jf,~lMjO):fJ JVA r:lt!l:llxl::;!3It''(, ;ft)t~f:N, ;fti*Jf:N, Wm1E1Hl"i~iiiE*H*l111. 1..., IIIfo71(O)WJl§illi~1ltrlttO)f~~'j!:@.IlX:O)
~f~:a-1lJi G1.I'I::"9;" C C b I::, cEiJ:-1:t :/O)~xxHe;t!ll U'::o :<jqlf~1l:j1!H;t, A1.1,B9:j~JliLO)iJ:It'!Ili?)'!'W~'l\·*<R· KfmJt;m:wtil'iJe5i: 1... t.::~l!,~**<:;l?;"o 7j(1J/i:l)l'H;:;!3lt ;"Ca2 + cMg2+O)~[ijJlli:iI'~ <, :fJ JVA r O)l!t~mEO)~&~jil':iFIJtg~ :I1.t.::o ri"OOOOrJqO)lmO):lt!l:llxl;:
Jt~, IIIfof:NO)-1:t :/!Jj'tg[iI'l!l'; <, pHiI'j',lijlt' CIt' -') !f-'j.r~I;t, A1.1,EI9iJ:)~~! J: I) b :fJ Jv 7. r 0) § l!.~~fl:iI'::k eo It' c:it;t G :11.;"0 it.::, cEiJ:-1:t :/O)JIXx{;tCacMg<:7 -1 TA C iJ: I), K+, Na+, Cl-, SO.,'--slHfifFr+1Wi~W.~~(DINH;t77 A C iJ:·::d.:: o WJl§Rst
"CTJV(Na+ r v--tj--) H§It" ;ffll,Jf:MJO)ij1z:ttit;jlf C -1 :t :/O);fll'tJl§5i::j~HlIJil'H~JE U'::o iFrn~ij!l:ttit~m:(;tCa2+iI'irH?fr < , "'JIt'<:SO,z-, NHj+<:;l?-=> t.::o ~1'f(::iiiEA 1... t'::DIN {;tiF II"12. 3 kg ha-1<:, -f0)-') -G86%{;t, ;fi1!~-±m~1i~*"'O)l*:j'j.ilqfE~
~:11., ;flff~c±mlJ1Z~!W(;:?i:)J*t'l91;:'fljm~:I1. '(It't.:: o
"" - ry - J" : 1;ifo71(, ijlz:ttit%f, ~5tJIXx, :fJ Jv 7. r, :Rr.~i*
1. Introduction about SUbtropical karst forests is poorly documented. Karst ter
rain accounts for about 12% of the world's land area or about
Nutrients dynamics in forest ecosystems have been widely 20 million km", and is mainly distributed in the Mediterranean
evaluated in relation to the effects of atmospheric deposition, Sea area, Eastern Europe, the Middle East, Southeast Asia,
and the mechanisms involved in hydrochemical processes in Southeast America, and the Caribbean region (Yuan 1997).
different ecosystem components. So far many important in- Thus, understanding the hydrochemistry and nutrient budgets
sights have been derived from studies in Europe (Escudero et in karst areas is important with respect to the large area and
aI., 1985; Draaijers and Erisman, 1995; Hardtle et ai., 2009), large population involved. In addition, karst environments are
North America (Paker 1983; Johnson et ai., 1991; Mitchell et extremely fragile. Compared to other forest ecosystems, nutri-
ai., 1992; Johnson et al., 2009), and Asia (Haibara and Aiba, ents cycling may be more vulnerable in karst forest ecosystems.
1982; Zeng et aI., 2005; Kaneko et aI., 2007; Muramoto et ai., The soil in karst regions is usually thin, scattered, and poor in
2007; Fang et al., 2008). Although many studies have been nutrients because of the low soil-forming capability of the
conducted in temperate forests and tropical forests, information highly weather-leaching and soluble bedrock (Wang 2002).
* ill!if.1t' 7JljlillJ~l'i;j(:7'G (Con'esponding Author) : T 183-8509 r{frJlm¥1IlJ3-5-8 :l!tffiJ~I'k:'j':Ij!,\:'j':!t1': Faculty of Agriculture, Tokyo University of Agricul· ture and Technology 3-5-8 Saiwaicho, Fuchu, Tokyo 183-8509, Japan E-mail: [email protected]
1 United Graduate School of Agriculture Science, Tokyo University of Agriculture and Technology 2 Faculty of Agriculture, Tokyo University of Agriculture and Technology 3 Guizhou Academy of Forestry Sciences, China 4 College of Forest Resources and Environment, Nanjing Forestry University, China
(Received 30 May 2011, accepted 31 August 2011)
-61
lH.:j;:i'zJ1ll53 (2), 2011
At present, it is still not so clear how the nutrient fluxes in
rainfall, throughfall, stemflow, soil water, and stream water af
fect the dynamics of the karst area and ecosystem. It is diffi
cult to quantify the changes and budgets of bulk precipitation
chemistry before and after entry into karst forests, but it is im
portant to estimate these parameters in order to provide a sci
entific basis for restoring karst vegetation and for preventing
rock desertification. Existing studies in this typical ecosystem
are mainly related to the chemical characteristics of bulk pre
cipitation and karst groundwater (Padilla and Pulidobosch,
1995; Han and Liu, 2001; Li et al., 2004; Dong et al., 2005;
Han et al., 2010). However, the fluxes and budget of water
dissolved nutrients through the ecosystem have not been well
integrated. The relatively high porosity and permeability of
carbonate rock underneath the karst areas (Wang et al., 2004),
make it difficult to accurately measure output (stream and deep
drainage) water volumes. Therefore, approximate values are
used for output water volumes according to information in the
literature and observations.
