Box 122

The atmospheric CO2 level has fluctuated wildly over the past 400 million years. The variation can be attributed to changes in gross primary production caused by such events as volcanism and changes in the orientation of the earth's axis that affected the climate. Over the past 200 years, as atmospheric CO2 has risen by over 100 ppmV, plants have responded by decreasing stomatal density. The inverse relationship between atmospheric CO2 concentration and stomatal density has been used as a paleobarometer to estimate past atmospheric CO2 concentration. The evolution of trees provides an excellent fossil record of leaf structures, specifically stomata and epidermal cells. The environmental factors affecting the interpretation of stomatal density are minimized by including the relationship of stomatal index (percentage stomates over stomates plus epidermal cells). Retallack (2001) used the genus Ginkgo, with a fossil record dating to the late Triassic period (229 Mya), and extended the record to the Permian using the stomatal index of plant species with morphologies similar to that of Ginkgo.

box 12.2 (Continued)

box 12.2 (Continued)

Age (my BP)

increasing atmospheric CO2. Extensive areas of cultivated land have recently been returned to trees and grasses promoting soil C sequestration. Nitrogen deposition from anthropogenic activities has increased plant production. Increases in rice production using flooded paddies and animal husbandry have contributed to increased CH4 emissions. These biological processes combined with considerable uncertainty in the gross rates of Ca and Mg dissolution from terrestrial sources may explain discrepancies in accounting in the global C budget.

ecosystem c cycling

Photosynthesis converts inorganic C (CO2) into organic C through gross primary production (GPP) (Fig. 12.3). Some of this carbon is returned to the atmosphere as plant-respired CO2; the remainder becomes plant biomass and is termed net primary production (NPP). Free-living autotrophic microbes, such as algae, also contribute to GPP and NPP. Net secondary production (NSP) is the consumption of NPP by fauna and microorganisms. The standing stock of C in an ecosystem is defined as GPP less the respiratory loss of autotrophs (photosynthesizers) and decomposers (heterotrophs) and is termed net ecosystem production (NEP). Free-living microorganisms and soil fauna consume (decompose) the majority of NPP. The process of decomposition occurs on the order of days to decades and is dependent on environmental conditions and the quality of the plant material entering

CO,

Net Ecosystem Production

FIGURE 12.3 Various components of gross primary production and net ecosystem production.

Net Ecosystem Production

FIGURE 12.3 Various components of gross primary production and net ecosystem production.

the soil. The selective preservation of some resistant plant constituents, such as lignin, and the activity of microorganisms produce precursors to humic substances described later in this chapter. Soil humic substances can persist for thousands of years and are important stable C pools making up two-thirds of terrestrial C stores. GPP, NPP, NSP, and NEP are processes usually measured in terms of g C m-2 year-1 or similar units.

The NEP of different ecosystems varies considerably depending on plant species, soil type, and climate. Gross primary production is often controlled by the ability of soil biota to release through decomposition essential nutrients such as N and P to sustain NPP. Figure 12.4 shows the size of various plant, microbial, and soil C and N pools in a hybrid poplar system. The activity of decomposers is required to release a small portion of the large soil N pool for NEP. Forests, including tropical, temperate, and boreal forests, have the largest NEP due to the accumulation of wood. Humid tropical ecosystems in general have the highest GPP of all terrestrial environments despite having a limited essential nutrient pool. They are characterized by high production and decomposition rates, where nutrients are

Carbon

Nitrogen

Leaves 213

Total tree 950 (973)

Branches 184 (208)

Stem 336 (406)

Microbial biomass C 103

Leaves 12.8

Leaves 213

Leaves 12.8

Branches 184 (208)

Stem 336 (406)

Branches 25 (45)

Total soil C 6864

Branches 25 (45)

