Institute of

Agriculture

Jennifer CarsonProfile

Jennifer grew up spending time on her uncle and aunt’s market garden in Spearwood but never considered a career in Agriculture. The possibility was first brought to her attention in first year uni when she saw “Soil Science” on the list of majors right above “Wool Science”and wondered who in earth majored in such obscure subjects. Four years later she graduated from the University of Western Australia with a Bachelor of Science majoring in Soil Biology. During university, Jennifer became interested in the challenge of increasing the sustainability of agriculture and in the possibility that organic farming might offer some solutions to this challenge. After university, she worked at Rick and Annie Dunn’s organic market garden in Perth for 18 months before working as a laboratory assistant at UWA and then travelling overseas. On returning to Perth, Jennifer commenced a PhD at UWA with the Soil Biology Group which she has recently completed.

Institute of

Agriculture

Influence of rock fertilisers on soil microorganisms in an organic pasture

Institute of

Agriculture

Jennifer Carson

Prof Lynette Abbott & Dr Deirdre Gleeson

Institute of

Agriculture

• Introduction: • Soil microorganisms and rock fertilisers in organic farming• Evidence that rock fertilisers may influence soil bacteria

• Experiments – Rock fertilisers influence bacteria because of their:• Composition• Particle size

• Implications:• Organic farming – Rock fertilisers can alter structure of bacterial communities• Soil biology – Influence of soil minerals on bacterial communities underestimated

Outline

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Agriculture

Microorganisms & rock fertilisers in organic farming

• Microbial processes improve soil fertility, but uncertain response to management

• Particularly important in organic farming

• Field trials in Australia

• Identify practices that improve soil conditions for microbial processes – Potential of rock fertilisers

• Rock fertilisers • Rock phosphate, basalt, mica, lime • Often poorly effective due to slow dissolution • Exception – sandy soils with pH<5

Introduction

Institute of

Agriculture

Hypothesise rock fertilisers affect microbial communities due to:

• Effect on plant growth & pH

• Composition

• Particle size

Evidence rock fertilisers affect microorganisms

• Increase biomass & activity when increase plant biomass & soil pH • In other habitats, composition of minerals affects microbes• Rock fertilisers can alter particle & pore size distribution, but not linked to effect on microorganisms

Introduction

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Agriculture

Composition of rock fertilisers

• Soil is spatial heterogeneous:• Microbes affected by properties of microhabitats• Microhabitats differ in mineral composition & size

• Mineral composition affects bacteria in other habitats • Different minerals colonised by distinct microbial communities• Preferential colonisation of minerals containing nutrients “hotspots” of activity

• Role of soil minerals overlooked• Microbial community on stones different from soil

“stonesphere”

D

c

Composition

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Agriculture

Hypothesis:• The microhabitats of different rock fertilisers in soil select distinct bacterial communities.

Experimental design:• 3 rock fertiliser treatments: mica, basalt and rock phosphate.

• 2 fraction treatments: rock fertilisers (>1 mm) & soil (<1 mm)

• 3 pasture treatments:T. subterraneum (clover), L. rigidum (ryegrass) and unplanted.

Composition

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Agriculture

10cm

With organic Organic matter matter removed

% Composition of soil & rock fertilisers (XRF). P2O5 CaO K2O MgO SiO2

Soil 0 0.1 0 0 99.7 Mica 0.5 1.6 3.9 3.2 73.2 Basalt 0.3 9.5 0.5 5.1 53.7 Rock P 34.7 50.0 0.1 0.3 8.5

Picture mica grains here

mica basalt rock phosphate

CompositionComposition of soil & rock fertiliser differed

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Agriculture

1. 1-2 mm grains rock fertilisers added to soil.

<1 mm:Soil fraction

>1 mm: Rock fertiliser fraction

2. Incubate in soil 10 w.

4. Total DNA extracted from fractions & region amplified by PCR.

Community structure (composition & relative abundance)

3. Sieve 1 mm.

CompositionMicrocosm experiment

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Agriculture

Statistics

• PCO (ordination): • Samples plotted in 2-D space. • Distance between samples shows difference.

• PERMANOVA• Permutational multivariate analysis of variance.• Treatment effects and pairwise comparisons.

• DISTLM (multivariate multiple regression):• Model variation in bacterial communities using soil properties• Modelled values for samples can plotted in 2-D space (RDA)

Composition

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Agriculture

Fraction treatments

• Bacterial communities in rock fertiliser and soil fractions

• Formed separate clusters • All pairwise comparisons were significant

Microhabitats of rock fertilisers selected bacterial communities with different structure to surrounding soil.

