“COVER CROPS AND THE SOIL
MICROBIAL COMMUNITY”
BOB KREMER
Microbiologist, USDA-Agricultural Research Service
Professor of Soil Microbiology,
University of Missouri
kremerr@missouri.edu
1
Cover Crop
. . . . crop grown primarily for the purpose of protecting and improving soil
between periods of regular crop production or between trees and vines in
orchards and vineyards [and agroforestry systems]. Brady & Weil 2008
Most multiple benefits of cover crops associated with microbial activity
Derived from establishment of rhizosphere environment by cover crop
Generation and maintenance of organic matter in soil
RHIZOSPHERE
Zone or volume of soil under influence of roots, extends > 5
mm from root and is the area of increased microbial activity
Range in Microbial Population
[cfu1 X 100 million per g soil or root]
Rhizoplane2
25 - 45
Rhizosphere
5 - 35
Non-rhizosphere
1 - 1.8
(‘Bulk soil’)
1 cfu
= colony-forming units
2 Rhizoplane
= plant root surface
Modified from Kennedy (2005)
Rhizosphere relationships
Plants Modify the Soil Microbial Community
20 - 50% of C fixed via photosynthesis released thru roots = Rhizodeposition
Added C stimulates growth and activity of microbial community
Many plants release specific organic molecules as signals for beneficial m/o’s
Legumes - flavonoids for symbiotic N-fixing rhizobia
Other signaling compounds for plant growth enhancing bacteria or
pathogen-suppressive microorganisms
Some plants release compounds that inhibit certain segments of microbial
community - i.e., Spring oats release avenacin that inhibits Mn-oxidizing bacteria
Mycorrhizae associate with 90% of plants (P availability, water uptake, pathogen
suppression;
Non-mycorrhizal plants include Brassicaceae (mustards, radish, canola)
SOIL - Biologically dynamic system
SOIL MICROORGANISMS (≥ 1 billion cells per g)
Microorganisms mediate transformations (microbial diversity)
nutrient cycling (C, N, P, S, Fe, Mn)
decomposition, CO2 evolution, C sequestration, SOM formation
detoxification of synthetic substances (‘bioremediation’)
Soil formation and structure, plant growth regulation, water quality
Bacteria
Algae
Fungi
Nodulated legume cover crops
house N-fixing rhizobia -
Mycorrhizal hyphae on
rhizoplane explore rhizosphere
soil for P, water, other nutrients
Although non-mycorrhizal, this
forage radish enhances soil
porosity, eliminates detrimental
microorganisms thru release of
cyanide-producing compounds
Unique DNA “fingerprints” differentiate soils under various
management systems, different SOM content/composition.
Restored grassland ~ 10 yr
Continuous corn ~ 15 yr
Extracted
soil DNA
profiles
SOM Formation is mediated thru Decomposition
[Each function can be measured
as a soil quality indicator]
CO2
**
[Improved water movement]
Pathways mediated by soil microorganisms through enzymatic activity!
Soil Ecology Factors
Living Cover Crops
Actively growing roots deliver C and nutrients to rhizosphere m/o’s
Microbial functions include nutrient transformation, decomposition, plant
growth regulator synthesis; Degrade/inactivate synthetic chemicals
Active rhizobia & mycorrhizae symbiosis; associative N fixation
Building soil structure
Cover Crop Residues
Organic matter inputs, building microbial biomass
Provide mineralizable nutrients; decomposition active
Increased microbial diversity; potential pathogen & weed suppression
Improved soil aggregate stability
Increased number, diversity, activity of soil micro-, macro fauna
Extensive root systems of some native prairie plants
Soil conservation implications of organic matter:
OM supports diverse microbial community; microorganisms mediate
soil aggregation for optimum soil structure characteristics
The ‘microbial
glue’ that aids in
formation of soil
aggregates
Source: USDA-ARS Sustainable Agriculture Systems Laboratory, Beltsville, MD
Cover Crop - Microbial Ecology Interactions
Primarily Evaluated in a Soil Quality Assessment Perspective
Soil Quality - functional ability of soil to support optimal biological activity
and diversity for plant and animal productivity, to regulate water flow and
storage, and to provide an environmental buffer
Indicators Used in Soil Quality Assessment:
(Relevant for Cover Crop Impact Assessment)
• Aggregate Stability (water transport, low erosion, microbial habitat, C
storage)
• Soil Organic Matter (C) (key factor influencing biological activity, soil
structure, soil fertility, plant nutrition)
• Soil Enzyme Activity (Soil microbial activity, nutrient transformation,
management impacts on decomposition)
[Soil Health: continued capacity of soil as a vital living system whereby
plant and animal growth and environmental quality is sustained; a holistic
approach in which plant, animal, and human health is promoted]
Oats as a Cover Crop for Managing Winter Annual Weeds
Fall-seeded oats
into standing
soybean or corn
Winter-killed mulch suppresses
winter annual weed growth
Corn no-tilled into
oat residue in
subsequent season
Oats as a Cover Crop for Managing Winter Annual Weeds
Oat cover cropping
improved soil quality –
increased soil aggregation
and soil microbial activity in addition to suppressing
winter annual weeds
Pecan Agroforestry System with Kura Clover as Alley Crop
Chariton County, MO
Kura clover stand six years after
establishment in alleys (2002)
Kura clover stand in alleys with
adjacent row of young pecan trees in
September 2006
Kura clover used as cover crop and source of hay while pecan
trees mature before producing nut crop
Kura Clover – Characteristics
•
Speer & Allison. 1985. Econ.