In the present study, we determined the concentration and
flux of nutrients in bulk precipitation, through fall, and stem
flow, and estimated the flux of nutrients in stream water in a
subtropical natural evergreen and deciduous broad leaf mixed
forest in Maolan, a subtropical karst area in southwest China.
The objectives of the study were fourfold: (1) to quantify the
ions inputs for a natural forest ecosystem; (2) to determine
changes that occur in the chemical composition of precipitation
that passes through the canopy; (3) to analyze fluxes of ions
changes that occur in the surface soil layer; (4) to calculate the
input-output budgets in the forest ecosystem. To clarify these
values in the natural karst forest may give us useful informa-
III Karst area
(a) o ,
N
tion to restore degraded karst forest ecosystems.
2. Study area and methods
2.1 Site description
The study was conducted at the Guizhou Academy of For
estry's Karst Forest Ecosystem Research Center, situated in the
Maolan National Nature Reserve in southeast Guizhou Prov
ince, southwest China (25°09'20"-25°20' 50''N, 107°52' 10"-
108°05' 40"E) (Fig. 1). The area covers 2,000 km' and includes
mountains composed of jagged carbonate rock that are 90%
covered by forests. A subtropical mountainous monsoon cli
mate dominates the study area at elevations between 400 and
1,000 m, characterized by a mean annual temperature of
14.3°C and mean annual relative humidity of 83%. Mean an
nual precipitation was 1,672 mm from September 2007 to Au
gust 2009, with 52% falling in summer (June-August) (Fig. 2).
The original soils are Mollie Inceptisols with a sharp lithic
contact mostly within a profile depth of 20-30 cm. The domi
nant evergreen species are Platycarya longipes, Carpinus Pll
bescem, Celtis tetrandra, Symplocos adenopus, and dominant
deciduous species are Cyclobalanopsis glallca, Acer cinnClmo
mifolium, Symplocos adenopus.
2.2 Water sampling
We studied the hydrochemistry of two typical 20 m x 30 m
plots (plot I and plot 2) located mid-slope on a hill (altitude,
785 m and 840 m). Characteristics of the vegetation in the for
est and surface soil properties are listed in Table 1. Precipita
tion, throughfalJ, stemflow and stream water were collected bi
weekly from September 2007 to August 2009. Bulk precipita
tion was collected using a bulk precipitation collector, set at a
height of I m above the ground on an open area near both
108 0 E
4 29 0 N
25 0
90 l80km 108 0 E
(b)
Fig. 1. Karst area of China (a) and the location of the study area (b)
-62-
Jpn J For Environ 53 (2), 2011
400
S 8 '-' = Q
200 :c C<l ;i:
.9-'-' ~ :.. ~
0
2007 2008 2009
Fig. 2. Monthly precipitation from September 2007 to August 2009
Table 1. Vegetation and surface soil properties for two plots in the karst area
Vegetation
Ratio of evergreen to deciduous (stems %)
Mean tree height (m)
Mean tree diameter at breast height (cm)
Stand density (stems ha-I)
Surface soil properties (at a depth ofO-20cm)
Bulk density (g cm -3)
pH(H20) values
Total C (g kg-I)
Total N (g kg-I)
CIN
plots. In each study plot, four throughfall collectors were
placed randomly 1 m above the ground. Each bulk precipita
tion and throughfall collector consisted of two connected patts,
a polyethylene funnel (30 cm in diameter) and a polyethylene
container (25 L). The collectors were opaque in order to mini
mize light penetration. A sterilized glass wool filter-plug was
placed in the mouth of each funnel to prevent insect, leaf, and
detritus contamination. Stemflow water was collected from the
dominant species (Platycarya longipes and Cyclobalanopsis
glauca) in each plot. Stemflow collectors installed under four
trees having an average diameter (DB H) of 11 cm (two each
of Platycarya longipe and Cyclobalanopsis glauca) were con
structed with 2.5 cm diameter polyethylene hoses by gluing
polyethylene collars (5 cm in width ) and twining round the
trees with the flow passing into a 25 L polyethylene container
(Fig. 3) (Oyat'zun et aI., 2004). Stream. water samples were
collected from an adjacent stream. All water samples were
stored immediately in refrigerators at the Management Bureau
of Maolan National Nature Reserve, returned to Tokyo Univer-
Plot I Plot 2
68:32 62:38
8.3 9.7
11.0 11.6
2500 3111
1.15 0.95
7.1 7.0
43.3 33.0
3.8 3.3
11.5 11.1
sity of Agriculture and Technology and stored at 4°C.
