The differential assimilation of nitrogen fertilizer compounds by soil microorganisms

Abstract

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.

Keywords: 15N stable isotope tracing (SIP); amino acids; ammonium; immobilization; nitrate; urea.

Figure 1.

Time–course plots of AA Δ¹⁵N values revealing ¹⁵N assimilation into individual AAs in the six treatments. (A) RM-¹⁵NO₃⁻ , (B) WA-¹⁵NO₃⁻ (error bars at t = 16 and 32 days are coloured to aid differentiation), (C) RM-¹⁵NH₄⁺, (D) WA-¹⁵NH₄⁺, (E) RM-¹⁵N-U, and (F) WA-¹⁵N-U. RM and WA refer to the two different soils from the two sites, RM and WA and the three amendments were potassium nitrate (K¹⁵NO₃), ammonium chloride (¹⁵NH₄Cl), and urea (CO(¹⁵NH₂)₂). Error bars are ± SE (n = 3). See Supplementary Fig. S3. For individual figures for each AA in each treatment for additional clarity.

Figure 2.

Time–course plots of AA % ¹⁵N_R incorporations revealing ¹⁵N assimilation into individual AAs in the six treatments, alongside pie charts of the relative percentage of retained ¹⁵N in each AA pool this represented, based on the plateau partitioning of ¹⁵N in each total hydrolyzable AA pool (derived from simple exponential regressions of the % ¹⁵N_R incorporated into AAs over time; Equation 1). (A) RM-¹⁵NO₃⁻ (error bars for Ala and Gly are coloured to aid differentiation), (B) WA-¹⁵NO₃⁻ (error bars for Glu, Asp, and Ala are coloured to aid differentiation), (C) RM-¹⁵NH₄⁺, (D) WA-¹⁵NH₄⁺, (E) RM-¹⁵N-U, and (F) WA-¹⁵N-U. Error bars are ± SE (n = 3). Adapted from Charteris (2019).

Figure 3.

Percentage of ¹⁵N incorporated into the total hydrolyzable AA pool for all treatments, labelled with the plateau % ¹⁵N_R incorporations determined by simple exponential regressions. (A) Percentage of applied ¹⁵N and (B) Percentage of the ¹⁵N still present in the soil or ‘retained’ at that time. Error bars are ± SE (n = 3), the error bars of the WA-¹⁵NO₃⁻ treatment are highlighted in red as the bar at t = 32 days is large and otherwise difficult to distinguish. Adapted from Charteris (2019).