Micheal Castellano

1. Introduction
• 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 predict.
• While many studies have investigated the relationship between the size and quality of
rapid turnover SOM pools and N mineralization, few have investigated the role of the
SOM stabilization capacity in mediating N availability.
• Research suggests that pools of physico-chemically stabilized C can have a finite
capacity to store additional 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 as physico-chemically stabilized SOM pools become saturated, a
decreasing proportion of fertilizer N will be retained in stable SOM pools and more will
remain available for crop uptake (Fig 1B).
Integrating Soil Carbon Stabilization Concepts and Nitrogen Cycling
Hanna J. Poffenbarger1, John Sawyer1, 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 stable and
non-protected soil organic C (SOC) pools as
a function of C inputs; adapted from Stewart
et al. 2007. As C inputs increase:
• The amount of C stored in stable pools
reaches a plateau and
• Carbon inputs not transferred to stable
pools remain in non-protected pools.
The red line indicates the saturation level of
stable 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 stable pools
become saturated:
• The proportion of N inputs transferred to
stable pools decreases (green line),
• More N remains in non-protected pools
where it is more readily available to plants
(red line), and
• The C/N ratio of non-protected pools
decreases
Experimental set-up
• Two 7-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.
Approach
Soil organic matter gradients
• In two long-term N fertilization trials, the same N rates applied to the same continuous
corn plots for 15 years have resulted in a range of residue inputs to soil, providing a
gradient of SOM saturation in which to study N cycling (Fig 2).
Objective and research questions
• To determine how phyico-chemical C storage mechanisms, C storage capacity,
and C storage kinetics interact to affect inorganic N retention and crop N uptake.
1. Can SOM saturation deficit explain variation in inorganic N retention and N
mineralization?
2. Can SOM saturation deficit affect fertilizer N use efficiency in corn and offer a new
way to interpret variation in agroecosystem N retention and mineralization?
Fig 2. Annual
aboveground residue
inputs in response to
long-term N fertilizer
rates (left panels) and
SOC in response to
average annual
aboveground residue
inputs (right panels) at
two sites in Iowa, USA.
Error bars represent ±
one standard error. A
quadratic or quadratic-
plateau model was fit
to residue input in
response to historic N
rate. A saturation
model was fit to SOC
in response to residue
inputs.
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.
Measurements
• Soil samples (0-15 cm) were collected at the corn fifth-leaf stage, and additional
samples will be collected at silking and physiological maturity.
• Corn biomass samples and grain yield will be collected at corn physiological
maturity.
• Soil samples will be analyzed for inorganic N (NH4
+-N + NO3
--N) concentrations
and fractionated into stable and non-protected SOM pools:
• Stable
• Microaggregate-occluded particulate organic matter
(POM; >53 µm)
• Mineral-associated SOM (<53 µm)
• Non-protected
• Coarse POM (>250 µm)
• Fine POM outside microaggregates (>53 µm)
• Corn tissue and each SOM pool will be analyzed for organic C and N
concentrations and 15N abundance.
• Fertilizer N Use Efficiency (FNUE) will be calculated using the “difference” and
“direct” methods.
• The difference method calculates FNUE as the difference in corn N
content in the fertilized and zero-N subplots divided by the fertilizer N
applied.
• The direct method is based on the proportion of 15N applied that is
recovered in corn aboveground biomass.
This project is supported by Agriculture and Food Research Initiative Competitive Grant no. 2014-
67019-21629 from USDA National Institute of Food and Agriculture.
Preliminary findings
Soil inorganic N concentrations
• In the zero-N subplots at Chariton, inorganic N concentrations at the fifth-leaf stage were
positively related to historic N rates. Otherwise, long-term N rates had little effect on soil
inorganic N concentrations in the zero-N and optimum-N subplots at the fifth-leaf stage.
Fig 3. Soil inorganic N
concentrations in response
to historic N fertilizer rates
and average annual
aboveground residue
inputs. Measurements
were taken at the fifth-leaf
stage in the zero-N and
optimum-N subplots at two
sites in Iowa, USA. Error
bars represent ± one
standard error. Linear
models were fit to the
relationships when slopes
were significantly different
than zero (P<0.05).
0%
100%
Total Soil Organic Matter in Stable Pools
(large ← Soil C Saturation Deficit → small)
N Inputs
Transferred to
Stable Pools
(g N g-1 N inputs)
Non-protected Pool
(g N kg-1 soil)
and
Net Nitrification
RelativeUnits
B.
A.
𝑦 = 3.63 + 0.058𝑥 − 0.00014𝑥2
𝑖𝑓 𝑥 < 183
𝑦 = 9.69 𝑖𝑓 𝑥 > 183
𝑦 = 1.78 + 0.034𝑥 − 0.000043𝑥2
𝑦 =
𝑥
0.020 + ( 𝑥
48.49)
𝑦 =
𝑥
0.0041 + ( 𝑥
43.16)
𝑦 = 11.85 + 0.018𝑥 𝑦 = 10.50 + 0.73𝑥