To measure ions movement in water within forest soil, ion
exchange resin (IER)-filled columns were installed on the sur
face soil layer. The IER-filled columns made of polyvinyl
chloride tubing (2 cm high; 6 cm diameter) were filled with
about 50 ml (30 g in wet weight) (Haibara et al., 1990; Fang
et aI., 2011) mixed IER (MB-1; Organo Corp., Tokyo, Japan)
and sealed at both ends with a nylon mesh (bottom, 0.5 mm;
top, 1 mm) fabric to avoid root penetration and introduction of
impurities from litter, invertebrates, and other sources. This
volume of IER is sufficient to collect cations/anions equivalent
to about 140 kmolc ha-1, assuming equal amounts of cations
and anions. In plot 1, thirty pairs of IER-filled columns in
polyvinyl chloride tubes were buried, randomly 1-2 m apart,
on the surface of and under the soil (5 cm deep) to facilitate
calculations per unit area. These columns were covered with
the natural soil layer and remained buried from September
2008 to August 2009.
-63-
Fig. 3. Stemflow collector
2.3 Chemical analysis
The pH was determined with a pH meter (PP-15; Sartorius,
Japan) after sampling. The water samples were passed through
0.20 .um membrane filters (DISMIC-13cr; Toyo Roshi Kaisha
Ltd., Japan) prior to determining Ca2+, Mg2+, K+, Na+, NH4+,
Cl-, N03-, and S042- by ion chromatography (HIC-6A; Shi
madzu, Japan) within I month after sampling. The quality of
chemical analyses was checked by including method blanks,
repeated measurements of internal and certified reference sam
ples and by inter-laboratory tests. IER-filled columns were col
lected from the field, placed in separate polyethylene columns,
and maintained at low temperatures. IER samples (4 g) were
extracted twice using 100 mL of 2 M KCl, and then were
shaken for 1 h and filtered. The sample volume was adjusted
to 250 mL by adding 2 M KCl to analyze for DIN (N03- and
NH4+). N03- and NH4+ concentrations were determined, respec
tively, by the phenol disulfonic acid and indophenol blue
methods using a flow injection auto analyzer (FIU-300N; Jasco
Inc., Tokyo, Japan). IERs were also extracted with 100 mL of
1M NH4Ac (pH 7.0) and filtered to analyze for Ca2+, Mg2+,
Na+, and K+ by atomic absorption spectrophotometry (AA-
6700G; Shimadzu, Kyoto, Japan), as described by Haibara et
al. (1990).
-64
2.4 Statistical analyses
Water volume was computed by converting each volume
collected biweekly to a depth (in mm) based on the total sur
face area of the collection funnels and summing each biweekly
measurement to equate to an annual total. The stemflow values
were scaled to a unit area basis from the crown area of the
sample trees. We used the empirical temperature-based
Thornthwaite method to estimate the output of precipitation
from evapo-transpiration, and then estimated annual output
(stream and deep drainage) water volumes (Palmer and Havens,
1958; Hargreaves and Samani, 1982; Trajkovic 2005). Annual
output fluxes of ions were calculated by multiplying annual
mean concentrations of ions in stream water and annual output
water volume. The annual volume-weighted mean concentra
tions of ions in precipitation, throughfall, and stemflow for the
period from September 2007 to August 2009 were calculated
by weighting the concentrations, measured at biweekly inter
vals, with the accumulated water flux for that period. Annual
ion fluxes (cmolc ha-1 yr-1) were computed by multiplying bi
weekly measurements of ion concentrations and total collector
volume, dividing by the ratio of total collector funnel area to
the number of hectares and summing biweekly flux estimates
per year. Spring, summer, autumn, and winter were defined as
March-May, June-August, September-November, and
December-February, respectively.
Differences in ion concentration and flux were identified by
one-way analysis of variance using SPSS 11.5 software for
Windows. Differences were considered statistically significant
at p < 0.01. Dry deposition ion estimates were acquired using
the canopy budget model (Parker 1983; Draaijers and Erisman,
1995; Zeng et aI., 2005; Muramoto et ai., 2007). The average
volumes of stemflow were less than 3% of precipitation
(Table 2). Considering the minor contribution of stemflow, this
factor was omitted in the calculation of the total input flux.
