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Nutrient cycling in terrestrial ecosystems
Nutrient availability and uptake is necessary for plant growth, in addition to the
availability of water, sunlight, and CO2. There are a number of elements that are
essential nutrients, but we will focus on a subset of N, P, Ca and K. The sources and
cycling of each of these elements differ, and together they illustrate an important range of
processes in biogeochemical cycling.
Primary productivity – definitions
Primary production is a measure of the amount of biomass (or, alternatively, carbon)
fixed (i.e. reduced to organic C from CO2) by the photosynthetic organisms in an
ecosystem. (In a few unusual ecosystems, like those around deep sea hydrothermal vents,
the primary production is chemosynthetic rather than photosynthetic). Ecologists and
biogeochemists define several different categories of primary production. For now, we
will be concerned with two.
Gross Primary Production (GPP): the total mass of carbon fixed per unit time in an
ecosystem. In terrestrial systems, GPP and NPP are usually expressed per unit area. For
example, “kg C m-2 day-1”.
Net Primary Production (NPP): because plants and other primary producers respire a
fraction of the carbon they fix, NPP is often a more interesting quantity. NPP is the net
accumulation of biomass (or mass of C) in an ecosystem per unit time. It is a measure of
the addition of new growth in a system, not counting losses to herbivores, fire etc. In
terrestrial systems it may be expressed per day, per season, or per year, and is usually
normalized to a unit area. For example, NPP may be “kg ha-1 yr-1”, where ha = hectare
(100 m × 100 m = 10,000 m2).
NPP and biomass
The “standing crop of biomass” is the mass of biomass per unit area in an ecosystem, not
to be confused with NPP. Because forests have invested heavily in “inert” wood
structures that hold their photosynthetic apparatus up to obtain light they have high
values for biomass per unit area. Table 5.2 from Schlesinger (handout) tabulates data for
NPP and the standing crop of biomass from many different types of ecosystems.
Compare, for example, NPP and biomass per unit area (columns 3 and 5) in three very
different terrestrial ecosystems: tropical rain forest, savanna, and tundra. While NPP in
the rain forest is ca. 3× the savanna value, rain forest biomass ≈ 11× savanna. The ratios
of rain forest/tundra are 14 for NPP and 67 for biomass. Rain forests are indeed more
productive (less limited by water and temperature than savanna or tundra), but they have
much higher standing crop values.
Globally, total NPP in the marine environment (column 4) is roughly half that of the
terrestrial environment (although these numbers are uncertain) but note that total
terrestrial biomass ≈ 470× marine biomass (column 6). Marine plankton don’t need to
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build large non-photosynthetic structures, thus they have much lower biomass/NPP
ratios.
Nutrient requirements of forests:
Because the range of terrestrial environments and the species that inhabit them is
quite large, generalizations about nutrient demands and uptake have to be made carefully.
Fortunately, there are some patterns that appear over different systems. Table 1
compares the carbon to nutrient ratios for different forest types. Despite differences in
NPP and biomass, there is reasonable agreement in average values (the study averaged a
number of systems, typically about 12 for each forest biome – the scatter isn’t shown
here, but it’s significant). Ratios like this (C/P, C/N, etc.) are sometime known as
“Redfield ratios”, although this term strictly applies to marine plankton dominated
ecosystems. The regularity of these ratios in comparable systems suggests that plants
need a more or less fixed quantity of nutrients in order to process new carbon and create
new growth.
It’s also interesting to note that, when expressed in percentages, some types of
biomass in the major forest systems are similar. For example, all have 60 - 66% “bole”,
and 16-22% root biomass. Branch and leaf allocations vary more widely. Obviously a
grassland has little woody biomass (bole and branch) and almost all is root and leaf, so
not all biomes follow these rules.
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Table 1 – biomass and nutrient ratios in six major forest types (Vitousek et al ., 1988).
Forest type
Northern
conifer
Temp broad
deciduous
Giant temp
conifer
Temp broad
evergreen
Tropical
closed forest
Tropical
wood/grass
land
% total biomass
Leaf
Branch
Bole
Roots
Mass ratio
C/N
C/P
N/P
4.5
10.2
62.8
22.6
143
1246
8.71
286
1.1
16.2
63.1
19.5
165
1384
8.40
624
2.5
10.2
66.4
20.8
158
1345
8.53
315
2.7
14.7
66.2
16.5
159
1383
8.73
494
1.9
21.8
59.8
16.4
161
1394
8.65
107
3.6
19.1
60.4
16.9
147
1290
8.80
Biomas
s t/ha
233
Nutrient sources
N, P and Ca have different sources in the environment. Phosphorous is almost entirely
derived from rock weathering, calcium (and K) from weathering and atmospheric
deposition, and fixed N from in situ bacterial nitrogen fixation and atmospheric
deposition (and maybe weathering). The importance of atmospheric deposition,
weathering, and in situ fixation varies significantly among ecosystems. It is interesting to
compare the inputs of nutrients to an ecosystem with the demands placed by new growth.
Our mass balance approach to watershed chemistry compares atmospheric inputs to
stream outputs. We assume that the system is at steady state (inputs = outputs), and then
attribute differences in those fluxes to either missing sources or sinks within the
watershed. For example, if stream loss of K+ exceeded K+ input in rainwater, we would
perhaps attribute the “extra” potassium to weathering of K-bearing minerals (such as
micas and feldspars) in the watershed. In this way we can get estimates of the major
fluxes (mass/year) of nutrients into the ecosystem(s) within the watershed.
Measurements of NPP (despite the difficulties) can be combined with data on the
chemistry of the new growth to estimate the “growth requirement” for nutrients. For
example, if a tree puts on 1 kg of new carbon with a C:N ratio of 140, then it must have
taken up 7.1 g of fixed N. Collecting the data for these mass balance calculations for
NPP is laborious, but it has been done in a number of study areas.
