Effects of Soil Texture on Belowground C (1)

Effects of Soil Texture on Below ground Carbon and Nutrient Storage in a Low land Amazonian Forest Ecosystem

3 1 Wh en d ee L. Silver, 4 * Jason Neff, Megan McGrod d y, Ed Veld kam p ,

2 Mich ael Keller, 5 an d Raim u n d o Cosm e

1 Dep artm en t of En viron m en tal S cien ces, Policy, an d Man agem en t, Un iversity of Californ ia, 151 Hilgard Hall, Berk eley,

Californ ia 94720, US A ; 2 T h e In tern ation al In stitu te of Trop ical Forestry, US DA Forest S ervice, Call Box 25000 Rio Pied ras, Pu erto Rico 00928, US A ; 3 Dep artm en t of Biological S cien ces, S tan ford Un iversity, S tan ford , Californ ia 94305-5020, US A ;

4 In stitu t fu er Bod en k u n d e u n d Wald ern aeh ru n g, Un iversitaet Goettin gen , Bu esgen w eg 237077 Goettin gen , Germ an y; 5 EMBRA PA A m az oˆn ia Orien tal, S an tare´m , Para´, Braz il

A BSTRACT

Soil textu re plays a key role in below grou n d C en zym e activity. Th e Cen tu ry m odel w as able to storage in forest ecosystem s an d stron gly in flu en ces

predict th e observed tren ds in su rface soil C an d N in n u trien t availability an d reten tion , particu larly in

loam s an d san ds bu t u n derestim ated C an d N pools

h igh ly w eath ered soils. We u sed field data an d th e in th e san ds by approxim ately 45% . Th e m odel Cen tu ry ecosystem m odel to explore th e role of soil

predicted th at total below grou n d C (0–20 cm depth ) textu re in below grou n d C storage, n u trien t pool

in san ds w ou ld be approxim ately h alf th at of th e sizes, an d N flu xes in h igh ly w eath ered soils in an

clays, in con trast to th e 89% w e m easu red. Th is Am azon ian forest ecosystem . Ou r field resu lts

discrepan cy is likely to be du e to an u n derestim a- sh ow ed th at san dy soils stored approxim ately 113 Mg C h a -1

tion of th e role of below grou n d C allocation w ith to a 1-m depth versu s 101 Mg C h a in

low litter qu ality in san ds, as w ell as an overestim a- clay soils. Coarse root C represen ted a large an d

tion of th e role of ph ysical C protection by clays in sign ifican t ecosystem C pool, am ou n tin g to 62%

th is ecosystem . Ch an ges in P an d w ater availability an d 48% of th e su rface soil C pool on san ds an d

h ad little effect on m odel ou tpu ts, w h ereas addin g N clays, respectively, an d 34% an d 22% of th e soil C

greatly in creased soil organ ic m atter pools an d pool on san ds an d clays to 1-m depth . Th e qu an tity

produ ctivity, illu stratin g th e n eed for fu rth er in tegra- of labile soil P, th e soil C:N ratio, an d live an d dead

tion of m odel stru ctu re an d tropical forest biogeo- fin e root biom ass in th e 0–10-cm soil depth de-

ch em ical cyclin g.

creased alon g a gradien t from san ds to clays, wh ereas th e opposite tren d w as observed for total P, m in eral

Ke y w o rd s: roots; soil carbon ; cen tu ry m odel; soil N, poten tial N m in eralization , an d den itrification

textu re; biogeoch em istry; tropics.

I NTRODUCTION

ecosystem s by affectin g th e ability of soils to retain

C, w ater, an d n u trien t ion s (Jen n y 1980). For th ese Soil textu re exerts a stron g in flu en ce on m an y

reason s, soil textu re is also a key param eter in

h ydrologic an d biogeoch em ical processes in forest m odels of terrestrial biogeoch em istry, w h ich gen er- ally sh ow th at soil organ ic m atter (SOM) in creases

Received 3 March 1999; accepted 27 Au gu st 1999.

lin early w ith clay con ten t at region al an d global

*Corresponding author; e-mail: w silver@n atu re.b erk eley.ed u

scales (Parton an d oth ers 1993; Sch im el an d oth ers

W. L. Silver an d oth ers

1994). Despite th e w ell-dem on strated im portan ce processes varied in relation to soil textu re w h ile of soil textu re, m an y qu estion s rem ain abou t th e

h oldin g lan dscape position , clim ate, an d cover type role of textu re in below grou n d C allocation , soil C

relatively con stan t. Ou r site occu rred at th e con tact storage, ion exch an ge capacity, an d ecosystem scale

betw een a relic deposition al su rface an d an u plan d processes, su ch as prim ary produ ctivity an d decom -

lan dscape, all of w h ich is n ow u plan d, Tierra Firm e position . Th is is especially tru e for low lan d tropical

forest w ith little or n o topograph ic variation . forests, w h ich exh ibit con siderable spatial h eteroge-

We w ere also in terested in u sin g a m odelin g n eity in soil textu re at both local an d region al scales

approach to test ou r u n derstan din g of th e processes (Cu evas an d Medin a 1986, 1988; Matson an d Vi-

con tribu tin g to pattern s in below grou n d C an d tou sek 1987; Moraes an d oth ers 1995).

n u trien t pools in tropical forests. Soil textu re is Th e in teraction s of soil textu re an d biogeoch em i-

often u sed as a prim ary param eter con trollin g SOM cal cyclin g are com plex. Clay soils can facilitate th e

stabilization in biogeoch em ical m odels. To exam in e form ation of passive C pools w ith slow tu rn over

th e role of textu re in greater detail, an d to test tim es du e to th e ph ysical protection of SOM by clay

m ech an ism s by wh ich textu re in flu en ces th e biogeo- m in erals (Ch risten sen 1992). Clay soils also ten d to

ch em istry of a m oist low lan d tropical forest, w e

h ave h igh er cation exch an ge capacity, n et prim ary u sed th e Cen tu ry biogeoch em istry m odel (Parton produ ctivity (NPP), an d litter decom position rates in

an d oth ers 1987) to sim u late th e forest on th e th e tropics u n der n atu ral con dition s (Ueh ara 1995).

differen t soil textu ral classes at ou r site. Th e Cen tu ry San dy soils are often associated w ith h igh fin e root

m odel origin ally w as developed for u se in tem perate biom ass in tropical forests du e to greater C alloca-

grasslan ds an d agricu ltu ral system s bu t h as sin ce tion to roots for n u trien t an d w ater captu re (Klin ge

been exten ded an d re-param eterized for u se in 1973b, 1975; Cu evas an d Medin a 1988). San dy soils

tem perate an d tropical forests (San ford an d oth ers m ay also h ave slow er litter tu rn over rates du e to

1991; Com in s an d McMu rtrie 1993; Vitou sek an d n u trien t an d w ater lim itation s on decom position

oth ers 1994, Raich an d oth ers 1997), as w ell as for (Cu evas an d Medin a 1986).

an alyses of global biogeoch em ical dyn am ics (Sch im el Soil textu ral properties vary in respon se to w eath -

an d oth ers 1997). Wh ereas th e applicability of erin g rates an d th e in itial m in eralogy of th e paren t