Parker (1983) also considered that the stemflow contribution
was insignificant, since it amounted to < 5% of the total input
flux. We calculated the net throughfall deposition (NTF) to de
termine the canopy's total effect on deposition as follows:
NTF = TF - PD = DD + CE (1)
where TF = throughfall deposition, PD = precipitation depo
sition (wet-only deposition), DD = dry deposition, and CE =
canopy exchange.
In theory, precipitation deposition (wet-only deposition) is
precisely measured using automatic samplers that are covered
by a lid during dry periods and open whenever precipitation is
detected by a sensor. Automatic collectors have the drawback
of being expensive and requiring a power source. Therefore,
the precipitation deposition (wet-only deposition) was not
monitored in the current study due to power limitations in the
fields. Chen and Mulder (2007) compared ion concentrations
Jpn J For Environ 53 (2), 2011
Table 2. Average annual amounts of water (mm), pH and volume-weighted mean concentration of ions in
bulk precipitation, throughfall, and stemflow, and mean concentration of ions in stream water between
September 2007 and August 2009
Water Ca2+ Mg2+ pH
mm
Bulk average 1672 5.8 29.4 4.8
precipitation S.E: 25.7 0.0 0.2 0.3
average 1293 6.0 158.1 74.1 Throughfall
S.E. 39.8 0.0 10.0 13.9
average 40 6.2 165.1 65.8 Stemflow
S.E. 5.2 0.1 23.9 8.7
Stream average 708 8.3 1276.3 1053.0
water S.E. 25.7 0.0 4.0 41.4
as.E. = Standard error
in precipitation collected using wet and bulk samplers over 2
years in a neighboring site (Leigongshan Nature Reserve in
southeast Guizhou Province, about 100 km north of the current
study site, Fig. 1) and found that ion concentrations in bulk
precipitation depositions were approximately the same as in
precipitation deposition (wet-only deposition), suggesting a
small contribution of dry deposition. Therefore, bulk precipita
tion deposition was used in place of precipitation deposition
(wet-only deposition) in the current study.
Canopy ion leaching (e.g., Ca2+, Mg2+, and K+), expressed by
the dry deposition factor (DDF), was estimated using Na+ as a
tracer for annual dry particle deposition and throughfall meas
urements (Ulrich 1983; Matzner et al., 2004). In the canopy
budget method, Na+ is assumed to be inert with respect to the
canopy, that is, neither uptake nor leakage occurs. Experimen
tal studies support the assumption that Na+ is not leached from
the tree canopy, because of Na+ is not a major plant nutritional
element and should not be taken up selectively and cycled by
forests (Lindberg and Lovett, 1992; Veltkamp and Wyers,
1997). Furthermore, particles containing are assumed to
equivalent to that of Ca2+, Mg2+, K+, NH4+, Cl-, N03-, and
SOl- (Bredemeier 1988; Draaijers and Erisman, 1995).
DDF = (TF - PD)NJPDNa (2)
dry deposition of Ca2+, Mg2+, K+, NH4+, Cl-, N03-, and S042-
is then calculated as bulk precipitation deposition multiplied by
DDF:
DD, = PDx x DDF
CE, = TFx - PDx - DD,
(3)
(4)
K+ Na+ NH4+ Cl- N03 SO/- Inorganic N
/lmolc I-I
3.9 7.9 23.4 6.1 2.7 24.9 26.1
0.4 1.6 1.0 0.7 0.1 1.5 1.0
85.5 21.3 16.1 28.9 14.6 61.1 30.7
7.2 3.8 3.7 2.7 3.8 2.0 2.8
127.8 5.9 12.9 17.4 7.5 56.3 20.4
21.2 0.3 1.1 1.7 2.1 2.2 2.0
0.7 14.0 0.3 21.2 18.7 111.4 19.0
0.3 1.0 0.0 3.6 3.4 9.7 3.4
3. Results
3.1 Bulk precipitation, throughfall, stemflow, and
stream water chemistry
Mean annual bulk precipitation was 1,672 mm, and monthly
values ranged from 0 to 370 mm (Fig. 2). More than 54% of
the annual precipitation fell from June to August, increasing
gradually in early June, and peaking in middle and late July,
and decreasing in early August. The lowest rainfall was re
corded in winter (December, January, and February).
Differences in the estimated annual average amounts of
water, pH, and ion concentrations (Ca2+, Mg2+, K+, Na+, NH4+,
Cl-, N03-, and S042-) in bulk precipitation, throughfall, stem
flow, and stream water in two plots were not statistically sig
nificant (p > 0.05). Therefore, the means were used in all
analyses. Twenty of percent of the precipitation was estimated
to be intercepted by the canopy (Table 2), and 77% of the pre
cipitation reached the forest floor as throughfall. The annual
average-volume of stemflow was less than 3% of precipitation.
Average bulk precipitation pH values were slightly higher than
those of natural rainwater (pH = 5.6) (Oyarzun et al., 2004).
