How Soil Microbes Transform Plant Residues into Soil ...
Transcript of How Soil Microbes Transform Plant Residues into Soil ...
How Soil Microbes Transform
Plant Residues into Soil Organic
Matter to Maintain Healthy Soils
Dr. Bobbi Helgason
Associate Professor, Soil ScienceUniversity of Saskatchewan
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Where does soil organic matter
come from?
https://theconversation.com/soil-carbon-is-a-valuable-resource-but-all-
soil-carbon-is-not-created-equal-129175
In agroecosystems most
new organic carbon
comes from plants (and
organic amendments).
Energy held by plant
carbon fuels other
important processes
(e.g. nitrogen cycling).
Graphic: J. LavalleeAlberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Why do microbes matter for SOM?
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
“Soil C sequestration is a ‘win–win’ option, a ‘no-regrets’ option, because a soil with more organic C, we say, is a better soil, a more productive soil”…“But now we encounter a disquieting constraint—the benefits of organic matter arise, not from its accumulation, but from its decay.”
Henry Janzen, AAFC Lethbridge
How does plant organic matter
become soil organic matter?
ODE TO ROT (J. Updike 1985)
“ ‘Let there be rot’, and hence bacteria and fungi sprang into existence to dissolve the knot of
carbohydrates photosynthesis achieves in plants, in living plants…
…Dead matter else would hold the elements in thrall -- nitrogen, phosphorus...” Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Microbial decomposition
Soil organic matter supports biota
Increased microbial
abundance and diversity
Improved soil
functioning
=
healthy soil!
Microbial Abundance = capacity
Microbial diversity = insurance
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Soil biota: ecosystem services
Decompose & cycle organic matter
Regulate nutrient availability
Suppress pests and disease
Maintain soil structure & hydrology
Exchange gas and store carbon
Detoxify soil
Control plant growth
The new paradigm of SOM formation
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Emerging view of soil
organic matter:
• Microbial accessibility
• Focus on C flows rather
than stocks
…a changing paradigm
The contentious nature of soil organic matterLehmann and Kleber, 2015 Nature
What affects SOM loss?
3 SOM destabilizing factors:
1) Desorption from minerals• Soil pH, increased moisture
2) Release from aggregates• Tillage, freeze-thaw, wet-dry cycles,
bioturbation
3) Increased biotic metabolism• Quality of litter, quality of soil
organic matter, microbial carbon use efficiency
Bailey, et al. 2019. Env. Res. Lett. 14: 020034
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
The contentious nature of soil organic matterLehmann and Kleber, 2015 Nature
Organic matter supports healthy soils
https://www.quartoknows.com/blog/quartohomes/2015/04/22/sustainable-gardens-and-organic-matter/
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Practices that can build SOM
Continuous cropping
Reduced physical disturbance
Diverse cropping rotations
Balanced nutrient management
Organic amendment application
Cover cropping
Not all soil organic matter is created
equally
https://www.freepik.com/premium-photo/fertile-loam-soil-suitable-
planting-soil-texture_4882973.htm
https://theconversation.com/soil-carbon-is-a-valuable-
resource-but-all-soil-carbon-is-not-created-equal-129175Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
https://theconversation.com/soil-carbon-is-a-valuable-resource-but-all-soil-carbon-is-not-
created-equal-129175
Particulate organic matter
(POM):
• protected within
aggregates
Mineral-associated organic
matter (MAOM):
• protected by binding with
clay minerals
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Not all soil organic matter is created
equally
Tools for tracing carbon in soil and
microorganisms13C tracer: stable isotope of carbon with 6 protons and 7 neutrons
https://deskarati.com/wp-content/uploads/2015/07/Carbon-Isotopes.jpg
DNAPhospholipid fatty acid (PLFA)
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Tillage as a destabilizing factor
Fall 2007, microcosms set up (Lethbridge + Ottawa)
Barley residue (13C at ca. 10 atom %) added either:
a. incorporated (0-10cm)
b. surface applied
(unlabeled residues every fall)
mixed residue surface applied residue
Collaborators: Drs. Henry Janzen, Ben Ellert and Ed GregorichAlberta Institute of Agrologists Helgason Webinar Dec 3, 2020
https://deskarati.com/wp-
content/uploads/2015/07/Carbon-Isotopes.jpg
Tillage as a destabilizing factor
2 years after 13C barley residue added
Lethbridge (drier, cooler):• Less residue remains when
incorporated
Ottawa (wetter, warmer): • residue decomposes faster
• decomposition is less affected
by residue incorporation
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020Helgason et al. 2014 Soil Biology and Biochemistry
Tillage as a destabilizing factor
“Ordination” visualizes differences between complex communities
LethbridgeOttawa
Different microbes are active in NT vs. CT.
