1. Dynamics of aggregate stability and biological binding agents
Salwan AL-Maliki

2. Introduction CO2 emissions and global warming World population Hunger
Soil degradation – important constraint on food production
What is soil structure and why is it so important?
Mechanisms of structural aggregation

3. CO2 emission and global warming

4. World population

5. Numbers experiencing hunger

6. Soil degradation
Soil degradation is a major threat to food production & increased atmospheric CO2.
The causes of soil degradation are:
erosion, loss of soil structure, soil compaction.
loss of soil organic matter and biodiversity.
overgrazing.
agricultural activities.
deforestation.

7. Causes of soil degradation

8. An example for soil degradation

9. What is soil structure and why is it so important?
Soil structure is rearrangement of primary soil particles into aggregates.
Soil aggregates are the basic unit of soil structure with a major influence on the physical , chemical, and biological properties.
Soil aggregates are created-stabilised by SOC, biota, ionic bridging, clay and carbonates.
Highly aggregated structure provides soil stable against erosion and soil degradation with benefits for productivity.
Aggregation improves soil C sequestration and mitigates increases in atmospheric CO2 concentrations.

10. Mechanism of aggregation
Fungal length: The fungal hyphae length led to joining micro-aggregates with macro-aggregates and as a result of creating a good soil structure. Fungi could also produce a glycoprotein and glomalin which are very efficient for aggregate stability increases.
Fungal hyphae
Microbial biomass carbon is a part of the organic matter in the soil constitutes living microorganisms and their products. It is acting an important role in developing soil aggregate stability and productivity of plant. The products of microbial decompositions of plant tissue such as fats, lignins, and waxes, mucilages which are very essential for structure stability.
Microbial biomass & activity
Earthworms

11. Mechanism of aggregation
Organic matter: The additions of fresh organic matter induce the soil microbial activity and create different binding agents such as fungal hyphae, Polysaccharides which contribute to aggregate stability improvements.
Organic matter
The micro-aggregate are created by bonding of C-P-OM, where C: clay particles, P polyvalent metal (Fe, Al, Ca), OM: organo-metal complex.
Polyvalent metals
clay Fe, Al, Ca organic matter
Earthworm: The mechanism of stable aggregate by earthworms is firstly in ingestion of soil, breakdown of organic matter, mixing of these fractions with mucus and eject them as a cast. These processes will increase stable aggregates.
polysaccharides, fats, waxes, mucilage.

12. Mechanism of aggregation

13. Aims of study
To investigate the factors responsible for stabilizing and destabilizing soil structure.
To assess the effect of earthworms on aggregate stability and associated microbial activity.
To investigate the effect of fungal length, microbial biomass carbon, organic carbon and basal respiration on aggregate stability.

14. Methods and materials - three phases
The first experiment investigated the stability of earthworms casts incubated for up to 26 days.
At various points (0, 10, 20 & 26 days) during this incubation, aggregate stability, microbial biomass carbon, fungal length and clay dispersibility were measured.

15. Methods and materials
The second experiment investigated the effect of earthworms and different types of organic matter at two depths over incubation periods of 40, 80 and 100 days.
Treatments were control, hay, hay + worms, straw, and straw + worms.
Aggregate stability, clay dispersibility, Microbial biomass carbon, hyphal length and organic carbon were measured.

16. Source of soil for the 2nd experiment

17. Methods and materials
The third experiment studied the effect of earthworms and organic matter on opencast mining soil.
Treatments were control, hay, and hay + worms
Measurements included fungal length, microbial biomass carbon, organic carbon and basal respiration in >2mm aggregates and <2 mm soil.
Incubation periods were 25, 50, 75 and 100 days
Aggregate properties were measured initially on bulk soil then after further incubation of stable aggregates

18. First experiment - changes in cast stability & microbial indices with time.

19. First experiment-PCA (casts)

20. Conclusion of the first experiment
Initial increases in stability were associated with higher microbial indices.
Later improvements in cast stability followed decreases in these indices.
Possible causes – a biotic processes (age hardening) leading to simultaneous physical protection of C with enhanced efficiency of stabilisation.

21. Second experiment, Interaction plot between treatment and incubation for stability %, aggregate > 2mm.
P value= 0.003
80 days
100 days
ca=control, h1a=worms+hay , ha=hay, s1a=worm+straw
sa = straw
40 days

22. Second experiment, Interaction plot between treatment and depth for the %stability
P value= 0.007
Top
Ca=control, h1a=worms +hay , ha=hay, s1a=straw + worms,
sa = straw
Bottom

23. Second experiment, Interaction Plot between depth and treatment for MBC Mg Toc Kg-1.
P value= 0.89
Top
Ca=control h1a=worm +hay, ha=hay, s1a=straw + worms, sa = straw
Bottom

24. Second experiment - Interaction Plot between treatment & incubation for MBC Mg Toc Kg-1.
P value= 0.33
40 days
Ca=control, h1a=worms +hay , ha=hay, s1a=straw + worms,
sa = straw
80 days
100 days

25. Second experiment - interaction between treatments & incubation for fungal length (m/g)
P value = 0.000
40 days
Ca=control, h1a=worms +hay , ha=hay, s1a=straw + worms,
sa = straw
80 days
100 days

26. Second experiment - %organic carbon

27. Conclusion of the second experiment
Earthworm increased stability but only if there is food (plus hay and straw ). Earthworms can ingest aggregates and destabilise them.
Earthworms increased the benefits of organic inputs on stability to a greater extent near the surface.
Earthworms decreased hyphal length but increased MBC initially; they generally increased hyphal length later but had limited effect on MBC.

28. Third experiment – treatment and time effects on % aggregate stability

29. Third experiment - treatment and time effects on hyphal length

30. Third experiment - treatment and time effects on basal respiration for bulk soil aggregates

31. Third experiment – treatment and time effects on microbial metabolic quotient

32. Third experiment – PCA for stability and potential binding agents

33. Conclusion of the third experiment
Earthworms increased stability by increasing MBC, fungal length and organic carbon in aggregates.
Earthworms reduced basal respiration in aggregates – reduction in CO2 in atmosphere by protecting organic matter within aggregates.
Without earthworms, CO2 emissions were initially higher for hay treatment - rapid decomposition of added organic matter but reversed in longer term.
Earthworms increased the efficiency of aggregate microorganisms by reducing metabolic quotient.

34. General conclusions
Micro- and macro-aggregate stabilisation mechanisms differ.
Responses to treatments varied between soils (e.g. hay + worms treatments % and %, expt 2 and 3 respectively.
Fungal length and MBC are important stabilising agents of casts & aggregates.
Earthworms + hay increased MBC, fungal length and organic carbon in aggregates and in soil.

35. Future work
Studying the temporal effects of different organic matter types with earthworms on aggregate stability and microbial indices.
Including the effect of elevated CO2 litter on soil aggregation and microbial activity.

36. Thanks For your attention
To Dr . John Scullion and Dr . Gareth Griffith for support.
To Iraqi government for its funding.