Cen tu ry h as been tested exten sively for tem perate m aterial or th e deposition of m aterial from stream s,

ecosystem s, less testin g of th e m odel h as been don e ru n off, an d erosion . In gen eral th erefore, soil tex-

for tropical ecosystem s (bu t see Vitou sek an d oth ers tu re ten ds to ch an ge at local scales alon g topo-

1994; Gijsm an an d oth ers 1996; Raich an d oth ers graph ic gradien ts an d at lan dscape or region al scales

1997). Ou r m otivation for u sin g Cen tu ry is tw ofold. associated with ch an ges in paren t m aterial or weath -

First, w e exam in e th e ability of th e m odel to captu re erin g rates. Th e n atu re of soil-form in g processes

th e tren ds w e observe alon g ou r soil textu re gradi- th at lead to spatial h eterogen eity in soil textu re are

en t. Th is exercise is h elpfu l both becau se it allow s u s also gen erally associated w ith differen ces in m icro-

to evalu ate ou r u n derstan din g of textu ral con trols an d m acroclim ate, vegetation , an d h ydrologic in -

on tropical forest biogeoch em istry, an d becau se it pu ts an d exports m akin g it difficu lt to con trol for

represen ts a validation test for a key param eter textu re w h ile h oldin g several oth er en viron m en tal

in flu en cin g th e beh avior of th is m odel. Secon d, w e variables con stan t.

exam in e th e poten tial role of feedbacks am on g In th is stu dy, w e u se both an em pirical an d a

textu re, n u trien ts, an d w ater in th e m odel by m odelin g approach to exam in e th e relation sh ip of

exam in in g m odel sen sitivity to ch an ges in N, P, an d soil textu re to pattern s in below grou n d C an d

w ater availability.

n u trien t pools an d N tran sform ation rates in a low lan d Am azon ian forest. Th e Am azon basin is a geologically old region th at is diverse w ith regards to

M ETHODS

soil textu re (Fu rch an d Klin ge 1978; Moraes an d oth ers 1995). Pattern s in biogeoch em ical an d eco-

Site Descrip tion

logical processes w ith soil textu re h ave been de- Th e stu dy w as con du cted in th e Tapajos Nation al scribed alon g topograph ic gradien ts in Am azon ian

Forest (TNF) located 50 km sou th of San tarem , Para, forests (Cu evas an d Medin a 1986, 1988; Matson

Brazil. Th e region h as a m ean an n u al tem peratu re an d Vitou sek 1987; Livin gston an d oth ers 1988;

C, an d receives approxim ately 2000 m m of Vitou sek an d Matson 1988; Medin a an d Cu evas

of 25 o

rain per year w ith a dry season lastin g from May 1989), an d larger, region al-scale gradien ts (McKan e

th rou gh October (Parrotta an d oth ers 1995). Th e an d oth ers 1995; Moraes an d oth ers 1995). In th is

1000-h a stu dy site w as located on an old, n early flat, stu dy, w e w ere in terested in h ow below grou n d

erosion al rem n an t plateau (plan alto) w ith a very

Soil Textu re Effects on Below grou n d C in a Tropical Forest

lim ited drain age n etw ork form ed on sedim en ts of th e Barreiras form ation . Th e u n derlyin g bedrock geology con sists of slan ted beds of sedim en tary rocks deposited as a flu vial-lacu strin e sequ en ce. Th e soil textu ral differen ces at th is site m ost likely reflect th e u n derlyin g sedim en tary bedrock th at con sists of altern atin g san dy an d clayey beds.

Soil textu ral classification of th e larger stu dy area w as determ in ed by field in spection of sam ples from 551 con trol poin ts spaced at 50-m in tervals in a regu lar pattern th rou gh ou t th e 1000-h a area (5 x 2 km ). In total, tran sects covered approxim ately 28 km . A lim ited n u m ber of poin ts w as skipped be- cau se of th e difficu lty of access in clu din g a sm all area of poorly drain ed soil. Soils w ere divided in to tw o categories: (a) th ose h avin g clay an d clay loam textu re (fou n d at 375 poin ts); an d (b) th e rem ain der

h avin g coarser textu res (m ain ly san d an d san dy loam fou n d at 176 poin ts). Assu m in g th e regu lar tran sect pattern is represen tative of th e area, approxi- m ately 68% of th e area su rveyed is on clays or clay loam s (u ltisols an d oxisols) an d 32% on san ds an d san dy loam s (u ltisols).

Field Sam p lin g We sam pled su rface soils (0–10 cm depth ), forest

floor, an d fin e root biom ass alon g six 10 x 60–m tran sects located perpen dicu larly to a 400-m gradi- en t from san d to clay soils. Tran sects w ere located at

Figu re 1. Soil textu re m ap of th e stu dy site in th e Tapajos

0 an d 50 m (san ds), 150 an d 200 m (san dy loam to Nation al Forest, Para, Brazil. Th e m ap w as gen erated from clay loam ), an d 350 an d 400 m (clays) alon g th e textu re sam ples collected every 50 m alon g 0.5-km tran sects th rou gh th e research area. Th e textu re tran sect

textu ral gradien t (Figu re 1). Addition al sam ples of w as located at th e con tact betw een san ds an d clays in forest floor an d fin e root biom ass w ere taken on

block 6. Addition al sam ples w ere collected in h igh clay san ds an d clays to facilitate com parison of th ese tw o

an d h igh san d soils.

textu ral extrem es (see below ). To estim ate soil

ch em ical an d ph ysical properties an d root C, N, an d P to 1 m depth , w e took sam ples from five large 3 x

w ith h eat an d H 2 O 2 to rem ove organ ic m atter an d

1 x 1–m qu an titative soil pits ran dom ly located at 0, w ith NaHMP as a dispersal agen t to m in im ize

50, 200, 350, an d 400 m alon g th e textu re gradien t foam in g. Soil pH w as determ in ed on fresh soils in a an d 18 addition al 1 x 1 x 1–m soil pits w ith n in e

slu rry of 4 g in 8 m L deion ized H 2 O. each on san d an d clay soils.

