E c o sy ste m s (2 0 0 4 ) 7: 2 7 5 -2 8 5
D O I: 1 0 .1 0 0 7 /S 1 0 0 2 1 -0 0 3 -0 2 3 6 -7
Ec o sy s t e m s
) 2 0 0 4 Springer-V erlag
Composition, Dynamics, and Fate of
Leached Dissolved Organic Matter
in Terrestrial Ecosystems: Results
from a Decomposition Experiment
Cory C. Cleveland,^* Jason C. Neff,^ Alan R. T o w n s e n d , a n d Eran Hood"^
^Institute o f A rctic a n d A lp in e Research, University o f Colorado, Boulder, Colorado 80309, USA; ^Earth Surface Processes Team,
US Geological Survey, Denver, Colorado 80225, USA; ^Department o f Environm ental, Population a n d Organismic Biology,
U niversity o f Colorado, Boulder, Colorado 80309, USA; "‘D epartm ent o f Environm ental Science, U niversity o f A laska Southeast,
Juneau, A laska 99801, USA
A bstract
Fluxes of dissolved organic m atter (DOM) are an
im p o rtan t vector for th e m ovem ent of carbon (C)
an d n u trien ts b o th w ith in a n d b etw een ecosystems.
However, alth o ugh DOM fluxes from throughfaii
an d th ro u g h iitterfaii can be large, little is know n
ab o u t th e fate of DOM leached from p lan t canopies,
or from th e litter layer into the soil horizon. In this
study, o ur objectives w ere to determ ine th e im por­
tance of plant-fitter leachate as a vehicle for DOM
m ovem ent, a n d to track DOM decom position [in­
cluding dissolve organic carbon (DOC) an d dis­
solved organic nitrogen (DON) fractions], as w ell as
DOM chem ical an d isotopic dynam ics, during a
lo ng-term laboratory incubation experim ent using
fresh leaves an d litter from several ecosystem types.
The w ater-extractable fraction of organic C was
hig h for all hve p lan t species, as w as th e biodegrad­
able fraction; in m ost cases, m ore th a n 70% of the
initial DOM w as decom posed in the hrst 10 days of
th e experim ent. The chem ical com position of the
DOM changed as decom position proceeded, w ith
h um ic (hydrophobic) fractions becom ing relatively
m ore ab u n d a n t th a n n o n h u m ic (hydrophilic) frac­
tions over time. However, in spite of proportional
changes in hum ic an d n o n h u m ic fractions over
time, o u r data suggest th a t bo th fractions are readily
decom posed in th e absence of physicochem ical re ­
actions w ith soil surfaces. O ur data also show ed no
changes in th e
signature of DOM during d e­
com position, suggesting th a t isotopic fractionation
during DOM uptake is n o t a signihcant process.
These results suggest th a t soil m icroorganism s p ref­
erentially decom pose m ore labile organic m olecules
in the DOM pool, w hich also ten d to be isotopically
heavier th a n m ore recalcitrant DOM fractions. W e
believe th a t the interaction b etw een DOM decom ­
position dynam ics a n d soil sorption processes con­
tribute to the
enrichm ent of soil organic m a t­
ter com m only observed w ith depth in soil prohles.
K ey w ord s: dissolved organic m atter (DOM); dis­
solved organic carbon (DOC); decom position; iso­
topic fractionation; hum ic substances; leaf litter;
carbon-isotope ratio.
an d n u trien ts b o th w ith in a n d b etw een ecosystems.
DOM fluxes from litter an d surface organic h o ri­
zons to low er soil layers also play a role in soil
heterotrophic activity (Zsoinay a n d Steindi 1991;
Jandi an d Soiins 1997). F urtherm ore, DOM is a
source of n u trien ts to soil organism s th ro u g h the
m ovem ent of dissolved organic nitrogen (DON),
phosphorus (DOP), an d sulfur (DOS) in litterfall
I n t r o d u c t io n
Fluxes of dissolved organic m atter (DOM) are an
im p o rtan t vector for th e m ovem ent of carbon (C)
R e c e iv e d 9 S e p te m b e r 2 0 0 2 ; a c c e p te d 13 M a r c h 2 0 0 3 ; p u b lis h e d o n lin e 2
A p ril 2 0 0 4 .
*Corresponding author; e-m ail: c o ry .c le v e la n d @ c o lo ra d o .e d u
275
276
C. C. Cleveland an d others
an d throughfaii into soils (Qualls an d others 1991).
Over long-tim e scales, DOM fluxes th ro u g h soil are
responsible for solid soil organic m atter (SOM) for­
m atio n via sorption of DOM onto m ineral surfaces
in spodosols (McDowell an d W ood 1984; M cDowell
an d Likens 1988). One central issue involved in
b o th th e form ation of SOM an d th e fate of DOM is
w h e th e r DOM is readily decom posable. A lthough
th e processes involved in the physical sorption of
DOM across soil types a n d ecosystems are reaso n ­
ably w ell k n o w n [for exam ple, see Nodvin and
others (1986) an d M cBride (1994), a n d Kaiser and
others (2001a), th e biodegradability of DOM has
received less atten tio n [but see Qualls an d Haines
(1992); Yano an d others (1998), a n d M cA rthur and
Richardson (2002)].