Unplanted

L. rigidum

Mica Basalt Rock phosphate

Composition

Institute of

Agriculture

Unplanted

L. rigidum

Mica Basalt Rock phosphate

Rock fertiliser fraction

• Bacterial communities in microhabitats of different rock fertilisers

• Formed separate clusters • All pairwise comparisons were significant

Microhabitats of mica, basalt & rock phosphate selected bacterial community with distinct structure.

Composition

Institute of

Agriculture

Unplanted

L. rigidum

Mica Basalt Rock phosphate

Soil fraction

• Bacterial communities in soil with different rock fertilisers applied

• Formed clusters • Fewer pairwise comparisons were significant

Applying different rock fertilisers to soil also influenced bacterial community in bulk soil.

Composition

Institute of

Agriculture

Element composition

• Composition of rock fertilisers & soil minerals explained variance in structure of bacterial communities:

• Unplanted: 44% by P, Mg & Na• L. rigidum: 35% by P, K & Mg

Bacterial community structure partly determined by nutrient content of rock fertilisers and soil minerals.

Composition

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Agriculture

Implications

• Soil microorganisms in organic farming:

• Rock fertilisers create new microhabitats in soil that select bacterial communities with distinct structure

• Partly due to their elemental content

• Effect of soil minerals on microorganisms:

• Mineral types in microhabitats may affect bacterial community structure

• Mineral heterogeneity may contribute to spatial variation in soil bacteria

Composition

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Agriculture

Particle size & spatial isolation

• Rock fertilisers can alter particle & pore size distribution but not linked to change in bacterial communities

• Theory predicting texture affects diversity of microbial communities• When not connected by water, diversity increasesTheory predicts:

• Evidence: • Texture• Water content

Particle size

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Agriculture

Hypothesis: • Texture & water content affect structure and diversity of bacterial community.

Experimental design:•2 texture treatments:

• 100% sand & 10% silt+clay • Ground quartz (<10 μm) so altered particle size not composition

• 6 water potential treatments:• Between -15 cm and -55 cm • Potential so knew size saturated pores

Particle size

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Agriculture

Column experiment

• Soil column connected to water reservoir

• Added 5 cm segments, packing to constant bulk density

• Soil columns saturated & adjusted to water potential of -10 cm at column base

• Incubated for 1 w at 25°C.

• Analysed structure & diversity bacterial communities

56 60 55 50 45 40 35 30 25 20 1510

Potential (cm)

51

6168 7788

102 122 153 204306

Largest water-filled

pores (μm)

Particle size

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Agriculture

Texture treatments

• Bacterial communities from different textures:

• Plotted in separate regions of graph• All pairwise comparisons significant

• Bacterial diversity was not affected by texture

Texture affected bacterial communities due to changes in physical properties of soil

Supports theory

10% silt+clay

100% sand

35

15

25

15

2025

4555

5540

40

Particle size

10% silt+clay 100% sand

Institute of

Agriculture

35

15

25

15

2025

30

4555

5540

Dry Wet

Water content treatments

• Bacterial communities from ‘wet’ and ‘dry’ soil:

• Plotted in separate regions of graph• All pairwise comparisons significant, except at 56% WFPS

• Bacterial diversity was higher in dry soil.

Water content influences diversity of complex bacterial community in field soil.

40

Particle size

10% silt+clay 100% sand

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Agriculture

Texture

• Difference in physical properties of soil explained 38% variance in structure of bacterial communities

Silt+clay content 13%Water content 15%Water in pores:

122-153 μm 3.2%88-102 μm 3.8%68-77 μm 2.6%

Variation in bacterial communities statistically related to changes in physical properties of soil.

Particle size

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Agriculture

Implications

• Soil microorganisms in organic farming:• Applying rock fertilisers with small particle size may alter the structure of bacterial communities• Partly due to their effect on physical properties of soil

• Effect of soil minerals on microorganisms:• Texture influences bacterial community structure (not only by altering chemical properties) by altering physical properties• Texture influences spatial isolation of bacterial communities in soil

Particle size

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Agriculture

Influence of rock fertilisers on soil microorganisms

Implications and future research

• Rock fertilisers have potential to influence soil bacterial communities Future research examine if changes benefit production

• Nutrient content of rock fertilisers may create “hotspots” of microbial activity

Future research examine if also “hotspot” of nutrient availability

• Rock fertilisers may increase microbial activity by improving physical conditions in soil