We m easu red den itrification en zym e activity (DEA) accordin g to Tiedje (1994) w ith som e m in or

Su rface Soil Ph ysical an d Ch em ical

ch an ges becau se of th e h igh activity m easu red. A Prop erties alon g th e Tran sects

10-g sam ple of field m oist soil w as placed in a Soils w ere sam pled from th e 0–10-cm depth by

250-m L Erlen m eyer flask an d 25 m L of a solu tion u sin g a 2.5-cm diam eter soil corer. We took m u ltiple

con tain in g 1 m M glu cose, 1 m M NaNO 3 an d 1 g L-1 core sam ples at each of five ran dom ly selected

ch loram ph en icol w as added. Th e flasks th en w ere poin ts alon g each tran sect yieldin g 30 aggregate soil

evacu ated an d flu sh ed six tim es w ith He to produ ce sam ples. Sam ples w ere refrigerated im m ediately

an an aerobic en viron m en t. Acetylen e (m ade from after collection an d u n til an alyses cou ld be con -

calciu m carbide) w as added to a fin al con cen tration du cted (approxim ately 3–7 days). Roots an d litter

of 10 kPa. Th e flasks w ere in cu bated du rin g a w ere m an u ally rem oved from field m oist sam ples,

30-m in an aerobic assay on a rotary sh aker (100 w h ich th en w ere passed th rou gh a 2-m m sieve. Soil

rpm ). Head space gas w as sam pled after 10, 20, 30, textu re was determ in ed u sin g a Bou you cos h ydrom -

an d 40 m in u tes an d an alyzed for N 2 O on a gas

ch rom atograph w ith electron captu re detector (Gee an d Bau der 1986). Sam ples w ere pretreated

eter in a soil su spen sion of 50 g of soil in 1 L of H 2 O

(Keller an d Rein ers 1994). Th e DEA w as calcu lated

W. L. Silver an d oth ers

from th e lin ear in crease in N 2 O takin g th e dissolved

Fin e Root Biom ass

N 2 O in to accou n t by u sin g th e Bu n sen relation sh ip. Fin e root stan din g stocks (0–10 cm depth ) w ere Poten tial n et N m in eralization an d n et n itrifica-

sam pled from five ran dom location s alon g each tion were m easu red u sin g aerobic laboratory in cu ba-

tran sect (n 5 30) by u sin g a root corer of 4.1 cm tion s (Hart an d oth ers 1994). Tw o 15-g su bsam ples

in side diam eter (Vogt an d Perrson 1991; Silver an d of field m oist soils w ere w eigh ed in to plastic Nal-

Vogt 1993). Cores w ere refrigerated u n til th ey w ere gen e bottles. On e set of su bsam ples w as im m edi-

processed by sortin g live an d dead roots by size class ately extracted w ith 100 m L of 2 M KCl; th e oth er

(less th an or equ al to 2 m m diam eter; greater th an set w as covered w ith perforated parafilm an d in cu -

2–5 m m diam eter) from w ash ed sieves. Th is tech - bated at am bien t tem peratu res (approxim ately 25 o C) n iqu e yielded few sam ples in th e greater th an for 7 days before extraction . Net n itrification an d N 2–5-m m size class, so th ese data are n ot reported m in eralization w ere an alyzed an d calcu lated accord-

h ere. In stead, w e u se data from th e large qu an tita- in g to Hart an d oth ers (1994). tive pits described below to estim ate th e coarse root Sam ples for oth er ch em ical an alyses w ere air-

dried an d grou n d to pass th rou gh a 2-m m sieve. fraction . Root sam ples w ere dried at 65

C an d Approxim ately 5-g sam ples w ere extracted sepa-

w eigh ed to determ in e m ass. We took 10 addition al rately w ith 55 m L 1 M NH

4 Cl for exch an geable

ran dom ly located sam ples for fin e root stan din g

cation con cen tration s an d w ith 50 m L of NH

F for

stocks (less th an or equ al to 2 m m diam eter) on

extractable P (Olsen an d Som m ers 1982) by u sin g a both san d an d clay soils (n 5 20). To ch aracterize vertical vacu u m extractor (Joh n son an d oth ers

pattern s in fin e root stan din g stocks by depth , w e 1991). Sam ples w ere an alyzed for exch an geable Ca,

took th ree root cores from both th e 20–30-cm an d Mg, K, Al, Mn , Fe, an d P on a Direct Cu rren t Plasm a

th e 30–40-cm depth s each on san d an d clay soils Spectraspan V spectroph otom eter, Fu llerton , CA

(n 5 12). All cores w ere processed as above. Root C USA at th e In tern ation al In stitu te of Tropical For-

an d N w ere m easu red on a C.E. In stru m en ts CN estry (IITF). For total C an d N, sam ples w ere

an alyzer at U.C. Berkeley, an d root P w as m easu red regrou n d to a fin e pow der an d an alyzed u pon

at IITF after a H 2 O 2 -H 2 SO 4 digest (Parkin son an d com bu stion by u sin g a Fison s CN an alyzer, Su ffolk,

Allen 1975).

UK. Stan dard referen ce m aterial, procedu ral blan ks, an d replicate sam ples w ere ru n for qu ality assess-

Coarse Roots

m en t at th e rate of 1 per 10 sam ples. Su bsam ples of all soils w ere dried at 105 o

A total of 23 large qu an titative pits w ere excavated

for coarse root biom ass. We sam pled th e five pits con ten t. All data reported h ere are on an oven dry

C to determ in e m oistu re

located at 0, 50, 200, 350, an d 400 m alon g th e soil soil basis. Bu lk den sity of th e 0–10-cm depth w as

textu re gradien t, as w ell as 18 1 x 1 x 1–m pits w ith determ in ed at th ree ran dom poin ts on each of th e

n in e each on san d an d clay soils. Each pit w as six tran sects alon g th e gradien t (n 5 18) by u sin g a

sam pled for root biom ass at th e 0–10-cm , 10–40- kn ow n volu m e bu lk den sity corer. Sam ples w ere

cm , an d 40–100-cm depth s. In th e 0–10-cm depth , dried at 105 o

C to a con stan t w eigh t an d w eigh ed for w e separately w eigh ed th e m ass of fin e roots th at bu lk den sity determ in ation s.

cou ld be easily separated from soil in th e field. Th is tech n iqu e u n dou btedly exclu ded a proportion of

Forest Floor Mass an d Elem en tal Con ten t th e very fin e (less th an 1 m m ) roots an d root Th e forest floor w as sam pled at five ran dom loca-

fragm en ts, w h ich are better sam pled u sin g th e tion s alon g each tran sect (n 5 30), an d at 10

corin g tech n iqu e m en tion ed above. At th e tw o addition al ran dom location s each on clay an d san dy

deeper depth s, fin e root biom ass w as low, so w e soils by u sin g a 15 x 15–cm tem plate (in side area).

took sam ples of total root biom ass com bin in g all size Th e forest floor con tain ed recen t litter an d h u m ified

classes togeth er. Roots w ere clean ed of adh erin g soil organ ic m atter. Sam ples were dried at 65 o

an d w eigh ed in th e field by u sin g a sprin g balan ce to determ in e m ass, grou n d in a Wiley m ill, an d

C, weigh ed

(650 g), an d su bsam pled for m oistu re determ in a- redried. Grou n d sam ples w ere predigested in H 2 O 2 tion s. Sam ples w ere dried at 65 o

C an d w eigh ed to

determ in e m ass, an d to estim ate a con version factor block digestor (Lu h Hu an g an d Sch u lte 1985) an d

an d th en digested in con cen trated HNO 3 by u sin g a

from field w et w eigh t to oven dry w eigh t. Grou n d an alyzed at Boston Un iversity for Ca, Mg, K, Al, P,

su bsam ples w ere predigested in H 2 O 2 an d digested Fe, an d Mn at IITF an d for total C an d N. Stan dard

in H 2 SO 4 for total P an d an alyzed on a DCP at IITF referen ce m aterial (citru s an d apple leaves), proce-

(Parkin son an d Allen 1975). Addition al su bsam ples du ral blan ks, an d replicate sam ples w ere ru n for

w ere an alyzed for C an d N on a CN an alyzer at qu ality assessm en t at th e rate of 1 per 10 sam ples.