DOM fluxes th ro u g h terrestrial ecosystems are
substantial a n d likely to play im portant roles in
in tern al ecosystem C retranslocation an d in the
m o v em en t of C from litter to soils (Kalbitz and
others 2000; Neff a n d A sner 2 0 0 f ). Previous w ork
has sh o w n th a t litter contains a highly soluble frac­
tion th a t m ay m ove into soils w ith rainfall (Gosz
an d others 1973 McDowell an d Likens 1988; M eyer
an d others 1998). In addition to C, DOM contains
hig h concentrations of N, P, an d o th er elem ents,
an d th e m o v em en t of DON a n d DOP into soil re p ­
resents a large source of available nutrients. The
relationship b etw een litter solubility an d DOM flux
to soils has led tw o groups to include the solubility
of organic m atter as a central controlling factor in
m odels of terrestrial elem ent cycling (Currie and
Aber 1997; Neff an d A sner 2 0 0 f). The underlying
rationale for linking solubility w ith decom position
is th a t litter b reakdow n is controlled by a m ixture of
physical, chemical, an d biological processes. H ow ­
ever, th ere is little inform ation on th e direct re la­
tionship betw een chem istry, solubility, a n d th e sub­
seq u en t biological availability of DOM across
ecosystems an d vegetation types.
U ncertainty over biological versus chem ical an d
physical controls on DOM decom position also lies at
th e h e a rt of a n ongoing debate over th e m ech a­
nism s b eh in d th e often n o ted enrichm ent of soil
carbon
values w ith soil depth (Ehleringer and
others 2000; G arten an d others 2000; Kaiser and
others 2001a). Several investigators have suggested
th a t isotopic fractionation during decay, including
th a t of DOM, should leave behind isotopicaiiy e n ­
riched sources of C as com pared to the original
SOM pool (Stout an d others 1981; N adeihoffer and
Fry 1988), an d th a t variation in
can therefore
provide a potential index of the ex ten t to w hich a
given organic m atter pool has b een transform ed by
m icrobial activity [for exam ple, see B aiesdent and
M ariotti (1996)]. However, there is substantial iso­
topic variation am ong different com pound types
w ith in p lan t m aterial [for exam ple, see W edin and
others (1995)], w hich in tu rn leads to isotopic vari­
ation in DOM prior to any decay. Thus, o th er stu d ­
ies have suggested th a t variation in physical stabi­
lization across the range of isotopic values could
help produce the observed depth prohles [for ex ­
am ple, see Ludwig an d others (2000) an d Kaiser
an d others (2001a)]. ft is therefore critical to eval­
uate the potential inhuence of m icrobial decom po­
sition on th e isotopic com position of DOM from the
additional in h u en ce of sorption processes.
In this study, o u r objectives w ere to determ ine
the im portance of plant-fitter leachate as a vehicle
for DOM m o v em en t into soil an d to track DOM
decom position [including b o th dissolved organic
carbon (DOC) a n d DON fractions], as w ell as its
chem ical an d isotopic dynam ics, during a long-term
laboratory incubation experim ent in w hich th e in ­
huence of soil sorption processes w ere rem oved.
The rationale for exam ining b o th the chem ical an d
isotopic com positions of DOM is that, over th e tim e
scales of soil form ation, DOM decom position m ay
contribute to the developm ent of the
patterns
com m only observed in soil prohles.
M
ethods
Study Sites and Sample Collection
W e investigated DOM solubility an d the dynam ics
of DOM decom position by using b o th live foliage
an d senesced litter of species from tem perate and
tropical ecosystems. Foliar sam ples from the te m ­
perate tree species {Abies lasiocarpa; subalpine hr)
w ere collected from a subalpine forest (3400 m)
located on N iwot Ridge (40°03'N, 105°36'W ), 40
km n o rth w est of Boulder, Colorado, on the eastern
slope of th e Rocky M ountains. Samples of A. lasio­
carpa w ere collected from individuals located along
a 50 X 5-m transect by clipping sm all branches from
trees at a height of f.5 m . Foliage was rem oved
from large stems, a n d stem s an d o th er w oody m a ­
terial w ere discarded prior to extraction. Litter sam ­
ples of A. lasiocarpa w ere obtained from th e forest
hoor along the sam e transect. Foliage from tw o
herbaceous alpine species {Acomastylis rossii, alpine
avens; an d Deschampsia cespitosa, tufted hairgrass)
w as also collected on N iw ot Ridge, b u t w as collected
from a single 20 X 20-m plot located in the alpine
tundra. Litter sam ples w ere collected following
p lan t senescence from a n adjacent 20 X 20-m plot.
All sam ples w ith in a vegetation type w ere bulked
prior to extraction.
Com position, Dynamics, an d Fate of Terrestrial DOM
Foliar sam ples from tw o tropical tree species
{Caryocar costaricense a n d Hieronyma alchorneoides)
w ere collected from a prim ary tropical rainforest
located in th e n o rth w e st corner of the Drake River
Valley on th e Osa Peninsula in southw estern Costa
Rica (8°43'N, 83°37'W ). Foliage w as collected from
th e canopy (approxim ately 35 m ) of th ree individ­
uals of each species by climbing trees an d excising
small branches. Litter of H. alchorneoides was sam ­
pled from th e forest floor below each individual
sam pled lo r foliage. Litter of C. costaricense w as n o t
collected because it is very difficult to identify,
w hereas litter of H. alchorneoides is easily identified
on th e forest floor. Samples w ith in a single vegeta­
tion type w ere bulked prior to extraction.
Experimental Design
For each vegetation type, six subsam ples (of bulked
samples) of approxim ately 50 g (dry-w eight equiv­
alent; 70°C) green foliage or senesced litter w as cut
into small pieces (less th a n 4 cm^), placed into 2-L
glass beakers, a n d extracted for 1 h in 1 L of deion­
ized w a te r at 25°C. Following extraction, leachate
w as preflltered th ro u g h a 0.5-m m -m esh sieve and
sterile filtered using 0.45 p.m MCE (cellulose ac­
etate-cellulose nitrate) filters. Pairs of leachate
w ere pooled, resulting in th ree leachate sam ples of
approxim ately 1.8 L each lor each vegetation type.
Samples w ere im m ediately sam pled lo r initial DOC,
total nitro g en (TN), inorganic N (NH4 ^ -I- N O j^),
an d S^^C-DOC.
