Principles of terrestrial ecosystem ecology.pdf

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Table 8.1. Major sources of nutrients that are absorbed by plants. nutrient demand by vegetation. The balance of nutrients required to support maximal growth is similar for most plants (Ingestad and Ågren 1988). Any nutrient present in less than the optimal balance is likely to limit growth, so plants invest preferentially in absorption of the nutrients that most strongly limit growth. Nutrients that accumulate in excess of plant requirements are absorbed more slowly. Nutri- Grain yield (g m -2 ) 600 500 400 300 200 100 0 0 United States 1974 1950 Australia India 1969 1966 1963 United Kingdom Nutrient Movement to the Root 177 Source of plant nutrient (% of total) Nutrient Deposition/fixation Weathering Recycling Temperate forest (Hubbard Brook) Nitrogen 7 0 93 Phosphorus 1 89 Potassium 2 10 88 Calcium 4 31 65 Tundra (Barrow) Nitrogen 4 0 96 Phosphorus 4
178 8. Terrestrial Plant Nutrient Use less than 1%) of the belowground volume, nutrients must first move from the bulk soil (i.e., the soil that is not in direct contact with roots) to the root surface before plants can absorb them. Diffusion Diffusion is the process that delivers most nutrients to plant roots. It is the movement of molecules or ions along a concentration gradient. Nutrient uptake and mineralization provide the driving forces for diffusion to the root surface by reducing nutrient concentration at the root surface (uptake) and increasing the concentration elsewhere in the soil (mineralization). Mineralization and other inputs to the pool of soluble nutrients are the main controls over the quantity of nutrients available to diffuse to the root surface (see Chapter 9). Cation exchange capacity (CEC) of soils also influences the pool of nutrients available to diffuse to the root and the volume of soil that the root exploits. Soils with a high CEC store more available cations per unit soil volume— that is, they have a high buffering capacity—but retard the rate of nutrient movement to the root surface through exchange reactions (see Chapter 3). The root therefore draws more nutrients from a given volume of soil in soils with a high CEC but acquires these nutrients more slowly. The net effect of a high cation exchange capacity is to increase the supply of nutrients available to a root under conditions of high base saturation—that is, where the exchange complex has abundant cations. Anion exchange capacity is generally much lower than cation exchange capacity (see Chapter 3), so most anions, like nitrate, diffuse more rapidly in soils than do cations. Some anions, like phosphate, however, tend to precipitate readily from the soil solution and therefore diffuse slowly to the root surface. Rates of diffusion differ strikingly among ions, due to differences in charge density (i.e., the charge per unit hydrated volume of the ion). Charge density, in turn, depends on the number of charges per ion and the hydrated radius of the ion. Divalent cations, like calcium and magnesium, are bound more tightly to the exchange complex and diffuse more slowly than do monovalent cations, such as ammonium and potassium. Ions of a given charge also differ slightly in diffusion rates because of differences in radius and number of water molecules that are loosely bound to the ion. Soil particle size and moisture determine the path length of diffusion from the bulk soil to the root surface. Ions diffuse through water films that coat the surface of soil particles. The higher the water content and the smaller the particle size, the more direct the diffusion path from the bulk soil to the root surface. Moist soils therefore permit more rapid diffusion than dry soils, and soils with a high clay content allow more rapid diffusion than do coarse-textured sandy soils. Each absorbing root creates a diffusion shell, or a cylinder of soil that is depleted in the nutrients absorbed by the root. This diffusion shell constitutes the zone of soil directly influenced by plant uptake. The root accesses a relatively large volume of soil for those ions that diffuse rapidly. Nitrate, for example, which diffuses rapidly, is typically depleted in a shell 6 to 10mm in radius around each absorbing root, whereas ammonium is depleted over a radius of less than 1 to 2mm, and phosphate is depleted over a radius of less than 1mm. It therefore takes a higher root density to fully exploit the soil for phosphate or ammonium than for nitrate. The root densities in many ecosystems are high enough to exploit most of the soil volume for nitrate but only a small proportion of the soil volume for ammonium or phosphate. The major way in which a plant can enhance uptake of ions that diffuse slowly is to increase root length and therefore the proportion of the soil that it exploits. Mass Flow Mass flow of nutrients to the root surface augments the supply of ions. Mass flow is the movement of dissolved nutrients to the root surface in flowing soil water. Transpirational water loss by plants is the major mechanism that causes mass flow of soil solution to the root surface. Mass flow can be an important mechanism for supplying nutrients that are abundant in the
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F. Stuart Chapin III Pamela A. Mats
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Preface Human activities are affect
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Contents Preface . . . . . . . . .
