Polyacrylamide as an organic nitrogen source for soil microorganisms with potential effects on inorganic soil nitrogen in agricultural soil

Linear polyacrylamide (PAM) is gaining considerable acceptance as an effective anti-erosion additive in irrigation water. The potential effects of repeated PAM application on soil microbial ecology and the potential for biotransformation of this polymer in soils are not completely known. Untreated and PAM-treated soils (coarse-silty, mixed, mesic Durixerollic Calciorthids) were collected from agricultural fields near Kimberly, ID. Soils were analyzed to determine the effects of PAM treatment on bacterial counts and inorganic N concentrations and the potential for PAM biotransformation. Culturable heterotrophic bacterial numbers were significantly elevated in PAM-treated soil for the plot planted to potatoes; this effect was not observed in the plot planted to dry pink beans. Total bacterial numbers, determined by AODC, were not altered by PAM treatment in any of the soils sampled. Polyacrylamide-treated soil planted to potatoes contained significantly higher concentrations of NO₃/^- and NH₃ (36.7 ± 2.20 and 1.30 ± 0.3 mg kg^-1, respectively) than did untreated soil (10.7 ± 2.30 and 0.50 ± 0.02 mg kg^-1, respectively). For bean field soil there was no difference between treated and untreated soil inorganic N concentrations. Enrichment cultures generated from PAM-treated and untreated soils utilized PAM as sole N source, but not as sole C source. While the monomeric constituents of PAM, acrylamide and acrylic acid, both supported bacterial growth as sole C source, the PAM polymer did not. Enrichment cultures that used PAM for N exhibited amidase activity specific for PAM as well as smaller aliphatic amides. Utilization of PAM for N, but not for C, indicates that ultimately PAM may be converted into long chain polyacrylate, which may be further degraded by physical and biological mechanisms or be incorporated into organic matter.