A lter initial sam pling, the three sets of leachate
from each vegetation type w ere transferred to 2.4-L
am ber bottles equipped w ith a n air-intake p o rt and
outlet. Samples w ere inoculated w ith 1 mL of a
w ater-diluted ( 1 0 ^^) soil sam ple from each system
(lor exam ple, leachate from tropical species was
inoculated w ith tropical soil). The w ater-diluted soil
sam ple w as p repared by transferring 1 g of fresh soil
to 9 mL of deionized w ater, vortex m ixing, and
transferring 1 mL of the diluted sam ple to an o th er
9-mL deionized w ater. D ilution w as sequentially
repeated u n til a 1 0 ^^-lold dilution h ad been
achieved. DOC added via the inoculum was negli­
gible. Inoculated bioreactors w ere capped an d a e r­
ated using pressurized atm ospheric air an d an airstone to p rev en t anoxia w ith in the system during
th e experim ent. At regular intervals, reactors w ere
sam pled for DOC, TN, an d inorganic N. All sam ples
w ere filtered using G eknan A/E (Gelm an "Science,
Arm Arbor, MI, USA) glass liber filters (1.0 p.m).
Concurrently, total sam ple volum e of each bioreac­
to r w as m easured, a n d deoinized w ater w as added
as necessary prior to sam pling to account lor the
277
difference betw een th e total am o u n t of sam ple re ­
m oved during previous sam pling events a n d the
initial volum e of solution in each vessel (that is,
w ater losses from evaporation w ere replenished).
DOC Fractionation
Filty-m illiliter subsam ples of leachate from each re ­
actor (sam pled at days 0 an d 1 0 0 ) w ere pooled and
fractionated into hydrophobic (hum ic) a n d h y d ro ­
philic (nonhum ic) fractions by using analytical scale
colum n chrom atography w ith XAD - 8 Am berlite
resin (Sima-Aldrich, Inc, St. Louis, MO, USA)
[sensu T hurm an (1985)]. W hat w e refer to as the
hum ic fraction is com posed of lulvic an d h um ic
acids an d sorbs to the XAD - 8 resin. The hum ic
fraction w as determ ined by back-eluting th e XAD - 8
resin w ith 0.1 N sodium hydroxide an d m easuring
the DOC concentration of the eluate alter acidifica­
tion to pH 2.0 w ith concentrated phosphoric acid.
The n o n h u m ic fraction is a heterogeneous class of
substances th a t passes th ro u g h th e XAD - 8 resin.
The n o n h u m ic fraction is com posed p redom inantly
of hydrophilic organic acids a n d low m olecular
w eight com pounds, including carbohydrates, carboxylic acids, an d am ino acids (T hurm an 1985).
The n o n h u m ic fraction w as calculated by m easu r­
ing the DOC concentration of the effluent from the
XAD - 8 resin. The su m of the DOC m easured in the
hum ic an d n o n h u m ic fractions w as typically b e ­
tw een 95% an d 105% of DOC in th e original sam ­
ple (that is, before fractionation).
Analytical Methods
Foliar an d litter C a n d N com position w as analyzed
using a Carlo Erba EA 1110 elem ental analyzer (CE
Elantech, Lakewood, NJ, USA) alter sam ples w ere
ground to a line pow der (40 m esh), a n d lor lignin
an d o ther p lan t products by using th e m eth o d of
Van Soest an d W ine (1968). Plant leachate DOC
an d total dissolved nitrogen (TDN) w ere m easured
using a Shim adzu TOC 5050A com bustion analyzer
(Shim adzu, Kyoto, Japan). Inorganic N (NH4 ^/
N O j^) was determ ined colorim etrically on a n Alpkem autoanalyzer (OI Analytical, College Station,
TX, USA). 8 ^^C-DOC was m easured on days 1 and
too of the experim ent. At each tim e point, 250-mL
subsam ples of leachate from each vessel w ere fil­
tered to 0.45 p.m a n d transferred to am ber vials,
acidified to pH 2 by using 0.5 M H 2 SO4 [to liberate
any dissolved inorganic carbon (DIC) from sam ­
ples], an d th e n re tu rn ed to n eu tra l pH (approxi­
m ately 7.0) by using fresh, 1 N sodium hydroxide.
Frozen sam ples w ere placed on a freeze dryer until
reduced to a line pow der. S^^C of freeze-dried DOC
278
C. C. Cleveland an d others
T able 1.
Intiai Foiiage a n d Litter C hem istry
Component (%)
Vegetation Type
Soluble
Ceil Contents
Hec, Bpr,
Cel, Lgn, Re
Hec, Bpr
Cel, Lgn, Re
Cel
Lgn, Re
Hieronyma alchorneoides
H. alchorneoides (litter)
Caryocar costaricense
Abies lasiocarpa
A. lasiocarpa (litter)
Acomastylis rossii
A. rossii (litter)
Deschampsia cespitosa
D. cespitosa (litter)
24.7
31.0
55.1
53.9
36.6
65.4
78.0
28.4
28.4
75.7
69.4
45.3
46.5
63.8
35.0
22.3
72.1
72.0
3.8
4.3
8.1
13.8
14.3
16.2
7.9
35.4
34.3
71.9
65.0
37.2
32.7
49.4
18.8
14.4
36.7
37.7
22.4
20.1
13.7
14.6
20.6
14.5
10.5
28.2
34.5
49.5
44.9
23.5
18.2
28.9
4.4
3.9
8.5
3.2
Hec, kemicellulose; Bpr, hound proteins; Cel, cellulose; Lgn, lignin; a n d Re, recalcitrants.
T able 2.
