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.

• 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 CyclingHanna J. Poffenbarger1, John Sawyer1, Daniel Barker1, Daniel Olk2, Johan Six3, and Michael J. Castellano1

1Agronomy Department, Iowa State University2 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

curve).

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),

and

• The C/N ratio of unprotected pools decreases

0%

100%

Max. Capacity

(N & C Saturation)

Total Soil Organic Carbon in Protected Pools(large ← Soil C Saturation Deficit → small)

N Inputs

Transferred to

Protected Pools

(g N kg-1 N inputs)

Unprotected Pool

(g N kg-1 soil)

and

Net Nitrification

Rela

tive U

nits

B.

Protected Pools(ie. g silt + clay C kg-1 soil;

g micro-agg POM C kg-1 soil) Carb

on in e

ach P

ool

(g k

g-1

so

il)

Carbon Inputs at Steady State

A.

SaturationDeficit

Experimental set-up

• 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.

Approach

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

inputs.

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

mineralization?

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

each site.

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.

Measurements

• Soil samples (0-15 cm) were collected at the corn fifth-leaf stage and analyzed for

inorganic N (NH4+-N + NO3

--N) concentration.

• 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.

Results

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

N concentrations

(left axis, green

points) and grain

yields (right axis,

black points) in

response to historic

N fertilizer rates

and average annual

aboveground

residue inputs 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<0.10).

Conclusions

• Our preliminary results suggest that the SOM gradient generated by long-term N

fertilizer rates had little effect on N availability to corn.

Future Work

• Soil samples collected at the corn fifth-leaf stage, silking, and physiological

maturity will be fractionated into protected and unprotected SOM pools:

• Unprotected

• Coarse particulate organic matter (POM; >250 µm)

• Fine POM outside microaggregates (>53 µm)

• Protected

• Microaggregate POM (>53 µm)

• Mineral-associated SOM (<53 µm)

• Corn tissue and each SOM pool will be analyzed for organic C and N

concentrations and 15N abundance.