Microbial utilization of heterocyclic nitrogen from atrazine

Radiotracer studies have provided indirect evidence that heterocyclic N in atrazine [2-chloro-4(ethylamino)-6-(isopropylamino)-1,3,5-triazine] may be utilized by microorganisms as a source of N; however, no data are available from investigations using ¹⁵N as a tracer. Studies were conducted to examine microbial utilization of atrazine ring-¹⁵N in soil and pure cultures of Pseudomonas sp. strain ADP, Agrobacterium radiobacter J14a, or bacterium M91-3. Atrazine [U-¹⁵N-ring] was synthesized from labeled urea. To investigate the effect of exogenous N on atrazine degradation, pure cultures were supplemented with unlabeled atrazine and labeled N as (NH₄)₂SO₄, KNO₃, urea, or glycine. Under C-limiting conditions, cells of Pseudomonas sp. strain ADP incorporated side chain-N into biomass and liberated ring-N as NH₄. When N was limiting, ring-N was also recovered in biomass. Degradation of atrazine by Pseudomonas strain sp. ADP and A. radiobacter was unaffected by the presence of exogenous N, whereas no degradation occurred with bacterium M91-3 in media containing urea or NH₄-N. The fate of double-labeled [U-¹⁴C, U-¹⁵N-ring]-atrazine (15 mg kg^-1) was examined after incubation of a Bloomfield soil (sandy, mixed, mesic Psammentic Hapludalf) amended with NH₄- or NO₃-N (75 mg kg^-1), with or without an inoculum of Pseudomonas sp. strain ADP (1.06 × 10¹³ cells kg^-1). In uninoculated soil, mineralization of atrazine-¹⁴C was inhibited by inorganic N, whereas in the inoculated soil, 87% of the atrazine was mineralized, regardless of the N treatment. The C/N ratio was much lower for amino acids isolated from soil than for the atrazine ring, suggesting preferential assimilation of N over C by microorganisms.