C arbon (C) a n d Nitrogen (N) Com position of Foliage, Litter, an d Leachate Samples
Solubility
Vegetation Type
%c
%N
C-N
c (M-g/g)
N (fxg/g)
DOC-TDN (DOM)
Hieronyma alchorneoides
H. alchorneoides (litter)
Caryocar costaricense
Abies lasiocarpa
A. lasiocarpa (litter)
Acomastylis rossii
A. rossii (litter)
Deschampsia cespitosa
D. cespitosa (litter)
52.1
51.3
51.4
52.4
52.1
46.4
48.0
45.8
47.8
2.4
1.4
2.0
1.1
0.8
1.8
0.8
1.7
1.1
21.7
36.6
25.7
47.6
65.9
25.8
60.0
26.9
43.5
1612.2 ± 230.4
3139.3 ± 617.5
8353.9 ± 2867.0
3574.1 ± 452.0
1594.3 ± 255.8
29108.6 ± 1299.1
29608.9 ± 1966.9
5137.4 ± 1065.7
7335.9 ± 487.2
815.3 ± 79.2
299.4 ± 29.4
271.6 ± 92.9
56.4 ± 3.7
104.3 ± 13.7
792.0 ± 55.8
498.9 ± 30.5
551.8 ± 84.2
530.0 ± 17.7
2.0 ± 0.1
10.43 ± 1.1
35.6 ± 22.4
63.3 ± 3.9
15.2 ± 0.5
36.8 ± 1.4
59.3 ± 1.9
9.3 ± 1.1
13.8 ± 0.5
Values are m ean ± 1 standard deviation o f tripiieate samples. DOC. dissolved organie earioon; DOM. dissolved organie matter; a n d TDN. total dissolved nitrogen.
w as analyzed using a Carlo Erba EA 1110 elem ental
analyzer coupled to a n Isoprim e m ass spectrom eter
(M icromass International, M anchester, UK). Isoto­
pic results are expressed m standard n o tatio n (S^^C)
m parts p er th o u san d ( % o ) relative to the standard
Pee Dee Belem nite, w h ere S^^C = [(^^sampie''^stan­
dard) —I] X 1000, an d R is the m olar ratio
The average standard deviation based o n analysis of
replicated sam ples w as ± 0.87.
R esults
Litter Chemistry and Solubility
Initial concentrations of th e w ater-extractable total
organic C, N, lignin, a n d C tractions obtained from
foliage an d litter used in the incubations differed
(Tables 1 a n d 2). C-N ratios w ere higher in litter
th a n foliage for ail species. A lthough concentrations
of lignin a n d o th er forest-product C fractions w ere
variable betw een vegetation state (foliage versus
litter), foliage a n d litter from w oody species h ad
higher lignin concentrations th a n herbaceous spe­
cies (Table 2). W e found only a w eak relationship
b etw een DOM solubility an d iignin-N ratios of the
original p lan t m aterial (data n o t show n).
DOC a n d TDN varied w idely b etw een species and
vegetation state (Table 2). Based o n a single extrac­
tion, fluxes of DOC ranged from a low of 0.08% of
dry biom ass (797.17 ± 127.89 p.g C g^^ A. lasiocarpa
litter) to a high of 2.11 % of dry biom ass (21,149.23
± 1404.98 p.g C g ^ \ A. rossii litter). Initial TDN
concentrations ranged from 28.28 ± f .85 p.g C g^^
from A. lasiocarpa litter to 530.04 ± 17.68 p.g g^^ in
D. cespitosa litter.
279
Com position, Dynamics, and Fate of Terrestrial DOM
160
H. alchorneoides
500
C. costaricense
140
400
120
300
100
H. alchorneoides {\\\Xer)
200
60
100
40
20
20
40
60
80
100
A. lasiocarpa
E
O
O
Q
20
40
60
80
100
120
_ 2.5
0
20
40
60
80
100
120
40
1500
80
200
-J
O)
0
120
A. lasiocarpa (litter)
70
__
A. rossii
60
150
1000
50
25
20
40
100
30
0.5
50
CQ
500
20
- 0.5
0
20
40
60
80
100
1500
120
30
25
0
20
40
60
80
100
120
30
300
0
20
60
80
100
400
120
.
D. cespitosa {WXter)
350
250
40
30
25
300
1000
20
200
150
15
200
D. caespitosa
500
20
250
150
100
10
100
50
0
20
40
60
80
100
120
5
50
0
0
20
40
60
80
100
120
20
40
60
80
100
120
Time (days)
Figure 1. Time course of dissolved organic carbon (DOC, ■), total dissolved nitrogen (TDN, A), and total organic nitrogen
(O) decomposition in bioreactors with inoculated leachate. Inorganic N values may be calculated as the difference between
TDN and organic N. Symbols are the mean of triplicate samples, and error bars are ± 1 standard deviation.
Biodegradability of Foliage- and Litterderived DOM
The biodegradable fraction of the DOM was large
for all species (Figure 1). In m ost cases, DOC con­
centrations w ere depleted by m ore th a n 50% in the
first 4 days and by m ore th a n 70% in the first I I
days (Figure 1). By the end of the decom position
experim ent, less th a n 10% of the original DOC
rem ained in the extracts from C. costaricense, A. la­
siocarpa (foliage and litter), A. rossii (foliage and
litter), and D. cespitosa (foliage and litter). Only the
leachate from the tropical species, H. alchorneoides
(foliage), had a final DOC concentration th at was
m ore th a n 10% of the initial DOC concentration.
TDN dynam ics in the reactors closely resem bled C
dynam ics early in the experim ent (Figure I). At the
beginning of the experim ent, TN concentrations d e­
clined initially in all of the reactors (due to m icro­
bial im m obilization of N), b u t concentrations recov­
ered as the experim ent progressed (Figure I).
F urtherm ore, TDN was dom inated by DON (Figure
I) initially, but, as the experim ent progressed, o r­
ganic forms of N becam e depleted and TN was d o m ­
inated by inorganic N species (Figure I).
