Successional processes

Hypothesis: Climate influences the rate and trajectory of succession by altering disturbance regime and the abundance of key species.

Task S7 Characterize soil microbial community

composition among successional stages and seasons in floodplain and upland ecosystems.

How do plant, animal and microbial communities change through succession and what are the consequences for ecosystem

processes?

Studying plant-microbial interactions in the cycling of soil C and N

(The black box approach)

Soil C and N levels are determined by the balance between organic matter inputs and losses due to decomposition, erosion and leaching.

Plant inputs– Litterfall– Root turnover– Exudation

Soil microbial community– Decomposition– Formation of organic matter

Schimel et al 2006

Microbial contributions to soil C storage

What role does microbialcommunity composition play insoil C sequestration?

• Microbial growth efficiency• Recalcitrance of microbially-

derived organic matter

How does community composition change across successional development?

• Substrate availability• Substrate quality

Six et al 2006

Proposed research

Assess soil microbial composition and biomass along floodplain and upland chronosequences using PFLA analysis.

WHY?

In order to develop and test hypotheses about the role of soil microbes in C cycling in forested ecosystems of interior Alaska, we need to have empirical observations of how community structure varies over time and space.

PLFA

• Unlike CF methods, PLFA is useful as a proxy for living and possibly active biomass– Phosphate group is quickly consumed upon cell death– Not found in storage products– Found in relatively constant proportion of the biomass

• Great structural diversity, coupled with high biological specificity

Taxonomic groupsFatty Acid Microbial Group15:0i, 17:0i, 15:0a, etc.. Gram positive bacteria

cy17:0, cy19:0, 18:111c Gram negative bacteria (also cy19:0 gm+)

10 Me18:0, 10 Me17:0, 10 Me16:0

Actinomycetes

18:26,9, 18:19c Fungi

20:4 6 Protozoan

16:1 5 Arbuscular mycorrhizal fungi

18:18c Methanotrophs

Experimental design

• All major stages of succession in FP (n=5) and UP (n=3) communities

• 3-5 replicate stands per stage• 3 sampling periods

– May– Mid July– Late September

• 2 horizons– O (integrated organic)– A (mineral)

• 50 cores composited from each 30m x 30m plot

• 2+ years??

Predictions

Broader patterns• Microbial biomass ↑ along the

chronosequence.• FP: Microbial community shifts

from bacterial-dominated to fungal-dominated over succession. UP: ↓ B:F.

• Potential for vertical stratification in community structure as a function of substrate availability and water-filled pore space.

Seasonal patterns• Bacterial:Fungal ↓ seasonally.

Successional processes

Hypothesis: Climate influences the rate and trajectory of succession by altering disturbance regime and the abundance of key species.

Task S8Determine the direct and interactive effects of

soil resources, microclimate, and microbial symbionts on the cumulative nitrogen fixation through succession by alder in floodplain and

upland ecosystems.

How do plant, animal and microbial communities change through succession and what are the consequences for ecosystem

processes?

Physiological ecology of the Alnus-Frankia-EMF tripartite

A. tenuifolia – a key player in the N economy of floodplain forest ecosystems in interior AK

Persists throughout successional development

How important are coordinated changes in ectomycorrhizal and Frankia associations of

alder in enabling species persistence and N fixation

capacity throughout succession?

Objectives

1. Identify EMF composition and functional traits in Alnus tenuifolia across a 200 year floodplain chronosequence

2. Characterize the ecophysiology of host selection for EMF in response to N and P fertilization in field plots, and in response to controlled partner choice experiments in the greenhouse.

10 15 20 25 30 35

Leaf N:P Ratio

-2.5

-2.0

-1.5

-1.0

-0.5

Leaf

15 N

(‰)

Hypothesis

Alder shifts associations with ectomycorrhizal species based on

variation in plant demand for N and P, combined with the availability and forms of

these nutrients in soil.

Objective 1: Describe EMF community composition and functional traits across succession

Prediction: Successional nutrient gradients favor selection of different fungal species across successional stages.

Task 1 - Extract DNAs from randomly subsampled EM root tips (control plots) and identify fungal associates through PCR and sequence analysis of the ITS region

Seasonality of mycorrhizal development

Task 2 - Determine whether the activities of key enzymes related to nutrient acquisition vary among fungal associates and successional stages

Acid phosphatase and phytase activity in single root tips using methylumbelliferone (MU)-labelled fluorescent substrate analogues.

Objective 2: Characterize host selection of EMF in response to N and P fertilization

Prediction: N fertilization will have the greatest effect on N-mobilizing EMF species and enzymes in late succession, while P fertilization will down-regulate acid phosphatase activity primarily in early succession

Task 1 - Extract and sequence DNAs from randomly subsampled EM root tips across N and P ammended plots

Task 2 – Controlled greenhouse experiment to examine the capacities of the dominant alder EMF species to mobilize different forms of P, organic vs. inorganic.

Field study

3 successional stages Alder, balsam poplar, white spruce

3 sampling periods

June, mid-July, early September 3 stand replications

20m x 20m plot divided into 16 5m x 5m subplots

Results (to date)

• Overall EMF diversity appears low– Strong core to core as well as site to site

variation– Most sites ‘appear’ to be dominated by <6

morphotypes with several ‘rare’ morphotypes mixed within.

• Fine root development delayed in alder relative to other taxon