integrating soil carbon stabilization concepts and ... integrating soil carbon stabilization...
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• In the U.S. Corn Belt, soil organic matter (SOM) is typically the largest source of N
for corn (Zea mays L.) and the largest sink of fertilizer N. However, there is
tremendous variation in N mineralization from SOM and inorganic N retention in
SOM among individual crop fields, making site-specific optimal N rates difficult to
• Research suggests that protected SOM pools have a finite capacity to store
added C (Fig 1A). Because soil C and N concentrations are well-correlated and
biologically-linked, the concept of C saturation may also apply to soil N.
• We hypothesize that a decreasing proportion of fertilizer N will be retained in SOM
as stable pools become saturated, causing a greater proportion to become
available for crop uptake (Fig 1B).
Integrating Soil Carbon Stabilization Concepts and Nitrogen Cycling Hanna J. Poffenbarger1, John Sawyer1, Daniel Barker1, Daniel Olk2, Johan Six3, and Michael J. Castellano1
1Agronomy Department, Iowa State University 2 National Laboratory for Agriculture and the Environment, USDA-ARS
3Department of Environmental Systems Science, ETH-Zurich
Fig 1A. Theoretical behavior of protected and
unprotected soil organic C (SOC) pools as a
function of C inputs at steady state; adapted
from Stewart et al. 2007. As C inputs increase:
• The amount of C stored in protected pools
reaches a plateau (solid black curve) and
• Carbon inputs not transferred to protected
pools remain in unprotected pools (dashed
The red line indicates the saturation level of
protected pools. Unsatisfied storage capacity is
termed “saturation deficit”.
Fig 1B. Conceptual model of N retention and N
mineralization as a function of the saturation
deficit (Castellano et al. 2012). As protected
pools become saturated:
• The proportion of N inputs transferred to
protected pools decreases (green line),
• More N remains in unprotected pools where it
is more readily available to plants (red line),
• The C/N ratio of unprotected pools decreases
(N & C Saturation)
Total Soil Organic Carbon in Protected Pools (large ← Soil C Saturation Deficit → small)
(g N kg-1 N inputs)
(g N kg-1 soil)
R e la
ti v e U
n it s
Protected Pools (ie. g silt + clay C kg-1 soil;
g micro-agg POM C kg-1 soil) C a rb
o n i n e
a c h P
o o l
Carbon Inputs at Steady State
• Two 2-m2 subplots were established within replicate continuous corn plots for all N
fertilizer rates at both sites.
• One subplot received the agronomic optimum N rate with 15N so that the
fertilizer can be traced into various soil and plant pools.
• The second subplot received zero N fertilizer.
Soil organic matter gradients
• Different N rates applied to continuous corn within long-term N fertilization trials
have imparted differences in organic matter inputs to soil (Fig. 2).
• At the Ames site, SOC reached a plateau with increasing N rates and residue
inputs, reflecting SOC saturation.
• At the Chariton site, SOC increased linearly with increasing N rates and residue
Objective and research questions
• To determine how SOC storage capacity affects inorganic N retention, N
mineralization, and corn N use efficiency.
1. Can SOC saturation deficit explain variation in inorganic N retention and
2. Does SOC saturation deficit affect fertilizer N use efficiency in corn?
Fig 2. Soil organic C
concentration as influenced by
long-term N fertilizer rates and
average annual aboveground
residue inputs at two sites in
Iowa, USA. Error bars represent
± one standard error. Curves are
asymptotic C saturation models
or linear models fit to the data
(Stewart et al. 2007). Vertical
dashed lines represent
agronomic optimum N rates for
Stewart, C.E., K. Paustian, R.T.
Conant, A.F. Plante, and J. Six.
2007. Soil carbon saturation:
concept, evidence and evaluation.
Biogeochemistry 86: 19–31.
Castellano, MJ, JP Kaye, H Lin, and JP Schmidt. 2011.
Linking Carbon Saturation Concepts to Nitrogen
Saturation and Retention. Ecosystems 15: 175–187.
Stewart, C.E., K. Paustian, R.T. Conant, A.F. Plante,
and J. Six. 2007. Soil carbon saturation: concept,
evidence and evaluation. Biogeochemistry 86: 19–31.
• Soil samples (0-15 cm) were collected at the corn fifth-leaf stage and analyzed for
inorganic N (NH4 +-N + NO3
• Corn grain yields were collected at corn physiological maturity.
This project is supported by the William T. Frankenberger Professorship and the Agriculture and
Food Research Initiative Competitive Grant no. 2014-67019-21629 from USDA National Institute of
Food and Agriculture.
Soil inorganic N concentrations and grain yields
• In the zero-N subplots at Chariton, inorganic N concentrations at the fifth-leaf stage
and grain yields were positively related to historic N rates and residue inputs.
Otherwise, long-term N rates had little effect on soil inorganic N concentrations and
grain yields in the zero-N and optimum-N subplots.
Fig 3. Soil inorganic
(left axis, green
points) and grain
yields (right axis,
black points) in
response to historic
N fertilizer rates
and average annual
residue inputs at two
sites in Iowa, USA.
Error bars represent
± one standard
error. Linear models
were fit to the
than zero (P250 µm)
• Fine POM outside microaggregates (>53 µm)
• Microaggregate POM (>53 µm)
• Mineral-associated SOM (