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shed�and�can�be�a�significant�reservoir�for�some�elements.
Living�organisms�consist�of�a�bewildering�variety�of�organic
Table 13.3. Elemental Compo-
compounds� (we� will� discuss� some� of� these� is� Chapter� 13).
sition of Dried Plant Matter
From� a� geochemical� perspective,� it� is� often� satisfactory� to
Element Percent Element ppm
approximate� the� composition� of� the� biomass� as� CH2O� (for
C 49.65 V 1
example� the� composition� of� glucose,� a� simple� sugar,� is
N 0.92 Mn 400
C6H12O6).��A�better�approximation�for�the�composition�of�land
O 43.2 Cr 2.4
plants� would� be� C1200H1900O900N25P2S1� (Berner� and� Berner,
Total 93.77 Fe 500
1996).��A�great�many�other� nutrients*,�however,� are� essential
Element ppm Co 0.4
for�life� (for� example,� Mg�and�Fe�are� essential� photosynthe-
Li 0.1 N i 3
sis;�Mo�is�essential�for�nitrate�reduction)�and�are� taken� up�by
plants� in� smaller� amounts.� � These� can� be� divided� up� into B5 Cu 9
macronutrients,�which�occur�in�plants�at� concentrations�in�ex- N a 200 Zn 70
cess�of�500�ppm�and�include�N,�P,�K,�Ca,�Mg,�and�S,�and�micro-
Mg 700 Se 0.1
nutrients,�which�occur�at� lower�concentrations�and�include�B ,
A l 20 Rb 2
Fe,� Mn,� Cu,� Zn,� Mo,� Co,� and� Cl.� � Other� elements� are� also
S i 1500 Sr 20
taken�up�by�plants�and�stored�in�tissue�even�though�they�play
P 700 Mo 0.65
no� biochemical� role� (so� far� as� we� know),� simply� because
S 500 Ag 0.05
plants� cannot�discriminate� sufficiently� against� them.� �Table
K 3000 B a 30
13.3�lists� the� concentrations� of� the� elements� in� dried� plant
Ca 5000 U 0.05
matter.��One�should�be�aware,�however,�that�the�actual� com-
Ti 2
position� of� plants� varies� widely;� grasses,� for� example,� can
contain�over�1%�SiO2�(>4600�ppm�Si). From�Brooks�(1972).
*
�A�nutrient�as�an�element�or�compound�essential�to�life�that�cannot�be�synthesized�by�the�organism�and
therefore�must�be�obtained�from�an�external�source.
568 January�25,�1998
W. M. W hit e Geochemistry
Chapter 13: Weathering, Soils, and Stream Chemistry
There�are�four�sources�of�nutrients�in�an�ecosystem:�the�atmosphere,�dead�organic�matter,�water,� and
rock.��The�atmosphere�is�the�obvious�direct�source�of�CO2�and�O2�and�the� indirect� source�of�N�and�H2O
in�terrestrial�ecosystems.��However,�it�may�also�be�the�direct�or�indirect�source�of�a�number�of�other�nu-
trients,�which�arrive�either�in�atmospheric�dust�or�dissolved�in�rain.��Plants�are�able� to�take� up�some
of�these�atmosphere-delivered�nutrients�directly� through� foliage;� most,�however,� cycle�through� the
soil�solution�and�are�taken�up�by�roots,�which�is�the�primary�source�of�nutrients.��Plants�cannot�take� up
nutrients�from�dead�organic�matter�or�from�rocks�directly:�nutrients�from�these�sources�must�first�be�dis-
solved�in�the� soil� solution.��Because�equilibrium�between� surface� adsorbed� and� dissolved� species� is
achieved� relatively� quickly,� elements�adsorbed�on�the� surfaces�of�oxides,� clays,� and� organic� solids
represent�an�intermediate�reservoir�of�nutrients.��For�example,�phosphrous,�often�the� growth-limiting
nutrient,�is�readily�adsorbed�on�the�surface�of�iron�oxides�and�hydroxides.��For�this�reason,�the� surface
properties� of�soil� particles� are� an�important� influence� on� soil� fertility.� � Even� in� relatively� fertile
soils,�however,�the�concentration�of�key�nutrients�such�as�phosphorous�may�be� effectively� zero�in�the
soil�immediately�adjacent�roots,�and�the�rate�of�delivery�to�plant�may�be�limited�by�diffusion.
In�most�ecosystems,�particularly�mature�ones,�detritus,�that�is�dead�organic�matter,� is� the� the� most
important�source�of�nutrients.��In�the�Hubbard�Brook�Experimental� Forest,� for�example,� this� recycling
supplies�over�80%�of�the�required�P,�K,�Ca,�and�Mg�(Schlesinger,� 1991).� �For�the� most�part,� this� recy-
cling�occurs�as�leaf�tissue�dies,�falls�to�the� ground,�and�decomposes.��However,�some�fraction� of�nutri-
ents�are� recycled�more�directly.� �Nutrients� may�be�leached� from� leaves� by� precipitation,� a� process
called�translocation.��Translocation�is�particularly�important�for�K,�which�is� highly� soluble�and�con-
centrated�in�cells�near�the� leaf� surface,�but�it� can�be�important� for�other� elements�as� well.� �Nutrient
loss�from�leaves� by�leaching� increases�in�the� order� K>>P>N>Ca.� In� addition� to� nutrient� recycling
through� translocation� and� detritus,� plants� also� recycle� nutrients� internally� by� withdrawing� them
from�leafs�and�stems�before�the�annual�loss�of�this�material�and�storing�them� for�use�in�the� following
season.��For�this�reason,�the�concentration�of�nutrients�in�litterfall�is�lower�than� in�living� tissue.� �Not
surprisingly,� the� fraction� of� nutrients� recycled� in� this� way,� and� overall� nutrient� use� efficiency,� is
higher�in�plants�living�on�nutrient-poor�soils�(Schlesinger,�1991).
Rainwater�passing�through�the�vegetation�canopy�will� carry�not�only�nutrients�leached� from�foli-
age,� but�also� species�dissolved� from�dust�and�aerosols�(together� called� dry�deposition)� deposited� on
leaves.� Fog�and�mist�will� also� deposit� solutes�on�plant� leaves.� �The� term�occult deposition� refers� to [ Pobierz całość w formacie PDF ]

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