1. Structure and Functions of the Soil Food Web: Understanding Healthy Soils
The organisms involved
How they interact
What they do
Monitoring food web condition
Food web management
Howard Ferris
Department of Nematology
University of California, Davis
November, 2005

2. Soil Food Web Structure – Resource Effects
Heat and CO2
Organic Source
Resources
Carbon is respired and Energy is used by each organism in the web
The amounts of Carbon and Energy available determine the size and activity of the web

3. Linear Food Chains

4. Linear Food Chains
and Trophic Cascades
But consider..
Bottom up effects
Omnivory

5. A more likely structure…
The Trophic Network
Or Food Web

6. Functional Guilds

7. Soil Food Web Structure – Biotic Effects

8. Positive and Negative Feedback in Food Web Structure
Sinorhizobium meliloti and bacterivore nematodes
0 nematodes
5 nematodes
With twenty nematodes
Fu et al. 2005

9. Soil Food Web Structure – Environmental Effects
Environmental heterogeneity
Zones and
Gradients:
texture
structure
temperature
water O2
CO2
NO3
NH4
minerals
Separate
metacommunities?

10. Food Web Structure - Patchiness
Resource distribution
Spatial heterogeneity
Organism motility
Omnivory
Strong and weak links
Microsite asynchrony
Viewing resolution

11. Structure and Function: Abundance and Diversity
consumers
resource
Functional Group Diversity
Functional Complementarity
Loreau, Oikos 2004
Strength of
Function or
Service
Functional Group Diversity
Functional Redundancy
Consider the food web as having two major sections: entry level organisms and their consumers.
Abundant and diverse entry level, low biomass and lack of diversity of predators. No regulatory effect.
Abundance and diversity of both levels.
Diversity may imply functional redundancy at both levels.
An enhancement of that concept is complementarity; each organism has unique attributes and contributes incrementally to the service.

12. Soil Food Webs - Function
Decomposition of organic matter
Cycling of minerals and nutrients
Reservoirs of minerals and nutrients
Redistribution of minerals and nutrients
Sequestration of carbon
Degradation of pollutants, pesticides
Modification of soil structure
Community self-regulation
Biological regulation of pest species
Functions of the food web
Many functions, depending on the subjective interests of the assessor. At a societal level, the decomposition of organic matter and cycling of minerals and nutrients are of paramount importance. Others include:
Decomposition of organic matter
Cycling of minerals and nutrients
Redistribution of minerals and nutrients in space and time
Reservoirs of minerals and nutrients
Sequestration of carbon
Detoxification of pollutants
Modification of soil structure
Community self-regulation
Biological regulation of pest species
We measure the beneficial effects of soil organisms in relation to these functions. One keystone species or guild may directly produce the
beneficial effect a. The beneficial effect may result from the functioning of the web with that guild present

13. Soil Food Web Structure and Function - the need for indicators
Structural and Functional Resilience of the soil food web is determined by connectance redundant and resilience of the web. Guilds vary in their sensitivity to environmental disturbance.. Often guilds at higher trophic levels in the web are more sensitive than those at lower trophic levels. Disturbance of the web may result in reduction of abundance of predators of opportunistic species. Consequently, upon enrichment with organic matter or plant growth, opportunistic herbivore and decomposer species may increase unregulated resulting in damage to the primary producer (plant) or immobilization of nutrients or other examples of functional instability.

14. The Nematode Fauna as a Soil Food Web Indicator
Herbivores
Bacterivores
Fungivores
Omnivores
Predators

15. Why nematodes as bioindicators?
Occupy key positions in soil food webs
Standard extraction procedures
Identification based on morphology
Clear relationship between structure and function
The most abundant of the metazoa
Each sample has high intrinsic information value
Nematode faunal analysis provides a useful tool in assessing this structure, function, and probably the resilience of the soil food web because:
The most abundant of the metazoa
Occupy key positions in soil food webs
Standard extraction procedures
Identification based on morphology
Clear relationship between structure and function
Each sample has high intrinsic information value :

16. Functional Diversity of Nematodes

17. Enrichment Indicators Structure Indicators
Rhabditidae
Panagrolaimidae
etc.
Short lifecycle
Small/ Mod. body size
High fecundity
Small eggs
Dauer stages
Wide amplitude
Opportunists
Disturbed conditions
Aporcelaimidae
Nygolaimidae
Long lifecycle
Large body size
Low fecundity
Large eggs
Stress intolerant
Narrow amplitude
Undisturbed conditions
Enrichment Indicators
Structure Indicators
Cephalobidae
Aphelenchidae, etc.
Moderate lifecycle
Small body size
Stress tolerant
Feeding adaptations
Present in all soils
Basal Fauna

18. Testable Hypotheses of Food Web Structure and Function
Disturbed
N-enriched
Low C:N
Bacterial
Conducive
Maturing
N-enriched
Low C:N
Bacterial
Regulated
Enriched
Ba1
Enrichment index
Structured
Fu2
Degraded
Depleted
High C:N
Fungal
Conducive
Matured
Fertile
Mod. C:N
Bact./Fungal
Suppressive
Fu2
Basal
Ba2
Om4
Om5
Basal
condition
Ca3
Ca4
Ca5
Fu3
Fu4
Fu5
Ba3
Ba4
Ba5
Structure index
Ferris et al. (2001)

19. Food Web Analyses Tomato Prune Systems Orchards Yolo Co. Yuba Co.50
100
Structure Index
Enrichment Index
Prune
Orchards
Yuba Co.
Mojave
Desert
Tomato
Systems
Yolo Co.
Redwood
Forest and
Grass
Mendocino Co.