At the Hubbard Brook experimental forest in New Hampshire an extensive set of data
have been collected for many years. Table 1 compares the growth requirements for five
nutrients with the input fluxes from weathering and atmospheric deposition. In all cases
except P, the growth requirements greatly exceed the inputs. Either the system is rapidly
depleting its nutrient stores, or the forest recycles its nutrients effectively. Recycling of
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nutrients is critical to ecosystems, because in virtually every case the inputs of one or
more (often all) nutrients are woefully insufficient to sustain the levels of NPP observed.
Table 2: Annual nutrient requirements for temperate hardwood forest at Hubbard Brook,
New Hampshire, and potential sources for them (after W. H. Schlesinger,
Biogeochemistry, 1991, Academic Press, New York), p. 144.
Element
N
P
Growth
requirement
115.6
12.3
(kg ha-1 y-1)
Potential % of growth requirement from:
New inputs
Atmospheric
deposition or
18
0
fixation
Rock
0
13
weathering
Recycled sources
Re-adsorption
31
28
Detritus
69
81
turnover
K
Ca
Mg
67.3
62.2
9.5
1
4
6
11
34
37
4
86
0
85
2
87
This recycling takes two basic forms: “Readsorption” or “translocation” of nutrients
occurs within a tree. As biomass is converted from leaf to twig to bole, nutrients are
removed from the woody tissues and transported to the leaf tissues, thus conserving
nutrients internally that are not needed for structural purposes but are needed for
photosynthetically active ones. For example a chaparral system (Table 3) in southern
California has an overall C/N = 157, similar to the average forest data of table 1. But,
leaves contain much more N than does wood, and leaf biomass has a much lower C/N
than wood or the weighted average, which is dominated by the large woody parts of the
trees. The same pattern holds for P.
foliage
live wood
total live
biomass, g/m2
553
5929
6563
C/N
67.4
181.9
157.3
C/P
1455
2440
2271
The second type of recycling occurs when plant litter (dead leaves, etc.) falls to the forest
floor and is decomposed. Decomposition releases nutrients to the soil, where they can be
taken up again by plants before they are flushed out and lost in the stream discharge.
When soil microbes respire dead organic matter they release the carbon as CO2 but retain
a significant fraction of the nutrients (especially N and P) in order to fuel their own
growth. The process of N and P incorporation into soil microbial biomass is called
“immobilization” and soil microbes can be an important reservoir of N and P.
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“Turnover” times of nutrients in forest ecosystems
Clearly we can establish different “turnover” or residence times of an element in a forest
ecosystem depending on how we define the problem. For example, the same chaparral
data used above shows how turnover times for the nutrients in live plant tissues and litter
(the reservoir) are markedly different with respect to inputs (atmospheric deposition and
fixation) than they are when calculated with respect to uptake for new growth. Enough N
is added to the system from deposition and fixation to replace the plant and litter pool of
N once every 262 years, but the plants (including dead tissue) replace their N every 7.6
years. Thus the average N must cycle through the system 262/7.6 = 34 times before
being lost from the system. Recycling is highly efficient!
These data and their analysis highlight the contrast between the relatively slow
“external” biogeochemical cycling in an ecosystem and the very rapid “internal” cycling.
While the figures vary, rapid internal recycling is a prominent characteristic of all
biogeochemical systems. It is necessary because the biota cannot afford to lose nutrients
anywhere near as rapidly as they take them up. Note that in the chaparral system, the N
and Ca content of detritus (dead wood and litter) are almost as high as in the live tissues.
A temperate deciduous forest that drops its leaves annually might lose several percent of
its nutrient pool each year. Such losses need to be recouped if the system is going to
maintain high levels of NPP.
Table 3) Nutrient data for a chaparral scrub stand (data from Schlesinger, 1991, p. 159)
Pools
Ca, g/m2
N, g/m2
foliage
live wood
4.5
8.2
28.99
32.6
rep tissues
0.32
0.62
total live
dead wood
litter
33.81
5.58
26.1
41.42
6.28
20.5
total dead
soil?
total
31.68
26.78
65.49
68.2
Fluxes
Inpu ts
Ca, g/m2/y
deposition
N, g/m2/y
0.19
Grow th r eq.
0.15 total req
fixation
0.11 reabsorption
weathering?
net
total
Tau, yr
0.19
Ca
0.26
N
overall
345
262
growth
8.9
5.2
w/ readsorp
8.9
7.6
5
Ca, g/m2/y
N, g/m2/y
7.39
13.11
0
4.15
7.39
8.96
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Soil and detritus as nutrient stores
While a substantial quantity of nutrients is stored in live biomass, forest detritus and soil
are also very important and can be larger pools. Nutrients in litter will be released upon
decomposition or converted to bacterial biomass (immobilized). Organic compounds and
clays in the soil can also retain nutrients through adsorption onto ion exchange sites. The
ion exchange capacity of a soil is an estimate of its capacity to retain cations (or anions)
in this way. Nutrients that are held by ion exchange processes are said to be part of the
“plan-available” nutrient pool of the soil. For example, K in feldspar minerals is
typically not plant-available because it is tightly held in the silicate lattice. However, K+
adsorbed onto a clay particle might be easily exchanged for another cation, and thus be
considered “plant-available.” Thus the type and composition of minerals (and organics)
in the soil plays a critically important role in its ability to store and recycle nutrients.
Table 5.2 is from W. H. Schlesinger, Biogeoochemistry: An Analysis of Global Change.
Academic Press, 1991, p. 121.
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