Stan ford Un iversity.

Soil Textu re Effects on Below grou n d C in a Tropical Forest

Soil Ch em ical an d Ph ysical Prop erties su m m ed by depth to produ ce pool estim ates for to 1 m Dep th

specific depth in tervals. We h ad n o bu lk den sity

Soil textu ral properties, bu lk den sity, soil d 15 N, an d

estim ates for su bsu rface loam soils, an d so th ese soils w ere exclu ded from th is an alysis.

soil cation , P, C, an d N con ten t w ere determ in ed in Soils w ere sam pled for d 15 N from vertical profiles 1-m deep qu an titative soil pits located alon g th e soil at 0 m (san d), 200 m (loam ), an d 350 m (clay) alon g textu re gradien t an d in addition al pits located on th e gradien t. We took on e sam ple from th e su rface san ds an d clays. An aggregate soil sam ple w as to 2-cm , 2–5 cm , 5–10 cm depth s an d su bsequ en t collected at th e 0–10-cm , 30–40-cm , an d 90– 10-cm -depth in crem en ts to 1 m in depth . Addi- 100-cm depth s from pits located at 0, 50, 200, 350, tion al sam ples (n 5 4–6) w ere taken from th e an d 400 m alon g th e gradien t (n 5 15) for textu ral su rface an d 90–100-cm depth s in th e san ds (0 m )

properties, exch an geable cation con cen tration s, ex- an d clays (350 m ). Soils w ere air-dried im m ediately tractable an d total P, an d pH. Soil pH w as deter-

C to a con stan t m in ed in w ater as above, an d in a slu rry of 4 g of soil

after field sam plin g an d again at 60 o

w eigh t. Soils th en w ere grou n d to a pow der an d in 8 m L of 1 M KCl. Soil textu re, exch an geable

an alyzed for d 15 N at th e Natu ral Resou rce Ecology cation s, an d extractable P w ere m easu red as above.

Lab at Colorado State Un iversity on a VG Isoch rom To estim ate total P con cen tration s, w e digested

Mass Spectrom eter, Fran klin , MA, USA w ith a

approxim ately 5 g of soil in H 2 O 2 an d con cen trated

dedicated Carlo Erba sam ple preparation system .

H 2 SO 4 by u sin g a block digestor (Parkin son an d Th e 15 N abu n dan ce is expressed in delta u n its Allen 1975). Solu tion s w ere an alyzed at IITF. Stan -

relative to th e 15 N/ 14 N ratio (R) of atm osph eric N dard referen ce soils, replicate sam ples, an d blan ks

( 15 N air/ 14 N air 5 0.0036765). Th e d 15 N is calcu lated w ere u sed for qu ality con trol.

as d 15 N 5 [(R sam ple /R stan dard )-1] x 1000]. Total C an d N w ere estim ated from su bsam ples

collected at 10-cm -depth in crem en ts (0–100 cm )

Mod el Ap p lication

from th e pits at 0 m (san d), 200 m (loam ), an d 350 We u sed th e Cen tu ry biogeoch em istry m odel (Par- m (clay) alon g th e textu re gradien t. We also took ton an d oth ers 1987) for ou r m odelin g exercise. sam ples from six addition al pits each on clay an d Cen tu ry h as a th ree-com partm en t SOM m odel con - san d soils at 7 cm , 40 cm , an d 100 cm depth . We fit a sistin g of active, slow an d passive pools of organ ic logarith m ic cu rve to th ese valu es an d in terpolated m atter. Tem peratu re, m oistu re, an d n u trien t con - th e soil C an d N con cen tration s for th e in term ediate strain ts con trol th e m ovem en t of C from plan t pools 10-cm -depth in crem en ts. Th e m ean squ ared residu - to SOM pools, as w ell as betw een SOM pools. In als for fitted C cu rves w ere 0.98 (60.01) for clays

addition , th e m ovem en t of C from th e slow to an d 0.94 (60.03) for th e san ds, an d for N th ey w ere

passive organ ic m atter pools in creases proportion -

0.95 (60.02) for both san ds an d clays. All sam ples ally w ith clay con ten t. We in itialized Cen tu ry by w ere an alyzed on a CN an alyzer at U.C. Berkeley

u sin g param eters from tropical forest sim u lation s an d in clu ded replicate sam ples an d referen ce soils.

don e by Vitou sek an d oth ers (1998) an d Raich an d We in terpolated total an d extractable P con cen tra-

oth ers (1997) w ith rain fall an d tem peratu re data tion s by depth by u sin g th e valu es from th e 0–10-

from th e TNF. Th e tropical forest param eterization s cm , 30–40-cm , an d 90–100-cm sam ples from pits

differ from th e origin al grasslan d an d forest m odels alon g th e textu re gradien t. Total P did n ot vary

in th e stoch iom etry of C:N ratios an d th e relatively sign ifican tly w ith depth in th e clays, so w e u sed a

h igh er N in pu ts. On e of th e m ain biogeoch em ical m ean valu e for all depth s. Total P follow ed a

differen ces between tem perate system param eteriza- decreasin g pattern w ith depth in th e san ds (r 2 5 tion s an d th e TNF is th e lack of paren t m aterial P in

0.84 6 0.01). Total P data reported h ere represen t a Brazil. For th ese ru n s, w e in itialized th e m odel w ith sm all sam ple size (n 5 4) an d th u s sh ou ld be view ed

n o paren t m aterial P an d h igh P sorption capacity to w ith cau tion .

sim u late th e very low P con dition s of th is low lan d Bu lk den sity by depth w as sam pled on san ds

tropical forest.

(50 m ) an d clays (350 m ) at 10-cm -depth in cre- Ou r m ain objective w as to evalu ate th e sen sitivity m en ts by u sin g a kn ow volu m e bu lk den sity corer.

of th e Cen tu ry m odel to soil textu re an d to com pare Roots an d rocks w ere rem oved, an d th e sam ples

th e m odel sim u lated textu ral effects w ith pools of C w ere dried at 105 o

an d elem en ts th at w e cou ld m easu re at ou r site. To w eigh ed to determ in e m ass. To estim ate th e m ass of

C to a con stan t w eigh t an d

perform th is sen sitivity stu dy, w e ‘‘tu n ed’’ th e m odel soil C, N, an d P to a depth of 1 m , w e m u ltiplied th e

to ach ieve good correspon den ce betw een m odeled con cen tration s by depth w ith th e bu lk den sity

an d m easu red SOM carbon for ou r clay site. Th ere estim ation s in san d an d clay soils. Valu es w ere

are a n u m ber of w ays to adju st Cen tu ry, bu t on e of

W. L. Silver an d oth ers

Table 1. Exch an geable Cation s, Extractable P, Total C an d N, an d Soil Ph ysical Properties a in th e Tapajos Nation al Forest, Para, Brazil

