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Transformations of Organic Matter Solubilized by Anhydrous Ammonia

Rice Research and Extension Center, Univ. of Arkansas, P.O. Box 351, Stuttgart, AR 72160

J. T. Gilmour and P. M. Gale

Dep. of Agronomy, Univ. of Arkansas, Fayetteville, AR 72701

*Corresponding author.

ABSTRACT

While it has been established that liquid anhydrous NH₃ (LAA) solubilizes soil organic C, the transformations of the solubilized organic C have not been defined. In order to determine the mechanisms of these transformations, LAA was applied to a Drummer silty clay loam (Typic Haplaquoll) in the laboratory. At 1, 5, 12, 20 and 35 d after LAA application a set of LAA-treated and nontreated (control) samples were analyzed for soil inorganic N, NH₃-N volatilized, soil pH, water-soluble organic C (WSOC) and CO₂-C evolved. Biomass C and total hydrolyzed C were estimated. The WSOC concentration increased from 25 mg C kg^–1 prior to LAA application to 282 mg C kg^–1 on the first day after LAA application. By 35 d after LAA application, the WSOC had decreased to 89 mg C kg^–1. Dissimilation, CO₂-C evolved, was an important conversion mechanism for the organic-C solubilized by the LAA. The LAA-treated soil had a cumulative CO₂-C production of 630 mg C kg^–1 compared to about 200 mg C kg^–1 for the control soil over the 35 d after LAA application. Estimations of assimilatory conversion of WSOC to biomass C at microbial efficiencies (E) ranging from 0 to 0.6 indicated that assimilatory conversions could range from 0.25 to 1.5 times the dissimilatory (CO₂-C) conversions, respectively. Total hydrolyzed (CO₂ + biomass + WSOC)-C directly attributable to the LAA at efficiencies of 0 and 0.4 were 445 mg C kg^–1 and 701 mg C kg^–1, respectively. In a separate experiment it was determined that soil pH had no significant effect on the decrease in WSOC concentration. Consequently, transformations of WSOC were attributed to the microbial processes of assimilation and dissimilation.

Contribution from the Univ. of Arkansas Rice Research and Extension Center and the Dep. of Agronomy, Univ. of Arkansas, Fayetteville, AR 72701. Project ARK-01047, Published with the approval of the Director of the Arkansas Agric. Exp. Stn. Supported in part by the Tennessee Valley Authority.

Received for publication July 6, 1987.