20. Management of Food Web Structure: Carbon Pathways and Pools
Herbivore
Fungal
Nematodes occur at all levels in the soil food web and so provide a convenient indicator of food web structure.
The food web is fueled by carbon from plants, either by direct consumption, absorption of exudates, or decomposition of detritus.
Omnivory
Decomposition
Bacterial
channelized
reticulate

21. Carbon Channel Management
Bacterivore Channel
“Fast Channel”
Moisture
Low C:N, labile substrates
High respiration and turnover
Mineralization of nutrients
Major predators are protozoa and nematodes
Herbivore Channel
“Intermediate Channel”
Host status and defense mechanisms
Damage to host
Substrate respiration and immobilization, excretion and defecation
Major predators are fungi and nematodes
Fungivore Channel
“Slow Channel“
High C:N, lignin, cellulose, resistant substrates
Conservation of carbon, greater web structure
Major predators are microarthropods and nematodes

22. Structure and Function
Regulatory function
Nutrient
function O Pr B F P O Pr B F P
When the bacterial channel is active, there is strong mineralization service.
When omnivore biomass is low, the regulation function is reduced.
When the bacterial channel is less active, mineralization and contribution to soil fertility may be lower.
When omnivore biomass is larger, the regulation service is stronger.

23. Inflow Channel Analysis
Inflow channel analysis provides an image of how resources are entering the web.
Herbivore channel predominating, not sustainable.
Fungal channel predominating; carbon conserved but not highly productive.
Bacterial channel predominating; productive, but carbon not conserved for higher trophic levels and their regulatory service.

24. Succession C supplied Resource transformation Community structure
shifts
Ferris and Matute (2003)

25. Resource transformation
Channel Index:
a weighted ratio of fungivore and bacterivore nematodes
higher CI indicates more fungal
Ferris and Matute (2003)

26. what if? Food Web channel management:
Nature and timing of carbon sources
what if?
In a pot experiment:
1. Different carbon sources amplify bacterial and fungal inflow channels but resources to higher trophic levels depleted after 6 months,
2. What would resources to higher levels look like if organic sources were applied every three months?
Data adapted from
McSorley and Frederick

27. Food for the Soil Food Web
external sources
rhizodeposition
old root death C
herbivory
herbivory
external sources
rhizodeposition
old root death C

28. Soil Food Web Management - Experiment and Observation
C:N = 8.3:1
C:N = 8.5:1
C:N = 4:1
C:N = 6:1
Mineralization potential of fungal- and bacterial-feeding nematodes

29. Sustainable Agriculture Farming Systems Project

30. Soil Food Web Management - experiment
Cover crop
Irrigation
temperature
moisture
activity T0 M0

32. Organically-managed for 12 years
How Fragile is the Food Web? 50
100
Sampled 2000
Organically-managed for 12 years
Sampled 2001
After Deep Tillage
Enrichment index
Structure index
Structure index
Berkelmans et al. (2003)

33. Nematode Sensitivity – Mineral Fertilizers
Concentration (mM-N)
0.1 1
Standardized Counts 50
100
150
200 X
c-p 1
c-p 2
c-p 3
c-p 4
c-p 5
Ammonium sulfate
0.5
0.05
0.02
Nematode guild
Tenuta and Ferris (2004)

34. Disadvantages: ↓Productivity ↑Continuous ↑Monitoring ↑Rotation ↑Risk
? Equipment
? Economics
? Labor
? Knowledge base
Advantages:
↓Tillage
↓Fossil fuel
↓Dust
↓Leaching
↑Soil structure
↑Porosity
↑Food web structure
↓Pesticides
↓Mineral fertilizers
↓Weeds and herbicides
↓Water
↓Costs
↑Sustainability
Surface mulch with designed C:N ratio
Pest resistance/tolerance
Undisturbed bed
Buried drip
Transplant inoculants:
Entomopathogenic nematodes
Rhizobia
Flourescent pseudomonads
Nematode-trapping fungi

35. Some References
Bongers, T., H. Ferris Nematode community structure as a bioindicator in environmental monitoring. Trends Ecol. Evol. 14,
Ferris, H., T. Bongers, R. G. M. de Goede A framework for soil food web diagnostics: extension of the nematode faunal analysis concept. Appl. Soil Ecol. 18,
Ferris, H., M.M. Matute Structural and functional succession in the nematode fauna of a soil food web. Appl. Soil Ecol. 23:
Loreau M Does functional redundancy exist? Oikos 104:
Neher, D. A., T. R. Weicht, D. L. Moorhead, R. L. Sinsabaugh Elevated CO2 alters functional attributes of nematode communities in forest soils. Funct. Ecol. 18:
Tenuta, M., H. Ferris Relationship between nematode life-history classification and sensitivity to stressors: ionic and osmotic effects of nitrogenous solutions. J. Nematol. 36:85-94.
Yeates, G. W., R. Cook Nematode fauna of three Welsh soils under conventional and organic grassland farm management. Pp in R. de Goede and T. Bongers, eds. Nematode Communities of Northern Temperate Grassland Ecosystems. Giessen, Germany: Focus Verlag.
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