The differential assimilation of nitrogen fertilizer compounds by soil microorganisms

The differential soil microbial assimilation of common nitrogen (N) fertilizer compounds into the soil organic N pool is revealed using novel compound-specific amino acid (AA) ¹⁵N-stable isotope probing. The incorporation of fertilizer ¹⁵N into individual AAs reflected the known biochemistry of N assimilation—e.g. ¹⁵N-labelled ammonium (¹⁵NH₄⁺) was assimilated most quickly and to the greatest extent into glutamate. A maximum of 12.9% of applied ¹⁵NH₄⁺, or 11.7% of ‘retained’ ¹⁵NH₄⁺ (remaining in the soil) was assimilated into the total hydrolysable AA pool in the Rowden Moor soil. Incorporation was lowest in the Rowden Moor ¹⁵N-labelled nitrate (¹⁵NO₃⁻) treatment, at 1.7% of applied 15N or 1.6% of retained 15N. Incorporation in the ¹⁵NH₄⁺ and ¹⁵NO₃⁻ treatments in the Winterbourne Abbas soil, and the ¹⁵N-urea treatment in both soils was between 4.4% and 6.5% of applied 15N or 5.2% and 6.4% of retained 15N. This represents a key step in greater comprehension of the microbially mediated transformations of fertilizer N to organic N and contributes to a more complete picture of soil N-cycling. The approach also mechanistically links theoretical/pure culture derived biochemical expectations and bulk level fertilizer immobilization studies, bridging these different scales of understanding.