Helgason et al. 2014 Soil Biology and Biochemistry
*ordination
Changes in organic matter composition? Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Residue type affects decomposition
Why does canola residue produce higher than expected
soil N2O emissions?
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Akeem Shorunke, MSc(Rich Farrell co-supervisor)
Soil microbes prefer canola residue
Akeem Shorunke, MSc(Rich Farrell co-supervisor)
Canola residues are particularly tasty; drives a microbial feeding frenzy
O2 gets consumed so microbes use NO3- instead (i.e. denitrification)
Most of the N2O-N came from the soil (not the residues) “priming”
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Each residue type formed a different
community structure
Microbial community structure
What is the net effect on
soil organic matter
formation?
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Akeem Shorunke, MSc(Rich Farrell co-supervisor)
N2O emissions
How does crop diversity affect SOM?
Tiemann et al. 2015 Ecology LettersAlberta Institute of Agrologists Helgason Webinar Dec 3, 2020
How does crop diversity affect SOM?Long-term cereal monoculture vs. diverse rotations
AAFC New Rotation Experiment (Swift Current est. 1987)
continuous wheat vs. wheat-canola-wheat-pea (n=3)
AAFC Totten Rotation Experiment (Harrow est. 2001)
continuous corn vs. corn-soybean-winter wheat (n=4)
www.winnipegfreepress.com
Dr. Jennifer TownAAFC, Saskatoon RDC
AAFC collaborators E. Gregorich, C. Drury, R.
Lemke, L. Phillips
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
How does crop diversity affect
microbial communities?
Location Stage Date
Harrow, ON Early Vegetative (EV) June 21 Anthesis (AN) August 3 Post-harvest (PH) December 5 Swift Current, SK Early Vegetative June 6 Anthesis July 20 Post-harvest September 12
Biogeochemistry
PO4, NO3, total N,
total and organic
C
Microbial enzyme
activities
Organic Matter
Characterization
Lignin (lignin phenols)
Amino sugars
Available C (CO2
respiration; 38d)
Microbial community
characterization
DNA sequencing
Phospholipid fatty acids
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Dr. Jennifer TownAAFC, Saskatoon RDC
Wheat: little difference in soil microbial community
structure and diversity
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Bacteria
Fungi
Dr. Jennifer TownAAFC, Saskatoon RDC
Diversity
bacteria fungi
Seasonal progression
Community structure
Corn: soil microbial community structure differed
(diversity did not)
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Bacteria
Fungi
Diversity
bacteria fungi
Seasonal progression
Dr. Jennifer TownAAFC, Saskatoon RDC
Community structure
Diverse crop rotations resulted in
different soil functioning
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Dr. Jennifer TownAAFC, Saskatoon RDC
Crop rotation affected SOM quality
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Dr. Jennifer TownAAFC, Saskatoon RDC
Diverse crop rotations resulted in
different soil function
Aboveground diversity
Belowground diversity function
interact to improve
fertility and soil organic
matter
Crop rotation affected nutrient
cycling, microbial community
structure and SOM composition
corn > wheat
Plant matter quality is important for fertility Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Dr. Jennifer TownAAFC, Saskatoon RDC
Organic matter: what about roots?
upload.wikimedia.org/wikipedia/commons/a/a7/4_Seasons_Roots.jpg
Roots can contribute up to 80% as much C as aboveground biomass (Fan et al. 2019)
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
What is the big deal with roots?
Rhizodeposition can account for up to 30%
of net photosynthate C
Philippot et al. 2013. Nat Rev Microbiol 11:789-799
https://doi.org/10.1371/journal.pbio.2006352.g002
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Emerging evidence that root C is preferentially stabilized(e.g. Sokal et al., 2019)
Intercropping, cover cropping
Organic matter: what about roots?
How does microbial stabilization of
root carbon change with depth in the
soil profile?
The role of functional traits of wheat,
canola and lentil roots on microbial soil
C and N cycling in the rhizosphere
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Mostafa Hojati
Kehinde Adams (aka “Adams”)
Organic matter supports healthy soils
Alberta Institute of Agrologists Helgason Webinar Dec 3, 2020
Soil organic matter provides
resilience to stress
Photo: Soil Sc ience Society of America ACSESS Digital Library
Soil management practice
Reduced physical disturbance
Continuous cropping
Diverse cropping rotations
Cover cropping
Balanced nutrient management
Organic amendment application