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Effects of Carbon Source on Immobilization and Chemical Distribution of Fertilizer Nitrogen in Soil¹

ABSTRACT

Three C sources (glucose, a phenolic glycoside, and catechol) were added to soil along with ¹⁵N-labeled (NH₄)₂SO₄ in an incubation study to determine effects of aliphatic and phenolic substrates on transformations of fertilizer N. Microbial utilization of the C substrates resulted in a rapid depletion of available mineral N, with maximum immobilization of soil and tracer N after 6, 10, and 20 d in the glucose-, phenolic glycoside-, and catechol-amended soil, respectively. A close relationship was observed between clay-fixed nonexchangeable ¹⁵NH⁺₄ and KCl-extractable (exchangeable + soluble) ¹⁵NH⁺₄ during incubation (r² = 0.91**). The nonexchangeable ¹⁵NH⁺₄ was readily released and immobilized during microbial utilization of the added substrates. Availability ratios indicated that the immobilized ¹⁵N at the point of maximum incorporation into the microbial biomass was from 3.5 to 8.0 times as susceptible to mineralization as the native soil N. Net mineralization of ¹⁵N was accompanied by a decrease in amino acid-¹⁵N, an indication that synthesis and decomposition of microbial protein were closely related to the level of available mineral N. Higher levels of acid-insoluble ¹⁵N in the phenolic glycoside- and catechol-amended soil were attributed to condensation reactions involving amino compounds and substrate-derived polyphenols. Overall, microbial transformations were more important than substrate effects in determining the distribution of immobilized ¹⁵N after 120 d of incubation.

¹ Contribution from the Agric. Res. Branch, NFDC-TVA, in cooperation with the Dep. of Agronomy, Univ. of Illinois, Urbana.

² Soil Chemist, F-137 NFDC, Muscle Shoals, AL 35660, and Professor, Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801, respectively.

Received for publication November 4, 1986.