M eters alon g th e Textu re Grad ien t

Soil Prop erty

400 Bu lk den sity (g cm 23 )

1.23 (0.09) ac 1.02 (0.02) c San d (% )

37 (1.0) c 38 (1.1) c Clay (% )

60 (1.1) c 60 (1.3) c Silt (% )

3 (0.5) 2 (0.5) pH (H 2 O)

4.26 (0.18) bc 4.19 (0.13) c Total C (% )

2.66 (0.11) 2.26 (0.06) Total N (% )

0.18 (0.01) 0.17 (0.01) Extractable P (µ g g 21 )

3.96 (0.25) c 3.01 (0.18) d Ca (cm ol 1 kg 21 )

0.07 (0.01) 0.06 (0.01) Mg (cm ol 1 kg 21 )

0.11 (0.01) b 0.08 (0.01) a Mn (µ g g 21 )

1.72 (0.20) b 1.74 (0.29) b Al (cm ol 1 kg 21 )

3.25 (0.13) d 2.56 (0.25) e

a 0–10 cm depth. Values are means 6 1 SE (in parentheses). Rows with different lowercase letters identify statistically significant differences among sites at the 95% level. Magnesium

concentrations were below detection (,) by the analytical instrumentation (0.013 cmol 1 kg 21 ).

th e m ost sen sitive aspects of th e m odel is th e stron g in terpolated NH 4 F-P an d total soil P con cen tration s poten tial N lim itation to prim ary produ ctivity (Vi-

(see above). For th e loam soil, w e u sed th e 0–10-cm tou sek an d oth ers 1998). By adju stin g N in pu ts, w e

bu lk den sity m easu rem en t becau se th ere w ere n o cou ld alter th e steady state con ten t of SOM for th e

data for th e deeper soil depth s. On ly clay an d san dy clay soil to ach ieve a good fit to ou r observed data.

soils w ere m easu red for root C, N, an d P. Coarse root Th e equ ilibriu m valu es of N addition s requ ired to

data w ere collected from th e 0–10-cm , 10–40-cm , obtain reason able SOM C an d N valu es for th e clay

an d 40–100-cm depth s. We estim ated th e m ass of site w ere addition s of 2.2 kg N h a -1 y -1 . Wh en th e

coarse root C, N, an d P for th e 0–20-cm depth by m odel in pu ts w ere set for th e clay site, w e altered

addin g on e-th ird of th e valu es for th e 10–40-cm on ly th e bu lk den sity an d textu re to reflect th e

depth to th e valu es for th e 0–10-cm depth . Becau se con dition s in th e loam an d san dy soils.

root biom ass ten ded to decrease w ith depth , th is is On ce Cen tu ry w as param eterized for th e clay site,

likely to be a sligh t u n derestim ation of th e pool sizes w e th en com pared th e equ ilibriu m soil an d root C,

of root C, N, an d P.

N, an d P con ten ts from th e m odel to m easu red valu es for san d an d loam soils. To system atically

Statistical An alyses

explore th e m odels sim u lated lim itation s to plan t Statistical an alyses w ere perform ed u sin g Systat grow th , an d con cu rren tly exam in e th e sen sitivity of

(Wilkin son 1990). Data w ere log tran sform ed w h en Cen tu ry to w ater an d n u trien t availability in th e

n ecessary to m eet th e assu m ption s for an alysis of san ds, w e ran experim en ts w h ere w e dou bled N

varian ce (ANOVA). On e-w ay an d tw o-w ay ANO- in pu ts (from 2.2 to 4.4 kg h a -1 y -1 ), P in pu ts (from

VAs w ere u sed to determ in e sign ifican t differen ces

1.1 to 2.2 kg h a -1 y -1 ), an d rootin g depth (60–120 cm w ith in variables alon g th e gradien t, an d w ith tex- depth ). Becau se w e cou ld n ot adju st w ater u se by

tu re an d depth . Pairw ise com parison s u sin g th e vegetation in th e m odel, w e u sed rootin g depth as a

Least Sign ifican t Differen ces protocol w ere per- rou gh su rrogate for w ater access an d u se, becau se

form ed to determ in e w h ere sign ifican t differen ces rootin g depth h as been sh ow n recen tly to correlate

occu rred. Pearson correlation s w ere u sed to exam - w ith ecosystem -level w ater u se in season ally dry

in e relation sh ips am on g soil textu re an d soil an d tropical forests (Nepstad an d oth ers 1994).

root properties. We also u sed sim ple an d stepw ise Th e Cen tu ry m odel ou tpu t u ses a fixed depth of

m u ltiple lin ear regression s to iden tify tren ds w ith 0–20 cm . To com pare th e Cen tu ry m odel ou tpu t

soil textu re. We perform ed t-tests to exam in e differ- w ith th e field data, w e u sed ou r m easu red bu lk

en ces in soil, root, an d forest floor C, N, an d P den sity, C an d N valu es, an d th e m easu red an d

betw een san d an d clay. Residu als from all an alyses

Soil Textu re Effects on Below grou n d C in a Tropical Forest

w ere ch ecked for n orm ality an d h om ogen eity of Table 2. Correlation Coefficien ts of th e varian ces (Steel an d Torrie 1980). We report sign ifi-

Relation sh ip of Soil Ch em ical an d Ph ysical can t differen ces at th e 95% level u n less oth erw ise

Properties, Fin e Root Biom ass, an d Forest Floor n oted. Valu es reported in th e text are m ean s fol-

Biom ass to Clay Con ten t in th e Tapajos Nation al low ed by stan dard errors.

Forest, Para, Brazil Variab le

Clay (% )

R ESULTS

Ca cm ol 1 kg 21 20.32

Ph ysical an d Ch em ical Prop erties

K cm ol 1 kg 21 0.47

of Su rface Soils

Mn µ g g 21 20.57 Th e percen t san d an d clay varied sign ifican tly in th e 1 Al cm ol kg 21 0.91*

0–10-cm depth alon g th e textu re gradien t (Table 1 Pµgg 21 20.84*

an d Figu re 2) ran gin g from 80% (61) san d to 60% 0.21

C (% )

(61) clay. Extractable soil P con cen tration s w ere 0.49*

N (% )

0.35 sign ifican tly n egatively correlated w ith clay con ten t

Live roots (g m )

(r 2

20.56* of clay con ten t an d exch an geable K con cen tration s

5 0.70, P , 0.01; Table 2), an d th e com bin ation

Dead roots (g m 22 )

20.06 (r 2 5 0.81, P , 0.01). Th e correlation w ith K m ay be

Forest floor (kg h a 21 )

20.73* du e to som e K occlu sion by th e 1:1 clays, alth ou gh

C:N

0.88* K is likely to be sorbed less stron gly th an P (Grah am

20.26 also occu rred with P an d exch an geable Al con cen tra-

NH -N (µ g g an d Fox 1971). A sign ifican t n egative relation sh ip 21 4 )

0.60 tion s (r 2 5 0.57, P , 0.01). Extractable soil P w as

NO 3 -N (µ g g 21 )

Net n itrification (µ g g 21 d 21 )

0.70* Net N m in eralization (µ g g 21 d 21 )

Bu lk den sity (g cm 23 )

*P , 0.01.

very low in clays soils (3.0 µ g g -1 ), an d in creased by

a factor of 3 in th e san ds (Table 1). Th ere w ere n o sign ifican t tren ds in total C or N

con cen tration s alon g th e soil textu ral gradien t in th e 0–10-cm soil depth , bu t th e C:N ratio decreased sign ifican tly from 18.4 in th e san ds to 13.6 in th e clays (Figu re 2). Total C an d N in th e soil w ere positively an d sign ifican tly correlated alon g th e

gradien t (r 2 5 0.73; P , 0.01). Th e soil C:P extractable an d N:P extractable ratios both in creased dram atically from san ds to clay alon g th e textu re gradien t (Fig- u re 2), an d w ere sign ifican tly positively correlated w ith clay con ten t (Table 2). Total C pools in th e su rface soils ran ged betw een 23 (60.6) an d 38 (67.2) Mg C h a -1 an d did n ot follow a sign ifican t tren d w ith soil textu re.

