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  • 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 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

    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

    R e la

    ti v e U

    n it s

    B.

    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

    (g k

    g -1

    s o

    il)

    Carbon Inputs at Steady State

    A.

    Saturation Deficit

    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 (P250 µm)

    • Fine POM outside microaggregates (>53 µm)

    • Protected

    • Microaggregate POM (>53 µm)

    • Mineral-associated SOM (

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