Throughfall pH never fell below 5.8, reaching 6.6 in May
2008 (not shown). Relative charge concentrations (Ilmolc 1-1) of
ions in bulk precipitation levels were the following: Ca2+ >
DIN (N03- + NH4+) > S042- > Na+ > Cl- > Mg2+ > K+. The
concentrations of all ions but NH4 + in throughfall were higher
than those in bulk precipitation. Concentrations of Ca2+, Mg2+,
K+, Na+, Cl-, N03-, and S042- in bulk precipitation increased
by 5, 15, 22, 3, 5, 5, and 3 times, respectively, after passing
through the canopy. Three ions, Ca2+ + Mg2+ + K+ accounted
for more than 89% of the cations in throughfall, similar to
-65-
their levels in stemflow.
In stream water, Ca2+ and Mg2+ accounted for 99% of the
cations. These high concentrations of Ca2+ and Mg2+ found in
stream water are consistent with the mean annual pH of 8.3.
3.2 Dry deposition and canopy exchange
Calcium ion had the largest dry deposition, followed by
S042- and NH4+, which were, 96% and 88% of the Ca2+ level,
respectively (Fig. 4). With respect to dry deposition of DIN,
the level of NH4+ was nearly 10 times higher than the N03-
level. Annual fluxes of canopy exchange for total ions meas
ured were 1.6 times higher than dry deposition. Calcium ion,
K+, and Mg2+ were among the most abundant ion exchanged in
the canopy. Annual canopy exchange of Ca2+, Mg2+, and K+
accounted for 73%, 92%, and 92% of ion input in throughfall,
respectively. Annual canopy exchange of NH4+ represented a
net absorption in all seasons. The sum of dry deposition and
canopy exchange in spring and summer was three times higher
than in autumn and winter. Relative seasonal dry deposition
decreased in the order, spring > summer > winter > autumn,
and canopy exchange values decreased in the order, spring >
summer> autumn > winter. In contrast, N03-, Cl-, and Na+
dry deposition and canopy exchange levels were lower and
varied little with the seasons.
3.3 Leaching from soil and stream water nutrient flux
Annual input (atmospheric deposition) of DIN and SOi--S
were estimated to be 12.3 kg ha- I yr- I and 12.3 kg ha- I yr- I,
respectively (Table 3). DIN export via stream water, only 1.6
kg ha- I yr- I, was lower than the input to the forest floor via
precipitation and dry deposition via throughfall. Thus, 87% of
the deposited N was retained in the soil and vegetation. Am
monium output was extremely low (0.003 kg ha- I yr- I). Sulfur
output amounted to 11.7 kg ha- I yr- I, indicating that the forest
ecosystem retained sulfur at the rate of 0.6 kg ha- I yr- I (about
5% of the total input) annually. Calcium ion and Mg2+ stream
water export values were 10 and 47 times higher than the total
input (precipitation + dry deposition of throughfall) to the for-
45
l~ Spring • Dry deposition
30 Jl 0 Canopy exchange
15 Jl • • 0 - .../I
-15 D -30
45
jJJ Summer
30
D JJ 15 • • 0 - D ....cJ = 0
-15 ., -30 " .c -" 45
1
Autumn 0 E 30
D ~ 15 ~
0 0 c: .:: 0 = .... 0 -15 ~
C -30 ::> 0 E 45
1
Winter < 30
15 In 0 0 --CJ -L:J ------
-15 -30
120 80 40 0
-40 cr NO,- so,'-
-80 Ions
Fig. 4. Seasonal dry deposition and canopy exchange flux
*Spring, summer, autumn, and winter were defined as March-May, June-August, September-November, and December-February, respectively.
Table 3. Fluxes and input-output budgets for major nutrients
Ca2+ Mg2+ K+ Na+ NH/-N Cra N03 -N SO/--S Inorganic N Parameters
kg ha- I yr'i
Inputb 17.8 1.9 6.2 6.0 11.1 6.7 1.2 12.3 12.3
Soil water 147.3 37.6 34.6 5.3 4.5 tr 9.8
Ocm (5.7 )' (2.2 ) (3.5) (0.2 ) (0.3 )
Soil water 159.3 50.7 20.2 5.5 18.6 tr 24.1
Scm (5.5) (1.9) (2.4) (0.2) (1.5 )
Output d 175.9 88.3 0.1 2.1 0.0 4.8 1.6 11.7 1.6
(1.2) (1.6) (0.0) (0.2) (0.0) (1.0 ) (0.4 ) (0.2 )
Input-output -158.2 -86.5 6.1 3.9 11.1 1.9 -0.4 0.6 10.7
budget
" Not observed in soil water b Input = Precipitation + Dry deposition of through fall, where dry deposition of precipitation was neglected , Standard en'or U Output = Stream water + Deep drainage, where ion concentrations of deep drainage were assumed to equivalent to that of stream water
-66-
Jpn J For Environ 53 (2), 2011
est floor (Table 3). The annual K output was relatively low
(0.1 kg ha-I yr-I). Calcium ion and Mg2+ represented the ma
jority of elements in both in the soil surface (0 cm and 5 cm)
and in output water. NH4+-N levels did not vary at depths from
o to 5 cm. Nitrate was more abundant than NH4+-N at a depth
of 5 cm. Nitrate decreased from a depth of 5 cm to stream
water. The level of K+ was lower in soil water at a depth of 5
cm than on the sutface. When the annual input-output budget
through water transport was evaluated, Ca2+ and Mg2+ had a
negative budget, whereas K+, Na+, Cl-, S042--S, and DIN had a
positive budget.