DOC Chemical Composition
The chem ical com position of the DOC (as d eter­
m ined by the XAD-8 fractionation scheme)
changed m arkedly from the beginning to the end of
the experim ent (Figure 2). Specifically, w ith the
exception of leachate from H. alchorneoides, w hich
was com posed of approxim ately 57% nonh u m ic
m aterial at both day 0 and day 100 of the in cu b a­
tion, there was a relative decrease in the proportion
280
C. C. Cleveland and others
experim ent (Figure 3), there was n o t a consistent
directional change in the isotopic com position of
the DOC following decom position.
100
D is c u s s io n
DOM Solubility
sb
■Si StJ
^ d
Q^
Si
' ^
Figure 2. Humic (solid bars) and nonhumic (dashed bars)
fractions of dissolved organic carbon at days 0 and 100 of
the incubation experiment.
of n o n h u m ic DOC in all leachates over the course of
the experim ent (Figure 2). The largest decreases in
the n o n h u m ic fraction of DOC following decom po­
sition w ere in leachate from the foliage of A. rossii
( - 4 5 % ), A. lasiocarpa ( - 3 7 % ), and C. costaricense
( - 3 1 % ), respectively. However, although the p ro ­
portion of DOC in the hum ic and n o nhum ic frac­
tions changed m arkedly over the course of the ex ­
perim ent, the concentrations of both hum ic and
n o n h u m ic DOC show ed strong declines betw een
days 0 and 100, indicating th at both fractions w ere
readily consum ed in the vessels (Table 3).
DOC Isotopic Composition
The 8^^C of the DOC did n o t change significantly
over the course of the experim ent, regardless of
species (P = 0.41; S tudent's ttest). Initial 8^^C-DOC
values ranged from —3I.3%o for H. alchorneoides
litter extract to —25.6%o in D. cespitosa litter extract,
and final 8^^C-DOC values ranged from -30.6%o
(H. alchorneoides litter) to —25.5%o (D. cespitosa lit­
ter). However, there w ere no significant differences
b etw een the 8^^C of the initially extracted DOC and
the DOC sam pled at the end of the experim ent from
any species. F urtherm ore, although there w ere
changes in the 8^^C of DOC over the course of the
Soluble organic m aterial enters the soil as leachate
from live and decaying aboveground biomass, and
evidence suggests th at these fluxes m ay be large
(Czech and K appen 1997; Kalbitz and others 2000).
Estimates of DOM fluxes via litterfall range from
1% to 19% of total litterfall C flux and from 1% to
5% of n et prim ary productivity (Gosz and others
1973; Neff and Asner 2001). Potential litter solubil­
ity values (obtained in situ and in laboratory ex p er­
im ents) range from 5% to 25% of litter dry mass
and from 5% to 15% of litter C content (McDowell
and Likens 1988; Zsoinay and Steindl 1991). In this
study, w ater-soluble DOM was extracted from foli­
age and litter from tem perate and tropical ecosys­
tems. DOC solubility ranged from a low of 0.08% of
dry biomass (797.17 ± 127.89 p.g C g“ \ A. lasio­
carpa litter) to a high of 2.11% of dry biomass
(21,149.23 ± 1404.98 p.g C g“ \ A. rossii litter) from
a single extraction. In m ost cases, foliage produced
m ore DOM th a n litter of the same species. H ow ­
ever, in all cases, litter-extractable DOM was at least
45% of foliage-extractable DOM, and, for tw o spe­
cies (A. lasiocarpa and D. cespitosa), concentrations of
DOM extracted from litter w ere higher th a n co n ­
centrations extracted from foliage of the same spe­
cies.
The high potentially soluble fraction of senesced
litter has been show n in several studies (Yavitt and
Fahey 1986; M oller and others 1999). The com bi­
n ation of a highly soluble and biodegradable o r­
ganic fraction in litter suggests th at DOM leaching
likely plays an im portant role in the delivery of
labile C and nutrients to surface m icrobial co m m u ­
nities. The losses we observed (that is, up to 2% of
litter C mass in a single leaching event) is a signif­
icant flux of m aterial ou t of the litter layer.
Traditional conceptual and sim ulation m odels of
mass loss in litter decom position have focused on
litter-chem istry control over decom position p ro ­
cesses. In a new m odel th a t links decom position to
the flux of soluble m aterial out of litter, Currie and
Aber (1997) explicitly linked decom position to the
generation of soluble organic m aterial in the litter
layer. O ther m odels of decom position processes also
implicitly or explicitly consider solubility as a co n ­
trol over decom position. In the C entury ecosystem
m odel, the flux of m aterial from litter to soil mi-
Com position, Dynamics, and Fate of Terrestrial DOM
Table 3.