Nitrogen pools an d flu xes varied sign ifican tly w ith soil textu re. Am m on iu m -N con cen tration s w ere sign ifican tly low er in th e loam soils th an in th e

Figu re 2. Soil ch em ical an d ph ysical properties in th e san ds (P 5 0.05), w h ereas NO 3 -N w as sign ifican tly 0–10-cm depth alon g a soil textu re gradien t in th e Tapajos

h igh er in th e loam soils th an in th e san ds (Figu re 3). Nation al Forest, Brazil. (A) Textu re; (B) C:N ratio; (C) N:P

Net n itrification rates in creased sign ifican tly from ratio; (D) C:P ratio. To calcu late ratios, w e u sed total C an d

0.68–0.72 µ g g -1 d -1 in th e san ds to 1.91–2.29 µ g N an d extractable P.

g -1 d -1 in th e clays (Figu re 3). Net n itrification rates

W. L. Silver an d oth ers

Forest Floor an d Root Biom ass Forest floor m ass alon g th e textu re gradien t aver-

aged 6.9 (60.7) Mg h a -1 (Table 3). Alon g th e gradien t, forest floor C con cen tration s w ere greater on th e san ds th an on th e oth er soil types, an d forest floor N an d P con cen tration s decreased sligh tly bu t sign ifican tly from san ds to clays. Wh en com parin g ju st th e tw o textu ral extrem es (n 5 40), forest floor

C, N, an d P con ten t w ere sign ifican tly greater on san ds th an on clays (Table 4). Forest floor C:N an d C:P ratios w ere very sim ilar on san d an d clays soils.

Stan din g stocks of total fin e root biom ass (live plu s dead) in th e 0–10-cm depth decreased sign ifi- can tly from approxim ately 4.5 (60.4) Mg h a -1 in th e san dy soils to less th an 1.9 (60.4) Mg h a -1 in clays alon g th e textu re gradien t (Figu re 4). Both live an d dead fin e root biom ass follow ed th e sam e tren d w ith soil textu re; live fin e root biom ass w as very low in all soil types (0.05–0.4 Mg h a -1 ). San dy soils h ad sign ifican tly greater live an d dead fin e root biom ass to a depth of 40 cm th an th e clays (Figu re 4). Total fin e root biom ass w as sign ifican tly n egatively corre- lated w ith N m in eralization rates an d positively

correlated w ith extractable P pools (r 2 5 0.48; P , 0.05). Fin e root C, N, an d P con ten t w ere sign ifi- can tly greater on san dy soils th an on clays in th e top

Figu re 3. In itial m in eral N con cen tration s A, poten tial

10 cm of soil (Table 4).

n et m in eralization an d n itrification B, an d den itrification en zym e activity (DEA) C in th e 0–10-cm soil depth alon g

Total root biom ass to a depth of 1 m ran ged from

11 to 188 Mg h a a soil textu re gradien t in th e Tapajos Nation al Forest, -1 in th e five pits alon g th e textu re Brazil. Clay con ten t in creased alon g th e gradien t.

gradien t (Figu re 4) an d w as greatest in th e 0–10-cm depth at all sites. Th ere w as a gen eral pattern of in creasin g total root biom ass alon g th e gradien t

explain ed m ost of th e tren d in n et N m in eralization rates, w h ich also gen erally in creased sign ifican tly

from san ds to clays. Th e exception w as at m eter 50 from san ds to clays, w ith th e exception of th e last

(san ds) w h ere w e en cou n tered an extrem ely h eavy clay soil site alon g th e gradien t (Figu re 3). Den itrifi-

root (u p to 15 kg m -2 w et w eigh t) th at pen etrated cation en zym e activity in creased sign ifican tly from

from n ear th e su rface to 1 m in depth . Com parin g all

77 (623) n g g -1 h -1 in th e san ds to 317 (624) n g g -1

22 pits on san d an d clay soils (in clu din g th e tw o clay

h -1 in th e clays an d w as sign ifican tly positively an d tw o san d pits alon g th e textu re gradien t), th ere

w as sign ifican tly m ore coarse root C, N, or P in clays clay con ten t (Figu re 3 an d Table 2).

correlated w ith in itial NH 4 -N con cen tration s an d

to a depth of 10 cm , an d greater root P in 40–100-cm Con cen tration s of exch an geable Al in creased

depth on clays. Th e ratios of root C:N an d C:P w ere sign ifican tly from san ds to clays (Table 1), an d

h igh er on th e san ds th an on th e clays for all soil clay con ten t alon e explain ed 84% of th e variabil-

depth s (Table 4). Th e total root C pool to 1 m depth ity in exch an geable Al (Table 2). Exch an geable

w as approxim ately 26 6 8 Mg h a -1 on san ds an d n u trien t cation con cen tration s w ere very low in

17 6 2 Mg h a -1 on clays.

th e su rface soils alon g th e textu re gradien t (Table 1). Exch an geable Mg w as below th e detection lim it

Soil Prop erties to 1 m Dep th of th e an alytical in stru m en tation (1.6 µ g g -1 or

0.013 cm ol 1 kg -1 ). Pool sizes (kg h a -1 ) of m ost Total soil C pools to 1 m depth w ere very sim ilar on elem en ts follow ed sim ilar tren ds as th e elem en tal

clays (80 Mg C h a -1 6 3) an d san ds (81 Mg C h a -1 6 con cen tration s alon g th e gradien t. Soil pH w as

4; Table 4). Th ere w as sign ifican tly m ore C in th e sign ifican tly low er in th e loam soils th an in th e clays

su rface 20 cm of th e clays bu t greater C in low er 50 (Table 1).

cm in th e san ds (P , 0.10). Total N follow ed a

Soil Textu re Effects on Below grou n d C in a Tropical Forest

Table 3. Forest Floor Mass an d Elem en tal Con cen tration s alon g a Textu re Gradien t in th e Tapajos Nation al Forest, Para, Brazil

M eters alon g th e Textu re Grad ien t

Forest Floor Prop erty

400 Mass (g m 22 ) 839 (165)

42 (1.6) b 47 (1.6) ab N (% )

