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: luxiaoqiang2010@gmail.com

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)

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~**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-

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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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