4. Discussion
4.1 Bulk precipitation
The charge concentrations of major cations (Cal + + Mg2+ +
K+ + Na+ + NH4+) were greater than those of major anions
(Cl- + N03- + S042-) in bulk precipitation, throughfall, stem
flow, and stream water. The imbalance of charge is due to the
presence of ions that were not measured in our study, such as
HC03-, HCOO-, and CH3COO- (Han et aI., 2010). To enhance
our understanding of chemical compositions at a large regional
scale, we compared the annual deposition of ions in bulk pre
cipitation with those falling on major rural areas in neighbor
ing areas or provinces in China such as Hunan, Chongqing,
Guangdong, and Guizhou (Table 4). A higher pH value (5.8)
was measured in this study. Bulk precipitation with pH val
ues < 5.0 may result from anthropogenic emission of Sand N
in the other sites listed in Table 4. Ion concentrations in bulk
precipitation reported here were lower than those reported for
the other sites. The annual bulk precipitation input of S042-
that we determined in this study was also lower than those re-
ported for evergreen and deciduous broad-leaves trees in
Southwest China (Jin et al., 2006; Zhang et aI., 2006; Aas et
aI., 2007), East China (average 208 ~molc I-I: Chen 1993), and
South China (Aas et aI., 2007) (Table 4). Han et al. (2010)
suggested that higher pH values could result from dissolution
of the high Ca2+ content of windblown dust in Maolan. Cal
cium ion accounted for more than 76% of the total cations
measured, indicating that Ca2+ was the dominant neutralizing
cation and likely neutralized Maolan acid rain. A likely source
for high Ca2+ concentrations is the local soil (Han et aI., 2010).
Ammonium ion concentrations in precipitation accounted for
90% of DIN. The bulk precipitation NH4+ levels coincide with
ammonia emission. The presence of NH3 is intimately related
to soil features in the study area (pH> 7.0). The ratio of NH4+1
N03- in Maolan was obviously greater than in other areas, and
the lowest NH4+IN03- ratio was measured in Liuxihe (Guang
dong), adjacent to an intensively industrial region (Table 4).
This result suggests that the karst natural forest in Maolan has
not been polluted by industrial activities andlor vehicular traf
fic. Ammonia volatilizes easily and deposits relatively quickly
close to the emission sources because of its greater density
than air (Romualdas et aI., 2007). Potassium ion deposition in
Guizhou province remained constantly lower than in other
study areas and could result from dissolution of K+ in dust.
The relative scarcity of Na+ and CI- in precipitation might re
sult from the great distance of the Maolan karst region to any
large body of saltwater. We conclude, therefore, that bulk pre
cipitation is characterized by low salinity and high pH, and its
composition is strongly influenced by natural rather than an
thropogenic sources.
Table 4. Comparison of the concentration of major ions in bulk precipitation in Maolan with other rural areas in
southern China
Precipitation Ca2+ Mg2+ KT Na+ NH4+ Cl- NO] SO/-Location area pH References
(mm) f!molcl- I
Shaoshan (Hunan) Non-karst 1524 4.7 82.8 19.7 9.8 12.0 25.2 10.5 13.3 89.5
Zhang et
(112'91'E, 27'8TN) a1., (2006)
Tieshanping Jin et a1.,
(Chongqing) Non-karst 1036 4.2 78.2 53.0 11.8 3.7 96.0 13.4 41.2 215.5
(l04'41'E, 29'38'N) (2006)
Liuxihe (Guangdong) Non-karst 1621 4.6 41.0 9.0 12.0 33.0 13.0 18.0 9.0 86.0
Aas et a1.,
(l12'26'E, 27'55'N) (2007)
Leigongshan Aas et a1.,
(Guizhou) Karst 1788 4.5 27.0 7.5 5.0 8.5 27.0 6.0 15.0 56.5
(106'43'E, 26'38'N) (2007)
Maolan (Guizhou) 1672 3.9 7.9 23.4 6.1 2.7 24.8 This study Karst 5.8 29.4 4.8
(107'58'E, 25'15'N)
67-
~**1Lj~53(2),2011
4.2 Throughfall
In contrast to precipitation data, few data are available for
comparing throughfall and stemflow depositions in southwest
and southern China. Compared to bulk precipitation, the
chemical composition of throughfall and stemflow water is
generally altered with respect to most chemical elements. This
alteration is widely acknowledged to result from the washing
off of dry particulates and gases as well as canopy exchange
and release of ions from plant tissues or canopy uptake (Parker
1983). In the present study, the concentrations of Ca2+, Mg2+,
K+, Na+, Cl-, N03-, and S042- were greater in throughfall com
pared to bulk precipitation (Table 2).