Microbial biomass 19.4

Soluble C 76

Total soil C 6864

Total soil N 608

Inorganic N 33

FIGURE 1 2.4 The distribution of C and N in a 2-year hybrid poplar stand during July. Microbial N is similar to poplar N, showing the importance of the microbial biomass in influencing nutrient availability. The values in parentheses represent C and N amounts for November.

actively cycling through plant litter and soil biota, and less storage in soil as organic matter. Optimal temperature and moisture conditions for decomposition prevail in humid tropical environments, leading to relatively closed nutrient cycles in the absence of disturbance. In northern latitude ecosystems, lower temperatures lead to slower decomposition of plant litter. The accumulation of plant matter in forest floor litter layers can withhold from the soil solution essential nutrients, particularly N. Disturbance such as fire, a physical oxidative decomposition process, is often required to release nutrients to increase NPP. Plant community succession also leads to changes in nutrient cycling through changes in the quantity and quality of litter inputs or the nutrient requirements of succeeding plant and microbial communities.

composition and turnover of c inputs to soil

The quantity and quality of plant litter and microbial turnover to soil greatly influence the outcome of decomposition processes. The sizes of the standing leaf and microbial N pools are similar in most ecosystems, indicating the importance of soil organisms for influencing available N for plant uptake. Various biological constituents such as cytoplasm and cell wall material of plant, microbial, and faunal inputs decompose at vastly different rates (see Chap. 16 for more details on turnover). Decomposition models often relate the decomposability of soil inputs to their C to N ratio, to their N content, and to the amount of resistant material such as lignin or chitin that they contain. Other factors such as their polyphenol (a secondary plant metabolite) content may impact decomposition through phytotoxic interactions. The availability of nutrients in soil solution affects not only the quality and quantity of inputs but also the ability of decomposers to consume the inputs. Higher decomposition rates result from increases in available N (NH4 and NO-), which causes vegetative growth with low C:N ratio (high N content). Elevated CO2 can produce plant litter with a higher C:N ratio, making it more difficult to decompose.

Net primary production adds to soil litter or detritus composed of leaves, branches, reproductive structures, leachates, and belowground products such as roots, exudates, and sloughed root cells. Plants contain an array of cytoplasm and cell wall components that require a complex scheme of enzymes to decompose. Unique plant components include cellulose, hemicelluloses, phenylpropanoids (lignin), and polyphenols (tannins). Microorganisms produce unique cell wall structures (e.g., peptidoglycans) and pigments (e.g., melanin). Leachate C is generally more important in forest systems in which significant leaching of dissolved organic C from the canopy and litter layer can occur. Root exudates are low-molecular-weight carbohydrates and amino-type compounds (amino acids, amino sugars, and small peptides) typically exuded from roots and mycorrhiza. The turnover of the NPP and NSP is an important source of humic substances that contribute to SOM formation and maintenance.

The various components of plant inputs to soil vary greatly as a source of energy and nutrients for NSP. The majority of plant inputs are C polymers (e.g., cellulose) and hydrocarbons (e.g., lignin) that contain few essential nutrients to facilitate decomposition. Cytoplasmic constituents such as sugars, amino compounds. and organic acids comprise up to 10% of plant residue dry weight (Table 12.1). Cytoplasm proteins contain the majority of N in plant tissue. Proteins and peptides are hydrolyzed by proteases and peptidases to individual amino acids during decomposition (Fig. 12.5). These labile N sources and other C-containing compounds provide the initial energy source and nutrients to start the decomposition process. Proteins also contain significant amounts of S in the form of the amino acids cystine and methionine. The protein content of plant tissues ranges from 1% in cell walls to 22% in meristematic regions and seeds. In leaves,

TABLE 12.1 Percentage of Cytoplasmic and Cell Wall Components in Plants (Adapted from Horwath, 2002)

Plant component % of total

Waxes and pigment 1

Amino acids, sugars, nucleotides, etc. 5

Starch 2-20

Protein 5-7

Hemicellulose 15-20

Cellulose 4-50

Lignin 8-20

Secondary compounds 2-30

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