1.66 (0.1) c 1.77 (0.3) c C:N

27 (2.1) 31 (8.0) P (m g g 21 )

0.53 (0.03) a 0.37 (0.05) b Ca (m g g 21 )

5.45 (0.82) 4.49 (0.79) Mg (m g g 21 )

1.67 (0.18) 1.74 (0.45) K (m g g 21 )

1.62 (0.25) 1.16 (0.12) Mn (m g g 21 )

0.25 (0.02) 0.33 (0.13) Al (m g g 21 )

6.31 (0.82) d 3.66 (0.66) cd

Values are means 6 1 SE (in parentheses). Rows with different lowercase letters identify statistically significant differences among sites at the 95% level; n 5 30.

differen t pattern th an C (Table 4). Th ere w as sign ifi-

Mod el Resu lts

can tly m ore total soil N in clays to 1 m (8.5 Mg N Th e Cen tu ry m odel sim u lation of th e soil textu re

ha -1 6 0.4) th an in th e san ds (6.6 Mg N h a -1 6 0.4), gradien t predicted greater soil C, N, an d P in clays

du e prim arily to th e sign ifican tly greater pools of N th an in loam s or san ds, respectively (Table 7). in th e top 50 cm in th e clays. Total C an d N Cen tu ry u n derestim ated th e soil C an d N pools by con cen tration s in th e su rface soils w ere low er w h en 45% an d 44% , respectively, on th e san ds, bu t sam pled from th e large qu an titative pits th an from captu red th e tren ds of low er C an d N in th e san ds th e core sam ples. It is u n clear w h at cau sed th ese th at w e fou n d w ith ou r sam ples from th e large differen ces, bu t th ey cou ld resu lt from th e sm aller qu an titative pits. Cen tu ry h as several P pools in clu d- sam ple size an d differen ces in th e area sam pled. We

estim ated a total below grou n d C pool (forest floor in g m in eral P, SOM P, secon dary P, occlu ded P, an d plu s roots plu s soil) of 113 Mg h a -1

paren t P. It is som ew h at difficu lt to com pare th ese 101 Mg h a -1

C on th e san ds to

C on th e clays (Table 4). m odel pools directly to data from P fraction ation Total soil P con cen tration s an d pools in th e 0–10

sch em es (Gijsm an an d oth ers 1996), an d w e lack cm depth follow ed th e reverse tren d of extractable

th e data for a com preh en sive evalu ation of th e P. Alon g th e soil textu re gradien t, total soil P

Cen tu ry P m odel; h ow ever, th ere are several in ter- con cen tration s in creased by a factor of 4 to 5 from

estin g pattern s w orth n otin g. We report th e m od- san ds to clays (Table 5). Total P con cen tration s

eled SOM P fraction from Cen tu ry in com parison to ten ded to be greater at th e deeper soil depth s in th e

ou r NH 4 F extractable P, forest floor P, an d total P san ds bu t follow ed n o tren d in th e loam or clays

valu es. In terestin gly, th e vast m ajority of th e P in soils (Table 5).

th e m odel sim u lation s w as located in th e SOM P Th ere w ere few tren ds in soil textu re w ith depth

pool rath er th an in secon dary or occlu ded P. Field alon g th e textu ral gradien t (Table 5). San d con ten t

m easu rem en ts of NH 4 F extractable P an d forest floor decreased sligh tly (-10% ) w ith depth in th e san d

P sh ow ed approxim ately th ree tim es m ore labile P pits w ith a correspon din g in crease in clay con ten t.

in th e san ds th an in th e clays. In con trast, m odeled P Exch an geable n u trien t cation an d extractable P

m in eralization (an in dex of labile P) follow ed th e con cen tration s gen erally decreased w ith depth in

opposite pattern an d ran ged from 0.15 g P m 2 y -1 in th e soil (Table 5).

th e san ds to 0.2 g P m 2 y -1 in th e clays. Th e m odel Delta 15 N valu es ran ged from 6.2 to 12.7. San ds

sim u lation predicted a th reefold in crease in SOM P

from san ds to clay, sim ilar to th e 2.7-fold in crease su rface soils (Table 6). Th ere w ere n o statistically

h ad sign ifican tly low er d 15 N th an clays in both th e

w e m easu red for total P in san ds versu s clays at th is

sign ifican t pattern s in d 15 N betw een th e su rface an d

depth . Th e average pool size of total P m easu red on

1 m depth an d n o stron g pattern s w ith depth in th e th e clays (398 kg h a -1 ) w as con siderably greater san d or loam soils. In th e clays, th e top 10 cm of

th an th e m odel SOM P ou tpu t (129 kg h a -1 ), an d

in clu sion of paren t, occlu ded, or secon dary P pools deeper soil depth s (Table 6).

m in eral soil w ere sligh tly depleted in d 15 N relative to

in th e m odel do n ot ch an ge th is pattern .

W. L. Silver an d oth ers

Table 4. Forest Floor, Root, an d Total Soil C, N, an d P Pools by Depth in San d an d Clay Soils in th e Tapajos Nation al Forest, Para, Brazil

Forest floor Litter C

Litter N

Litter P

Litter C:N

Litter C:P 1030 (102)

0–10 cm Soil C

Soil N

Soil P

Soil C:N

Fine root C

Fine root N

Fine root P

Coarse root C a 6.00 (3.25)

Coarse root N

Coarse root P

Root C:N

Root C:P 1888 (116.3)

10–40 cm Soil C

Soil N

Soil P 277.5 (5.30)

Soil C:N

Figu re 4. Th e distribu tion of fin e an d coarse root biom ass Total root C

in th e Tapajos Nation al Forest, Brazil. A Th e proportion of Total root N

live an d dead fin e roots in th e su rface soils alon g th e soil Total root P

textu re gradien t. Error bars represen t 61 SE. B Th e Root C:N

distribu tion of live an d dead fin e root biom ass by depth to Root C:P

40 cm in san d an d clay soil types. C Total root biom ass by 40–100 cm

depth in five 3 x 1 x 1–m qu an titative soil pits alon g th e Soil C

soil textu re gradien t. Th e h igh apparen t biom ass at m eter Soil N

50 w as h eavily w eigh ted by a sin gle large an d h eavy root Soil P

th an passed th rou gh all th ree depth s. Soil C:N

Total root C

Th e m odel gen erally estim ated fin e an d coarse Total root P

Total root N

root C con ten t w ith in 20% of th e m easu red valu es, Root C:N

except for overestim atin g fin e root C in th e clays by Root C:P

67% . Th e m odel closely predicted fin e root N an d P 0–100 cm

con ten t in clays, bu t u n derestim ated th ese in th e Total below-

san ds. We m easu red greater coarse root N an d P in ground C

Total below- both clay an d san dy soils th an Cen tu ry predicted,

bu t Cen tu ry accu rately captu red th e tren ds of greater Total below-

ground N

coarse root N an d P in clays th an in san ds (Table 7). ground P

Cen tu ry u n derestim ated th e total below grou n d C (root plu s soil) in th e san ds by 53% . Abovegrou n d

a P 5 0.05. C and N values are in Mg ha 21 ; values for P are in kg ha 21 . Fine roots

biom ass C on san ds w as predicted to be 20% low er

are #2 mm diameter; coarse roots are .2 mm diameter. Asterisks signify statistically significant differences between soil texture types by using a 2 sample

th an on clays, w h ereas th e total forest C pool w as

t-test. Standard errors are in parentheses.