An important proportion of atmospheric DIN deposition can
be retained within the canopy, especially NH4+, suggesting that
the uptake of NH/ by the canopy might be balanced by leach
ing of base cations. Ammonium ion exchange in the canopy
possibly results from absorption and is at the same level (50%)
for input as that taken up by the forest canopy (Jin et al.,
2006; Chen and Mulder, 2007). Lovett and Lindberg (1984)
speculated that NH4+ might be converted to N03- by epiphytic
bacteria in addition to the well documented active uptake
within the canopy. Ammonium ion uptake greatly exceeded
that of N03- in a rural area (Romualdas et al., 2007). Intensive
K+ enrichment was a striking finding of our study. Potassium
ion enrichment in throughfall is usually explained by a high
rate of K+ leaching by acids from leaf tissue (Lindberg et al.,
1986). Abundant Ca2+ enrichment in throughfall fluxes was
rather surprising, since the field studies of Alcock and Morton
(1985) and Amezaga et al. (1997) demonstrated that Ca2+
might be retained within canopies in non-karst areas.
4.3 Dry deposition and canopy exchange
It is very important to distinguish dry deposition and canopy
exchange because dry deposition represents an input to the
ecosystem, whereas canopy exchange is a transfer within the
ecosystem. Moreover, knowledge of the relative contributions
of various constituents to total nutrients input can lead to a
better understanding of the source of those substances. The
canopy budget method is useful for estimating the contribution
of dry deposition and canopy leaching or uptake, to net
throughfall water (Mayer and Ulrich, 1974; Parker 1983; Ul
rich 1983). We separated the contributions of dry deposition
and canopy exchange to throughfall flux using the canopy
budget model (Fig. 4). In this study, annual dry deposition was
less important than canopy exchange as a source of deposition,
probably because of low-level dry cation or anion deposition
associated with soil dust and particles in natural forests (Law
son and Winchester, 1979a). Jin et al. (2006) reported that an
nual dry deposition (for Ca2+, Mg2+, K+, Na+, Cl-, N03-, and
SO/-) was higher than canopy exchange in similar forests in
Chongqing (China), the United States, and European countries.
These data differ from our data. Potassium ion was identified
as the most abundant canopy-leaching ion in throughfall (Bre
demeier 1988; Cappellato and Peters, 1995; Gonzalez-Arias et
aI., 2000), whereas Ca2+ was the most abundant ion in this
study. Canopy Ca2+ was exchanged at an annual rate of 118
cmolc ha- l, whereas the value for K+ was 94 cmole ha- l (Fig. 4).
Both ion exchange rates were much higher than the levels
measured in the similar forest types considered by Jin et al.
(2006). The high concentrations of Ca2+ in leaf tissue in karst
compared to non-karst areas need to be considered (Zhu et al.,
2003). Parker (1983) demonstrated that an internal cycling
pathway controls K+ levels, and throughfall leaches its supply
by 60%-90% compared to the 92% leaching observed in the
present study.
Both dry deposition and canopy exchange can vary with
seasons, because these processes depend on the forest's exter
nal environment and biology (Lovett et al., 1996). The highest
throughfall enrichment occUlTed in the spring or the summer,
partially from plant growth and relatively high precipitation.
Annual canopy exchange was 1.6 times higher that of annual
dry deposition. Canopy exchange in winter was, as expected,
lower than that in other seasons. Our study implicates canopy
NH4+ as a net sink in all seasons. More intensive uptake of
NH4+ than N03- was reported by Hansen (1996) and Balestrini
and TagliafetTi (2001). Uptake by the canopy is countered by
anion leaching (Draaijers and Erisman, 1995). Major anthropo
genic or oceanic sources are located far from the karst area's
natural forest that we studied. Therefore, canopy exchange
should control throughfall NH4+ flux. However, S042- showed
little interaction with canopies, and enrichment of S042- by
throughfall primarily results from wash-off of accumulated dry
deposition (Lawson and Winchester, 1979b; Lindberg et ai.,
1986).
4.4 Nutrient budget and leaching from soil
Because of the relatively high porosity and permeability of
carbonate rock in the karst areas, it is difficult technically to
quantify water output volumes. We used the Thornthwaite
method to estimate output water volumes to provide a rough
nutrient output for this study. Many comparative studies in the
past determined that the Thornthwaite method generally gave
over- and under-estimations of water output volumes.
