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 hferris@ucdavis.edu November, 2005

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

Linear Food Chains

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

A more likely structure… The Trophic Network Or Food Web

Functional Guilds

Soil Food Web Structure – Biotic Effects

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

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

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

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.

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

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.

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

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 :

Functional Diversity of Nematodes

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

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)

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.

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

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

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.

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.

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

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

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. 1999.

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

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

Sustainable Agriculture Farming Systems Project 1988-2000

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

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)

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)

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

Some References Bongers, T., H. Ferris. 1999. Nematode community structure as a bioindicator in environmental monitoring. Trends Ecol. Evol. 14, 224-228. Ferris, H., T. Bongers, R. G. M. de Goede. 2001. A framework for soil food web diagnostics: extension of the nematode faunal analysis concept. Appl. Soil Ecol. 18, 13-29. Ferris, H., M.M. Matute. 2003. Structural and functional succession in the nematode fauna of a soil food web. Appl. Soil Ecol. 23:93-110. Loreau M. 2004. Does functional redundancy exist? Oikos 104:606-611. Neher, D. A., T. R. Weicht, D. L. Moorhead, R. L. Sinsabaugh. 2004. Elevated CO2 alters functional attributes of nematode communities in forest soils. Funct. Ecol. 18:584-591. Tenuta, M., H. Ferris. 2004. 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. 1998. Nematode fauna of three Welsh soils under conventional and organic grassland farm management. Pp. 305-313 in R. de Goede and T. Bongers, eds. Nematode Communities of Northern Temperate Grassland Ecosystems. Giessen, Germany: Focus Verlag. More information: http://plpnemweb.ucdavis.edu/nemaplex