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Fertilization Effects on Physically Protected Light Fraction Organic Matter

Research Branch, Agriculture & Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6

C. F. Drury

Research Branch, Agriculture & Agri-Food Canada, Harrow, Ontario, Canada N0R 1G0

B. H. Ellert

Research Branch, Agriculture & Agri-Food Canada, Lethbridge, Alberta, Canada T1J 4B1

*Corresponding author (gregoriche{at}em.agr.ca).

ABSTRACT

Aggregation influences the decomposition of soil organic matter by limiting the accessibility of C substrate to microbes and fauna. We evaluated whether the turnover of light fraction organic matter (LF-OM) in arable systems was affected by its location in the soil matrix and whether long-term fertilization affected LF-OM turnover. Organic C and natural ¹³C abundance were measured on the free and physically protected LF-OM of soils that had been under continuous corn (Zea mays L.) for 32 yr. Fertilized corn soils had >2.5 times as much free LF-OM material as their nonfertilized counterparts; most of this LF-OM was derived from corn (a C₄ plant). In contrast, most of the physically protected LF-OM was derived from C₃ vegetation, indicating that it had been incorporated into aggregates before the start of the experiment. Assuming that organic matter turnover follows first order kinetics, the estimated half-life of C₃-C in the physically protected LF-OM was twofold longer than that in the free LF-OM. These data indicated that fertilization did not affect the turnover of either the free or the physically protected LF-OM; however, the location of organic matter in soil aggregates was a key factor in its susceptibility to decomposition.

Received for publication January 17, 1996.

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