Historical US Corn Grain Yields
Do you think
Bouma et al. (2016) documented large differences in soil C and function for Dutch fields
mapped as the same soil type but w/ long-term differences in management.
Soil genotype vs. phenotype???
crops crops grassland
1.7% OM 3.3% OM 5.0% OM
Soil Changes After Sixty Years of Land Use in Iowa
Jessica Veenstra, Iowa State University, 1126 Agronomy Hall, Iowa State
University, Ames, IA 50010
Soils form slowly, thus on human time scales, soil is essentially a non-renewable
resource. Therefore in order to maintain and manage our limited soil resources
sustainably, we must try to document, monitor and understand human induced
changes in soil properties. By comparing current soil properties to an archived
database of soil properties, this study assesses some of the changes that have
occurred over the last 60 years, and attempts to link those changes to natural and
human induced processes. This study was conducted across Iowa where the
primary land use has been row crop agriculture and pasture. We looked at
changes in A horizon depth, color, texture, structure, organic C content and pH.
Hill top and backslope landscape positions
have been eroded & now have thinner topsoil w/ less SOM
Catchment areas now have deeper topsoil w/ more SOM
Veenstra and Burras resampled 82 soil profiles on IA
farms with historical descriptions and classified the
soils according to the US, Canadian and FAO-WRB
taxonomic systems. 11 to 33% of the pedons originally
classified as Black soils (e.g., Prairie soils)
no longer classified as Black soils.
The current amount of OM in a soil
= the long-term balance between organic
matter inputs and outputs
grown in place,
imported from off-farm
lost to erosion
So why hasn’t modern agriculture had a
more favorable impact on SOM?
grain, stover and roots each comprise
~1/3rd of the total biomass
A 200 bu/a
of roots and
to ~ 1 %
weight of an
High yielding grain crops return lots of residues
Living organisms Microbial
often > 75%
Most of the C in
This transformation is a major
focus of current SOM research
Practices that enhance crop yield
also impact a soil’s metabolism
When there is
I eat more!!
Have you read this classic commentary?
Journal of Soil and Water Conservation - 1995
What are the symptoms of
the HIGH CARB DIET
that most US soils
There has been a surge in SOM research
since DeLuca’s commentary –
what have we learned???
The next slides review
some of the most
We have known that SOM is a complex mixture
of living, dead and very dead OM
for a LONG TIME
Adapted from Magdoff and Weil (2003)
The authors present a conceptual framework -
the microbial carbon pump (MCP) to demonstrate how
microorganisms are an active player in soil C storage.
The MCP couples microbial production of organic
compounds to their further stabilization, which the
authors define as the entombing effect (EE).
Nature Microbiology - 2017
Microbes take apart organic matter but they also synthesize it
ex vivo modification = extracellular enzymes attack and transform
plant residues, resulting in accumulation of plant-derived compounds
that are not readily assimilated by microorganisms
in vivo turnover = cell uptake of organic substrates –> biosynthesis
resulting in deposition of compounds produced by microbes
Through these 2 pathways, compounds are produced that are more
resistant to further degradation and/or more readily stabilized by
interactions with the soil mineral matrix.
A growing body of evidence indicates that the in vivo pathway
is MORE important for C stabilization than ex vivo modification
2 major pathways by which microorganisms
influence C stabilization
Net stabilization of C when EE > PE
Priming effect =
The long-term storage of organic forms of
N in soils was classically attributed to
chemical complexity of plant residues that
retarded microbial degradation. Recent
advances have revised this framework,
with the understanding that persistent soil
organic N consists largely of chemically
labile, microbially processed organic
compounds. Chemical bonding to
minerals and physical protection in
aggregates are more important to long-
term preservation of these organic
compounds than molecular complexity…
Soil fungi can form mineral-stabilized SOM
not only by modifying SOM but also by
synthesizing mineral surface reactive metabolites
Adapted from Carter (2002)
mineral protected OM
What regulates the
abundance of these
Can the protective
Compartments of varying connectivity
between SOM and the mineral matrix
physically protected OM
Geoderma - 2016
The proportion of residues physically
protected within aggregates decreases and
priming effects increase when aboveground
C inputs increase - leading to lower rates
of long-term C stabilization when
aboveground residue additions are high.
of why high crop
yields often do NOT
result in much
change in SOM???
High above ground residue inputs = low efficiency of SOM stabilization
(Shahbaz et al., 2016)
Functionally diverse perennial and cover
cropped rotations increased both C input
and SOC concentrations, potentially by
exploiting niches in time that would
otherwise be unproductive, that is,
increasing the “perenniality” of
Permanganate oxidizable C better reflected
practices that promote organic matter
accumulation or stabilization and therefore
can be a useful indicator of long-term
soil C sequestration.
Conversely, mineralizable C (aka soil
respiration during a short incubation) better
reflected practices that promote organic
matter mineralization and therefore can be a
useful indicator of short-term soil nutrient
Measuring what matters = understanding connection between form and function
CO2 burst test
(CO2 production for 24 hrs
after wetting dry soil)
In fall 2017, students in my Soil Properties class brought in paired soils
(Crop field vs Fence Row) from their farms. In all cases, the fence row
soils had higher soil respiration in 24 hrs after wetting.
The SituResp method is a reliable method for
performing a $low cost$ assessment of soil
microbial activity that could be used for soil
< 1/10th the cost of SOLVITA
(Thoumazeau et al, 2017)
Cuvettes containing pH indicator Cresol Red in agar gel can
provide a cheap yet sensitive measurement of respiration
A standardized test of soil digestive ability???
Research based on alkali extraction assumes that
large recalcitrant humus molecules are created by a
process of humification; however, these humic
substances appear to be artifacts of the extraction
process and have not been observed in soil using
modern analytical techniques.
Nature - 2015 High profile rejection of
traditional humus concepts
At the next Soil Science Society of America meeting
(January 2019 in San Diego), there will be a day-
long special session focused on whether soil humic
research has any meaning/value.
There will be a morning debate between proponents
and opponents of humic acid extraction and
characterization and afternoon poster session and
oral session for volunteered presentations.
The two sides will each write a review paper using a
shared theme for publication in the Journal of
Environmental Quality, and likely more review
papers will follow.
personal communication w/ Dan Olk
• We analyzed data from 60 published studies and
global databases with > 50,000 measurements.
• A 1% increase in C increased plant available water by
only 1.2% of soil volume on average.
European Journal of Soil Science - 2017
water in the
top 3 feet =
1/36” = 2.8%
Everyone knows that SOM increases plant available water but is this true?
Model developed using data from > 2000 soils
useful tool for teaching soil water concepts
(infiltration & percolation)
and rooting depth/volume
(from which crops acquire water)
are far more responsive to↑C
than water holding capacity
Saturation of capacity
(Dick and Gregorich, 2004)
Impact on synthesis
and stabilization of
Fields or parts of fields with the lowest OM content
(relative to their potential) will benefit the most from
practices that build SOM.
Is there a practical way to predict a soils potential?
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