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Contents ix Changes in Storage . .
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Contents xi Root Uptake Properties
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Contents xiii Disturbance . . . . .
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1 The Ecosystem Concept Ecosystem e
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Figure 1.1. Examples of ecosystems
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Community ecology Population ecolog
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ing from biotically driven changes
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The pool sizes and rates of cycling
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and alters patterns of vegetation d
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Figure 1.5. Direct and indirect eff
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many aspects of ecology, hydrology,
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Energy (W m -2 ) 1 1 0 1 0 1 0 1 Ab
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The Atmospheric System Atmospheric
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ecular O2 to form O3. The absorptio
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Hadley cell Hadley cell Ferrell cel
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early sailors as the doldrums. Subs
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phere, extends to depths of 75 to 2
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tures in Great Britain and western
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when the water vapor condenses to f
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Solar flux 1.4 1.2 1.0 0.8 0.6 0.4
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Figure 2.16. Time course of the ave
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Radiative forcing (W m -2 ) Warming
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January September Sun March pattern
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access to light compete effectively
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the top? How does each of these atm
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(Dokuchaev 1879, Jenny 1941, Amunds
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Organic carbon (%) Erosion likely g
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erosion dominating on steep hillslo
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conductivity of soils. Groundwater
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chemical weathering through their c
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Although most of the transfers in s
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leached material from above, it is
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opment, so these soils have weak de
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y roots and bacteria are important
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Table 3.4. Sequence of H + - consum
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new chemical conditions cause them
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72 4. Terrestrial Water and Energy
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98 5. Carbon Input to Terrestrial E
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100 5. Carbon Input to Terrestrial
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124 6. Terrestrial Production Proce
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228 10. Aquatic Carbon and Nutrient
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11 Trophic Dynamics Introduction Al
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246 11. Trophic Dynamics People, fo
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248 11. Trophic Dynamics Consumptio
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250 11. Trophic Dynamics Plant defe
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252 11. Trophic Dynamics Biomass Te
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254 11. Trophic Dynamics promote ra
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256 11. Trophic Dynamics eating rat
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258 11. Trophic Dynamics 4 th 3 rd
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260 11. Trophic Dynamics terrestria
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262 11. Trophic Dynamics R R trophi
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264 11. Trophic Dynamics food chain
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266 12. Community Effects on Ecosys
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282 13. Temporal Dynamics An emergi
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284 13. Temporal Dynamics Small rod
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286 13. Temporal Dynamics regime (H
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288 13. Temporal Dynamics successio
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290 13. Temporal Dynamics Stage Lif
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292 13. Temporal Dynamics that redu
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294 13. Temporal Dynamics Soil carb
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296 13. Temporal Dynamics Nutrient
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298 13. Temporal Dynamics are aband
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300 13. Temporal Dynamics leaf area
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302 13. Temporal Dynamics account f
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304 13. Temporal Dynamics 6. How do
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306 14. Landscape Heterogeneity and
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15 Global Biogeochemical Cycles The
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tion of surface waters. On daily to
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Crowley 1995). An improved understa
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therefore limits the long-term rate
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important for understanding the rec
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Terrestrial N fixation (Tg yr -1 )
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The addition of limiting nutrients
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has a significant atmospheric compo
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cled. Evaporation and precipitation
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Figure 15.11. Trends in (A) world p
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which cycles are soil pools and flu
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Our use, mismanagement, and uninten
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mycorrhizal or nitrogen-fixing mutu
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Major human impacts on the natural
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promote long-term sustainability of
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Coral reef ecosystems have recently
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Table 16.3. Examples of services pr
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anthropogenic change and to sustain
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Abbreviations an nutrient productiv
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Abbreviations 373 NEE net ecosystem
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Glossary A horizon. Uppermost miner
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Biomass. Quantity of living materia
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Coriolis effect. Tendency, due to E
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Exoenzyme. Enzyme that is secreted
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Inceptisol. Soil order characterize
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Microbial loop. Microbial food web
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Phototroph. Nitrogen-fixing microor
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simply to the greater number of spe
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Sun leaf. Leaf that is acclimated t
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Color Plate I monthly averages of t
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Plate 3. The global pattern of net
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