(Trajkovic 2005; Kondoh 1994) and Xu and Chen (2005) cal
culated an average relative etTOr of 6.3% when comparing the
calculated versus the observed evapo-transpiration rates. Based
on this relative error, the annual stream water amount in our
research area was adjusted to be 708 mm and occupying 42%
of the annual precipitation (1,672 mm). The result was compa
rable to the report by Larssen et al. (1998) of 47% precipita
tion in another forested small catchment nearby in Guizhou
Province. Using throughfall as a measure of the total deposi-
-68-
Jpn J For Environ 53 (2), 2011
tion of nutrients may be problematic because there is leaching
and uptake by the canopy. So, bulk precipitation (wet-only
deposition) and dry deposition of throughfall were employed
as inputs and the stream water and deep drainage were consid
ered as outputs in the current study.
The retention of DIN in the cun'ent study reached 10.7 kg
ha- l yr- l, lower than other study areas in central-southern and
southern China (average 26.2 kg ha- l yr- l: Du et ai., 2008; 32-
34 kg ha- l yr- l: Fang et at., 2008). A comparative study of
input-output N budgets for 65 forested catchments and plots
across Europe (Dise and Wright, 1995) demonstrated that an N
deposition threshold is about 10 kg ha- l yr- l in input fluxes,
above which significant N03--N leaching could start to occur
from forests, and that with deposition exceeding 25 kg ha- l
yc l, all ecosystems start to leach large quantities of nitrogen.
The DIN input of 12.3 kg ha- l yr- l at the study site has ex
ceeded the rate of 10 kg ha- l yc l, but within the latter thresh
old of 25 kg ha- l yc l. Export of DIN (1.6 kg ha- l yr- l
) by
stream water was much lower than its input to the forest" floor
via bulk precipitation and dry deposition by throughfall. Spe
cifically, soil and vegetation retained 86% of the deposited N.
The small amount of N leaching observed in the study forest
may have been caused by water movement during the growing
season when precipitation was intensive (Fang et at., 2008).
This finding suggests that nitrogen saturation in the study for
est may have not occurred.
Annual input of sol-oS was relatively low. Only 5% of the
total deposited SOl--S was retained annually, suggesting that
S042- -S retention may be attributed to weak soil adsorption.
The large differences between Ca2+ and Mg2+ input and output
flux in the karst region studied here suggests the presence of
internal sources in stream water. Calcium ion and Mg2+ derived
primarily from carbonate weathering are important factors. The
chemical composition of stream water is controlled primarily
by carbonates rather than silicates or by evaporative weather
ing (Han and Liu, 2001). The relatively low stream water K+
output (0.1 kg ha- l yr- l) indicated significant retention within
the soil or uptake by vegetation. Potassium ion uptake reached
99% of its total input annually. Plants utilize a large amount of
K+, and its depletion upon output might indicate that soil K de
ficiency results from the presence of carbonate rock. Potassium
ion recycling within the forest through bulk precipitation and
dry deposition of throughfall should efficiently provide a sup
ply for plant growth. The natural forest in the karst areas could
improve K+ availability and efficiency of use (Han et at.,
2010).
5. Conclusions
We attempted here to quantify nutrient fluxes in the karst
natural forest ecosystem in Maolan, southwestern China to un-
derstand nutrient dynamics in a region that receives small
amounts of nutrients from the atmosphere (Table 4). The few
sources of DIN and K and their low output indicated efficient
inner recirculation that promotes plant growth. Degradation of
this tightly regulated and sensitive system could decrease DIN
and K+ supplies or absorption. Elevated concentrations of Ca2+
and Mg2+ suggest that they are derived from an internal hydro
logic system. Calcium ion and Mg2+ derived predominantly
from carbonate weathering played a major role in regulating
chemical compositions in precipitation, throughfall, and stream
water from the karst regions. Their abundance can neutralize
acidity, and maybe activate and improve the availability of cat
ions. These processes should enhance absorption of adequate
nutrients by plants, thereby accelerating vegetative succession
and increasing the resistance to acid deposition.
Acknowledgements
This study was conducted with the cooperation of Guizhou
Academy of Forestry's Karst Forest Ecosystem Research Cen
ter of China. We gratefully acknowledge Dr. Ye Tian from
Faculty of Silviculture, Nanjin Forestry University for his en
thusiastic help in the field investigation and for his valuable
comments. We also thank Dr. Rieko Urakawa from Tokyo
University of Agriculture and Technology for her help in ex
perimental analysis and Ms. Yanpin Gao, Ms. Qu Hu and all
the staff from Guizhou Academy of Forestry, and Mr. Wei
Luming from the Management Bureau of Maolan National Na
ture Reserve for his cooperation and providing the experimen
tal site for the field works. Dr. Yunting Fang at Tokyo Univer
sity of Agriculture and Technology provided constructive com
ments about the manuscript. This work was supported by MOE
study of Nitrate Nitrogen Loss caused by Nitrogen Saturation
at Forest surrounding the Tokyo Metroporitan Area.
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