A Grid-Based Autonomous Agent Model of Hierarchical Soil Structure and Bacterivorous Nematodes

Thomas Weicht1,3, Josef Gorres2, Daryl Moorhead3, and Deborah Neher1

1University of Vermont, Burlington, VT; 2University of Rhode Island, Kingston, RI; 3University of Toledo, Toledo, OH

INTRODUCTION• Previous study observed:

– Net Nmin decreased and Cmin increased as soils dried (1)– Bacterivorous nematode populations increased in dry soils (2)– Assumption:

• Soil pore size distribution is spatially random and represented by a parallel cylindrical pores.

–Problem: >20µm diameter nematodes have access to ≤ 6µm diameter water filled pores.

ACKNOWLEDGEMENTS :

Project funded by USDA-NRICGP 2005-3510716262

Literature Cited1. Gorres J, Savin M, Neher D, Weicht T, Amador J

(1999) Grazing in a Porous Environment: 1. The Effect of Soil Pore Structure on C and N Mineralization. Plant and Soil 212:75-83

2. Neher D, Weicht T, Savin M, Gorres J, Amador J (1999) Grazing in a Porous Environment. 2. Nematode Community Structure. Plant and Soil 212:85-99.

OBJECTIVE •Determine the impact of hydrologically isolated soil water on nematode mediated Nmin

•Energy and nutrient flows are linked inextricably through food webs.

•Modeling Study:•Parameterize growth and population dynamics models for a range of bacterivorous nematode species

•Selection of study nematodes will cover a range of criteria:•Optimum temperature range•Growth rate•Foraging strategy

•Characterize aggregate distribution and pore area surrounding aggregate.•Modeling approach:

•Spatially explicit grid based model where cells or clusters of cells represent aggregates and bulk soil, and the transition from cell to cell represents pore area and soil moisture interactions.

Nematode Life History Studies Four isolates currently in culture:•2 Acrobeloides sp. (Family: Cephalobidae)

•Differ in temperature optimums•Presumed specialists – long patch retention times•Poor swimmer•Higher optimum temperature isolate study in progress

•Data reported in this presentation

•1 Rhabditella sp. (Family: Rhabditidae)•Low temperature optimum•Presumed generalist / opportunist – short patch retention times•Swims well•Study in progress

•1 Pristionchus sp. (Family: Neodiploasturidae)•Low temperature optimum•Presumed generalist / opportunist – short patch retention times•Isolate may be replaced with one that differs from the Rhabditella sp.

Procedure•17 to 18 individuals per isolate are followed from hatch to death at 6 temperatures:

•Temperature treatments:•8, 12, 16, 20, 22 and 24°C

•All individuals were grown on 100µl of 0.5% water agar with 9 X 1012 bacteria per ml.•Photomicroscopic images were used to follow somatic growth at 12 or 24hr intervals and egg biomass at weekly intervals.•Reproduction was monitored daily.•Data were used to construct age and stage structure life history models.

•Somatic growth and population dynamics models are in process•Physiological parameters available from the literature will be used to complete energy flux models.

RESULTSA.Somatic Growth• Somatic growth followed a sigmoid pattern

with lower and upper asymptotes (fig. 1A)• Identification of developmental stages:

Observations of cuticles or lethargy provided indications of developmental stage transitions• Observations were correlated with

measurements• Length or the rate of growth decreases during the

molt

• Individuals that failed to develop into adults could not meet the energetic requirements to complete a molt (fig. 1B)• Failure to develop was either foraging or

temperature related.

B. Temperature Effects on Growth and Production•Temperature impacted somatic growth by decreasing the maximum adult length (fig. 2A) and similar size reductions were observed in juvenile stages.•Temperature regimes completed show no impact of temperature on average egg production (fig. 2B).

•However, continuing temperature regimes (16 and 12°C) have ca. 90 days of reproduction, and twice as long as completed regimes.•Egg volume seems to be unaffected by temperature, anecdotal. Analysis in progress.

C. Temperature Impacts on Development and Population Growth•Decreasing temperature increased the length of all developmental stages except J4 (fig. 3A).•Populations grow at temperatures above 10°C

•Duration of the juvenile stages seems to impact the rate of population growth greatest

•Sensitivity analysis will confirm this observation after collection of all data

CONCLUSIONS•Acrobeloides mortality is age dependent and occurs near the age other cohorts become adults regardless of the developmental stage.

•Includes observations on isolates outside this study•This observation can not be captured in Life tables or Leslie matrices, but may be incorporated in cohort or individual population dynamics models•Spatially explicit models could capture the energetics required to complete a molt

•Impacts of temperature on stage size and biomass•Demonstrate temperature impacts on respiration and partitioning of energy to respiration and growth

•Duration of juvenile period has the greatest impact on population growth

CONTINUING WORKNematodes

•Permanent mounts are in preparation for species identification on all isolates•For the Acrobeloides sp. presented, a maximum temperature needs to be determined•Temperature studies need to be completed for two other species in culture•Another contrasting species should be isolated for study

Soil Characterization and Modeling •Preliminary compression fracturing of soils and wet sieving are underway.

•Porsimeter data indicates that 30% of pore habitat in 50µm dia. aggregates is potentially available to nematodes

•A preliminary 2-dimensional grid based model with grids representing soil habitat was completed

•Computational efficiencies need to be worked out to develop a 3-dimensional model

•Nematode population studies need to be combined with data available in the literature to complete population dynamics and energy flux models

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Figure 1. A) Somatic growth at 12°C. Open boxes highlight molting events; B) Somatic growth at 8°C. Developmental stages highlighted above the line. The arrow refers to second molt (see inset)

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Figure 2. A) Average adult length at 5 temperature regimes. Error bar represent Standard Error; B) Average total egg production at 3 temperature regimes. Error bar represent Standard Error.

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Figure 3. A) Time line of average stage development.B) Calculated daily coefficient of population increase (λ).