(DOC)
281
Cliemical Com position of Extracted (Initial) and Decom posed (Final) Dissolved Organic Carbon
Initial DOC Composition
(mg/L)
Final DOC Composition
(mg/L)
DOC consumed
(mg/L %)
Vegetation Type
Humic Acid
Nonhumic
Humic Acid
Nonhumic
Humic Acid
Nonhumic
Hieronyma alchorneoides
H. alchorneoides (litter)
Caryocar costaricense
Abies lasiocarpa
A. lasiocarpa (litter)
Acomastylis rossii
A. rossii (litter)
Deschampsia cespitosa
D. cespitosa (litter)
46.8 ± 6.7
88.0 ± 17.6
141.4 ± 49.8
78.3 ± 10.8
32.3 ± 5.2
437.6 ± 22.3
787.7 ± 56.1
151.9 ± 29.6
217.1 ± 20.3
35.5 ± 5.1
67.8 ± 13.5
278.2 ± 98.0
104.6 ± 5.1
46.1 ± 7.4
1021.1 ± 52.0
701.3 ± 49.9
104.7 ± 20.4
160.5 ± 15.0
18.7 ± 1.3
14.7 ± 1.8
21.9 ± 3.4
13.6 ± 0.9
5.7 ± 0.7
12.2 ± 1.9
2.0 ± 0.5
0.5 ± 0.5
19.7 ± 1.9
24.9 ± 0.2
4.5 ± 0.5
10.9 ± 0.5
28.2 (60.1)
73.3 (83.3)
119.5 (84.5)
70.3 (89.7)
31.5 (97.4)
378.5 (86.5)
728.4 (92.5)
142.2 (92.5)
192.0 (88.4)
21.8 (61.6)
62.0 (91.6)
266.1 (95.6)
102.7 (98.1)
98.9 (99.0)
1001.4 (98.1)
676.4 (96.4)
100.2 (95.7)
149.6 (93.2)
8.0 ± 2.1
0.8 ± 0.8
59.1 ± 5.6
59.3 ± 0.5
9.7 ± 1.1
25.1 ± 1.1
Values are m eans ± 1 standard deviation o f duplicate samples.
o
CO
To
<M
5^
9^
I
P
I
y
I§■
I
II
I
II
I
II
I
I
:c;
Figure 3. 8^^C-DOC at day 0 (solid bars) and day 100
(dashed bars) of the incubation experiment. Values are the
mean of duplicate samples, and error bars are ± 1 stan­
dard deviation. None of the within-species differences
were significant at oo = 0.05.
erodes is controlled by the lignin-N ratio, w hich is
related to the hot-w ater-soluble fraction of litter
(Parton and others 1994). This relationship led Neff
and A sner (2001) to explicitly tie the generation of
DOC to the lignin-N ratio of vegetation. However,
in this study, we found only a w eak relationship
b etw een lignin-N and solubility. We found th at
solubility across plant species is m uch m ore variable
th a n lignin-N ratios. In other words, although sol­
ubility is often low in vegetation w ith high lignin-N
ratios, the supposed general relationship betw een
solubility and lignin-N is overw helm ed by the sig­
nificant variation in litter solubility across species.
A lthough our solubility data w ere n o t d eter­
m ined using a m ore com m on ho t-w ater extraction
(Parton and others 1994), we believe th a t our data
m ore realistically represent the flush of DOM into
soil following precipitation events. Accordingly, we
suggest th at the failure of simple relationships such
as lignin-N ratios to predict decom position rates in
a nu m b er of systems (Hobbie 2000; Hobbie and
Vitousek 2000) could be related to the role of sol­
ubility in litter decom position. A lthough litter
chem istry has been a central focus of litter decom ­
position studies, litter solubility has not. The large
variation and high absolute quantities in the soluble
litter fraction across species suggest th at m ore a t­
tention should he paid to the role of physical disso­
lution in controlling litter decom position rates.
DOC/DON Biodegradability
The distinction betw een biodegradable and nonhiodegradahle DOM is im portant in understanding
DOC hiogeochem istry, both conceptually and
m echanistically. The results of our decom position
experim ent suggest that, in the absence of com pet­
ing sinks for DOM, DOM from all species and veg­
etation states is highly biodegradable. A handful of
experim ents have attem pted to partition labile and
recalcitrant DOM fractions of plant leachate by ex ­
am ining biological decom position over a fixed p e ­
riod, and they have typically show n high initial
rates of decom position, followed by a rapid decline
282
C. C. Cleveland an d others
to a lower, constant rate (Zsoinay an d Steindl 1991;
Bossier an d Fontvieille 1993). In throughfaii, 18% 50% of th e DOM is considered biodegradable, and,
in litter leachate, the biodegradable fraction ranges
from 6% to 20% (Qualls an d Haines 1992; Yano
an d others 2000). However, m an y studies of bio­
availability h ave used held collection of DOM to
assess th e decom posability of DOM (Jandi an d Sollins 1997). Com parison of relatively low estim ates
of DOM bioavailability w ith the 70% -9 0 % disap­
pearance of DOM in this experim ent suggests th at
held collections m ay underestim ate the initial bioavaiiabiiity of DOM g enerated in the litter layer due
to rapid decom position following production.
In this experim ent, DOC decom position followed
a negative exponential relationship, b u t decom po­
sition rates did n o t decline signihcantiy u n til the
m ajority of DOC (that is, m ore th a n 70% ) h ad been
consum ed. O ur results are in contrast to studies of
soil DOM decom position th a t show a clear tw o-pool
decom position p attern w ith a signihcant, recalci­
tra n t residual DOM fraction (Zsoinay an d Stiendi
1991). One explanation for this discrepancy is the
absence in o u r experim ent of physical destabiliza­
tio n /desorption m echanism s in soils, w hich m ay
serve to generate a h u x of m ore hum ihed/recaicitra n t DOC to th e soil solution. The high biological
availability of DOC generated in litter breakdow n
m ay also help explain the strong relationships b e ­
tw een w ater-extractable DOC a n d m icrobial activ­
ity hi soils (Jandi an d Soihns 1997). B oth hi surface
soils th a t receive inputs from fitter dissolution and
in deeper soils w here o th er processes of DOC g en ­
eration are involved, it is clear th a t DOC huxes
should be closely tied to overall rates of heterotrophic activity in soils (Yavitt an d Fahey 1986; M c­
Dowell an d Likens 1988; Qualls a n d Haines 1992;
Brooks a n d others 1999).
