Terrestrial Environments

1
Lecture 4

• Terrestrial Environments
• Chapter 4

2
Scales of observation
field
pore
cellular
molecular
3
Soil Horizons

• Surface soil
• most biologically-active zone
• Vadose zone
• pore spaces are filled with gas and some liquid
• Saturated zone
• pore spaces are fill with liquid

4
Properties of soil and subsurface environment

• 3 phases
• solid (mineral),
• 40-50 by volume
• defines soil texture
• sand (2mm)
• silt (50 mm)
• clay (0.2-2.0 mm)
• pore space
• aqueous
• gas

5
Soil Textures
6
(No Transcript)
7

• Pores are conduits for movement of gases, liquid
  and microbial cells through the different soil
  horizons

Bacterial biofilm
Soil particle
8
Sites of ion exchange
9
Bacterial attachment to clay particles
10
Soil Microcosm
11
Surface colonization and biofilm formation
Clay particle
12
Soil Organic Matter

• Humic substances
• heterogenous polymers formed during process of
  decay of plant, animal and microbial biomass
• stable, long-term nutrients for soil microbes
• MW range from 700-300,000 daltons
• carry a net negative charge
• display a large surface area

13
Rhizosphere

• Volume of soil extending 1mm to several mm from
  root
• Contains high concentrations of organic and
  inorganic nutrients excreted by plant root
• The high nutrient
• levels promote
• higher densities
• of bacterial cells
• than in surrounding
• soil

14
Soil Water Potential

• Controls water movement in soil
• defined as work per unit quantity necessary to
  transfer an infinitesimal amount of water from a
  specific elevation and pressure to another point
  somewhere else in the porous medium
• units of pressure
• pascals, atmospheres, bars

15
Soil Water Potential
16
Soil Water Potential

• Matrix contribution
• decreases free energy of water
• causes potential to become more negative
• capillary forces hold water tightly to mineral
  surface in unsaturated soil
• potential of this water ranges from -31 to
  -10,000 atm
• large negative potential causes this water to be
  immobile

water
pore
17
Soil Water Potential

• As more water fills pore space, water potential
  is controlled by matric or capillary forces.
• Water bridges pore space between two particles.
• Potentials range from -0.1 to -31 atm
• this water moves slowly downward from large pores
  to smaller pores

unsaturated
pore
18
Soil Water Potential

• As more water is added, free water is formed.
• Free water is under control of gravity
• soil water potential of free water ranges from 0
  to -0.5 atm

Saturated pore
19
Oxygen effects

• Microbial activity is greatest when water
  potential is at -0.1 atm
• system at this potential is still unsaturated
• when system is saturated, oxygen cannot diffuse
  fast enough from gas phase (source) to liquid
  phase where microbes are active
• diffusion limitation

pore
gas
water
20
Consumption of oxygen in aqueous medium

• Bacteria consume oxygen as terminal electron
  acceptor to produce more cells (biomass)
• The additional biomass removes the dissolved
  oxygen faster than it can be replaced by
  diffusion from the gas phase

O2
O2
21
Effect of porous medium on composition of gas
phase
22
(No Transcript)
23
Vadose Zone

• Unsaturated oligotrophic environment that lies
  between surface soil and saturated zone
• low in organic carbon (lt0.1)
• when vadose zone extends deep below the surface,
  water and contaminants from surface takes a long
  time to get to saturated zone (aquifer)
• Yucca Mountain has a vadose zone that is 600m
  thick
• this is where they want to store spent nuclear
  waste for 10,000 years

24
Saturated zone

• This is were the worlds aquifers exist
• gt50 of United States freshwater supply is in
  saturated subsurface layers
• Also is low in organic carbon
• saturated zones can be sandwiched between
  unsaturated layers
• confined aquifers within 300m of surface are
  termed intermediate aquifers
• these have flow rates slower than aquifers closer
  to surface
• these are the ones that supply most of the
  potable water we depend on

25
Microbiology of subsurface

• Bacterial densities in surface soil 108cells/g
  dry soil
• patchy distribution
• niches defined by mineral, liquid, and gas phase
  of porous medium
• Bacterial densities in subsurface105-107cells/g
  dry soil

26
Cell biomass based on different measurement
techniques