30% low er on san ds th an on clays. Cen tu ry pre- dicted th at total forest C w ou ld in crease from san ds to loam s to clays (Table 7).

Soil Textu re Effects on Below grou n d C in a Tropical Forest

Table 5. Soil pH, Textu re, Exch an geable Cation s, Table 6. Mean Delta 15 N by Depth in Th ree Soil Extractable (P ex ) an d Total P by Depth in Five Large

Pits a alon g a Soil Textu re Gradien t in th e Tapajos Qu an titative Soils Pits a Nation al Forest, Para, Brazil

Loam s Clays Soil

M eters alon g th e Textu re Grad ien t

San d s

M eter 200 M eter 350 Prop erty

Dep th

M eter 0

8.59 (0.49) b 0–10 cm depth

9.08 10.02 8.38 pH (KCl)

9.55 10.11 9.18 pH (H 2 O)

9.57 12.49 12.64 Total P (µg g )

6.58 9.54 Mg (cmol 1

9.62 kg )

Ca (cmol 1 21

6.62 6.72 12.74 K (cmol 1 kg 21 )

9.45 (0.20) a Mn (µg g )

Al (cmol 1 kg 21 )

0.99 1.10 2.57 2.20 3.28 a 3 3 1 3 1 m.

10–40 cm depth

Replicate samples were taken from the 0–2-cm and 90–100-cm depths. For these

pH (KCl) 4.07 4.16 3.93 3.96 3.93 depths, standard errors are in parentheses. Different lowercase letters indicate statistically significant differences between sand and clay by using replicate samples pH (H 2 O)

4.28 4.18 4.91 4.25 3.91 within depths. There were no statistically significant patterns with depth within Sand (% )

75.22 80.48 51.52 44.79 37.12 sites (P , 0.05). nd, no data.

Total P (µg g 21 )

Pex (µg g 21 ) ,

21 Ca (cmol on th e clays th an on th e oth er tw o soil types (Table 1 kg ) 0.01 0.01 0.08 0.00 0.01 Mg (cmol 1 kg 21 )

7). Th ese m odel resu lts agree w ith ou r experim en tal K (cmol 1 kg 21

data th at sh ow h igh er DEA, m in eralization , an d )

m ore en rich ed d N w ith depth on th e clays th an on Al (cmol 1 kg 21 )

1.53 1.27 1.87 2.40 2.31 th e san ds.

40–100 cm depth Dou blin g th e rate of N in pu ts in th e san ds in - pH (KCl)

4.23 4.18 3.96 3.88 3.93 creased th e m odel soil C con ten t from 12.7 to 26.3 pH (H 2 O)

4.21 4.35 4.28 4.50 4.21 Mg h a -1 , w h ereas dou blin g th e P in pu ts in creased Sand (% )

75.27 73.32 50.14 44.91 39.31 th e soil C by on ly 0.4 Mg h a -1 (Table 7). Dou blin g Clay (% )

23.77 22.38 49.86 52.69 61.69 th e fu n ction al rootin g depth from 60 to 120 cm Silt (% )

0.95 4.30 0.00 2.40 0.00 depth also h ad on ly a m in im al im pact on soil C Total P (µg g 21 )

pools in san ds w ith an in crease of on ly 0.5 Mg h a -1 Pex (µg g 21 )

79.42 78.25 128.36 n a

(Table 7).

Ca (cmol 1 kg 21

Mg (cmol 1 kg 21 ) ,

K (cmol 1 kg 21 )

0.01 0.01 0.01 0.01 0.01 D ISCUSSION

Mn (µg g 21 ) ,

Th e Effects of Soil Textu re on Soil

Al (cmol 1

kg 21 )

1.07 1.40 1.50 1.77 2.54 Nu trien t Pools

a 3 3 1 3 1 m.

Th e pool sizes an d distribu tion of soil P an d N varied

Values represent one pooled sampled per depth. Samples differ from those in Table 1. , signifies below detection of the analytical instruments. na 5 not

sign ifican tly alon g th e 400-m soil textu re gradien t

available.

in th is forest. San dy soils h ad m ore extractable P th an th e loam an d clay-rich soils. In w ell-aerated soils su ch as th ese, P is easily com plexed w ith exch an geable Al an d Fe in th e m in eral soil, sh ow n

Th e Cen tu ry m odel also in clu des param eteriza- by th e sign ifican t n egative correlation of exch an ge- tion s of losses du e to leach in g an d volatilization .

able Al an d P alon g th e gradien t as clay con ten t Nitrogen losses gen erally follow ed th e sam e tren ds

in creased. Th ere w as sign ifican tly greater total P as for total SOM N pools w ith h igh er rates of N

w ith in creasin g clay con ten t alon g th e gradien t, leach in g on th e clays th an on th e loam s or san ds

probably du e to th e form ation of Fe an d Al ph os- (data n ot sh ow n ), an d h igh er rates of trace gas loss

ph ates an d organ ically bou n d P, n eith er of w h ich

W. L. Silver an d oth ers

Table 7. Cen tu ry Model Sim u lation an d Measu red Valu es of th e Below grou n d C, N, an d P Pools an d N Trace Gas Flu x in th e Tapajos Nation al Forest, Brazil

Clays Pools or

San d s

San d y Loam s

Flu xes

CENTURY M easu red Soil organ ic m atter C

CENTURY

M easu red

CENTURY

M easu red

Mg h a 21 12.7 22.9 24.0 31.4 31.2 29.8 Soil C 2 3 N

na na Soil C 2 3 P

Mg h a 21 26.3 na

na

na

na na Soil C 2 3 w ater

Mg h a 21 13.1 na

na

na

na na Soil organ ic m atter N

Mg h a 21 13.1 na

na

na

Mg h a 21 0.96 1.70 2.12 2.44 2.90 2.79 Soil organ ic m atter P

129.1 5; 398 Fin e root C

kg h a 21 44.1 5; 148

2.24 1.34 Fin e root N

Mg h a 21 1.78 1.94 2.11 na

0.03 0.03 Fin e root P

Mg h a 21 0.02 0.08 0.03 na

1.06 1.01 Coarse root C

kg h a 21 0.84 2.07 0.99 na

10.9 8.89 Coarse root N

Mg h a 21 8.63 10.63 10.2 na

0.07 0.24 Coarse root P

Mg h a 21 0.06 0.16 0.07 na

1.58 6.23 Below grou n d C

kg h a 21 1.25 4.72 1.48 na

44.3 40.0 Abovegrou n d C

Mg h a 21 23.1 35.5 36.3 na

88.6 na Total forest C

Mg h a 21 69.8 na

83.0 na

132.9 na Trace gas loss

Mg h a 21 92.9 na

na

kg N h a 21 y 21 1.07 na

1.31 na

1.42 na