O ur results are consistent w ith conceptual m o d ­
els of DOM decom position th a t suggest C decom ­
position is tightly coupled w ith N availability. In this
experim ent, DON decom position dynam ics closely
resem bled those of DOC. In all of th e reactors, TN
w as initially dom inated by DON species, an d the
decom position of DOC was clearly fueled by N m in ­
eralized from organic forms. In species w ith Initially
hig h DOC-DON ratios, N was efhciently retained in
m icrobial biom ass over the course of the experi­
m ent, as suggested by consistent decreases in total
"free" N (organic a n d inorganic) in the system
th ro u g h o u t decom position. In DOM w ith relatively
low DOC-DON ratios, TDN in th e system decreased
initially, but, as DOC concentrations decreased,
DON w as m ineralized to inorganic N species, w hich
dom inated TN by the end of the experim ent.
It is also n o tew o rth y th a t the relationship b e ­
tw een DOC an d DON decom position m ay be in flu ­
enced m ore by individual species th a n by the o v er­
all N status of a w hole system . For exam ple, the
alpine species A. rossii a n d D. cespitosa are codom i­
n a n t on m u ch of th e alpine tu n d ra at Niw ot Ridge
(considered a n N-lim ited system ), yet the decom ­
position of DOM from each species is strikingly
different. Specihcally, m icrobial decom position of
leachate from A. rossii (C-N = 60) appears to be
tightly constrained by available N, w hereas th e d e­
com position of D. cespitosa (C-N = 9) leachate ra p ­
idly liberates large quantities of inorganic N. Thus,
soil m icrobial com m unities receiving inputs of
DOM via leachate from A. rossii m ay be N limited,
w hereas m icrobes receiving inputs from D. cespitosa
m ay by C limited, suggesting th a t individual species
exert strong controls on DOM decom position dy­
nam ics (B ow m an an d others 2003).
DOC Chemical Composition
DOC chem istry m ay play an im portant role in d e­
term ining th e balance betw een decom position, sta­
bilization, an d leaching losses of DOM. In o u r study,
tracking the relative abundance of the hum ic an d
n o n h u m ic fractions of DOC during decom position
provided inform ation about h o w the chem ical ch ar­
acter of bulk DOC is influenced by heterotrophic
activity. Past research has show n th a t hydrophobic
acids are (relatively) biologically recalcitrant and
dom inate in soil organic horizons, b u t are effec­
tively rem oved (via chem ical sorption) from so lu ­
tion as w ater m oves vertically th ro u g h m ineral soil
horizons. In contrast, hydrophilic acids are consid­
ered m ore biologically available for decom position,
m ove easily th ro u g h soil prohles, a n d are th e d o m ­
in an t form of DOM deeper in soil prohles [see ref­
erences in Easthouse an d others (f992)].
In the absence of com peting sinks for DOM (for
exam ple, chem ical sorption in soil), w e observed a
relative increase in th e am o u n t of DOC in th e h u ­
m ic fraction over th e course of this experim ent,
suggesting th a t the n o n h u m ic fractions of the DOC
w ere preferentially decom posed by th e m icroorgan­
isms in the incubation. M oran a n d Hodson (1990)
have previously show n th a t h um ic substances su p ­
p o rt signihcantiy less m icrobial grow th th a n do
n o n h u m ic substances extracted from the sam e e n ­
vironm ent. However, M oran an d Hodson (f990)
also suggested that, although m icrobial grow th was
less on hum ic C sources th a n o n n o n h u m ic C
sources, th e hum ic DOC fraction w as readily used
by n atu ra l assemblages of m icroorganism s as an
energy source.
O ur results corroborate the hndings of M oran
Com position, Dynamics, an d Fate of Terrestrial DOM
an d H odson (1990) in th a t w e observed shifts in the
relative abundance of h um ic versus n o n h u m ic sub­
stances over th e course of the experim ent, w ith
n o n h u m ic DOC fraction being utilized to a greater
ex ten t th a n th e hum ic DOC fraction. However, our
results also provide support lo r the idea th a t the
h um ic DOC fraction represents an im portant, p o ­
tentially biodegradable C source to m icroorganism s.
In som e cases, m ore th a n 90% of the hum ic frac­
tion w as biologically consum ed in the reactors.
However, alth o ugh o u r data suggest th a t a large
pro p o rtio n of hum ic substances are potential C
sources for m icroorganism s, previous w ork has
sh o w n th a t th e hydrophobic n a tu re of hum ic sub­
stances leads to greater sorption in soil horizons,
relative to n o n h u m ic (hydrophilic) substances
(Qualls an d Haines 1991). The lack of soil sorption
m echanism s in o u r incubation vessels likely in ­
creased th e ability of the m icrobial com m unity to
access th e h um ic DOC fraction, and, w ith the ab ­
sence of soil sorption as a com peting sink for DOM,
h um ic fractions becam e labile substrates for biolog­
ical decom position. Neff an d A sner (2001) sug­
gested th a t th e com petition b etw een physical an d
biological rem oval m echanism s for DOC in surface
soils is a central feature of th e balance b etw een C
stabilization an d decom position an d a n im p ortant
control on SOM C concentrations. O ur results fu r­
th e r suggest that, in surface m ineral soils, DOC
sorption m ay rem ove otherw ise biodegradable o r­
ganic m atter from solution. This highlights the im ­
portance of com petition b etw een physical an d bio­
logical processes in determ ining the fate of p lan t C
in soils.
DOC Isotopic Composition
A com m only observed p h en o m en o n in soil is a S^^C
en rich m en t of the bulk soil organic m atter w ith
d epth (G arten an d others 2000; Powers and
Schlesinger 2002). W h eth er isotopic enrichm ent
can provide inform ation on th e processes involved
in SOM decom position has n o t b een fully ex ­
plained. Part of th e enrichm ent w ith depth is likely
related to historical changes in th e S^^C content of
th e atm osphere [for exam ple, see Fung (1991)], b u t
th e atm ospheric changes are n o t nearly sufficient to
explain th e am o u n t of isotopic change seen in
m an y soil prohles. Thus, if SOM
en rich m en t is
related to th e degree of decom position as suggested
by several au th o rs (AAgren an d others 1996; Gleixn e r an d others 1999), th e n these isotopic data could
provide useful insight into the relative rates of d e­
com position across ecosystems. M echanistic frac­
tio n atio n during decom position has b een observed
in several studies. For exam ple, Herm an d Chapela
283
(2000) show ed that, w h e n grow n on S^^C-labeled
sucrose, fractionation occurs during sugar uptake
by basidiom ycete fungi, resulting in fungal
enrichm ent. Hogberg an d colleagues (1999) also
dem onstrated th a t m ycorrhizal fungi receiving C
from h o st species are enriched in
relative to
th eir host.