Botany 39:165
Taylor & Smith. 1998. Adv.
Agronomy 63:143
Perennial growth habit – rhizomes (underground stems) allow
rapid spreading and re-growth
•Biological nitrogen fixation
•Hardy – survives harsh winters
•Suppresses weeds
• High forage quality – grazing and hay
• Conserves soil, improves soil structure
• Supports bees for honey production
• May be managed as living mulch crop
• Valuable seed crop
Aggregate stability increased in alleys relative to check sites
% Water-stable Aggregates
50
LSD (0.05)=8.0
2002
2003
2004
2005
2006
45
40
35
30
25
20
15
10
5
0
C
ed
at
tiv
ul
it
m
ss
ra
G
w
Ro
ee
Tr
pe
lo
ks
ac
B
er
ld
ou
Sh
m
Su
Soil organic matter (Total Organic Carbon) increased during
Kura clover establishment, contributing to improved soil
fertility and plant productivity.
Soil Organic Carbon
TOC (g kg-1 soil)
30
2002
2003
2004
2005
2006
LSD (0.05)=7.50
25
20
15
10
5
va
te
d
s
C
ul
ti
G
ra
s
ow
Tr
ee
R
lo
pe
Ba
ck
s
ld
er
Sh
ou
Su
m
m
it
0
Soil Glucosaminidase Activity
Product (PNP)/g o.d. soil/hr
Increased activity => optimum N cycling including mineralization from
organic sources to supply available N to plants and microbial community
200
180
160
140
120
100
80
60
40
20
0
LSD (0.05) = 44.5
2002
2003
2004
2005
2006
C
ow
e
ed
at
tiv
ul
ss
ra
G
R
r
p
lo
ks
ac
de
ul
ee
Tr
B
o
Sh
it
m
m
Su
Source: Lin et al. 2009 Submitted to Soil Biology & Biochemistry
Atrazine Degradation (CO2)
Microbial Enzyme Activity
Rhizosphere Microorganisms on Cover Crop Species Contribute to
Pesticide Biodegradation and Removal from Soil
Maintenance of vegetative residues through cover
cropping, refuge areas, buffer strips, etc not only
restores organic matter but also provides habitats for
natural insect predators of weed seeds
‘Micro-insect’ larva attacking
Amaranthus (i.e., pigweed) seed
Osage County, MO
Cover crop management and effects on soil microbial communities
Little Information Available for Cover Crop Management Effects on
Soil Microbial Activity • No-till regular crop into standing cover crop - delays decomposition of residues
• Flail or rotary mowing - poor mulch distribution; initial rapid microbial activity
but decreases in long-term; subject to variations in soil moisture/temperature
•Sickle bar mowing; Rolling - even residue coverage; most consistent microbial
activity through regular crop growing season
• Perennial living cover crop - consistent microbial activity throughout season
and from year-to-year, soil quality improves
• Frost (freeze)-kill - consistent microbial activity; improves soil quality if followed
routinely in a rotational management scheme
• Chemical kill (burndown) - use with caution; glyphosate can alter soil microbial
diversity to favor buildup of root pathogens on dying roots; need to obey delay
period before no-till regular crop into killed residue
Sources: Zibilske & Makus 2009; Johal & Huber 2009
Buildup of potentially pathogenic fungi on cover crop
roots killed with herbicide as a burndown treatment
No Herbicide
Cover crop: 6-wk old soybean
Glyphosate Treatment
Management Strategies to Offset Adverse Effects of Glyphosate
Conventionally- managed
Orange Plantation
Citrus Management with
cover crop mulching Brazil
• Cover crops enhance microbial activity and SOM,
improving soil quality - important to sustained soil and
crop productivity and maintaining the environment
Component of Biological Soil Management that is linked
to Soil Quality
“Soil quality is considered as the major linkage between
conservation management practices and achievement of
major goals of sustainable agriculture”
Doran et al. 1999. Determinants of Soil Quality and Health. Pp. 17-36 in R. Lal (ed.) Soil
Quality and Soil Erosion. CRC Press, Boca Raton, FL.
GOAL:
STRATEGY:
Cover Cropping
Residue management
SOIL
QUALITY
Organic recycling
Integrated biological mgt.
Parr et al. 1992. Am. J. Altern. Agr. 7:5-11
Sustainable Agriculture
Resource conservation
Environmental health
Pest suppression
THANK YOU!