A lthough selective preservation of fungal derived
biom ass (w hich m ay be
enriched) m ay help
explain som e of the enrichm ent effect observed
w ith depth in soil prohles (G arten an d others 2000),
our data suggest th a t isotopic fractionation during
biological decom position of C com pounds is n o t a
signihcant process. However, there are several im ­
p o rta n t differences b etw een experim ents th a t d em ­
onstrate fractionation during decom position and
our results, w hich do not. First, decom position in
our reactor vessels is likely dom inated by bacteria
ra th e r th a n fungi. Second, in contrast to fungal
breakdow n on solid C substrates w h ere a su b stan ­
tial residual rem ains, the highly labile DOC in this
experim ent is nearly fully decom posed, thus red u c­
ing the likelihood of kinetic fractionation (Schimel
1993).
R esearch dem onstrating m icrobial fractionation
is typically conducted using p u re cultures grow n on
relatively p u re C sources (Herm an d C hapela 2000).
However, w e suggest that, w h e n the C source is a
m ixture of C com pounds (that is, a suite of hum ic
an d n o n h u m ic DOM), m icroorganism s do n o t p ref­
erentially utilize isotopicaiiy lighter C com pounds
as has b een suggested elsew here [for exam ple, see
Powers a n d Schlesinger (2002)]. F urtherm ore, al­
th o u g h o u r S^^C data m erely suggest this outcom e,
our DOC chem ical fractionation data m ay explain
w hy preferential decom position of S^^C-depleted
DOM does n o t occur. Specihcally, o u r data an d the
data of others suggest th a t although bo th hum ic
an d n o n h u m ic C fractions are technically available
for m icrobial decom position, the n o n h u m ic (hydro­
philic) fraction is m obile a n d supports high m icro­
bial grow th, w hereas th e hum ic fraction is pro n e to
sorption in the u p p er soil prohle (Kaiser a n d others
2001a). F urtherm ore, the relatively labile, h y d ro ­
philic fraction tends to be enriched in S^^C relative
to the hydrophobic, hum ic fraction (Hood 2001;
Kaiser an d others 2001a). Thus, w e believe th at
although isotopic fractionation via m icrobial d e­
com position m ay be observed using relatively p u re
C sources, soil C consists of a h eterogeneous suite of
organic substrates in w hich th e chem ical character
of th e organic C is m u ch m ore im portant th a n its
isotopic com position for determ ining m icrobial u ti­
lization a n d C uptake.
O ur data support th e suggestion of Kaiser and
284
C. C. Cleveland an d others
others (2001a) th a t variable m obility a n d sorption
of DOC w ith variable isotopic values plays a m ajor
role in producing isotopic en rich m en t w ith soil
depth. In o th er words, the greater m obility of the
isotopicaiiy enriched n o n h u m ic DOC, as opposed
th e greater sorption of the m ore ^^C-depleted h u ­
m ic fraction, should favor the m o v em en t a n d sub­
seq u en t hx atio n of m ore enriched C com pounds at
depth. S^^C en richm ent w ith soil depth is often
greatest in high-clay soils, as w ell as in h u m id tro p ­
ical systems, a n d w e suggest th a t these general
trends are consistent w ith th e hypothesis th a t dif­
ferential sorption of DOC drives m u ch of th e ob­
served patterns. Soils w ith higher clay contents
have a h ig h er sorption potential for DOC as com ­
p ared to coarser tex tu red soils (Neff an d Asner
2 0 0 f); thus, high-clay soils should m agnify a sorp­
tion-driven change in S^^C w ith depth. Similarly,
m an y tropical soils have extrem ely high clay con­
tents. These clay-rich soils, coupled w ith high p ro ­
ductivity an d h igh soil w a te r fluxes due to a b u n ­
d an t rainfall, should com bine to produce
exceptionally large changes in S^^C w ith depth, an d
indeed th e largest isotopic changes are seen in tro p ­
ical soil prohles (G arten an d others 2000). Finally,
w h ere isotopic fractionation during decom position
does occur, th e high rates of decay seen in tropical
systems will only fu rth er m agnify the p attern set up
by differential sorption of DOC.
ACKNOWLEDGEMENTS
W e th an k S. Hobbie for perform ing th e p lan t lignin
analyses, an d B. B ow m an for helping locate held
sites a n d collect sam ples on Niw ot Ridge. W e also
th an k H. a n d M. M ichaud of the Drake Bay W ilder­
ness Camp for providing held access an d logistical
support in Costa Rica, a n d th e O rganization for
Tropical Studies (OTS) an d th e M inisterio de Ambiente y Energia (MINAE) in Costa Rica for assisting
w ith research logistics. B riana C onstance helped
w ith all phases of the lab w ork, an d Valerie M orris
of th e INSTAAR Stable Isotope Lab assisted w ith the
carbon isotopic analyses. Thanks to Paul Brooks,
Jo n Carrasco, an d tw o anonym ous review ers for
providing helpful com m ents on early versions of
this article. This w ork w as supported th ro u g h a
grant from th e A ndrew W. M ellon F oundation
(A.R.T. an d J.C.N.) a n d NSF G rant DEB-0089447
(A.R.T.)
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